MEMORY, 2005, 13 (7), 759±772
Interrupted actions affect output monitoring and event-related potentials (ERPs) P. Andrew Leynes The College of New Jersey, Ewing, NJ, USA Jarret T. Crawford Rutgers University, New Brunswick, NJ, USA Martin L. Bink University of North Texas, Denton, TX, USA Memory for performed and interrupted actions was measured on source recognition and source recall tests in order to investigate output monitoring (i.e., memory for actions). Event-related potentials (ERPs) were recorded during the source recognition test to provide insight into the neural basis of output monitoring (OM). Source identification and recall of performed actions was greater than interrupted actions, thereby replicating the enactment effect. Examination of memory errors revealed that interrupted actions were more often mistaken as performed actions. The ERP data indicated that brain activity elicited by performed actions differed from interrupted and new actions. A clear difference in temporal onset of two ERP effects (i.e., a central-parietal and a frontal ERP difference) was observed, and it supports the previous hypothesis that two distinct processes support OM and source monitoring judgements. The pattern of frontal ERP differences suggested that interrupted actions prompted people to use more systematic decision processes overall to make OM judgements. Central-parietal ERP effects suggested that sensori-motor information was not recollected for interrupted actionsÐrather OM judgements were based on cognitive operations in this case.
Output monitoring (OM) refers to the process of discriminating performed actions from other types of actions (e.g., imagined, observed, or new); consequently it is one type of memory judgement that can have a profound impact on our daily lives. Failures of OM can range from embarrassing (e.g., telling the same story twice) to deadly (e.g., overdosing on medication) and can contribute to failures of prospective memory (i.e., fulfilling future intentions) if one believes that an intended act has already been
completed (Schaefer, Kozak, & Sagness, 1998). OM can be considered a special case of source monitoring (i.e., determining the origin of memory). For example, information may be acquired from an external source (i.e., written text) or it may arise from oneself in the form of thought or actions (i.e., an internal source). Therefore, determining if one actually performed an action or only planned to perform it is a type of source-monitoring discrimination based on two internal sources.
Correspondence should be addressed to P. Andrew Leynes, Department of Psychology, The College of New Jersey, P.O. Box 7718, Ewing, NJ 08628, USA. Email:
[email protected] This research was supported in part by Cooperative Agreement NCC2-1316 from NASA-Ames Research Center with the third author (MLB). The authors thank Brett Bersano, Alyssa Cairns, Melissa Croce, Kendrick Forsthoff, Tamika Francois, Lia Guimaraes, Eric Klein, Valerie Maher, Andrea Perger, and Vanessa Talarico for their help with collecting the data. We also thank John Jonides and one anonymous reviewer for helpful comments on an earlier draft of this paper.
# 2005 Psychology Press Ltd http://www.tandf.co.uk/journals/pp/09658211.html
DOI:10.1080/09658210444000377
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A theoretical basis of source monitoring, the Source Monitoring Framework (SMF; Johnson, Hastroudi, & Lindsay, 1993), has been developed by Johnson and colleagues and provides a strong foundation for studies of OM. According to the SMF, the origin of information is not directly retrieved (as in a source tag). Instead, source judgements are the result of a complex set of decision processes whereby qualitative memory characteristics (e.g., perceptual details, spatial and temporal information, semantic detail, affective information, motoric information, and cognitive operations) are activated, evaluated, and weighed to attribute memories to particular sources (Johnson et al., 1993). Two principal types of decision process (i.e., heuristic or systematic) govern the evaluation of the memory characteristics that are activated. Whether decision processes are heuristic, systematic, or some mixture of both kinds of processes depends on the combinations of qualitative characteristics that differentiate the sources being considered and the agenda of the decision maker. Heuristic decisions tend to be fast and non-deliberate, such as generally considering the average differences in qualitative characteristics of the various sources (e.g., more sensori-motor information indicates that the action was actually performed). Systematic decisions involve a more strategic analysis of activated information including such processes as retrieving additional supporting memories, discovering relations, and engaging extended reasoning (see Johnson et al., 1993 for an extended discussion of these decision processes). As the SMF suggests, the type of processes used in many OM situations may differ somewhat from those used when recalling or recognising actions. Memory for actions has been the subject of many investigations (see Cohen, 1989; Engelkamp, 1998, for reviews). The focus of this literature has been on the ``enactment effect'', which is the finding that recognition and recall of performed tasks is generally superior to that of actions that are learned verbally. Although this literature has some obvious overlap with OM, simply recalling actions does not appear to involve the exact same set of judgement processes as ascribing actions into different types (i.e., an OM judgement). For example, Kormi-Nouri (2000) found that although recall of performed and imagined actions was superior to actions that were encoded verbally (i.e., the enactment effect), no recall differences were observed between performed and imagined actions. However, Leynes and Bink (2002) found
that more performed than imagined actions were recognised and recalled when participants made an OM judgement (i.e., distinguishing an action as being ``performed'', ``imagined'', or ``new''). The fact that OM is affected by a manipulation that does not influence free recall of actions suggests that ascribing actions to a particular class of item (i.e., a source judgement) involves distinct, but probably overlapping, processes as compared with free recall or recognition of actions. Despite the many studies of action memory (see Leynes & Bink, 2002, for a previous review), little is known about the factors that affect OM, due to the general lack of studies that ask people to make OM judgements. However, there are recent ERP studies that have begun to investigate how people discriminate performed actions from other types of actions. In one study, Leynes and Bink (2002) asked participants to perform some actions and to imagine and plan to perform another set of actions at study. ERPs were recorded during a source recognition test that asked participants to determine if the test probe was a performed action or an imagined action, or was new. They observed ERP effects that were similar to other source-monitoring ERP studies (cf. Rugg, Schloerscheidt, & Mark, 1998; Senkfor & Van Petten, 1998; Wilding, 1999; Wilding & Rugg, 1996). An old/new difference emerged approximately 600 ms post-stimulus onset and was followed by ERP differences at frontal electrode sites. Most source-monitoring studies (which have predominantly used verbal materials) report an old/new effect around 600 ms, which tends to be largest at parietal electrodes. This effect has been hypothesised to reflect recollection of information stored in memory (e.g., Paller & Kutas, 1992; Paller, Kutas, & McIsaac, 1995; Wilding, 2000; Wilding & Rugg, 1996). The old/new difference in the Leynes and Bink study tended to have a more central distribution; consequently, the authors suggested that the more central distribution reflected activation of the motor cortex when sensori-motor information associated with imagining or performing actions was reactivated during the test (see also Senkfor, Van Petten, & Kutas, 2002 for a similar claim). Leynes and Bink (2002) also found that, compared with planned and new actions, performed actions elicited more positive ERPs at right frontal electrode sites and more negative ERPs at left frontal electrode sites. Frontal ERP differences, like those that are observed in many sourcemonitoring studies, are presumed to reflect post-
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retrieval processing (Leynes, 2002; Mecklinger, 2000; Ranganath & Paller, 2000; Wilding, 1999; Wilding & Rugg, 1997a), and are similar to the decision processes described in the SMF. Leynes and Bink appealed to the Cortical Asymmetry of Reflective Activity (CARA) hypothesis (Nolde, Johnson, & Raye , 1998) to explain the pattern of frontal ERP differences. The CARA hypothesis contends that right frontal lobes support heuristic processes, whereas the left frontal lobes are recruited to support more systematic decision processes. Thus, the authors suggested that performed actions were evaluated with more heuristic processes (because they elicited more positive ERPs at right frontal electrodes) due to the availability of strong sensori-motor information. In contrast, more positive ERPs at left frontal sites was taken as evidence that the left frontal lobes were recruited to support more systematic processes in order to evaluate planned and new actions. Senkfor et al. (2002) also conducted an ERP study of OM. They compared memory for performed, watched, imagined actions versus items encoded using a non-action task. In their study, people saw objects at study and either created and performed an action with the object, watched the experimenter perform an action with the object, merely imagined performing an action with the target object, or estimated the cost of the target object. At test, participants saw a picture of an object and made a source judgement based on the encoding condition (i.e., ``performed'', ``watched'', ``imagined'', or ``cost''). Although Senkfor et al. report a number of important behavioural and ERP differences between these conditions, the differences involving performed actions are of particular importance to the present investigation. They found that memory for events with action information (i.e., performed, watched, and imagined) elicited more positive ERPs at frontal electrodes than non-action items (i.e., cost) from 800 to 1400 ms post-stimulus onset. Memory for events with motion (i.e., performed and watched) differed from those actions without overt motion (i.e., imagined and cost) at parietal electrode sites approximately 1000 ms post-stimulus onset. Finally, performed actions elicited more positive ERPs than the other conditions at posterior electrodes 600±800 ms poststimulus onset. Senkfor et al. argued that these (and other) differences observed across these conditions reflect content-specific brain activity during retrieval of memories that ``vary only in
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the nature of the episodic memories that they trigger'' (2002, p. 416). The differences in ERP effects across the Leynes and Bink (2002) and the Senkfor et al. (2002) studies underscore the importance of interpreting OM results in terms of the SMF. For example, source judgements are influenced by aspects of the test context, such as the combination of sources (Marsh & Hicks, 1998) and cues available at test (Dodson & Shimamura, 2000), as well as the types of decision processes used at test (Johnson et al., 1993). Because these factors appear to also effect ERP activity (e.g., Leynes, 2002; Leynes, Bink, Marsh, Allen, & May, 2003), any differences in ERP effects observed between the Senkfor et al. and Leynes and Bink studies are likely due to the different test contexts and to the different types of decision processes prompted on each memory test. In fact, the general purpose of the present study was to manipulate test difficulty in order to further explore the importance of decision processes in OM judgements. According to the SMF, the process of discriminating between sources will be more difficult when they are similar, and there is evidence that this is true for OM (Foley & Ratner, 1998). The purpose of the present study was to extend the findings of Leynes and Bink (2002) by investigating OM when the modes of output were very similar. Towards this end, participants performed some actions and began to perform others that were interrupted by the experimenter before they could be completed. Using the same basic design as Leynes and Bink, we recorded ERPs during a source recognition test, and this phase was followed by two source recall tests in order to provide insight into the effects of interrupted actions on OM processes. Based on the available evidence and the SMF, we expected that discriminating between performed and interrupted actions would result in different OM processes than when performed and imagined actions are discriminated, because partially completed actions (i.e., interrupted) would also contain some vivid sensori-motor information. Therefore, sensori-motor information would not be as valuable for source discriminations (i.e., diagnostic) as when performed and imagined actions are considered. Although it is fairly straightforward to predict that discriminating between performed and interrupted actions would alter OM processes, exactly how this manipulation could affect OM processes, and by extension ERP activity which
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reflects these processes, is a little more ambiguous. Based on the SMF, OM processes could be altered in at least two different ways. First, making the actions more similar could alter the mix of decision processes (i.e., heuristic and systematic) used to evaluate the sources. Recall that Leynes and Bink (2002) argued that changes in frontal ERPs were evidence that different decision processes were used to evaluate performed actions as compared with imagined and new actions. In the case of the present study, the vivid sensori-motor information associated with interrupted actions might cause more systematic decision processes to be used to evaluate performed, interrupted, and new actions, which should eliminate the right and left frontal ERP differences observed by Leynes and Bink. Second, we would expect changes in centralparietal ERP effects if interrupted actions affect the information activated (or recollected) during OM because recollection of information has been linked with parietal old/new ERP differences (e.g., Paller & Kutas, 1992; Paller et al., 1995; Wilding, 2000; Wilding & Rugg, 1996). For example, we expect the central-parietal old/new effect would be larger for performed than interrupted actions if people recollect more information for performed actions. Central-parietal ERP differences might also be observed if interrupted actions alter the kind, rather than the amount, of information because there is some evidence that ERPs are sensitive to the type of information consulted at test (see Johnson, Nolde, Mather, Kounios, Schacter, & Curran, 1997, for a similar argument). For example, Rugg, Allan, and Birch (2001) provided evidence that parietal ERPs varied according to the type of encoding, and suggested that parietal ERPs are sensitive to the kind of ``retrieval orientation'' used at test, and Leynes et al. (2003) observed attenuated parietal ERPs when people based source judgements on cognitive operations (e.g., thoughts at the time of study) rather than modality. Thus for the present experiment, interrupted tasks might cause people to activate different kinds of information (e.g., cognitive operations) because sensori-motor information would not clearly distinguish actions; consequently, central-parietal ERP differences might be observed because the kind of information activated varies between sources. Analysing the effect of interrupted actions on behavioural responding should also be informative. On the source recognition test, we expected that more interrupted tasks would be mis-
attributed to performed actions (as compared with performed actions misattributed to interrupted actions) because the additional sensori-motor information should result in a greater tendency to believe interrupted tasks were actually performed.1 Performance on the source recall test has the potential to add to our understanding of the enactment effect, because Leynes and Bink (2002) observed a source recall enactment effect for performed versus imagined actions when a similar effect was not observed for mere recall of these types of actions (Kormi-Nouri, 2000). Furthermore, recall errors can provide additional insight into the factors that affect OM. For example, Leynes and Bink found that output-monitoring errors differed for performed versus imagined actions. If similar results are found in the present experiment, then that would be evidence that interrupting actions is an important factor in OM decisions.
METHOD Participants A total of 24 College of New Jersey students (7 males, 17 females), aged 17±22, volunteered in exchange for course credit. An incentive of $25 was offered to the subject who had the best memory and fastest reaction time among those tested. All participants were right-handed (Oldfield, 1971), and reported that they did not have a history of neurological disease.
Stimuli A set of actions were used as stimuli in the present experiment. The actions were taken from those used by Leynes and Bink (2002) and expanded to include a total of 150 actions. Each action was simple, three words long, and contained a unique verb and noun (e.g., ``bend the wire'', ``crack the egg'', etc.).2 Each character presented on the monitor extended 0.48 of visual angle vertically and horizontally. For each participant, the computer randomly assigned 50 actions to each of the three item types (performed, interrupted, or new). 1
This prediction is also based on the fact that two studies (Foley & Ratner, 1998; Leynes & Bink, 2002) have reported that more misattributions were observed for imagined actions. 2 A complete list of actions is available, on request, from the authors.
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The objects used to perform the actions were in plain view and were located in the central room in the laboratory suite.
Procedure During the study session, participants encountered 100 actions for a non-specific test of memory for actions. Participants were instructedÐprior to the study phaseÐthat some of the actions would be performed and that some would be interrupted (i.e., the experimenter would say ``STOP'' during the completion of the action). The experimenter read all actions to the participant. Next, the participant located the appropriate object and began to perform the action. When the action was to be performed, the experimenter allowed the participant to complete the action without any interference. When the action was to be interrupted, the experimenter let the participant initiate the action as much as possible and then stopped the participant before the action was completed.3 Before the study trials began, participants were allowed a few minutes to examine the objects to become familiar with the experimental materials. Immediately following the study phase, the experimenter described the ERP recording procedures and attached the electrode cap (see below). Therefore, there was an average delay of 50 minutes between the study phase and the source recognition test. The testing phase consisted of one source recognition test, which was completed first, and two recall tests (described below). After the electrode cap was attached, the participant entered a separate, electrically shielded room to record ERPs during the source recognition test (ERPs were not recorded during recall). During the source recognition test, the entire action phrase was presented one at a time in the centre of the computer monitor and remained on the screen until a response was registered. All 50 performed, 50 interrupted, and 50 new actions were presented in a random sequence. Participants were instructed to decide if the action was performed at study, was interrupted, or was new, and register their response by pressing one of
3 In all cases, the participant was allowed to locate the object and complete the task as much as possible before being interrupted. For example, if the action was ``complete the puzzle'', the participant located the puzzle, began to manipulate the puzzle pieces, and placed a few of the puzzle pieces before the experimenter interrupted the task.
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three marked keys on a computer keyboard. They used their right-hand index finger to respond ``interrupted'', the right-hand middle finger to indicate ``performed'', and their thumb to indicate ``new''. Following the source recognition test, participants re-entered the room used for the study phase; thus, the stimulus materials (i.e., objects) were in plain view for the two subsequent recall tests. Stimulus materials were returned to their original position before the recall test commenced in order to remove any obvious cues that could be used to determine if actions were previously performed. On the ``recall performed'' test, participants were asked to verbally recall (in any order) all of the tasks that they performed during the study phase. Participants were asked to verbally recall all of the tasks that were interrupted during the study phase on the ``recall interrupted'' test. For both recall tests, the experimenter recorded the actions on a sheet of paper that was obscured from the view of the participant. The order of the recall tests was counterbalanced across participants, and the experimental session was concluded and participants were debriefed after the second recall test.
ERP recording procedures ERPs were recorded during the source recognition test only. Potentials were sampled at a rate of 150 Hz from 29 Ag/AgCl electrodes mounted in an elastic cap (Neuromedical Supplies, Inc.) referenced to the left mastoid online. Electrode voltages were re-referenced offline to average of the left and right mastoids. Electrodes were placed over the frontal lobes (Fp1, Fp2, F7, F3, Fz, F4, F8, FC3, FCz, FC4), temporal lobes (FT7, FT8, T7, T8, TP7, TP8), parietal lobes (CP3, CPz, CP4, P7, P3, Pz, P4, P8), occipital lobes (O1, O2), and at the central position on the scalp (C3, Cz, C4). Vertical electrooculogram (vEOG) was recorded bipolarly using two Ag/AgCl electrodes affixed above and below the subject's left pupil. Horizontal electrooculogram (hEOG) was recorded bipolarly from identical electrodes and attached to the outer canthi of both eyes. Interelectrode impedance was below 5 kO. EEG and EOG signals were recorded with a Contact Precision Instruments amplifier with a 0.03±40 Hz frequency range (73 dB attenuation). During the test phase, EEG and EOG were sampled for 300 ms before the presentation of the action phrase for a duration of 2800 ms.
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ERP data processing The data were digitally filtered off-line using a 30 Hz lowpass filter (73 db/oct). Ocular artefacts were corrected using the algorithm developed by Semlitsch, Anderer, Schuster, & Presslich (1986). Trials on which ERP amplitudes exceeded + 150 mV were excluded from the analyses (M = 9%, S.D. = 13% equally distributed across source type).
RESULTS Unless specified otherwise, all effects reported in this article are significant at the a = 0.05 level.
Behavioural data Source recognition. The proportion of performed, interrupted, and new responses as a function of source type are displayed in Table 1. In a few instances (i.e., 14 cases out of 2400 total actions studied), people completed interrupted actions before the action was stopped by the experimenter. These trials were reclassified to performed actions; therefore, scores were adjusted for each person to reflect the true number of performed and interrupted actions. Although source recognition was at near ceiling levels, these results are entirely consistent with other investigations of memory for actions (Foley & Ratner, 1998; Goff & Roediger, 1998; Thomas & Loftus, 2002) as well as other ERP studies of source monitoring (Leynes & Bink, 2002; Ranganath & Paller, 2000; Senkfor & Van Petten, 1998). The first set of analyses revealed that correct identifi-
TABLE 1 Proportion of responses on the source test as a function of source type Source type Claim ``Performed'' ``Interrupted'' ``New''
Performed
Interrupted
New
.97 (.03) .01 (.02) .02 (.02)
.19 (.09) .75 (.13) .06 (.06)
.01 (.01) .01 (.01) .98 (.02)
The value in parentheses is the standard deviation. Bold numbers represent hits for each source type.
cations of all three items differed as a function of source, F(2, 46) = 79.97, MSE = .01. Post-hoc analyses revealed that both performed actions, F(1, 23) = 82.45, MSE = .01, and new actions, F(1, 23) = 83.28, MSE = .01, were identified at a higher rate than interrupted actions. The proportion of performed and new actions correctly identified did not differ, F(1, 23) = 1.5, p > .10. Detailed analyses of source-monitoring errors were conducted to provide additional insight into source-monitoring processes, because two kinds of errors may result on a source-monitoring test of this type. A source may be misattributed to the other source (i.e., a source confusion) or it may be called ``new'' (i.e., missed). Interrupted actions were misattributed (.19 vs .01) F(1, 23) = 80.40, MSE = .0047, and missed (.06 vs .02) F(1, 23) = 19.71, MSE = .0015, more often than performed actions. The difference in misattributions is similar to the finding that participants were more likely to falsely claim imagined actions were performed rather than that performed actions were imagined (Foley & Ratner, 1998; Leynes & Bink, 2002). In addition, errors for interrupted actions were not equally distributed because more interrupted actions were incorrectly labelled ``performed'' (.19) than ``new'' (.06), F(1, 23) = 44.02, MSE = .0041. One possible mechanism that could have influenced the pattern of source accuracy is a response bias that is often seen in source monitoring (Johnson et al., 1993), which is reflected in the false alarms (i.e., new test items incorrectly identified as one of the old sources). More specifically, Leynes and Bink (2002) found a bias to call false alarms ``planned'' (i.e., an it-had-to-be planned effect). However, no such bias was detected in the present experiment, F(1, 23) = 1, p > .10, although the overall false alarm rate was very low in this experiment. Response times for correct source judgements on the source recognition test were calculated and analysed. An analysis of variance revealed that reaction times varied as a function of source, F(2, 46) = 69.36, MSE = .03. Post-hoc analyses revealed that identification of performed (M = 1623 ms, SD = 208) and new sources (M = 1673, SD = 245) were equivalent, F(1, 23) = 1.54, p > .10. Thus, the overall difference in reaction times arose from the fact that the time to identify interrupted actions (M = 2149 ms, SD = 339) was longer than both performed actions, F(1, 23) = 90.01, MSE = .04, and new actions, F(1, 23) = 81.59, MSE = .03.
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TABLE 2 Proportion (standard deviation) of retrospective and output monitoring recall errors as a function of source recall test Source test
Recall performed Recall interrupted
Retrospective memory errors
Output monitoring errors
Omission
Wrong verb
Wrong noun
Misattribution
Intrusion
Repeat
Total errors
.165 (.106) .226 (.088)
.005 (.011) .005 (.010)
.003 (.007) .004 (.010)
.084 (.064) .008 (.012)
.002 (.006) .001 (.004)
.010 (.019) .008 (.016)
.269 (.107) .252 (.090)
Source recall. Proportions of correctly recalled actions were computed for both sources. Recall was measured by verbal recall of both types of actions on separate recall tests. Thus, two methods (i.e., liberal and strict scoring) were used to score verbal recall because participants could verbally reproduce an action but fail to recall the verbatim action phrase. For example, recalling ``jingle the chain'' when the action was rattle the chain would be scored as a hit using the liberal method but as an error using the strict method. More performed actions (M = .83, SD = .10) were recalled than interrupted actions (M = .69, SD = .08) t(23) = 6.88, when the liberal scoring method was used and when the strict scoring method was used (performed M = .67, SD = .10 vs interrupted M = .54, SD = .08) t(23) = 6.21. Thus, the ``enactment effect'' was observed using both scoring methods. Table 2 displays the proportion of source recall errors as a function of the type of error (i.e., retrospective or output monitoring error) and source recall test. Retrospective memory errors were classified into errors of omission (i.e., cases when the action was completely forgotten), ``wrong verb'' errors (i.e., cases when an incorrect action was recalled), and ``wrong noun'' errors (i.e., cases when an incorrect object was recalled). OM errors were classified into misattribution errors4 (i.e., cases when the actions belonging to the other source were falsely recalled), intrusions (i.e., cases when new actionsÐseen briefly during the source recognition testÐwere falsely recalled as old), and repeats (i.e., cases when actions were recalled 4 More specifically, misattributions for ``recall performed'' test are when interrupted actions were falsely recalled as performed. This error would refer to real-world situations when people failed to complete a task but believe that the action was completed (e.g., missing a dose of medication because you believe that you have already taken it). Misattributions for the ``recall interrupted'' test are when actions performed at study were mistakenly believed to have been interrupted. This kind of OM error could also be very serious in some cases (e.g., taking a dose of medication twice).
more than once during the test). Recall errors were analysed using a repeated measures ANOVA with factors of source recall test and type of error. The main effect of Source Test was not reliable, F(1, 23) < 1, p > .05, indicating that the same number of errors were made on the two source recall tests. However, a significant Source Test by Error interaction indicated that the type of error varied as a function of the source test, F(5, 115) = 19.26, MSE = .001, p < .001. Visual inspection of the errors in Table 2 suggested that this interaction arose from a greater number of omission errors when recalling interrupted actions and a greater number of misattributions when recalling performed actions. This observation was confirmed by a post-hoc contrast of omissions and misattributions and a separate comparison of all other errors (i.e., wrong verb, wrong noun, intrusions, and repeats). The Source by Error interaction was significant for the comparison of omissions and misattributions, F(1, 23) = 37.97, MSE = .003, p < .001, but not for the comparison of the other types of errors, F(3, 69) < 1, p > .05. Therefore, two important findings emerged from the analysis of recall errors. First, the enactment effect resulted from fewer omissions that were observed when performed actions were recalled. Second, recalling performed actions did not result in fewer errors overall because more interrupted actions were falsely recalled as performed actions. Thus, the pattern of OM errors on the recall test mirrors the pattern observed during the source recognition test in that interrupted actions were more likely to be mistaken as performed. On both recall tests, people rarely recalled incorrect verbs or nouns when they failed to recall the verbatim action phrase (see Table 2). Instead there was a tendency to recall similar verbs and nouns when recalling interrupted (similar verb errors: M = .158, SD = .068; similar noun errors: M = .059, SD = .040) and performed actions (similar verb errors: M = .138, SD = .056; similar noun errors: M = .064, SD = .047). These results are
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additional evidence that verbatim memory is dissociable from memory for the gist of the actions (Earles, Kersten, Turner, McMullen, 1999; Leynes & Bink, 2002).
ERP data ERPs were averaged from accurate test trials on the source recognition test to form three ERPs (i.e., performed, interrupted, and new); therefore, the few instances where participants completed interrupted actions were omitted from the ERP data analysis. A mean number of 43, 33, and 44 trials comprised the performed, interrupted, and new item ERPs, respectively. ERPs for incorrect source judgements were not analysed due to an insufficient number of trials to form reliable ERP averages (i.e., less than 16 trials). To quantify the ERP effects, seven amplitude measures were computed as the average activity over consecutive 200 ms intervals relative to the average activity 300 ms before the onset of each respective action (i.e., 600±800, 800±1000, 1000±1200, 1200±1400, 1400±1600, 1600±1800, and 1800±2000 ms after the onset of the test probe). These amplitude measures were analysed separately at 25 electrodesÐomitting the most extreme frontal (i.e., Fp1 & Fp2) and parietal electrodes (i.e., O1 and O2). Thus, an analysis of variance model that contained a factor for Source (performed/interrupted/new), Anterior/Posterior (AP) electrode placement (five levels front to back), and Left/Right (LR) electrode placement (five levels left to right) analysed the ERP amplitudes. Analyses incorporated the Geisser-Greenhouse correction for nonsphericity, and significant effects are reported with corrected degrees of freedom when appropriate. Figure 1 displays the grand average ERP data recorded during the source recognition test.5 The results of the omnibus analysis of ERP amplitudes revealed significant effects involving the factor of Source for the 600±800 [Source: F(1.7, 39.1) = 5.74], 800±1000 [Source: F(1.88, 43.24) = 6.70], 1200±1400 [Source X LR: F(3.04, 69.92) = 5.30; Source X AP X LR: F(4.8, 110.4) = 3.49], and 5
Visual inspection of the grand average ERP data in Figure 1 suggests that the ERP data differed for the conditions at an earlier time window (i.e., around 300 ms post-stimulus) than was analysed. To investigate this effect, we analysed the ERP activity from 250 to 350 ms post-stimulus onset using the same ANOVA models described earlier. None of the effects were significant from this analysis.
1400±1600 ms time intervals [Source X LR: F(2.64, 60.72) = 5.69; Source X AP X LR: F(3.84, 88.32) = 3.09]. None of the interactions involving the factor of source was significant after re-scaling (McCarthy & Wood, 1985) suggesting that differences in ERP topography resulted from differential activation of similar neural generators. The significant effects in the omnibus analyses were explored with pairwise comparisons of the ERPs elicited by the three sources using the same ANOVA model as the omnibus analysis. Table 3 presents the results of these post-hoc analyses as a function of the time interval. As is evident from inspection of Table 3, the performed ERPs were not reliably different from interrupted ERPs during the 600±800 ms interval. However, both performed and interrupted ERPs were reliably more positive than new ERPs, particularly at central electrode sites. During the 800± 1000 ms interval, a different pattern emerged. The performed ERPs continued to be more positive than new ERPs and were also more positive than interrupted ERPs, whereas interrupted and new ERPs did not differ. During the later intervals (i.e., 1200±1400 and 1400±1600 ms intervals), the performed and interrupted ERPs did not differ. The significant source by electrode location interactions in the post-hoc analyses indicated that both old sources elicited more positive ERPs at frontal electrodes (particularly at right frontal sites) and more negative ERPs at central-parietal electrodes (see Figure 1).
Summary of ERP effects The two typical ERP effects reported during source monitoring were detected in the analyses of the ERP data. Beginning approximately 600 ms after the onset of the test probe, both old sources elicited more positive ERP activity than new items (i.e., similar to the parietal old/new effect described in the introduction). However, this difference tended to be more centrally distributed in this experiment (cf. Leynes & Bink, 2002) and was reliable only for performed actions during the 800±1000 ms interval. From 1200 to 1600 ms, two additional ERP differences emerged: one at frontal electrodes and one at parietal electrodes. At the frontal electrodes, both old items elicited ERPs that were more positive than new, particularly at right frontal electrodes (i.e., the right frontal old/new effect typically observed in source-monitoring ERP
Figure 1. Grand average ERP activity recorded during the source recognition test at all 29 electrode sites. Positive microvoltage is plotted upward on all graphs in the figure. The top two graphs present uncorrected vertical and horizontal ocular activity. The remaining graphs are arranged according to the electrode placement on the scalp. Electrodes placed over frontal areas are placed at the top of the figure. Electrodes placed over the left hemisphere appear on the left side of the figure.
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TABLE 3 Significant effects from the pairwise post-hoc comparisons conducted to explore significant effects detected in the omnibus ERP analyses Time interval and factor
Comparison Performed vs interrupted
Performed vs new
Interrupted vs new
600±800 Source
±
F(1, 23) = 9.07**
F(1, 23) = 4.30a
800±1000 Source
F(1, 23) = 12.00**
F(1, 23) = 7.49*
±
1200±1400 Source 6 LR Source 6 AP 6 LR
± ±
F(1.56, 35.88) = 6.29* F(3.20, 73.60) = 3.47*
F(1.56, 35.88) = 8.42** F(2.56, 58.88) = 5.59**
1400±1600 Source 6 AP Source 6 LR Source 6 AP 6 LR
± ± ±
F(1.64, 37.72) = 8.82** F(1.40, 32.20) = 5.30* ±
± F(1.40, 32.20) = 9.53** F(2.08, 47.84) = 5.33**
a
AP = Anterior/posterior electrode placement factor in the analysis, LR = Left/Right electrode placement factor in the analysis, p value equals .05; *p < .05; ** p < .01; ***p < .001.
studies). This right frontal old/new effect was accompanied by a difference at parietal electrode sites that was characterised by more negative amplitudes for old items (relative to new). This parietal negativity tended to be larger at parietalcentral electrodes.
DISCUSSION The purpose of this experiment was to investigate the brain activity associated with OM for performed and interrupted actions. At study, participants performed some actions and began to perform an equal number of other actions, which were interrupted by the experimenter. Next, ERP activity was recorded during a source recognition test that required performed, interrupted, and new actions to be identified. The source recognition test was followed by two source recall tests: one that required recall of performed actions and one that required recall of interrupted actions. The behavioural data revealed that source recognition of performed actions was superior to interrupted actions and more performed actions were recalled. Thus, the ``enactment effect'' was observed on the source recognition and recall tests. These results indicate that the enactment effect does not arise simply from the presence of sensori-motor information in the memory trace because interrupted actions were partially performed. Instead, the process of completing an
action appears to distinguish the action in some way; thus, future studies should determine how much of the task must be performed in order to produce the enactment effect. Examination of OM errors on both the recognition and recall tests provided valuable insight into the basis for OM judgements. On the source recognition test, more interrupted actions were misattributed to performed actions than were missed. Similarly, OM errors on the recall tests revealed that interrupted actions were more likely to be falsely recalled as performed. Thus, both the source recognition and source recall data suggest that interrupted actions were often mistaken as performed actions. This pattern of errors differs from when memory for performed and imagined actions was tested (Leynes & Bink, 2002). Although cross-experimental comparisons warrant considerable caution, these observations suggest that the sensori-motor information associated with beginning an action leads to more OM confusions than do memory traces without overt sensori-motor information, such as imagined actions. Therefore, additional studies should investigate the effect of interrupting actions, because it may play a major role in the efficacy of OM. The ERP data provided additional insight into the cognitive processes that are used when making output-monitoring judgements. Recall that Leynes and Bink (2002) reported a centralparietal old/new effect that was followed by
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frontal ERP differences when people discriminated between performed, imagined, and new actions. Similar ERP effects (one centralparietal effect followed by frontal ERP differences) were observed in the present experiment. Because these two ERP effects appear to reflect different kinds of processing during remembering, the importance of the frontal ERP effects are discussed first and are followed with a discussion of the central-parietal ERP differences. Based on the SMF, we reasoned that making two sets of actions more similar (by interrupting one set of actions) could change the decision processes used to judge sources, thereby affecting the pattern of frontal ERP differences. In the Leynes and Bink (2002) investigation, any recollections of overt performance could be used to accurately judge an action as having been performed, which would, arguably, lead people to output monitor with more heuristic decision processes overall. However, sensori-motor information is present in both performed and interrupted memory traces; therefore, OM judgements of performed and interrupted actions should be based on a more systematic evaluation of memory. This hypothesis was somewhat supported because the right and left frontal ERP differences elicited by performed actions in the Leynes and Bink (2002) experiment were not observed in the present experiment. Although the frontal ERP differences observed in the present experiment and in the Leynes and Bink study are consistent with the CARA hypothesis, they highlight the need for future brain-imaging studies that directly manipulate source-monitoring decision processes to clarify the role of the frontal lobes in memory (cf. Leynes, 2002). Central-parietal ERP differences were also detected in the present experiment, and this difference was much larger for performed actions than interrupted actions. We believe this ERP difference reflects recollection (or activation) of sensori-motor information in the memory trace (cf. Leynes & Bink, 2002). Recently, Rugg et al. (2001) suggested that early parietal ERPs were sensitive to the kind of information retrieved at test. Similarly, Leynes et al. (2003, Experiment 2) observed attenuated parietal ERP effects when information activated by the test probe was misleading (i.e., modality cues) and when source judgements could be based on cognitive operations during encoding. Collectively, these results suggest that the parietal ERP differences may reflect the kind of information that is recollected.
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According to this argument, the central-parietal ERP effect in the present experiment was attenuated because people recollected less sensorimotor information for interrupted actions, because the sensori-motor information could not be used to effectively differentiate interrupted actions from performed actions. Instead, cognitive operations were activated and evaluated when interrupted actions were correctly identified. The behavioural data provided some support for this argument in that when interrupted actions were erroneously remembered (on both source recognition and source recall) they were more likely to be attributed to performed actions; therefore, the presence of sensori-motor information in interrupted actions was misleading when it was reactivated. Based on these findings, we believe that people based judgements of interrupted actions on the cognitive operations associated with encoding (i.e., remembering that the experimenter stopped the completion of the action) rather than on presence of sensori-motor information in the memory trace. It is important to recognise that these results are consistent with previous conclusions that parietal ERP differences reflect recollection of information (e.g., Paller & Kutas, 1992; Wilding & Rugg, 1997a). However, there appears to be a growing body of evidence that suggests these differences will vary when the context alters the kind of information that is recollected (Leynes et al., 2003; Rugg et al., 2001). Based on these recent findings as well as previous studies, we suggest that the parietal ERP differences reflect recollection of specific kinds of information. In most experimental situations to date, memory judgements have been based on sensory (or motoric) information and are, therefore, accompanied by a parietal (or central-parietal) old/new ERP difference. However, people may purposely fail to recollect this information when the information is potentially misleading (the present experiment and Leynes et al., 2003) or when the retrieval orientation is altered at test (Rugg et al., 2001). In these cases, the early ERP amplitudes may vary for individual items because some judgements may be based on recollection of sensory information, whereas other judgements may be based on alternative information (e.g., cognitive operations, affective information, or semantic information). The other finding of interest in the present study was a late parietal ERP effect that was observed beginning at 1200 milliseconds, which
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corresponded to the onset of reliable differences at frontal electrode sites. Both performed and interrupted actions elicited more negative ERPs than new. Similar effects have been reported during OM (Leynes & Bink, 2002) and during source monitoring (Johansson, Stenberg, Lindgren, & RoseÂn, 2002; Ranganath & Paller, 2000; Wilding, 1999; Wilding & Rugg, 1997a, 1997b). In each of these previous studies, this pattern of ERP activity was presumed to reflect distinct processes from those reflected in other ERP effects. Wilding and Rugg (1997a) suggested that the parietal negativity reflected response-related processes (e.g., reaction times). However, present data do not directly support this hypothesis because performed and new reaction times were equivalent yet performed ERP amplitudes were more negative (for similar results see also Johansson et al., 2002; Leynes & Bink, 2002). Although the process that subserves this parietal negativity remains elusive, two alternative explanations for this effect remain viable. Ranganath and Paller (2000) suggested that it might reflect response confidence, and Johansson et al. suggested that it might reflect activation of the anterior cingulate cortex which may be more or less active under conditions of response competition. Clearly, future studies are needed to delineate the process that gives rise to this ERP effect. We have argued that interrupting actions affects the kind of information activated during OM; however, one could argue that our results can be explained by a simple difference in the depth of encoding between performed and interrupted actions.6 According to this view, performed actions could have been viewed as a more important set of actions by our subjects and processed more deeply than interrupted actions. Although this account can explain some of the results, it cannot explain all of our findings. First, there was no reliable difference in source recall errors between performed and interrupted actions (although the pattern of errors differed for the two sources). If there were simple encoding differences between performed and interrupted actions, then we should have observed fewer errors when performed actions were recalled. Second, the entire pattern of ERP effects is inconsistent with those observed when items differ in depth of encoding. More specifically, the depth of encoding appears to modulate the size of the parietal old/ 6
We thank John Jonides for raising this issue.
new ERP differences; however, the late frontal and late parietal negativity ERP effects are not observed (Gonsalves & Paller, 2000; Paller & Kutas, 1992; Paller et al., 1995). The fact that we observed the late frontal and parietal ERP differences in the present experiment suggests that the cognitive processing during this OM test was more elaborate than the processing produced by items that vary in encoding depth. In addition, this argument is inconsistent with the previous literature on action memory. Cohen (1983) found that placing additional importance on some actions affected memory for actions encoded verbally (VTs) but not actions performed (subject performed tasks; SPTs), and levels of processing manipulations do not affect memory for SPTs (Cohen, 1981). In light of all of these considerations, our data are more consistent with our original predictions, based on the SMF, than they are with a depth of processing account. Nevertheless, this counterargument raises an important OM issue because performed actions obviously differ from other types of actions. Thus, future studies should not only try to understand OM retrieval processes but also isolate exactly how performed actions differ.
Conclusions The results of the present study suggest that OM is affected by incomplete or interrupted actions. When judging if actions had actually been performed, interrupted actions were often confused with performed actions on both source recognition and recall tests. These results suggest that beginning to perform an action interferes with OM processes, presumably because sensori-motor information in memory traces is misleading. This conclusion was further supported by the ERP data. The most important ERP finding was that the central-parietal ERP difference, which has been hypothesised to reflect activation of sensorimotor information (Leynes & Bink, 2002), was greater for performed than interrupted actions. This result was interpreted as evidence that recollection of sensori-motor information can be attenuated when it is potentially misleading and when alternative information (e.g., cognitive operations) can be used to inform source judgements. The present study and another investigation of ERPs and OM (Leynes & Bink, 2002) have contributed to our general understanding of how
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people judge actions that have been performed. The results of present study suggest that interrupted actions might be more likely than imagined actions to be confused with performed actions. At a practical level, this result suggests that minimising distractions might be an effective strategy to improve OM, which is particularly important when this kind of decision can have serious consequences (e.g., when taking medication). In addition, the ERP data across both studies suggest that OM judgements of performed actions are based on more heuristic decision processes when the actions differ in overt versus covert performance, whereas more systematic decision processes are deployed when discriminating between performed and interrupted actions. Thus, the latter type of discrimination may depend more on properly functioning left frontal areas. Although these advances are important, there are many more variables that affect output-monitoring judgements and are likely to also influence brain activity. For example, the similarity of the actions (Foley & Ratner, 1998), the actor who performs the actions (Markham, 1991), ageing (Guttentag & Hunt, 1988; Koriat, Ben-Zur, & Sheffer, 1988), and the number of times actions are imagined (Goff & Rodeger, 1998; Thomas & Loftus, 2002) are all factors that affect the ability to output monitor. Investigating the effect of these and other variables on OM and measures of brain activity will undoubtedly lead to a better understanding of source memory in general, and why people sometimes fail to complete intentions (Schaefer et al., 1998). Manuscript received 3 November 2003 Manuscript accepted 23 July 2004 PrEview proof published online 4 November 2004
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