NEUROREPORT
MOTOR SYSTEMS
Haptic texture a¡ects the kinematics of pointing movements, but not of eye movements Uta Sailer,CA Thomas Eggert,1 Jochen Ditterich, Marc Hassenzahl and Andreas Straube Ludwig-Maximilians-University, Department of Neurology, Center for Sensorimotor Research, Marchioninistr. 23, D- 81377 Munich; 1 Darmstadt University of Technology, Institute for Psychology, Steubenplatz 12, D- 64293 Darmstadt, Germany CA
Corresponding Author:
[email protected] Recieved 5 November 2002; accepted16 December 2002 DOI: 10.1097/01.wnr.0000059781.23521.ee
Discrepant ¢ndings on the degree of eye^ hand coupling suggest its dependence on the task. One task characteristic modulating this coupling may be the relevance of certain target attributes for each motor system. We tested this assumption by comparing eye and hand movements towards targets of di¡erent haptic texture, a target attribute which is behaviourally relevant only to the hand, not the eye. Pointing to a slippery target (fur) resulted in longer hand
movement time than to a rougher target (sandpaper). This e¡ect was due to an increased ratio of time spent in deceleration. In contrast, eye movement time was invariant across di¡erent haptic target textures. Thus, information about target texture is used c 2003 di¡erently by eye and hand. NeuroReport 14:467^ 469 Lippincott Williams & Wilkins.
Key words: Eye-hand coordination; Kinematics; Motor control; Texture
INTRODUCTION A number of studies have demonstrated that the degree of coupling between eye and hand is dependent on the task. Often, a change in the parameters of one motor system is associated with a change in the parameters of the other system. For example, changes in saccadic amplitude have been shown to transfer to the hand motor system [1,2]. In contrast, other authors have found saccadic behaviour not to be reflected in the hand motor system [3]. The same is true for the influence of hand movements on eye movements. In a pointing task the trajectory of the eye was influenced by a concomitant reach to the target [4]. However, this influence seems to vary with the task. Even under very similar experimental conditions, eye and hand reactions were sometimes very similar, sometimes not [5]. One task parameter to modulate the amount of information exchange between eye and hand may be the relevance of certain target attributes. Some might simply not be considered relevant for one motor system to be transferred from the other. In daily life, hand movements need much more detailed information about the target than eye movements [6]. For an accurate grasping movement, for example, information about various characteristics of the target object, such as its weight or texture, is indispensable. This information, however, is irrelevant for an eye movement towards the same object. The present study investigates the question of whether an exchange of information between eye and hand occurs
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when the information is relevant for one motor system only. If the coupling of eye and hand is dependent on the relevance of target attributes for eye and hand, one would expect that changes in target attributes relevant only to the hand should change only the parameters of hand movements, and not those of eye movements. To this aim, we asked subjects to look and point to (i.e. touch) targets of different surface texture, a target attribute relevant only for the execution of hand movements, not of eye movements. For accurate hand movements, the different friction of these surfaces has to be considered in order to avoid slipping, whereas this parameter does not play a role for eye movements. Using another condition we addressed the question of what causes the change in movement parameters, i.e. the target’s seen or its touched characteristics. Texture seems to be a highly salient object attribute for the haptic system, but less so for the visual system [7], leading Klatzky and colleagues to conclude that the haptic and visual systems have distinct encoding pathways. We tried to distinguish between the effect of haptic contact with the object and its visual appearance. Under one condition, the texture seen did not coincide with the texture eventually touched; under the other, the texture seen was identical with the texture touched. We hypothesised that only the physical contact of the hand with the target texture changes the parameters of hand movements because of its direct relevance for action.
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NEUROREPORT
Stimuli and procedure: Each subject sat in a darkened room 35 cm from a 15 inch flat-screen monitor with the right elbow resting on a padded support and the head stabilised by a chin rest. Two targets were affixed onto the screen at 7 6 cm left and right from the centre. The targets were two round brown patches 2.6 cm in diameter of the materials coarse-grain sandpaper and short-haired soft fur. A fixation spot for the eye was displayed at the centre of the screen. A spot of 5 mm diameter glued 1 cm below served as starting position for the finger to which the subjects were asked to return between trials. After a pseudorandomised interval of 100–500 ms the fixation spot for the eye was replaced by a 12 mm arrow pointing either to the right or left for 100 ms. The subjects were requested to look and point at the patch located in the direction of the arrow as fast as possible. Under one experimental condition the subjects touched the patches. This haptic and visual identification condition involved a control condition with two neutral brown cardboard patches instead of the fur and sandpaper patches, i.e. one cardboard patch to the left and one to the right of the centre. Under a different experimental condition, the visual identification only condition, a plexiglass pane was placed over the fur and sandpaper patches. The subjects saw the different textures of fur and sandpaper, but always touched the plexiglass pane. Each condition consisted of 20 trials to the right and 20 to the left in pseudorandomised order. Each subject participated in all experimental conditions. Order of conditions, stimuli position, and subject’s gender were counterbalanced. Under both conditions half of the subjects had the fur on the right side and the sandpaper on the left. For the other half of the subjects, it was the opposite. Pointing position was measured with an ultrasonic speaker 1 cm in diameter attached to the subject’s right index finger tip. The spatial 3D location of this speaker was measured at a sampling rate of 200 Hz by means of an ultrasonic device (Zebris). Eye movements were monitored with an infrared corneal reflection device (IRIS Skalar), its output was digitised at a rate of 1 kHz. The beginning and end of eye and hand movements was defined by means of velocity criteria (as described in [8]). Maximal latency for an eye or hand movement was 600 ms, minimal latency 80 ms. Only the movement towards the screen was analysed, corrective movements were not.
RESULTS The following movement parameters were investigated: latency (time between arrow onset and initiation of movement), movement time (time between movement onset and end), amplitude (distance between the movement’s start and end position), ratio of deceleration time (time from peak velocity to movement end) to movement time, peak velocity,
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end position variability (standard deviation of the movement’s end position). First, a separate 2 2 2 repeated measures ANOVA was performed for latency and movement time with the factors movement type (eye/hand), texture identification (haptic and visual/visual only), and material (fur/sandpaper). In this, as well as in all other ANOVAs, stimulus position (left/right) was introduced as a between subjects factor. Stimulus position had no effect in any of the analyses. No significant effects were found for latency, showing that texture identification or material affected the latencies neither of eye nor of hand movements. Not surprisingly, a main effect for movement type was found for movement time (F ¼ 378.83; df ¼ 1,10; p o 0.0001), indicating longer movement times for hand movements. There was also an interaction of identification with material (F ¼ 13.49; df ¼ 1,10; p o 0.01). Movement time for the fur and the sandpaper differed more when they were identified both haptically and visually than when they were identified only visually. Haptic identification resulted in longer movement time towards the fur, and in shorter movement time towards the sandpaper (Fig. 1). This interaction was, however, primarily due to hand movements, as indicated by a further interaction of movement type with identification and material (F ¼ 13.67; df ¼ 1,10; p o 0.01) with subsequent post hoc analysis (Scheffe´). Eye movement time in itself did not differ with identification or material.
eye movement time (ms)
Subjects: Twelve right-handed subjects (four women and eight men aged 26–41 years), with normal vision or vision corrected by contact lenses participated in the experiment. They were naive with respect to the purpose of the study.
55
fur
54
sandpaper
53 52 51 50 49 48 47 46 haptic and visual visual only identification 260
hand movement time (ms)
MATERIALS AND METHODS
U. SAILER ETAL.
250
fur sandpaper
240 230 220 210 200 190 haptic and visual visual only identification
Fig. 1. Mean eye and hand movement time and standard error of the mean (n ¼12), depending on type of identi¢cation and material. Top: eye movements; bottom: hand movements (note di¡erent scaling).
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NEUROREPORT
HAPTIC TEXTURE AFFECTS POINTING KINEMATICS
Movement time was further analysed by means of a 2 3 (movement type material) repeated measures ANOVA, the factor material containing the levels fur, sandpaper (haptic and visual identification condition only) and control. A main effect for movement type (F ¼ 362.24; df ¼ 1,10; p o 0.0001) indicated longer movement times for hand movements. More interestingly, a main effect for material (F ¼ 3.84; df ¼ 2,20; p o 0.05) showed that movement time was longest for the fur, shortest for the sandpaper, and intermediate for the control material. This main effect was primarily to due hand movements, as shown by an interaction of movement type and material (F ¼ 5.08; df ¼ 2,20; p o 0.05) with subsequent Scheffe´ test. Eye movement time was not changed by varying the texture of the target. This difference in movement time was not due to differences in amplitude, as shown by a one-factorial repeated measures ANOVA comparing hand amplitude towards the three materials fur, sandpaper (haptic and visual identification condition only) and control (F ¼ 1.52; df ¼ 2,22; n.s.). Hand movement amplitude was constant across the three materials touched. Next, we determined whether the increased hand movement time was due to a proportionally longer deceleration time or lower peak velocity. Movement time and peak velocity were each submitted to separate repeated measures ANOVAs with the single three-leveled factor material. There was a significant main effect for ratio of deceleration time to movement time (F ¼ 5.56; df ¼ 2,20; p o 0.05), showing the highest ratio for the fur, an intermediate ratio for the control material, and the lowest ratio for sandpaper. Thus, pointing to sandpaper resulted in a significantly smaller relative amount of time spent in deceleration (mean 7 s.d. 0.15 7 0.15, n ¼ 12) compared to that for fur (0.27 7 0.19, n ¼ 12) and the control patch (0.24 7 0.11, n ¼ 12; Scheffe´ p o 0.05). No effect for peak velocity was found. As subjects did not make a faster or slower movement depending on the texture of the target, the change in movement time was due to different amounts of time spent in deceleration. As the subjects consistently reported that they experienced the furry patch as slippery, they may have been less precise when hitting the fur than the other targets. Submitting end position variability to a repeated measures ANOVA with the single three-leveled factor material did not reveal any significant effects. Subjects thus maintained a constant landing position across targets of different textures.
DISCUSSION The present experiment showed that target attributes relevant to the hand motor system change the kinematics of only the hand, not of the eye. Hand movement time was longer towards the furry patch than towards the sandpaper patch. Eye movement time did not vary with target texture. Thus, the available information about the target is used in different ways for eye and hand movements. We suggest
that this is due to the differential relevance of haptic target texture to the motor systems of eye and hand. Fikes et al. [9], who found increased movement time for grasping movements with a slippery object, concluded that visually cued, but haptically relevant, characteristics of objects can have temporal consequences prior to contact (p. 329). However, visual cueing alone does not affect precontact movement time. When subjects saw the fur and sandpaper, but touched the plexiglass, movement times were not different. Thus, the movement was determined by what was touched (the behaviourally relevant information) and not by what was seen (the visually apparent information). The same applies to eye movements, because object texture that is irrelevant for accurate eye movements did not affect their kinematics. The variations in movement time were due to variations in the ratio of deceleration time to overall movement time. Functionally, the dependency of deceleration ratio on target texture may be related to different precision requirements [10]. To keep the finger from slipping, the force applied to the target has to remain below a certain threshold. Pointing to the fur requires higher precision, because the range of force has to be smaller in order to avoid slipping. To keep the force applied to the fur within this limited range, lower velocity right before contact may be necessary. This may have lengthened the deceleration phase. In contrast, for the sandpaper, subjects could simply aim straight ahead and let the material stop them.
CONCLUSIONS Overall, the data suggest that the eye and hand motor systems make selective use of the information that is behaviourally relevant for each. This became manifest as independent variation in eye and hand behaviour dependent on the apparent situation. For eye movements it would in fact be counterproductive to be slowed down by a slippery target texture. For hand movements, in contrast, this slowing down is essential in order to avoid slipping off the target. As the requirements for accurate movements differ, it seems efficient that eye and hand represent and use the available target information differently in order to ensure optimal performance.
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Acknowledgements:This research was supported by the Deutsche Forschungsgemeinschaft, SFB 462 Sensomotorik.We thank Judy Benson for copyediting the manuscript.
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