An Electroencephalographic Investigation of the Filled-Duration Illusion

The study investigated how the brain activity changed when participants were engaged in a temporal production task known as the “filled-duration illusion.” Twelve right-handed participants were asked to memorize and reproduce the duration of time intervals (600 or 800 ms) bounded by two flashes. Random trials contained auditory stimuli in the form of three 20 ms sounds between the flashes. In one session, the participants were asked to ignore the presence of the sounds, and in the other, they were instructed to pay attention to sounds. The behavioral results showed that duration reproduction was clearly affected by the presence of the sounds and the duration of time intervals. The filled-duration illusion occurred when there were sounds; the participants overestimated the interval in the 600-ms interval condition with sounds. On the other hand, the participants underestimated the 800-ms interval condition without sounds. During the presentation of the interval to be encoded, the contingent negative variation (CNV) appeared around the prefrontal scalp site, and P300 appeared around the parieto-central scalp site. The CNV grew larger when the intervals contained the sounds, whereas the P300 grew larger when the intervals were 800 ms and did not contain the sounds. During the reproduction of the interval to be presented, the Bereitschaftspotential (BP) appeared over the fronto-central scalp site from 1000 ms before the participants’ response. The BP could refer to the decision making process associated with the duration reproduction. The occurrence of three event-related potentials (ERPs), the P300, CNV, and BP, suggests that the fronto-parietal area, together with supplementary motor area (SMA), is associated with timing and time perception, and magnitude of these potentials is modulted by the “filled-duration illusion”

EEG investigations of duration discrimination: the intermodal effect is induced by an attentional bias.

Previous studies indicated that empty time intervals are better discriminated in the auditory than in the visual modality, and when delimited by signals delivered from the same (intramodal intervals) rather than from different sensory modalities (intermodal intervals). The present electrophysiological study was conducted to determine the mechanisms which modulated the performances in inter- and intramodal conditions. Participants were asked to categorise as short or long empty intervals marked by auditory (A) and/or visual (V) signals (intramodal intervals: AA, VV; intermodal intervals: AV, VA). Behavioural data revealed that the performances were higher for the AA intervals than for the three other intervals and lower for inter- compared to intramodal intervals. Electrophysiological results indicated that the CNV amplitude recorded at fronto-central electrodes increased significantly until the end of the presentation of the long intervals in the AA conditions, while no significant change in the time course of this component was observed for the other three modalities of presentation. They also indicated that the N1 and P2 amplitudes recorded after the presentation of the signals which delimited the beginning of the intervals were higher for the inter- (AV/VA) compared to the intramodal intervals (AA/VV). The time course of the CNV revealed that the high performances observed with AA intervals would be related to the effectiveness of the neural mechanisms underlying the processing of the ongoing interval. The greater amplitude of the N1 and P2 components during the intermodal intervals suggests that the weak performances observed in these conditions would be caused by an attentional bias induced by the cognitive load and the necessity to switch between modalities

Schematic representation of the experimental paradigm.

<p>The task was to discriminate empty intervals delimited by auditory (A) and/or visual (V) signals. In two sessions, the intervals were defined by two signals of the same modality (intramodal intervals: AA or VV) while in the other two, the signals were delivered from different modalities (intermodal intervals: AV or VA). Each trial began with a fixation point (FP), followed by the presentation of an empty interval (ISI= 450 or 550 ms) bounded by auditory and/or visual stimuli (33 ms each) (S1 and S2 = Stimuli 1 and 2). Then, after the presentation of a fixation point (FP), a visual instruction asked the participants to indicate whether the empty interval corresponded to the short or long interval by pressing respectively “1” or “2” on a Serial Response Box.</p

Electrophysiological activity recorded during the long interval.

<p>Data collected at prefrontal (FP1, FP2, FPz), fronto-central (F3, F4, Fz, FCz, C3, C4, Cz), parietal (P3, P4, Pz) and occipital electrodes (O1, O2, Oz) are presented.</p

Mean CNV amplitude.

<p>Mean amplitude recorded during AA, AV, VA and AA intervals at fronto-central (F3, F4, Fz, FCz, C3, C4, Cz) (A) and parietal (P3, P4, Pz) (B) sites. Bars are standard errors.</p

CNV time course over parietal sites.

<p>Mean CNV amplitude over lateral parietal electrodes (P3, P4) for successive time windows for the short (A) and long (B) intervals. Bars are standard errors. ERPs elicited by the short (C) and the long (D) intervals and averaged over lateral parietal electrodes (P3, P4).</p