Experiment 1

The first five experiments in this Empirical Section involved the same basic method: Participants were presented with Operant and Observational trials and returned numerical judgments of the interval between events in the sequences. In Operant trials, participants pressed a button at a time of their choosing, which resulted in the delivery of a 100ms 1kHz pure tone after a set interval. In Observational trials, participants did not perform any intentional or causal actions; instead, they were presented with an audible click, which was followed by the same 100ms 1kHz pure tone after a set interval. The click in these trials was recorded from the button box used on Operant trials and thus was identical to the noise made by the micro-switch when participants pressed the button in the Operant trials. At the end of each trial, participants had to estimate the length of the interval between their button press (Operant trials) and the tone or the click and the tone (Observational trials) via numerical estimation. This method has previously been used to successfully demonstrate a Binding effect between intended action and subsequent effects, while no binding occurred between unintentional action (e.g., rubber hand movement) and subsequent events (Engbert, Wohlschlager, & Haggard, 2008). However, this study used short (200-300ms) inter­ event intervals, while Experiment 1 employed intervals comparable to those of Haggard et al. (2002) in an attempt to replicate the reduced Binding shift over longer inter-event intervals. Since Operant trials involved intentional causal action and Observational trials did not, I hypothesized that participants would judge the inter-event intervals to be shorter in Operant trials than in relative Observational trials, with a greater degree of underestimation at shorter relative to longer intervals.

2.1.1.1 Method

Participants

Sixteen Cardiff University students (12 female), with a median age of 20, participated for half an hour and received £3 in payment.

Materials and Apparatus

The experiment was conducted on an Apple iMac, and programmed with Psyscope (Cohen, Macwhinney, Flatt, & Provost, 1993); the psyscope “Button Box” was used for timing o f stimulus delivery and as an input device in the Operant trials. Temporal estimates were entered via the keyboard.

Each experimental trial contained two events, separated by an interval. The first was either a button press, or a click in Operant or Observational trials, respectively; the second was a 100ms 1 kHz pure tone.

A green or red fixation cross at the centre of the screen denoted Operant or Observational trials respectively. Crosses were removed at the beginning of the first event of each trial.

Design and Procedure

The factors Interval (150, 250, 350, 450, 550 and 650ms) and Trial Type (Observational, Operant) were factorially combined in a within-subjects procedure. Operant and Observational trials were blocked so that participants experienced one trial of each interval per block. The order of intervals within a block was random, and Operant and Observational blocks alternated. Each participant worked on 10 Operant and 10 Observational blocks, thus providing 20 separate temporal estimates for each

interval, 10 Operant and 10 Observational trials. The nature of the first block (Operant vs. Observational) was counterbalanced between participants.

Each interval began with a display of a fixation cross at the centre of the screen. The cross was green on Operant and red on Observational trials. On Operant trials, pressing the green button on the response box cleared the fixation cross from the screen and triggered the relevant interval (150, 250, 350, 450, 550, or 650ms), after which the tone was delivered. On Observational trials, the red cross remained on the screen for a random interval between 1500 and 2000ms, after which the click was presented and the cross disappeared; the pure tone was the presented after the appropriate interval. The intervals used were the same as in the Operant condition however they were configured so that the interval began at the start of the "click" stimulus. This was done because we did not know when participants would begin subjectively timing the inter-event interval. Although they were instructed to judge the interval between events, had they commenced subjectively timing during the click stimulus when the inter-event interval began at the end o f the click, it would have resulted in a longer subjective inter-event interval than the objective inter event interval. As such the positioning of the objective inter-event interval at the start of the click stimulus results in a shorter subjective inter­ event interval should the subjective timing begin at any point after the start of the click, thus working against the hypothesis. Each trial ended with an on-screen prompt to provide an estimate (between 0 and 999ms) for the inter event interval.

Participants were informed that they would be partaking in a study of time perception. After giving written consent to participate in the study, participants were provided with a general outline of the experimental procedure, followed by written instructions specific to Operant or Observational blocks. At the start of Operant blocks, participants were informed that the appearance of the tone was wholly dependent on

their actions: Pressing the green button on the Button Box would produce a tone after a set interval. Instructions suggested that participants could delay their button press to see that delivery of the tone was wholly dependent on their action.

Prior to Observational blocks, instructions emphasized that participants were not required to press the key, but would rather passively observe two unrelated events. These instructions were presented at the beginning of each block, in order to facilitate clear discrimination between Operant and Observational blocks

2.1.1.2 Results and Discussion

All statistical analyses for this and all subsequent experiments in this Empirical Section adopted a significance level of 0.5. Each participant returned 10 temporal judgements for each Operant and Observational interval. Participant’s median judgements for each interval were used as the unit of analysis and are displayed in

1000 900 800 +-» | 700 o» z 6oo 500 200 - — ♦— Operant - - Observational 100 150 250 350 450 550 650

Inter-Event Interval (milliseconds)

Figure 2. Experiment 1. Mean Median Temporal Judgment Per Participant. (Error bars show ±1 Standard Error)

Inspection of Figure 2 shows that participants clearly distinguished between the various intervals, and suggests that Operant intervals were judged shorter than equivalent Observational intervals. Furthermore, it appears that this difference increases as a function o f interval duration. A 2x6 ANOVA found significant effects of Trial Type, F{ 1,15)= 14.00, MSF=31373.09, Interval, F(5,75)=31.66, MS£=16129.07, and a Trial Type x Interval interaction, F(5,75)=7.68, MSE=3092.34.

To investigate the interaction with increasing inter-event intervals, we conducted a Slope Analysis that has previously been used in studies examining the numerical estimation of temporal duration (Penton-Voak, Edwards, Percival, & Wearden, 1996). An individual regression slope across the numerical judgments of Operant and

Observational inter-event intervals for each participant was calculated, and these Operant and Observational slopes were then compared with Wilcoxon Signed Rank tests. The slope of numerical Temporal Judgments were significantly less steep, Z=3.411, with 15 participants returning a shallower slope in Operant intervals, with one tie.

Experiment 1 established temporal binding as a robust empirical phenomenon that is not specific to the Libet Clock Method, or indirect measures, but can be reproduced when subjective time is measured directly with a magnitude estimation method. However, the finding that the underestimation of operant relative to observational intervals appears to increase with inter-event intervals as a function of interval length (as demonstrated by or Slope Analysis) is at variance with Haggard et al. (2002b). At their maximum interval of 650ms, perceived tone shift had fallen to 16ms, from a 103 ms and 40ms shift at 250ms and 450ms respectively. These results, in contrast, suggest that perceptual shifts induced via intentional action are stronger when action and outcome are separated by a 650ms interval than when a 250ms interval is involved. The following experiment was aimed at testing the limits of the timeframe over which binding can occur given this surprising finding.