Entrained delta oscillations reflect the subjective tracking of time

Abstract

The ability to precisely anticipate the timing of upcoming events at the timescale of seconds is essential to predict objects' trajectories or to select relevant sensory information. What neurophysiological mechanism underlies the temporal precision in anticipating the occurrence of events? In a recent article, 1 we demonstrated that the sensori-motor system predictively controls neural oscillations in time to optimize sensory selection. However, whether and how the same oscillatory processes can be used to keep track of elapsing time and evaluate short durations remains unclear. Here, we aim at testing the hypothesis that the brain tracks durations by converting (external, objective) elapsing time into an (internal, subjective) oscillatory phase-angle. To test this, we measured magnetoencephalographic oscillatory activity while participants performed a delayed-target detection task. In the delayed condition, we observe that trials that are perceived as longer are associated with faster delta-band oscillations. This suggests that the subjective indexing of time is reflected in the range of phase-angles covered by delta oscillations during the pre-stimulus period. This result provides new insights into how we predict and evaluate temporal structure and support models in which the active entrainment of sensori-motor oscillatory dynamics is exploited to track elapsing time. We recently demonstrated that participants' accuracy at determining whether an auditory target occurred at the predicted time depends on the pre-stimulus alignment of oscillations in the delta-band. 1 In this initial magnetoen-cephalographic study, we tested the ability of 19 participants with normal hearing to perform a delayed target detection task. Participants were required to listen to iso-chronous sequences of 4 or 5 tones (400 Hz, 150 ms duration per tone, average beat rate: 1 Hz). The occurrence of the last tone of the sequence was manipulated so that it occurred either at the right time (Dt 0 : no delay) or too late (DtC: delayed by 75 ms or 150 ms). The task was to detect whether the last tone was delayed or not with regards to the beat. We primarily showed that before target occurrence, coupled delta (1-3 Hz) and beta (18-22 Hz) oscillations temporally align with upcoming targets and bias decisions toward correct responses. Consistent with earlier hypotheses, 2,3 this suggests that the brain predictively aligns ongoing neural activity so that incoming sensory events occur during an ideal neuronal excitability phase. Such an effect, however , was insufficient to provide a neurophysiologically plausible mechanism to explain how the brain evaluates durations in the time-order of seconds. Because of their implication in the predictive tracking of temporal regularities, 1,2,4-7 we conjectured that neural delta oscillations (1-3 Hz) could further be exploited to measure durations at the seconds timescale (in the order of 0.3-1 second). According to this idea, the entrained delta-band phase-course might be used as an "internal chronograph" to derive a subjective measure of duration. 8 In other words, elapsing time can be tracked through the accumulation of phase-information (i.e. the measure of a trigonometric distance) across time. Therefore , for a given physical duration preceding the target, trials that are associated with longer perceived durations should be associated with increased phase-evidence build-up (i.e., a larger range of phase-angles covered during the pre-stimulus period). When averaging trials, this should result in slightly faster (i.e., higher-frequency)