Exp Brain Res (2010) 203:465–470 DOI 10.1007/s00221-010-2199-1
R ES EA R C H N O T E
Temporal binding of action and eVect in interval reproduction GruVydd R. Humphreys · Marc J. Buehner
Received: 14 August 2009 / Accepted: 15 February 2010 / Published online: 4 May 2010 © Springer-Verlag 2010
Abstract We report two experiments demonstrating temporal binding between action and outcome (Haggard et al. 2002a) as measured in a temporal reproduction paradigm. Our results show that the eVect is empirically robust, does not rely on repeated presentation of Wxed intervals, truly aVects time perception, and persists across intervals much longer than in earlier demonstrations with the Libet Clock paradigm (Libet et al. 1983). Keywords Temporal binding · Time perception · Interval reproduction The timings of actions and their resultant eVects are critical for the success of many tasks from the leisurely (playing cricket) to the dangerous (driving on a busy motorway). Haggard et al. (2002a), however, demonstrated an implied shortening of the interval between action and outcome: Using the Libet clock paradigm (Libet et al. 1983), in which participants indicate the time of an event with respect to a fast moving clock-hand on a regular clock-face (2.56 s per full rotation), Haggard et al. asked participants to indicate the time of either their own intended button press, or a tone. These single event trials served as a baseline of judgment error. In operant trials, participants pressed the button at a time of their choosing, and this resulted in the presentation of a tone after some interval. Participants were asked to indicate by means of the Libet clock the time at which they either pressed the button, or heard the resultant tone. Relative to their respective baseline measures, the indicated time of these events had
G. R. Humphreys · M. J. Buehner (&) School of Psychology, CardiV University, Park Place, CardiV CF10 3AT, UK e-mail:
[email protected]
shifted: button presses occurred subjectively later, while the tone appeared earlier. In eVect the interval between related events was reported as being shorter than veridical. In forward models of motor control (Wolpert 1997), an intended movement creates an “eVerent copy” of itself that predicts the outcome of the movement. Ultimately, the predicted and actual outcomes of the movement are compared, and if the eVerent copy accurately predicted the outcome, the relationship between the two is strengthened. Haggard et al. (2002a, b) suggested that a strong relationship would lead to the temporal attraction of intended action and eVect: the Temporal Binding phenomenon. Given the above explanation of the observed shifts as part of a mechanism that aids learning of the relationship between intended action and eVect, it seems logical to assume that a pre-requisite for the appearance of the temporal shifts (and by extension shortening of the interval between them) is the presence of an intentional action. A great deal of empirical evidence supports this. For example, these binding shifts still occur when participants observe the experimenter depressing the button, but not when the button depresses of its own accord (Wohlschläger et al. 2003), nor when the button is depressed by a rubber hand (Wohlschläger et al. 2003). The temporal contiguity between action and eVect is an important cue in the post-experience attribution of agency (Wegner 2002). The introduction of a lengthy delay between action and eVect (without apparent knowledge of underlying mechanisms that may explain the delay, cf. Buehner and May 2002) would likely void any sense that the individual was a causal agent in the chain of events. Indeed, Haggard et al. (2002b) demonstrated a decrease in the amount of shift in the reported onset of the tone with increasing intervals (from 250 to 650 ms). Although the aforementioned experiments indicate that the Temporal Binding eVect is robust, as they were all
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conducted using the Libet clock there exists the possibility that the binding eVect is in some way an artifact of the procedure. Despite its venerable status, the paradigm is still subject to a great deal of both support and criticism. A recent article (Danquah et al. 2008), for example, suggests that participants’ judged timing of events is subject to systematic biases arising from the sensory modality through which they experienced the stimulus and the speed of the rotating clock hand. Our aim here was to test the generality and robustness of the binding eVect using a well-established procedure in time perception research: interval reproduction. If temporal binding indeed aVects the perception of the interval then it should be possible to observe these eVects directly and where they are supposed to happen: at the perception of the interval. Alternatively, temporal binding might only eVect shifts in event perception, without necessarily targeting the representation of the interval separating the constituent events. Stetson et al. (2006), for instance, induced temporal binding via an action-outcome task. On 60% of trials, the action generated an outcome after a Wxed interval, while the remaining 40% of trials decoupled the ‘outcome’ from the action, so that it occurred at a diVerent time than what participants had expected, including before the action itself. The Wxed action–outcome interval (implemented in 60% of trials) varied between blocks and was either 35 ms or 135 ms. Exploiting the unreliable temporal structure of this setup, Stetson et al. collected temporal order judgments for actions in relation to the ‘outcome’. Critically, they found that the point of subjective simultaneity had shifted forward in time in the 135-ms block relative to the 35-ms block: ‘outcomes’ on this block, which objectively happened after the action, were perceived to have happened before it. This illusory reversal of temporal order suggests that binding aVects representations of when in subjective time events happen, rather than representations of the intervals separating them. In fact, illusory reversals of action–outcome intervals would be hard to reconcile with a subjective shortening of the interval separating them, because it would imply that the interval subjectively turned negative. Whether temporal binding aVects the perception of events or intervals is of interest because the two imply diVerent theoretic implications. Shifts in event perception suggest that action–outcome binding is eVected via realignment of sensuo-motor time (see Kennedy et al. 2009; Stetson et al. 2006) and thus might lead to subsequent performance errors (Cunningham et al. 2001). Reduction in perceived interval length on the other hand suggests either a delayed awareness of causal actions (Buehner and Humphreys 2009), or a slowing of the internal clock (Wenke and Haggard 2009). Engbert et al. (2008) reported that verbal estimates reXect relative underestimation of action–outcome intervals.
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Humphreys and Buehner (2009) furthermore demonstrated that action–outcome intervals as long as 4 s are subjectively shortened as indicated via their verbal estimates. Intervals of this length are well outside the boundary of event shifts induced by sensual-motor realignment or the forward model of motor planning. Moreover, Humphreys and Buehner also reported that the extent to which subjective intervals are shortened increased proportionally with the length of the interval, suggesting indeed that the internal clock slowed down. While these studies suggest that the binding eVect is not simply an artifact of the Libet clock method and might implicate time perception, magnitude estimation is also prone to post-dictive response biases, such as responses being assigned values that conform to Wxed numbers ending in 00 and 50 (Wearden 2006), and other taskdemand induced distortions. Consequently, we sought to provide further evidence for the generality and robustness of the eVect, its persistence across longer intervals, and, speciWcally, the direct involvement of time perception. One further limitation of the previously discussed verbal estimation studies (Engbert et al. 2008; Humphreys and Buehner 2009) is that they all employed a Wxed number of repeating inter-event intervals. This is problematic for two reasons. Firstly, employing repetitions of Wxed intervals inevitably results in higher predictability between action and eVect, which in turn strengthens the association between the two, thus leading to temporal attraction (e.g. Haggard et al. 2002a, b). Secondly, repeated use of the same Wxed intervals might exacerbate response biases (cf. Wearden 2006), such that verbal estimates reXect categorical responses rather than continuous percepts of elapsed time. Our aim with this study was to provide a stronger and more convincing demonstration of subjective shifts in time perception by avoiding repetitions of predictable intervals. We modiWed the procedure used by Humphreys and Buehner (2009): Participants were exposed to temporal intervals between either their own action and a subsequent tone, or two unrelated tones and had to reproduce the relevant intervals by holding down a key. Over two experiments we demonstrated that participants reproducing the interval between action and eVect returned a greater degree of negative reproduction error in action–eVect sequences relative to observed unrelated sequences, suggesting they consistently underestimate the duration of the interval between the two related events.
Experiment 1 On operant trials, participants pressed a button, which was followed by a beep; on observational trials, a click noise (identical to that of the button press) was followed by a beep. On both, participants had to indicate the length of the
Exp Brain Res (2010) 203:465–470
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A Reproduction Error (ms)
Fig. 1 Experiments 1 (a) & 2 (b) mean reproduction error
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of the green cross for a random interval (1,500–2,000 ms) before the Wrst auditory stimulus (“click”) that was then followed by a tone, and again participants indicated the length of the interval by depressing the yellow button. Depression of the green button during observational trials had no eVect. Actual and reported interval length were recorded and entered into the analysis.
Participants
Results and discussion
40 CardiV University Undergraduates participated in this and another experiment not detailed here. They received £4 as payment. Materials, Design & Procedure All experimental runs were conducted on Apple iMacs running Psyscope (Cohen et al. 1993), and used the Psyscope “Button Box” for timing and input. Each participant experienced a 30-trial block each of both operant and observational trials. An experimental trial consisted of two events: a button press in operant trials or a click noise in observational trials, followed after a random interval (between 1200 and 1600 ms) by a 100 ms 1 kHz pure tone. The observational click stimulus (»120 ms duration) was recorded prior to the commencement of the experiment and consisted of a recording of the brief depression and release of a button on the Box, presented at a natural volume. The random inter-event interval commenced immediately after the end of this stimulus. On operant trials, the inter-event interval was triggered as soon as the button was pressed. To indicate the start of a new trial, a green cross was presented on-screen. This disappeared at the start of the Wrst event of each trial (button press or click noise). Participants were instructed that they would be partaking in a simple study of time perception. Operant and observational trials were blocked, and the order of blocks was counterbalanced. In an operant trial participants were instructed that pressing the green button on the button box would result in a tone after an interval, following which they were prompted to depress the yellow button for a duration equal to that of the interval between the button press and tone. Observational trials commenced with the display
Commencing a response before being indicated to onscreen resulted in the response not being recorded, with a mean of 0.93 misses per participant. For each remaining trial, the actual inter-event interval was subtracted from the participant’s reproduced interval to calculate the reproduction error; thus, while 0 error indicated an accurate reproduction of the interval, negative and positive errors respectively indicate under- and overestimation. Each participant contributed a mean operant and observational judgment error (based on the average error across the block) toward the analysis. Error scores falling two standard deviations outside the overall mean were excluded, resulting in 4 participant exclusions. The results for the remaining sample are displayed in Fig. 1a. Visual examination of Fig. 1a suggests that while participants slightly underestimated the interval in the observational condition (M = ¡11.35 ms), the interval was underestimated to a greater degree in operant trials (M = ¡174.33 ms) t(35) = 4.94, P < 0.05. As predicted, participants reliably underestimated the duration of the interval between their own operant action and subsequent eVect relative to observed events. This result suggests that temporal binding is a robust phenomenon, which can be measured with standard methods used in the time perception literature. Moreover, the interval range we employed (1200–1600 ms) corroborates our earlier observation (Humphreys and Buehner 2009) that binding occurs at super second intervals and is not limited to the millisecond level. While we instructed participants in the observational trials to judge the interval between the two events (i.e. from
inter-event interval by holding down a key for the appropriate time. Given the results of previous temporal binding studies, we hypothesized that participants’ reproductions in the operant condition would indicate a shorter reproduced interval relative to the observational condition.
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the end of the Wrst stimulus to the beginning of the second), they may in fact have commenced subjective timing the interval at any time during the presentation of the Wrst stimulus. Given that the objective inter-event interval commenced at the end of this click stimulus with its duration of approximately 120 ms, the comparison of operational and observational trials may be biased, as observed observational intervals might potentially then be longer (by up to »120 ms) than operant ones. Experiment 2 thus implemented a more conservative test of the binding hypothesis, with the observational interval beginning at the start of the Wrst stimulus. In eVect this assumes that participants likewise commence timing the interval at the very start of the click stimulus. Naturally, if participants begin timing later than that (i.e. anywhere within the 120 ms of the click stimulus), they would experience overall shorter intervals on observational than on operant trials. Because the error score is always calculated relative to the length of the random interval determined by the computer (irrespective of whether this interval begins at the beginning or end of the click stimulus), this would result in an increase of negative reproduction error in the observational relative to the operational trials. Thus, this change in method results in a much more conservative test of the binding hypothesis.
Experiment 2 Method Participants A new sample of 43 CardiV University undergraduates, participated in return for course credit. Materials, Design & Procedure As in Experiment 1, except that in the observational trials the random interevent interval (which would then be followed by the tone) commenced at the beginning of the click stimulus, not at the end. Results and discussion As before, commencing a response before being indicated to on-screen resulted in the response not being recorded, with a mean of 2.74 misses per participant. Three participants were removed from subsequent analysis because their returned judgment errors were more than two standard deviations from the mean. Figure 1b displays participants’ mean reproduction errors. As in Experiment 1 it appears that participants demonstrated a larger degree of negative reproduction error (underestimation) in the operant (M = ¡345.04) than the observational condition (M = ¡149.25), t(39) = 4.33,
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P < 0.05. Therefore, we once again demonstrated a binding eVect. Comparing the two experiments, we note that participants in Experiment 2 generally underestimated both intervals more strongly than participants in Experiment 1. One possibility, suggested by an anonymous reviewer, is that this simply reXects diVerences in subjective timing between the two participant cohorts. We certainly agree that the within-subjects eVect and its replication under such conservative conditions are of central interest here. Nonetheless, we Wnd it instructive to consider whether cross-experimental diVerences might have resulted in overall larger negative reproduction errors. A comparison of the two methods reveals a diVerence in total possible observational inter-event ranges to which participants were exposed to: In Experiment 1, the inter-event interval (randomly determined between 1200 and 1600 ms) commenced at the end of the Wrst observational stimulus (»120 ms click noise). If participants started timing anywhere during the duration of the click stimulus (including its onset), the interval range in observational trials would have been increased by up to 120 ms, resulting in a 1200– 1720 ms range). In contrast, the operant range was limited to 1200–1600 ms (assuming participants started timing at the moment of their keypress. In Experiment 2, the observational range was programed to commence at the start of the (»120 ms) click stimulus. Thus, participants commencing their subjective timing with the start of the Wrst stimulus would observe inter-event intervals between 1200 ms and 1600 ms. However, as we feel it more likely that participants started timing during this »120 ms stimulus the actual inter-event interval observed is likely to be shorter. Indeed, if participants started timing at the very end of the click stimulus, their observed range would be between 1080 and 1480 ms. It is not surprising, therefore, that our more conservative method resulted in more negative reproduction errors for observational trials relative to Experiment 1. Critically, however, operant intervals were likewise underestimated to a stronger extent relative to Experiment 1, even though operant trials were exactly the same in both experiments. This suggests that temporal binding is not only highly pervasive, but also context-dependent (see also Buehner and Humphreys 2009): Both experiments involved a comparison of operant and observational intervals; the latter were shortened in Experiment 2 relative to Experiment 1, while the former remained the same. We observed that participants’ percepts of the observational intervals (as measured by their temporal reproductions) reXected this objective shortening. Importantly, we also observed that representations of the operant interval changed in a way that preserved the appearance of temporal binding. In other words, operant intervals were perceived to be subjectively shorter
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than observational intervals, even though in all likelihood they were objectively longer. In other words, we argue that (a) operant intervals were evaluated in reference to observational intervals, and that (b) when this latter reference stimulus changed, operant evaluations changed accordingly.
General discussion Our results provide further evidence for the robustness of the temporal binding phenomenon even under the extremely conservative testing conditions of Experiment 2. Temporal binding thus is not conWned to the artiWciality of the Libet clock paradigm. Our method also moves research toward more direct means of observing the subjective shortening of action–eVect intervals, rather than implying this shortening from the shifting of single events. Furthermore, the demonstration of binding with inter-event intervals in excess of 1200 ms corroborates our previous Wndings (Humphreys and Buehner 2009) that binding continues to exist beyond the 650 ms limit implied by Haggard et al. (2002b). We began this article by contrasting two distinct possibilities how temporal binding might be eVected. One concerned shifts in event perception (Haggard et al. 2002a, b) and sensuo-motor realignment (Cunningham et al. 2001; Kennedy et al. 2009; Stetson et al. 2006), the other direct changes in time perception, possibly eVected via a slowing of the internal clock (Buehner and Humphreys 2009; Wenke and Haggard 2009). The former explanation draws on the concept of an eVerent copy of the motor action being compared to its subsequent eVect (e.g. Haggard et al. 2002a, b). Under this account, the match between predicted and sensed outcome leads to a strengthening of the relationship between action and eVect, which in turn induces a post-dictive shift in event perception, and thus, by extension, also a subjective shortening of the inter-event interval. There is one problem when applying this account to the current results: Earlier studies in support of the eVerent binding hypothesis always employed the same, constant, inter-event interval within a block of trials. This was necessary to ensure that an accurate and reliable eVerent copy could be created, which allowed an exact prediction of the eVect. Our experiments, in contrast, employed a variable random interval between 1200 and 1600 ms. Thus, the eVect could, at best, be predicted to fall within a 400 ms window. Given that the total size of reported shifts in event perception is at most around 100 ms under ideal conditions (Haggard et al. 2002a, b), it is unlikely that our paradigm enabled exact eVerent-based predictions. Likewise, temporal realignment (Stetson et al. 2006; Kennedy et al.
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2009) would also require adaptation to a fairly constant interval. Thus, we are tempted to endorse the second account, which assumes that in binding, time perception itself is aVected. Common models of timing involve a pacemaker that pulses at regular intervals, while an accumulator sums the number of pulses and thus indicates the amount of time passed since a reference point. (Gibbon et al. 1984). Perceived interval duration can be manipulated (e.g. Wearden 2008) arguably by increasing or decreasing the speed of this pacemaker, resulting in longer or shorter perceived intervals, respectively. With respect to our results, when participants learn that each button press results in a tone, an intentional movement may trigger a subsequent decrease in pacemaker speed. Support for this interpretation comes from Wenke and Haggard (2009) who demonstrated that temporal resolution during causal action–outcome intervals was impaired relative to passive control intervals. In their task, participants either actively pressed a key to generate a subsequent tone or observed a machine pulling their Wnger down followed by a tone. During the inter-event interval, two subcutaneous shocks were administered to the participants’ Wngers, and participants had to indicate whether they occurred simultaneously or in succession. Wenke and Haggard reasoned that a decrease in pacemaker speed would result in poorer temporal resolution and thus in impaired temporal discrimination. Indeed, they found poorer discrimination early on in cause–eVect intervals, but also found that discrimination performance (and presumably thus also pacemaker speed) returned to normal later on in the interval and before the occurrence of the eVect. A joint consideration of Experiments 1 and 2 reveals an apparent compensatory shortening of operant intervals aimed at preserving the qualitative binding relationship in spite of shortened observational reproductions. This suggests the presence of a cognitive element. A recent study has suggested a deep-rooted connection between pacemaker and information-processing elements in which the change in the speed of one can eVect the speed of the other (Wearden et al. 2010). Alternatively, the presence of causal intentional action might detract attention from time passing (perhaps toward the causal events delineating the interval), which would lead to comparatively more pulses being missed in operant than observational intervals (Zakay and Block 1997). In both cases, we would then observe a shortening of the judged interval between events in the operant condition relative to the observational condition. This interevent shortening may then aid in creating a post-dictive sense of agency as suggested by Wegner (2002), and is likely to occur even over the long intervals used here and by Humphreys and Buehner (2009). It seems more and more likely that temporal binding is an adaptive, powerful mechanism that aids in understanding
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the structure of real world action–eVect sequences and is not merely a response bias speciWc to the Libet clock. More recent results also suggest that temporal binding is a special form of causal binding (Buehner and Humphreys 2009), which can be observed in domains other than time, namely space (Buehner and Humphreys 2010). It is pertinent that we now examine the remaining questions surrounding the origins of the phenomenon, and methods such as those presented here will provide us with the tools to do so. Acknowledgments This research was funded by Engineering and Physical Research Council grant EP/C000444691/1, awarded to MJB.
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