Occipital Alpha-TMS causally modulates Temporal Order Judgements: Evidence for discrete temporal windows in vision

Abstract

Recent advances in neuroscience have challenged the view of conscious visual perception as a continuous process. Behavioral performance, reaction times and some visual illusions all undergo periodic fluctuations that can be traced back to oscillatory activity in the brain. These findings have given rise to the idea of a discrete sampling mechanism in the visual system. In this study we sought to investigate the causal relationship between occipital alpha oscillations and temporal order judgements (TOJ) using neural entrainment via rhythmic TMS. 10 participants received 5 TMS pulses (100 ms ISI) per trial leading to neural entrainment at 10 Hz. Directly following the TMS entrainment we presented a sequence of two distinct Gabor patches (68 ms ISI) at varying SOAs (25 to 158 ms after last pulse, 9 SOAs) and asked participants to report the temporal order of the Gabors. This dense sampling method allowed us to probe TOJ performance as a function of SOA: By collecting ~1035 trials per subject over the course of 3 sessions we calculated a TOJ time-series that covered 4/3 of an entrained alpha cycle. A subsequent power analysis of the TOJ time-series revealed a significant modulation at 10 Hz, only for sequences presented contralateral to the cortical TMS site. Furthermore the analysis of the 10 Hz components of individual TOJ time-series showed significant phase clustering between participants. In conclusion we find that certain phases of the entrained oscillation facilitate temporal order perception of two visual stimuli, whereas others hinder it. Our findings support the idea that the visual system periodically compresses information into discrete packages within which temporal order information is lost, in line with the idea of discrete perception. To our knowledge this is the first study providing causal evidence via TMS for a periodic modulation of time perception.

Acknowledgement: DAAD, ERC P-CYCLES