Multiple Spatial Representations Determine Touch ... - PsycNET

Journal of Experimental Psychology: Human Perception and Performance 2014, Vol. 40, No. 2, 784 – 801

© 2013 American Psychological Association 0096-1523/14/$12.00 DOI: 10.1037/a0034690

Multiple Spatial Representations Determine Touch Localization on the Fingers Stephanie Badde, Brigitte Röder, and Tobias Heed

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

University of Hamburg Touch location can be specified in different anatomical and external reference frames. Temporal order judgments (TOJs) in touch are known to be sensitive to conflict between reference frames. To establish which coordinates are involved in localizing touch to a finger, participants performed TOJ on tactile stimuli to 2 out of 4 possible fingers. We induced conflict between hand- and finger-related reference frames, as well as between anatomical and external spatial coding, by selectively crossing 2 fingers. TOJ performance was impaired when both stimuli were applied to crossed fingers, indicating conflict between anatomical and external finger coordinates. In addition, TOJs were impaired when stimuli were mapped to the same hand based on either anatomical or external spatial codes. Accordingly, we observed a benefit rather than impairment with finger crossing when both stimuli were applied to 1 hand. Complementary, participants systematically mislocalized touch to nonstimulated fingers of the targeted hand. The results indicate that touch localization for the fingers involves integration of several sources of spatial information: the anatomical location of the touched finger, its position in external space, the stimulated hand, and the hand to which the touch is (re)mapped in external space. Keywords: tactile, reference frames, remapping, body representation, coordinate conflict

(Clemens, De Vrijer, Selen, Van Gisbergen, & Medendorp, 2011; Holmes & Spence, 2004; Lloyd, Shore, Spence, & Calvert, 2003; Maravita, Spence, & Driver, 2003). This computation is referred to as tactile remapping (Driver & Spence, 1998). Tactile remapping has often been investigated with the temporal order judgment (TOJ) task (Sternberg & Knoll, 1973). In this task, participants have to indicate the temporal order of two tactile stimuli that are applied to different locations, usually one to each hand, in rapid succession. When participants adopt a “normal,” uncrossed hand posture, the minimal time interval between the stimuli required for judgments to be correct (termed just noticeable difference, JND), is about 40 – 60 ms (Shore, Spry, & Spence, 2002). When the hands are held in a crossed posture, however, performance is severely impaired, with the JND rising up to 4 times that of the uncrossed posture (Yamamoto & Kitazawa, 2001; Shore et al., 2002). This crossing effect has been attributed to the incongruence of the anatomical and external spatial coordinates of the tactile stimuli in the crossed posture (Azañón & Soto-Faraco, 2007; Cadieux, Barnett-Cowan, & Shore, 2010; Craig & Belser, 2006; Heed, Backhaus, & Röder, 2012; Kitazawa et al., 2008; Kóbor, Füredi, Kovács, Spence, & Vidnyánszky, 2006; Roberts & Humphreys, 2008; Röder, Rösler, & Spence, 2004; Schicke & Röder, 2006; Shore et al., 2002; Soto-Faraco & Azañón, 2013; Takahashi, Kansaku, Wada, Shibuya, & Kitazawa, 2013; Wada et al., 2012; Wada, Yamamoto, & Kitazawa, 2004; Yamamoto & Kitazawa, 2001). For instance, with the hands crossed, a tactile stimulus to the right hand (anatomical coordinates) is located in the left hemifield (external coordinates). Whereas there seems to be general agreement about the crossing effect being an indicator of tactile remapping (Heed et al., 2012; Röder et al., 2004; Shore et al., 2002; Yamamoto & Kitazawa, 2001), the functional mechanism causing the effect remains debated. Shore et al. (2002) suggested that the crossing effect results

When asked to describe a touch to their body, most people will refer to the body part at which they felt the sensation. At first sight, this description contains the most relevant information about the touch, namely, where on the body it occurred. The brain’s homuncular organization—that is, the spatial clustering of neurons according to the body’s surface in the primary and secondary somatosensory cortices—appears to reflect this fact (Disbrow, Roberts, & Krubitzer, 2000; Penfield & Boldrey, 1937; Yang, 1993). Yet, the location of a touch on the skin does not sufficiently describe its spatial location. For example, to explore the cause of the touch sensation, one might want to inspect it visually. To direct the eyes to the origin of the touch, the current posture of the touched body part must be taken into account (Buchholz, Jensen, & Medendorp, 2011; Groh & Sparks, 1996; Overvliet, Azañón, & Soto-Faraco, 2011). Therefore, to direct gaze to the touch, the brain must compute the touch’s external spatial coordinates from the two-dimensional coordinates on the skin (referred to hereafter as anatomical coordinates) and from posture information, which derives from vision, proprioception, and the vestibular system

This article was published Online First December 23, 2013. Stephanie Badde, Brigitte Röder, and Tobias Heed, Biological Psychology and Neuropsychology, University of Hamburg, Hamburg, Germany. This work was supported by the German Research Foundation (DFG) through the German-Chinese Research Training Group CINACS, DFG GRK 1247/1 and 1247/2. T.H. is supported by the Emmy Noether Program of the DFG. We thank J. Möller and S. Röper for help with data acquisition. Correspondence concerning this article should be addressed to Stephanie Badde, Biological Psychology and Neuropsychology, University of Hamburg, Von-Melle-Park 11, 20146 Hamburg, Germany. E-mail: stephanie [email protected] 784


MULTIPLE SPATIAL REPRESENTATIONS OF TOUCH

from the conflict between the anatomical and external coordinates of touch. More specifically, these authors proposed that solving the reference frame conflict prolongs processing of the tactile stimuli, which, in turn, hampers the temporal order judgment about the two stimuli (Cadieux et al., 2010). In accordance with the hypothesis of coordinate conflict, it has been shown that both reference frames are concurrently maintained in the brain (Buchholz et al., 2011; Heed & Röder, 2010). In contrast, Yamamoto and Kitazawa (2001; see also Kitazawa, 2002) have suggested that TOJ can only be made on the remapped, external representations of the tactile stimuli. More recently, these authors have introduced the motion projection hypothesis (Kitazawa et al., 2008). In short, they proposed that the crossing effect arises because of the incorrect reconstruction of the temporal order of the two stimuli based on apparent tactile motion in the wrong direction. This motion vector is constructed based on the (external) spatial coordinates of the two stimuli that are initially mapped to the wrong hand. Consistently, recent fMRI results demonstrated activation near visual motion areas (perisylvian cortex) during tactile TOJ in crossed and uncrossed postures, but not in a tactile numerosity judgment task (Takahashi et al., 2013). Finally, some authors have referred to the idea of a default posture representation of the body to explain TOJ crossing effects (e.g., Azañón, Longo, Soto-Faraco, & Haggard, 2010; Longo, Azañón, & Haggard, 2010), which are suggested to arise from additional processing demands related to deviation of the body from its default posture, such as the hands lying in the contralateral hemifield. Crossing effects in TOJ are not restricted to the hands. For example, Schicke and Röder (2006) reported crossing effects between hands and feet. More recently, Heed et al. (2012) demonstrated TOJ crossing effects between different fingers. This result showed that touch to the fingers is remapped into external space with respect to finger position (see also Riemer, Trojan, Kleinböhl, & Hölzl, 2010), contrasting with earlier suggestions that the fingers differ from the hands in that their posture is not taken into account for touch processing (Benedetti, 1985, 1988; Haggard, Kitadono, Press, & Taylor-Clarke, 2006). In addition, Heed et al. (2012) proposed that touch to a finger is localized by integrating spatial information about the stimulated finger and the hand to which it belongs. In one experiment, they manipulated crossing separately for hands and fingers: participants performed TOJ about tactile stimuli applied to the little fingers. In the first condition, both hands and fingers were located in their anatomical hemifield. In the second condition, the hands were crossed, and with them, the little fingers switched hemifields. However, in the third condition, the hands were crossed, but the little fingers were crossed back into their anatomical hemifield, thus, dissociating crossing for fingers and hands. Performance in this latter condition was intermediate between the entirely uncrossed and entirely crossed conditions, indicating separate crossing effects for fingers and hands. Therefore, it was suggested that hand and finger posture were integrated to localize the tactile stimuli. That hand and finger information about a touch are integrated is not self-evident. For example, it has been suggested that touch to the fingers may not be automatically assigned to a hand. In one study, participants were asked to detect a tactile stimulus at a previously designated finger; in some trials, an irrelevant second stimulus was simultaneously applied to another finger (Tamè,

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Farnè, & Pavani, 2011). When this second stimulus occurred on the other hand’s homologue finger of the first touch, it did not interfere with the detection task. In contrast, irrelevant stimuli either applied to the same hand or to a nonhomologous finger of the other hand led to detection errors. Thus, a second stimulus on the other hand was without effect if, and only if, the two stimulated fingers were homologue. These results were interpreted as implying that touch representation at the fingers is independent of the hands (Tamè et al., 2011). To resolve this issue, we designed a modified TOJ paradigm to test whether touch to a finger is mapped to a hand, and whether these touch-to-hand mappings influence touch localization at the fingers. In previous TOJ research, hand and finger position have usually been confounded: There were two possible stimulus locations, one finger at each hand. Consequently, a touch to the left finger was mapped to the left hand throughout posture conditions and reference frames. This confound can be avoided by applying the tactile stimuli to fingers of the same hand as well. If touch is localized at the fingers independent of the hands, no difference should arise between touch localization within one hand and touch localization between hands. However, if, as suggested, touch to the fingers is localized with respect to both, fingers and hands, relative location judgments should be more difficult when both stimuli are applied to fingers of one hand than when the stimuli are applied to fingers of both hands. This is because the touch-to-hand mapping would provide additional, redundant information for between hands TOJ, but not for within hands localization. Indeed, some authors reported a disadvantage for TOJ within hands compared to TOJ between hands (Craig, 1985; Sherrick, 1970). In the context of tactile remapping, the subsequent question then is whether touch is mapped to a hand solely based on anatomy, or whether the external position of the fingers also plays a role for touch-to-hand mapping. Haggard et al. (2006) reported that participants ability to identify the hand to which a stimulated finger belonged anatomically was influenced by hand posture. Identification of which hand had been stimulated took longer and was more error-prone when the fingers were interwoven compared with when the hands were stacked in a vertical posture, such that the fingers belonging to one hand were all placed next to each other (Haggard et al., 2006). Anatomical information influenced the identification of the touched hand as well, because performance was especially impaired for fingers, which were placed next to the homologous finger of the other hand. In sum, touch localization might rely on the anatomical and external finger coordinates of the tactile stimulus, as well as on the mapping of the touch to a hand, either in anatomical or in external space. So far, we have discussed conflicts in touch localization. However, independent of touch, finger crossing creates a conflict between spatial representations of the fingers established either with respect to the postural representation of the body, the postural schema (Head & Holmes, 1911) or in reference to the structural description of the body (Medina & Coslett, 2010; Rusconi, Gonzaga, Adriani, Braun, & Haggard, 2009; Schwoebel & Coslett, 2005). The term postural schema refers to the representation of the body’s current posture. Thus, it could be seen as the equivalent of the external reference frame. Even though there is an ongoing debate about the exact definition of this body representation (Holmes & Spence, 2006), the existence of a representation devoted to


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body posture is not refuted. For instance, recent fMRI studies have shown that limb posture (independent of touch) is represented in the posterior parietal cortex (PPC; Pellijeff, Bonilha, Morgan, McKenzie, & Jackson, 2006; Wada et al., 2012), a region that has also been associated with tactile remapping (Azañón et al., 2010; Bolognini & Maravita, 2007; Lloyd et al., 2003; Renzi et al., 2013). On the other side, the neuropsychological disorder autotopagnosia provides evidence for the existence of a structural description of the body, which corresponds to the tactile anatomical reference frame. Patients suffering from autotopagnosia are not able to point to body parts such as the elbow. However, their online sensory-motor representation of the body, including the postural schema, is intact as shown by successful completion of reaching and grasping tasks (Buxbaum & Coslett, 2001). Finally, there is evidence from healthy adults for a structural representation of the fingers. Rusconi et al. (2009) presented participants with four tactile stimuli in a given trial. Each hand received two stimuli, both to a different finger. Thereby, vision of the hands was occluded. Participants had to indicate whether the number of fingers between the two tactile stimuli was identical for the two hands or not. When touch had been presented to homologous fingers on the two hands performance increased. However, participants as well performed above chance in the other conditions, which required the use of a structural representation of the fingers. Furthermore, when the two hands were held in the same posture, a behavioral advantage compared to incongruent hands postures arose only when both tactile stimuli had been presented to homologous fingers. In contrast, performance was not influenced by posture, in conditions in which the structural representation of the body was necessary to solve the task, that is, when nonhomologous fingers were stimulated. The authors interpreted this selective effect of posture as indicating that the body structural description is independent of the postural schema (Rusconi et al., 2009).

Experiment 1 The present experiments addressed the role of anatomical and external space for touch localization at the fingers and hands. To this aim, we tested the effect of conflict between the different coordinates on tactile localization in the TOJ task. Participants made a choice about which of two tactile stimuli occurred first. In previous TOJ studies, the two possible locations were constant at least throughout an entire block (and, in most studies, throughout the entire experiment). In contrast, in the present experiment, two out of four possible locations were chosen randomly in every trial. The four possible stimulus locations were the two index fingers and two little fingers. Performance for all possible finger pairings was tested in two different postures, an uncrossed and a crossed posture. Crucially, in the crossed posture, only the two index fingers were crossed, whereas the little fingers remained in the same positions as in the uncrossed conditions. This design resulted in four different TOJ types, as is illustrated in Figure 1. We refer to the different TOJ types as different “grades of crossing.” TOJ between stimuli to the index fingers reflect a “full crossing,” because both stimulated fingers are crossed in the crossed posture condition. In contrast, we refer to TOJ between stimuli to the two little fingers as “indirect crossing,” because the two stimulated fingers did not change their position between uncrossed and crossed conditions, as crossing involved only the nonstimulated index fingers. Finally, TOJ between stimuli to an index and a little finger reflect a “partial crossing,” because only one finger, the index finger, changed position in the crossed conditions. These partial crossing TOJ involve either stimulation of one finger of each hand (partial crossing/between hands TOJ) or of fingers of the same hand (partial crossing/within hands TOJ). The level of crossing (full, partial, indirect), as well as the hands involved (with the two stimulated fingers belonging to the same hand or to

Figure 1. The four different types of temporal order judgments (TOJs) derived from the relation between the stimulated fingers (surrounded by dashes). The dots mark which of the properties in the leftmost column apply to this condition. The hypotheses laid out in the text predict impaired TOJ performance for TOJ conditions, which have one of the first four properties, when compared with TOJ conditions, which meet the no conflict criterion.



different hands), establish conflicts between the different spatial representations possibly involved in touch localization at the fingers (Figure 1). In general, we expected impaired TOJ performance whenever the stimulation involved a coordinate conflict, compared with conditions involving no coordinate conflict, which in turn should not differ from each other (compare Figure 1). In full crossing TOJ conditions, the anatomical and the external relative finger coordinates of both tactile stimuli are in conflict, when the fingers are crossed. Therefore, we expected a crossing effect, that is, performance should be impaired in crossed compared to uncrossed conditions for these TOJ (see Heed et al., 2012). In indirect crossing conditions, there is no conflict between anatomical and external coordinates, and, accordingly, we expected that TOJ performance should not be influenced by posture in this condition. If touch is mapped to a hand based on external finger coordinates (Haggard et al., 2006), two types of conflict can arise in the crossed posture for partial crossing/between hands TOJ. First, a tactile stimulus at a crossed index finger would then be mapped to two different hands: one in anatomical space and one in external space. For example, when the crossed right index finger is located next to the left hand in external space, a touch to that finger might be mapped to the left rather than to the right hand in external space. Second, as a consequence of such touch-to-hand (re)mapping based on finger posture, both tactile stimuli (one to the left little finger and one to the crossed-over right index finger) will be mapped to the same hand in external space. This might impair performance, based on the fact that hand coordinates would not provide redundant information for the relative location of the two stimuli, contrary to when the two stimuli are mapped to different hands. Because no conflict arises for these TOJ in the uncrossed posture, we expected a crossing effect in partial crossing/between hands conditions. Finally, in partial crossing/within hands conditions TOJ performance in uncrossed conditions should be impaired compared to uncrossed performance in the other TOJ types. This is because both stimuli will be mapped to the same hand in anatomical and in external space. Again, if the two stimuli are mapped to different hands, hand assignment provides redundant information for touch localization, which is, however, absent, if stimuli are mapped to the same hand. For partial crossing/within hands TOJ when the index fingers are crossed, the two stimuli would then be mapped to different hands, in external space, which will now provide redundant information, and, therefore, lead to better performance than in the uncrossed posture. However, this potential advantage in the crossed posture is accompanied by the same reference frame conflict which arises in partial crossing/ between hands TOJ: a tactile stimulus at a crossed index finger should be mapped to two different hands, one in the external and one in the anatomical reference frame. Therefore, performance in partial crossing/within hands TOJ should be impaired in both hand postures when compared to uncrossed between hands TOJ. In our experiments, responses were given by lifting the finger which was judged to have been stimulated first. Because there were four possible stimulus locations of which only two were actually stimulated in a given trial, participants could respond with a finger which had not been stimulated. We termed these errors phantom errors, referring to the fact that, somehow, participants felt stimuli at positions at which, in fact, no stimulation had occurred. This type of error is a new feature of the current TOJ design compared with previous TOJ studies, and we report here

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that they carried systematic information about tactile (mis)localization in both anatomical and external spatial coordinates. We analyzed phantom errors under two perspectives: the localization of touch with respect to the limbs and the spatial representation of the limbs independent of touch. We suggest that touch to the fingers is localized with respect to the fingers and hands (Heed et al., 2012), leading to problems when both stimuli are mapped to the same hand (discussed previously; Figure 1). Beneath the clear predictions regarding TOJ performance, this hypothesis can additionally be tested by analyzing whether phantom errors occur more frequently at fingers of the same hand as the target finger than at fingers of the other hand. In contrast, if tactile stimuli at the fingers were represented independently of the hands (Tamè et al., 2011), participants should make more phantom errors between homologous than between nonhomologous fingers, because signals from the homologous fingers would feed into the same representation of a certain finger type and, thus, be more difficult to differentiate in a TOJ task than tactile stimuli to nonhomologous fingers. Furthermore, we also tested, whether the assignment of touch to a hand changes with the external coordinates of the index fingers in the crossed conditions by comparing phantom errors at the target and the nontarget hand across posture conditions. Finally, comparisons of phantom errors at the index fingers between uncrossed and crossed postures are of special interest with respect to the representation of the limbs independent of touch. More specifically, they provide the opportunity to test the relationship between the postural schema of the body and the body structural description. Recall that, by definition, phantom errors occur at nonstimulated fingers. If the amount of phantom errors changed with finger posture, this change could not, by definition, be caused by tactile stimulation to that finger (because no stimulation occurred at that finger). Moreover, in the crossed posture, the two spatial representations of the index fingers are in conflict: The finger represented as left index finger in the body structural description is located between fingers of the right hand in the postural schema. Phantom errors, therefore, provided us with a unique window into the relationship between these two body representations.

Method Participants. Eighteen participants (15 women, age ⫽ 18 –36 years, mean ⫽ 25 years) volunteered for the experiment. All participants reported to have normal or corrected-to-normal vision and to be free of tactile impairments. Seventeen participants were right-handed; one was ambidextrous but favored the right hand (mean score ⫽ 77, range ⫽ 27–100 in the Edinburgh Handedness Inventory; Oldfield, 1971). In return for their attendance, participants received course credit or were compensated with 7 Euro/ hour. The experiment was conducted in accordance with the guidelines of the Declaration of Helsinki (World Medical Association, 2008). Apparatus. Participants sat at a table, resting their wrists and elbows either directly on the table surface or, if required for comfort, on little foam cushions. The index and little fingers of both hands were placed on response buttons. The ring and middle fingers were either positioned in between the two adjacent response buttons or flexed, so that they were situated underneath the palm of the respective hand (participants could choose the more


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comfortable position). Depending on the experimental condition, the fingers either rested on response buttons corresponding to their anatomical order (uncrossed conditions), or the index fingers were crossed (crossed conditions). A foam cushion was placed underneath one wrist to avoid skin contact between the hands in the crossed condition. For the same reason, a small patch of cotton was placed between the two index fingers in the crossed condition. The response buttons were housed in small cubes (height: 2, width: 3 ⫻ length: 6 cm). Distance of the response buttons (and, thus, of the fingers) was kept equal between all fingers over all conditions. To this end, the distance between them was adjusted to the smallest distance between any two adjacent fingers in the crossed posture. This assured that fingers were spaced equidistantly, as well as comfortably. Tactile stimulators (Oticon bone conductors, type BC 461– 012, Oticon Ltd., Milton Keynes, United Kingdom, sized about 1.6 ⫻ 1 ⫻ 0.8 cm) were taped to the four involved fingers, covering the whole fingernail and some proximate skin. For stimulation, they were driven with a frequency of 167 Hz (i.e., a square wave with cycle duration of 6 ms, including on and off phase) for 15 ms. The experiment was controlled by the software Presentation, version 14.5 (Neurobehavioral Systems, Albany, CA), which interfaced with custom built hardware to drive the stimulators and record responses. To shield off any auditory cues produced by the tactile stimulators, participants wore ear plugs as well as headphones playing white noise. Written instructions were displayed on a monitor 80 cm in front of the participant. Task. Participants performed TOJs (e.g., Shore et al., 2002; Sternberg & Knoll, 1973; Yamamoto & Kitazawa, 2001) of tactile stimuli. In each trial, two fingers were stimulated in rapid succession, and participants were asked to lift the finger that was stimulated first, regardless of its position. Responses had to be withheld until the second stimulus had been presented. No feedback about correctness was provided. Design. We report results in two separate sections. First, we describe results for trials in which participants made a response with one of the stimulated fingers. These trials are directly comparable to previous TOJ studies, and we, therefore, refer to them here as “regular TOJ.” Second, we then report results for the trials in which participants responded with a nonstimulated finger; that is, phantom errors. Regular TOJ performance. We manipulated four withinparticipants factors: the posture of the index fingers (factor: Crossing Status, levels: uncrossed and crossed); the Type of TOJ (factor: Type of TOJ, levels: full crossing, indirect crossing, partial crossing between hands and partial crossing within one hand, see introduction and Figure 1); the stimulus onset asynchrony (SOA) between the two tactile stimuli (factor: SOA, levels: ⫺300, ⫺110, ⫺50, 50, 110, 300, negative SOAs refer to trials in which the anatomically leftmost finger was stimulated first); and the finger type of the target finger (factor: Target Finger, levels: index and little finger). Phantom errors. The design of the experiment with four potential target fingers of which only two were stimulated in a given trial, led participants to systematically respond with nonstimulated fingers, referred to as phantom errors. The four manipulated within-participants factors, Crossing Status, Type of TOJ, Target Finger, and SOA, apply to the analyses

of phantom errors as well. However, phantom errors, by definition, cannot be made with the target finger (as the target finger is a stimulated finger). Hence, phantom errors could be better described by the relation between the finger making the phantom error and the target finger. Applying this logic, phantom errors could occur at the anatomically same hand or opposite hand (factor: Phantom Hand) compared with the target finger. Additionally, phantom errors occurred at both types of finger (factor: Phantom Finger, levels: index and little finger; Figure 2). Procedure. Every trial was 2,500 ms long. Trials with reaction times (RTs) shorter than 200 ms, trials without a response within trial duration, and trials containing a phantom error, that is, a response with a nonstimulated finger, were repeated at the end of each block. The experiment was divided into 16 blocks of 54 trials each. Each SOA was presented 12 times for each condition. The factors Type of TOJ, Target Finger, and SOA were randomized within blocks, whereas Crossing Status was randomized between blocks. There were eight consecutive blocks of one condition of Crossing Status, and condition order was counterbalanced across participants. Participants were encouraged to rest between blocks. The experiment took approximately 90 min. Data analysis. Before the analyses, trials with RTs shorter than 250 ms and longer than 1,500 ms were excluded (4.7% of all trials). Analysis of regular TOJ performance. Accuracy (1 ⫺ error rate) was assessed as dependent variable for each SOA. Mean values over all SOAs were calculated as aggregated measure of performance in each condition (Cadieux et al., 2010). To account for the factor SOA, we conducted a probit analysis (Finney, 1947; Shore et al., 2002). To this aim, TOJ responses were transformed into “rightmost first” responses, indicating whether or not the participant judged the anatomically rightmost finger to be stimulated first (e.g., Shore et al., 2002; Yamamoto & Kitazawa, 2001). Probits were calculated by applying the inverse normal distribution Z ⬃N (0, 1) (probit(prightmost first) ⫽ z N ⌽(z) ⫽ prightmost first) to the proportion of rightmost first responses for each SOA. The

Figure 2. The different types of phantom errors and their properties. Stimulated fingers are surrounded by dashes and stimuli are enumerated by temporal order. Phantom errors are symbolized by black circles.



resulting probits were linearly regressed onto SOA values ranging from ⫺110 to 110 ms. For longer SOAs, TOJ performance follows a Uniform rather than a Gaussian distribution; therefore, slopes were calculated only over short SOAs (Shore et al., 2002; Schicke & Röder, 2006; Spence, Shore, & Klein, 2001). The slope estimated by probit analysis has been shown to be a good measure of TOJ performance (Heed et al., 2012; Shore et al., 2002), with steeper slopes indicating better performance. For visual display, we modeled the proportion of “rightmost first” responses as a function of SOA (with negative SOAs indicating “leftmost first”-stimuli), with a sigmoid curve estimated with a constrained maximum-likelihood method (Wichmann & Hill, 2001a, 2001b) as implemented in the Psignifit toolbox for Matlab (http://www.bootstrap-software.org/psignifit/). First, repeated measures analyses of variance (ANOVAs) with factors Crossing Status and TOJ Type were conducted on probit slopes and accuracy to asses the crossing effect in all TOJ Types. After controlling for a significant two-way interaction, crossed and uncrossed conditions were compared for each TOJ type with paired t tests. In addition, based on our hypotheses (compare Figure 1), uncrossed TOJ performance in partial crossing/within hands TOJ was compared to uncrossed TOJ performance in the three types of between hands TOJ with paired t tests. Second, we tested for biases to respond with a specific finger dependent on our experimental factors. To this end, we analyzed the point of subjective simultaneity (PSS). However, this measure is strongly dependent on the fit of the psychophysical function and was highly variable in our sample. We, therefore, used an alternative, assumption-and distribution-free approach. We computed the average proportion of rightmost finger first responses for each experimental condition and subjected these scores to a repeated measures ANOVA with factors Crossing Status and Stimulated Fingers. Contrary to the PSS, the average proportion of rightmost finger first responses weighs all SOAs equally. It can be viewed an indicator of finger bias: values higher than 0.5 point to a tendency toward rightmost finger responses, whereas values below 0.5 indicate a leftmost finger bias. Analysis of phantom errors. Phantom errors, that is, responses with fingers that had not been stimulated in the trial, were quantified by calculating the relative proportion of responses with each of the four possible fingers (Figure 7). Statistical analysis focused on the two nonstimulated fingers in each condition and, thus, reflects modulation of phantom errors across conditions.

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First, we conducted a repeated measures ANOVA with factors Crossing Status, Phantom Finger, and Phantom Hand on phantom error probability averaged across phantom errors of all TOJ Types. The factor TOJ Type could not be included in the analysis, because TOJ types represented one particular combination of Phantom Finger and Phantom Hand. For example, in full crossing TOJ, phantom errors could only occur with the little fingers. Similarly, in within hand TOJ, phantom errors could only occur with fingers of the nontargeted hand. Second, a repeated measures ANOVA with factors Crossing Status and Target Finger was conducted on phantom error probabilities during partial crossing/within hands TOJ. This analysis was restricted to within hands TOJ, because this was the only condition in which phantom errors could be compared across target fingers without a confound with the targeted hand, that is, the factor Phantom Hand. Third, to further explore the influence of SOA on phantom errors with crossed index fingers, we ran a repeated measures ANOVA with factors Phantom Hand and SOA on phantom error probabilities from all TOJ types. All statistical analyses were conducted using R (R Development Core Team, 2011). Because most subsets of the dataset were unbalanced, we used Type III sums of squares in the ANOVAs (Shaw & Mitchell-Olds, 1993). If the assumption of sphericity was violated (indicated by Mauchly’s Test for Sphericity), we used Greenhouse-Geisser’s epsilon to correct the degrees of freedom. Significant interactions were followed up by sub-ANOVAs and paired t tests. Reported p values are adjusted for multiple comparisons according to Holm (1979). All reported results were significant at a Type I error level of 5% unless noted otherwise.

Results Analysis of regular TOJ performance. First, repeated measures ANOVAs with factors Crossing Status (levels: uncrossed or crossed) and TOJ Type (levels: full crossing, indirect crossing, partial crossing/between hands or partial crossing/within hands) revealed a main effect of Crossing Status, a main effect of TOJ Type and an interaction between Crossing Status and TOJ Type (Table 1; Figures 3 and 4). Planned post hoc comparisons with paired t tests confirmed a crossing effect in full crossing TOJ conditions and in partial crossing/between hands TOJ conditions. No significant crossing effect was evident in the indirect crossing condition. A crossing effect was also not found in the partial

Table 1 Results of the Statistical Analyses of Regular Temporal Order Judgment (TOJ) Performance in Experiment 1

Repeated measures analysis of variance Crossing Status TOJ Type Crossing Status ⫻ TOJ Type Paired t tests (crossed vs. uncrossed) TOJ Type: Full crossing TOJ Type: Indirect crossing TOJ Type: Partial crossing/between hands TOJ Type: Partial crossing/within hands Paired t tests (uncrossed within hand TOJ vs. uncrossed between hands TOJ) Partial crossing/within hands TOJ vs. full crossing TOJ Partial crossing/within hands TOJ vs. indirect crossing TOJ Partial crossing/within hands TOJ vs. partial crossing/between hand TOJ

Probit slope

Accuracy

F(1, 17) ⫽ 28.56, p ⬍ .001 F(3, 51) ⫽ 49.09, p ⬍ .001 F(3, 51) ⫽ 12.61, p ⬍ .001

F(1, 17) ⫽ 38.69, p ⬍ .001 F(3, 51) ⫽ 43.25, p ⬍ .001 F(1.95, 33.09) ⫽ 32.55, p ⬍ .001

t(17) ⫽ 6.18, p ⬍ .001 t(17) ⫽ 1.48, p ⫽ .156 t(17) ⫽ 2.92, p ⫽ .010 t(17) ⫽ 0.51, p ⫽ .619

t(17) ⫽ 7.57, p ⬍ .001 t(17) ⫽ 0.10, p ⫽ .921 t(17) ⫽ 4.22, p ⬍ .001 t(17) ⫽ 0.61, p ⫽ .550

t(17) ⫽ 6.60, p ⬍ .001 t(17) ⫽ 7.53, p ⬍ .001 t(17) ⫽ 9.65, p ⬍ .001

t(17) ⫽ 5.47, p ⬍ .001 t(17) ⫽ 7.82, p ⬍ .001 t(17) ⫽ 7.13, p ⬍ .001


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Figure 3. Average probit slopes and accuracy of temporal order judgments (TOJs) shown for all four types of stimulation, averaged over participants. Full crossing refers to stimulation of the index fingers, indirect crossing refers to stimulation of the little fingers and partial crossing refers to stimulation of an index and a little finger either of one or of both hands (Figure 1). Performance is shown for crossed (dark gray) and uncrossed (light gray) conditions. Error bars show SEs.

crossing/within hands condition. However, in this condition, uncrossed TOJ performance was significantly reduced compared to uncrossed TOJ performance in the three between hands TOJ conditions (Table 1; Figures 3 and 4). Second, a repeated measures ANOVA with factors Crossing Status (levels: uncrossed or crossed) and Stimulated Fingers (levels: both index fingers, both little fingers, index finger of the left/right hand and little finger of the right/left hand, index and little finger of the left/right hand) with the proportion of rightmost finger first responses as dependent variable revealed a main effect of Stimulated Fingers, F(2.62, 44.49) ⫽ 9.77, p ⬍ .001; Figure 5. Post hoc analyses (Figure 6) confirmed that full and indirect crossing TOJ as well as partial crossing TOJ within the left hand had a slight bias toward the anatomically rightmost finger. Partial crossing/between hands TOJ involving the left little and the right index finger showed a significantly stronger tendency toward the right index finger. In contrast, partial crossing TOJ between the left index finger and the right little finger as well as partial crossing TOJ within the left hand had a bias toward the leftmost finger (Figure 5). Viewed together, these results indicate that when TOJ were made between index and little fingers of different hands (partial crossing/between hands TOJ), TOJ were biased toward the index finger. In contrast, TOJ between index and little finger of the same hand (partial crossing/within hands TOJ) were biased toward the little finger (compare Figures 4 and 7). Analysis of phantom errors. First, a repeated measures ANOVA on phantom error probabilities with factors Crossing Status (levels: uncrossed or crossed), Phantom Finger (levels: index or little finger) and Phantom Hand (levels: same or other hand as the target finger) demonstrated main effects of all three factors, several two-way interactions and a significant three-way

interaction between all factors (Table 2, Figures 7 and 8). To further analyze these interactions, separate sub-ANOVAs were conducted for the two types of Phantom Finger. In other words, the effects of Crossing Status and Phantom Hand were analyzed separately for phantom errors at the little and at the index fingers. The sub-ANOVA on phantom errors at the little fingers revealed a significant main effect of Phantom Hand, but no effect of Crossing Status: Phantom errors at the little fingers were more frequent at the same hand as the target finger than at the other hand. In contrast, the sub-ANOVA on phantom errors at the index fingers showed a main effect of Crossing Status, a main effect of Phantom Hand and an interaction between the two factors. Post hoc pairwise paired t tests confirmed that, in the uncrossed conditions, phantom errors at the index fingers were more likely at the targeted hand than at the other hand. However, in the crossed conditions, the number of phantom errors at the index finger of the targeted and of the nontargeted hand did not differ significantly. Phrased differently, significantly more phantom errors occurred at the index finger of the nontargeted hand in crossed compared to uncrossed posture conditions (Figure 8). Second, a repeated measures ANOVA with factors Crossing Status (levels: uncrossed or crossed) and Target Finger (levels: index or little finger) on the probabilities of phantom errors in partial crossing/within hands TOJ conditions revealed a significant main effect of Crossing Status, F(1, 17) ⫽ 21.05, p ⬍ .001. Phantom errors were more likely in crossed than in uncrossed posture conditions (Figure 7, rightmost column). No significant effect of Target Finger was evident. Third, visual inspection of Figure 9 suggested that, despite a nonsignificant four-way interaction of the factors Crossing Status (levels: uncrossed or crossed), Phantom Finger (levels: index or little finger), Phantom Hand (levels: same or other hand as the target finger), and SOA (levels: ⫺300, ⫺110, ⫺50, 50, 110, 300), there might be a trend for an interaction between SOA and Phantom Hand for phantom errors at crossed index fingers. Such an effect would be interesting because only these phantom errors indicate the external representation of the nonstimulated fingers. To explore this possibility, we conducted a repeated measures ANOVA with factors Phantom Hand and SOA on phantom error probabilities at crossed index fingers. This analysis yielded a significant main effect of SOA, F(2.42, 41.08) ⫽ 20.50, p ⬍ .001 as well as a significant interaction between the two factors (F(2.07, 35.27) ⫽ 20.50, p ⫽ .031; Figure 9, top row, dark gray lines). Because the fourway interaction of the factors Crossing Status, Phantom Finger, Phantom Hand, and SOA was not significant, this analysis has to be regarded as exploratory.

Discussion In this experiment, we explored how different types of conflict between external and anatomical reference frames for finger location and touch-to-hand mapping affected touch localization. The two new features of the present experiment were that there were four, rather than just two, possible locations for tactile stimulation, and that only two of the four involved fingers (the index fingers) were ever crossed. This design allowed us to differentiate effects of full (both stimuli to the crossed fingers), partial (one stimulus to the crossed index



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Figure 4. Top row: Mean proportion of “index finger first”-responses as a function of SOA. Negative SOAs denote “little finger first”-stimulus pairs, and positive SOAs “index finger first”-stimuli. Sigmoid curves are fitted cumulative Gaussian distribution functions with variable asymptotes. Bottom row: Results of the probit analysis. Markers represent the probits of “index finger first”-responses, and lines depict the outcome of the associated linear regression. Error bars show SEs, and performance is shown for uncrossed (light gray) and crossed (dark gray) posture conditions in all panels.

fingers, one to the uncrossed little fingers) and indirect crossings (no stimulus to the crossed fingers, both stimuli to the uncrossed little fingers), and thus to disentangle conflicts between relative finger coordinates and conflicts resulting from mapping of touch to a hand (Figure 1). Moreover, the two stimuli were either presented to fingers of the same hand, or,

Figure 5.

alternatively, to fingers of different hands. Furthermore, extending the number of possible locations for touch led to the emergence of phantom errors, that is, responses with fingers which were not stimulated in the respective trial. The effect of finger crossing was directly related to the number touched fingers that were crossed. The crossing effect was largest

Mean proportions of “rightmost finger first” responses in Experiment 1. SEs are shown in parentheses.


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Figure 6. Results of the post-hoc analysis of the main effect of stimulated fingers on the proportion of “rightmost finger first” responses in Experiment 1. The data were analyzed with pairwise, paired t-tests. Reported p values were corrected for multiple comparisons following Holm (1979).

in the full crossing condition (TOJ between stimuli at the index fingers), intermediate in the partial crossing/between hands condition, and no crossing effect was evident in the indirect crossing condition (TOJ between stimuli at the little fingers). Additionally, when TOJ were conducted between stimuli at fingers of one hand, no crossing effect emerged, but TOJ performance in both postures was reduced compared to performance in uncrossed between hands TOJ conditions. Phantom errors at both, index and little fingers, showed a hand effect: Phantom errors were more frequent at fingers of the same hand as the correct response compared to fingers of the other hand. Moreover, phantom errors at the index fingers were influenced by posture. In the uncrossed conditions these followed the hand effect. However, in the crossed conditions these phantom errors were equally likely at the target and at the nontarget hand, leading to significantly more index finger phantom errors in crossed than in uncrossed posture conditions.

These results are consistent with the predictions we derived from our hypotheses (see Figure 1 for a list of potential coordinate conflicts). First, TOJ performance was impaired when the relative anatomical and external finger coordinates of touch were in conflict (full crossing TOJ). Second, TOJ performance was also impaired when both stimuli were applied to the same hand (partial crossing/within hands TOJ). Third, TOJ performance was impaired when both stimuli were mapped to the same hand in external space (partial crossing/between hands TOJ). Fourth, phantom errors at the little fingers were more likely at fingers of the targeted hand. We conclude, in sum, that touch localization was based on both anatomical and external finger coordinates as well as on touch-to-hand mapping in anatomical and external space. Finally, phantom errors at the index fingers were influenced by posture and targeted hand, indicating an influence of both the postural schema and of the body structural description on these mislocalization errors.



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Figure 7. Response probability with each possible finger (depicted with anatomical coding) for all stimulus onset asynchronies (SOA; negative SOA values refer to “leftmost finger first” stimuli). Data is presented separately for uncrossed (top row) and crossed conditions (bottom row) and all temporal order judgment (TOJ) types (Figure 1). For display, data from partial crossing conditions has been rereferenced across hands. “Regular” TOJ responses, that is, responses with stimulated fingers, are represented as filled circles. Phantom errors, that is, responses with nonstimulated fingers, are shown as open circles. Shading and size vary on different scales to highlight effects in both types of data. Gray shades are scaled linearly and, therefore, are rather associated with classical TOJ data. Size scaling is pseudologarithmic, amplifying differences between infrequent responses. As a consequence, the size distribution emphasizes the pattern of phantom errors.

However, there are several potential confounds which must be ruled out before discussing the details of our conclusions. First, one might argue that the TOJ effects reported here are, in fact, effects of the distance between the two stimulated fingers. Several authors have reported a decline of TOJ performance when uncrossed hands were positioned closer to each other compared to when the hands were positioned further apart (Kim & Cruse, 2001; Roberts, Wing, Durkin, & Humphreys, 2003; Shore, Gray, Spry, & Spence, 2005). Indeed, the three grades of crossing used in the present experiment also represent three different distances (full crossing, both index fingers–small distance, indirect crossing, both little fingers– big distance, partial crossing/between hands, index and little finger– intermediate distance). However, there was no significant effect of distance in accuracy and slope data for uncrossed TOJ performance. Rather, the modulation of touch localization in the different between hands conditions was restricted to the crossed conditions, ruling out a more general “distance effect.” Second, for the partial crossing/between hands conditions, a crossing effect may have emerged due to a shift of the psychophysical function in the crossed conditions, specific to each finger combination. If such a shift pointed in opposite directions for the two different finger combinations, a crossing effect would emerge. However, when data from partial crossing conditions are shown as a function of the finger type of the first stimulus (Figure 4), it becomes obvious that the crossing effect

was independent of such asymmetric shifts of the psychometric functions. In addition, a shift of the point of subjective simultaneity toward more “little finger first” responses in the crossed conditions is predicted by the account of Shore et al. (2002). These authors suggested that TOJ crossing effects arise because a remapped tactile stimulus needs increased processing time, because of the conflict between anatomical and external coordinates (Cadieux et al., 2010). In our experiment, only the index fingers changed their position. Thus, in the partial crossing conditions, processing time should be prolonged only for tactile stimuli to the index fingers, but not for touch to the little fingers. As a consequence, the PSS should be shifted toward the index fingers in the crossed posture, indicative of a higher proportion of responses with the little fingers. In contrast, TOJ biases toward either finger were not affected by crossing (Regular TOJ Analysis 2, Figure 5). Rather, TOJ were biased toward the index finger in both uncrossed and crossed conditions. This implies that processing of tactile stimuli to the crossed index fingers did not take longer than processing stimuli to the little fingers in these conditions. Yet, this result can still be explained by a “prolonged processing” account: If one assumes that the reference frame conflict in partial crossing conditions is different from the coordinate conflict in full crossing conditions. In partial crossing/between hands conditions conflict arises from the differences of touch-to-hand mapping in anatomical, respectively, external space (e.g., a touch to the index finger of the left


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Table 2 Results of the Statistical Analyses of Phantom Errors in Experiment 1


Phantom error probabilities Repeated measures ANOVA Crossing Status Phantom Hand Phantom Finger Crossing Status ⫻ Phantom Hand Crossing Status ⫻ Phantom Finger Phantom Hand ⫻ Phantom Finger Crossing Status ⫻ Phantom Hand ⫻ Phantom Finger Sub-ANOVA little fingers Crossing Status Phantom Hand Crossing Status ⫻ Phantom Hand Sub-ANOVA index fingers Crossing Status Phantom Hand Crossing Status ⫻ Phantom Hand Post-hoc analysis index fingers: pairwise paired t tests Crossing Status—uncrossed: same hand vs. other hand Crossing Status—crossed: same hand vs. other hand Phantom Hand—same hand: uncrossed vs. crossed Phantom Hand—other hand: uncrossed vs. crossed Note.

F(1, 17) ⫽ 11.71, p ⫽ .003 F(1, 17) ⫽ 31.37, p ⬍ .001 F(1, 17) ⫽ 20.18, p ⬍ .001 F(1, 17) ⫽ 16.73, p ⬍ .001 F(1, 17) ⫽ 9.84, p ⫽ .006 F(1, 17) ⫽ 0.06, p ⫽ .807 F(1, 17) ⫽ 9.44, p ⫽ .040 F(1, 17) ⫽ 0.03, p ⫽ .867 F(1, 17) ⫽ 12.44, p ⫽ .002 F(1, 17) ⫽ 0.20, p ⫽ .662 F(1, 17) ⫽ 15.19, p ⫽ .001 F(1, 17) ⫽ 14.46, p ⫽ .001 F(1, 17) ⫽ 14.95, p ⫽ .001 t(17) ⫽ 8.14, p ⬍ .001 t(17) ⫽ 1.58, p ⫽ .265 t(17) ⫽ 0.54, p ⫽ .595 t(17) ⫽ 4.91, p ⬍ .001

ANOVA ⫽ analysis of variance.

hand is mapped to the right hand in external space). In full crossing TOJ the conflict forms between the anatomical and external relative finger coordinates (anatomically left index finger located to the right of the other index finger). Phantom errors occurred in all conditions (see Figure 7) and were reliable across participants. The analysis of phantom errors revealed that these, unlike lapses, nevertheless occurred systematically. During debriefing, participants reported in all conditions difficulties in identifying which of the four fingers were stimulated, independent of the temporal order, indicating sensory confusion. Furthermore, the pattern of phantom errors cannot be explained by physical transmission of vibration between stimulus locations, because phantom errors occurred

even with fingers of a nonstimulated hand. In addition, motor confusion of homologous fingers (Tamè et al., 2011) seems unlikely, given that the probability of phantom errors with the index finger of the nontarget hand was not higher when the index finger was stimulated first compared to when the little finger was stimulated first (Phantom Error Analysis 2). Finally, even though, phantom errors during tactile TOJ have not been reported before, there have been a number of reports of spatial misperceptions of touch to the fingers of one hand (Braun et al., 2011; Overvliet, Anema, Brenner, Dijkerman, & Smeets, 2011). However, phantom errors are probably not related to misperceptions because of sensory overlap in the primary somatosensory cortices, as these are rather restricted to neighboring fingers (Schweizer, Braun, Fromm, Wilms, & Birbaumer, 2001; Schweizer, Maier, Braun, & Birbaumer, 2000).

Experiment 2

Figure 8. Probability of phantom errors with respect to the factors Phantom Hand (same or other hand as the target finger) and Phantom Finger; that is, the finger at which the error occurred (index or little finger). Probabilities are shown for uncrossed (light gray) and crossed (dark gray) posture conditions. Error bars show SEs.

In this experiment, partial crossing/within hands TOJ were tested in isolation and across a wider range of SOAs than in Experiment 1. In the previous experiment, within hands TOJ performance in both posture conditions was especially impaired when the index fingers rather than the little fingers were targeted. This may have obscured potential differences between crossed and uncrossed conditions, because of resulting shifts of the psychophysical curves toward the little fingers (Figures 4 and 5). The within hand TOJ condition is, however, of importance in our line of arguments that the hands play a pivotal role in the localization of touch to the fingers and, in addition, that the mapping of touch to the hands is performed using both anatomical and external finger coordinates. In the uncrossed conditions, both stimuli are mapped to the same hand in anatomical and in external space. If touch localization is influenced by these touch-to-hand mappings, performance should be impaired compared to uncrossed, between hands TOJ. This is because, for uncrossed be-



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Figure 9. Probabilities of phantom errors displayed with respect to the factors Phantom Hand (same or other hand as the target finger), Crossing Status (uncrossed or crossed index fingers), Phantom Finger (index or little finger), and SOA (negative SOA values refer to “leftmost finger first” stimuli). Error bars show SEs.

tween hands TOJ, the assigned hands provide redundant information: the touch to the leftmost finger is mapped to the left hand, and the touch to the rightmost finger is mapped to the right hand. However, for within hands TOJ no such redundant information is available. Crucially, in the crossed conditions, stimuli to fingers of the same hand are mapped to different hands in external space. Thus, in this experiment, touch-to-hand mapping provides redundant information only with reference to external space, and only when the fingers are crossed. Yet, this redundant information benefit for the crossed posture is accompanied by a reference frame conflict: A touch to an index finger is assigned to both hands, one in anatomical and one in external space. This conflict might impair TOJ performance in the crossed compared to uncrossed conditions, and, therefore, reduce the potential performance gain in the crossed conditions. To be able to investigate these hypotheses despite the large shifts of the psychophysical functions found in Experiment 1, we increased the range of SOAs. To further reduce task difficulty, participants conducted only partial crossing/within hands TOJ.

Method Participants. Twenty participants (16 women, aged 19 –33 years, mean⫽ 25 years) were recruited from the population of the city of Hamburg. All participants reported normal or corrected-tonormal vision and to be free of tactile impairments. All participants were right-handed (mean score ⫽ 86, range ⫽ 42–100 in the Edinburgh Handedness Inventory; Oldfield, 1971). Apparatus. The setup was identical to that of Experiment 1. Task. Participants performed TOJ of two tactile stimuli, which were always applied to index and little finger of one hand. They responded with the finger which was stimulated first. Design. Four within-participants factors were manipulated: the posture of the index fingers (factor: Crossing Status, levels:

uncrossed and crossed), the stimulus onset asynchrony (SOA) between the two tactile stimuli (factor: SOA, 50, 80, 110, 200, 300, 500, 700 and 1000 ms), the stimulated hand (factor: Hand, levels: right and left hand) and the finger type of the target finger (factor: Target Finger, levels: index and little finger). Only partial crossing/within hands TOJ were conducted in the experiment. Procedure. The experimental procedure was identical to that of Experiment 1. The experiment consistent of 12 blocks, of 64 trials each and took about 60 min. The factors SOA, Hand and Target Finger were varied within blocks, whereas Crossing Status changed every other block. No feedback was provided. Data analyses. Trials with RTs shorter than 250 ms and longer than 1500 ms were excluded from the analysis (5.2% of all trials). We compared probit slopes and accuracy between crossed and uncrossed conditions with paired t tests.

Results Regular TOJ performance. Paired t tests revealed significantly better TOJ performance in the crossed than in the uncrossed conditions for probit slopes, t(17) ⫽ 2.59, p ⫽ .019 and accuracy, t(17) ⫽ 2.45, p ⫽ .023; compare Figure 10. Phantom errors. Phantom errors occurred only rarely in this experiment and, therefore, are reported only descriptively. The result pattern was similar to that of the partial crossing/within hand TOJ conditions in Experiment 1. Phantom errors occurred only in the crossed conditions at the index finger of the nontargeted hand. The proportion of phantom errors at the index finger was comparable for both target fingers (index and little finger).

Discussion Experiment 2 extended one specific condition of Experiment 1, namely partially crossed fingers during TOJ within one hand. The

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performance was lower because there were a greater number of conditions. For example participants in Experiment 1 may have shifted their attention to the other hand, after they received the first touch (see Axelrod, Thompson, & Cohen, 1968; Craig, 2003; Craig & Busey, 2003; for hypotheses on the role of attention shifts in tactile TOJ). In Experiment 1, such attention shifts would have been useful in the majority of trials, because 75% of TOJ had to be made between fingers of two hands. Obviously, in Experiment 2, which comprised only within hands TOJ, such a switching strategy would have been highly disadvantageous.


General Discussion

Figure 10. Temporal order judgment (TOJ) performance in Experiment 2. Top row: Mean proportion of “index finger first”-responses as a function of SOA. Negative SOAs denote “little finger first”-stimulus pairs, positive SOAs “index finger first”-stimuli. Sigmoid curves are fitted cumulative Gaussian distribution functions with variable asymptotes. Bottom row: Results of the probit analysis. Markers represent the probits of “index finger first”-responses, and lines depict the outcome of the associated linear regression. Error bars show standard errors and performance is shown for uncrossed (light gray) and crossed (dark gray) posture conditions in both panels.

main result of Experiment 2 was that TOJ performance was better in crossed than in uncrossed posture conditions. In other words, we observed a crossing effect, but in the opposite direction than all previously reported crossing effects, with performance being superior in crossed over uncrossed conditions. Before this result will be set in context with the results of Experiment 1 in the General Discussion, we will discuss the differences between the Experiments 1 and 2. In both experiments TOJ were biased toward the little fingers. However, the shift of the PSS toward index finger first responses (which indicates a bias toward the little fingers) was smaller in Experiment 2 (PSS ⫽ 98 ms, for “index finger first” responses averaged across participants) than in Experiment 1 (PSS ⫽ 153 ms). Furthermore, in Experiment 2, a clear crossing benefit emerged, whereas in Experiment 1, such a reversed crossing effect for index finger first trials was not clearly evident. Crucially, in Experiment 1, performance for within hands TOJ was less accurate than in Experiment 2, t(34) ⫽ 2.20, p ⫽ .035, mean accuracy ⫽ 0.73 in Experiment 1 vs. mean accuracy ⫽ 0.80 in Experiment 2 (restricted to the SOAs used in Experiment 1). These differences, in finger bias and in accuracy, might explain why the crossing benefit was not evident in Experiment 1, but in Experiment 2. Most likely, in Experiment 1 TOJ

The goal of the present study was to identify the types of spatial representations involved in localizing touch to a finger. To this aim, we tested whether coordinate conflicts between the different spatial representations affected touch localization. In two experiments, participants judged the temporal order of stimuli applied to two out of four possible locations, the index and little fingers of either hand. While the two stimuli were presented to a random choice of two fingers in Experiment 1, in Experiment 2 the two stimuli always occurred at one hand only. To investigate the conflict between anatomical and external reference frames, we implemented a crossed finger posture. The crucial feature of the experiments was that only the index fingers, but not the little fingers were crossed in half of the trials. A crossing effect, that is, reduced performance in crossed compared to uncrossed posture conditions, was evident for TOJ between stimuli to the index fingers (full crossing TOJ). A smaller, albeit significant crossing effect was demonstrated for partial crossing TOJ between stimuli to the index finger of one and the little finger of the other hand (partial crossing/between hands). In contrast, posture did not affect TOJ between tactile stimuli to the little fingers (indirect crossing TOJ). TOJ between tactile stimuli to one hand (partial crossing/within hands) differed from the other three TOJ types. First, in the uncrossed conditions TOJ between stimuli to the index and little finger of one hand (partial crossing/within hand) were impaired compared to uncrossed TOJ between tactile stimuli to fingers of both hands. Second, Experiment 2, which comprised only this condition and tested a larger range of SOAs, revealed a crossing benefit for within hands TOJ: Performance was better in crossed than in uncrossed posture conditions. Additionally, we analyzed erroneous responses with nonstimulated fingers, which we termed phantom errors. These errors were more frequent at fingers of the same hand as the correct response than at the other hand. In other words, participants were more prone to confuse touch location between fingers of one a hand. When phantom errors occurred at the little fingers, this hand effect was not modulated by posture, that is, by index finger crossing. Similarly, the proportion of phantom errors at index fingers of the targeted hand remained constant across postures. In contrast, phantom errors at the index fingers of the other, nontargeted hand occurred more frequently in the crossed conditions than in the uncrossed conditions. These results have several implications with respect to the spatial representations underlying touch localization as well as the spatial representations of the fingers independent of touch.



Spatial Localization of Touch With Respect to Fingers and Hands In full crossing conditions, we observed a TOJ crossing effect. When both stimuli were applied to the crossed index fingers, the anatomical and external relative finger coordinates were in conflict for both tactile stimuli. For example, a touch to the anatomically left index finger, was applied to the rightmost of the fingers in terms of external space. Consequently, the crossing effect in full crossing TOJ supports the claim that touch to the fingers is localized with respect to anatomical as well as external finger coordinates (Heed et al., 2012; Riemer et al., 2010). In partial crossing/between hands TOJ conditions, we also observed a crossing effect. However, this effect was smaller than in full crossing conditions. In this condition, one touch was applied to the little finger, whose posture remained fixed throughout the experiment. The other touch was applied to the index finger of the other hand. Although, the index finger changed position during crossing, the relative finger coordinates of a touch applied to the index finger were congruent across anatomical and external space in both postures. For instance, a touch to the left index finger was always left of the other touch, independent of posture and reference frame, because, in this condition, the other touch was applied to the right little finger. Thus, for partial crossing/between hands TOJ, tactile remapping did not create a conflict between the relative anatomical and external coordinates of the index finger stimulus, as it did in the full crossing condition. Rather, we suggest that the crossing effect in partial crossing/between hands conditions indicates that a touch to a crossed index finger was (re)-mapped to the other hand. This touch-to-hand remapping leads to two distinct types of conflict (Figure 1). First, in the crossed conditions, a touch to the index finger is mapped to two hands, one in anatomical space and the other one in external space. Second, in external space, both tactile stimuli are mapped to the same hand. As a consequence, external touchto-hand mapping no longer provides redundant hand information to separate the two stimuli. Rather, for a touch to the crossed index finger, the external relative finger coordinates and the external relative hand coordinates are in conflict. For instance, the touch to the crossed index finger of the left hand would be located at the leftmost finger with respect to the other stimulus, but be mapped to the right hand due to its location in external space. In sum, we suggest that the crossing effect in partial crossing/between hands conditions is a consequence of the (re)mapping of a touch at the crossed index finger to the other hand. This suggestion is consistent with the results of Haggard et al. (2006), who demonstrated that touch to the fingers is mapped to a hand based on anatomical information and based on finger posture. Furthermore, our interpretation is supported by the crossing benefit, which was demonstrated in Experiment 2: Performance was better, rather than worse, in crossed compared to uncrossed conditions. In contrast to the above discussed partial crossing/ between hands TOJ, in Experiment 2, the two tactile stimuli were mapped to the same hand in the uncrossed condition. We suggest that in the crossed conditions, on the contrary, both tactile stimuli were mapped to different hands with respect to

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external space. Thus, for within hands TOJ, externally based touch-to-hand mapping provided redundant information in the crossed, but not in the uncrossed condition. The crossing benefit indicates that this advantage through redundant coding outweighed the conflict which most likely arose, in the crossed condition, between anatomical and external touch-to-hand mappings. In addition, the anatomical touch-to-hand mapping evoked yet a new type of coordinate conflict for one of the two stimuli. For instance, a touch to the left index finger was located at the rightmost finger of the stimulated finger pair. Thus, the relative finger coordinates were in conflict with the assigned hand. In contrast, the other stimulated finger, the left little finger in this condition, was, consistently, the leftmost finger of the pair. When taking into consideration all four of these pieces of spatial information (the relative finger coordinates and the hand coordinates of both stimuli), then responses with the little finger should be more frequent than responses with the index finger. Indeed, in within hand TOJ conditions participants responded more often with the little fingers. Further evidence stems from the analysis of nonregular TOJ responses. Phantom errors at both index and little fingers showed a hand effect: The probability for a phantom error was higher at the targeted hand, as compared to the other hand. Crucially, in our task, only the finger coordinates of touch, but not identity of the stimulated hands, were task relevant. Thus, the hand effect evident in phantom errors again suggests that each touch to a finger was also mapped to a hand. At the little fingers, this hand effect was not influenced by posture, suggesting that the anatomical touch-to-hand mapping of stimuli to these fingers was maintained independent of index finger location. We will discuss phantom errors with the index fingers in the context of body representation below. In sum, we conclude that touch to a finger is localized by integrating several pieces of spatial information: the anatomical identity of the touched finger, its position in external space, the hand to which the touch is mapped in anatomical space, and the hand to which the touch is (re)mapped in external space. These considerations are complemented by the finding that tactile intensity judgments were influenced by the structural representation of the fingers (Rusconi et al., 2009). In this experiment, four tactile stimuli, two to each hand were applied. Thereby, stimulus intensity was constant within one hand, but differed between hands. Participants reported which hand received the stronger stimulation. Consequently, the identity of the stimulated fingers was not task relevant. Still, performance differed between conditions in which homologous fingers of both hands were stimulated and conditions in which nonhomologous fingers of both hands were stimulated. Finally, the results from the indirect crossing conditions underline that adapting an unusual posture has no general effects on touch localization, since TOJ between the little fingers were not modulated by posture. We attributed impairments in TOJ performance to coordinate conflicts between different types of spatial representations. With respect to the present study, the motion projection hypothesis (Kitazawa et al., 2008) predicts that a stimulus to a crossed index finger is assigned to the index finger of the wrong hand. If one posits that the strength of the motion vector


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decreases when both stimuli are applied to the same hand, the motion projection hypothesis would be able to account for our data as well. This is because, TOJ performance was always impaired, when both tactile stimuli were anatomically or externally mapped to the same hand compared to touch-to-hand mappings of both stimuli to different hands. Notably, the conclusions drawn based on the motion projection hypothesis are the same as the conclusions we drew with respect to a coordinate conflict account (see Shore et al., 2002): Tactile localization was influenced by touch-to-hand mappings in anatomical and in external space. What differs between the motion projection account and our current proposal is merely the assumed mechanism behind the impairment of TOJ performance resulting from the conflict between reference frames.

Spatial Representation of the Fingers Independent of Touch So far, we have discussed the effects of coordinate conflicts on touch localization. However, crossing the fingers additionally induced a conflict between different body representations: The postural schema (Head & Holmes, 1911) and the body structural description (Schwoebel & Coslett, 2005; Rusconi et al., 2009). The postural schema keeps track of the position of the limbs in external space. In contrast, the body structural description always localizes the left index finger at the left hand. Thus, the body structural description corresponds to the anatomical reference frame. The phantom errors at the index fingers we observed in the present study represent a unique window into the relation between these two body representations, as they occurred at nonstimulated fingers. Phantom errors at the index fingers were influenced by posture. In the uncrossed conditions, phantom errors were much more likely at the targeted than at the nontargeted hand. However, in the crossed conditions, phantom errors were equally likely at index fingers of both targeted and nontargeted hands, that is, the number of phantom errors at the targeted hand remained constant whereas the number of phantom errors at the nontargeted hand increased with index finger crossing (see Figure 8). Moreover, phantom errors at index fingers of the nontargeted hand were not influenced by the type of the target finger. Thus, a simple confusion of the crossed index fingers cannot explain the increase of phantom errors at the nontargeted hand in the crossed conditions. We propose that in the crossed conditions the index fingers were assigned to the other hand in external space (body postural schema). Thus, phantom errors occurred, as well, when fingers of this externally closer hand were stimulated. In sum, we suggest that more phantom errors occurred at the index fingers in the crossed condition, because touch was mislocalized based on both body representations: the body structural description, which induced phantom errors with fingers of the anatomically same hand, and the postural schema, which defined the externally same hand. The coexistence of these two finger representations is in accordance with the results of Rusconi and colleagues (2009), who found no interaction between hand posture and the body structural description (Rusconi et al., 2009, Experiment 4). Several studies have provided evidence that the calculation of external spatial coordinates requires time. Previous studies ev-

idenced influences of an external reference in a time range of about 80 –190 ms measured from the onset of a tactile stimulus (Azañón & Soto-Faraco, 2008; Heed & Röder, 2010; Overvliet, Azañón, & Soto-Faraco, 2011). The pattern of phantom errors in the current study suggests that this time course extends to the representation of the postural body representation of the fingers. At an SOA of 50 ms, the hand effect in phantom errors was identical for crossed and uncrossed index fingers. In other words, posture did not influence phantom errors at this short SOA. Rather, phantom errors were determined by an anatomical reference frame. It is important that, at SOAs of 110 and 300 ms, the hand effect was inverted: In the crossed posture, more phantom errors were made with the index finger of the nontargeted hand (Figure 9). Thus, at longer SOAs, phantom errors with the index fingers reflected the use of external finger coordinates. Consequently, the postural schema first exerted an influence somewhere between 50 and 110 ms poststimulus, well in line with the time estimates regarding the external coordinates of touch derived from previous studies (Azañón & SotoFaraco, 2008; Heed, 2010; Overvliet, Azañón, & Soto-Faraco, 2011; Rigato et al., 2013; Soto-Faraco & Azañón, 2013). The interpretation of our results entails the existence of a very flexible hand representation. In this view, each index finger is actually assigned to two hands at the same time when the fingers are crossed: its anatomical hand in the body structural description and the hand located at the external location of the finger in the postural schema. As a seemingly paradoxical result, a hand could be represented as having two index fingers– one assigned on the basis of an anatomical reference frame, and a second one on the basis of an external reference frame. Although such double representation may seem counterintuitive, there is ample evidence that the representation of the body is rather loosely bound to human anatomy. Schütz-Bosbach, Musil, and Haggard (2009) investigated the role of self-touch in the representation of body structure. Participants stroked with one hand over some fingers of their other hand, which were interleaved with the fingers of the experimenter. When asked how many fingers participants thought were between the actively or passively stimulated fingers, participants underestimated the number of fingers in the central part of the hand. Participants’ responses could only be accounted for by misrepresentation of the number of fingers at a hand. Furthermore, some stroke patients perceive to own more arms or legs than normal (e.g., Bakheit & Roundhill, 2005; Halligan, Marshall, & Wade, 1993; McGonigle, 2002). A supernumerary arm can even be invoked in healthy participants using a variant of the rubber hand illusion (Ehrsson, 2009), and this manipulation affects reaching with the real arm (Newport, Pearce, & Preston, 2010). Thus, the brain’s representation of the body is surprisingly flexible, and, our proposal of a doubled representation of a finger has counterparts in similar phenomena for arms and legs. Indeed, the representation of the fingers in SI has been shown to be highly plastic (Braun, Schweizer, Elbert, Birbaumer, & Taub, 2000). However, the current results suggest that plasticity for finger representations must exist also in other cortical regions than SI, namely regions which integrate touch and postural information. Summed up, extending the well-known TOJ paradigm to a choice between four rather than two locations has allowed us to



differentiate multiple types of spatial coordinates contributing to touch localization. First, the current results corroborate the hypothesis that touch to the fingers is assigned to both fingers and hands and that information about both is integrated for a final location estimate. Second, our results indicate that touch is mapped to a hand based on both, anatomy and on the external location of the stimulated finger. Further, both the anatomically and the externally based touch-to-hand mapping were used for tactile localization. Finally, our results demonstrate that at least two different body representations of the fingers, the postural schema and the body structural description, were concurrently active.

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Received July 29, 2012 Revision received September 9, 2013 Accepted September 13, 2013 䡲