The Chronometry of Time Processing in Visual and Premotor Cortices

Time is the most elusive dimension of everyday experience; we cannot touch nor see time nevertheless time is embedded in any sensory experience of the world and we can surely perceive it. How time is extracted from sensory inputs, how it is processed and represented in the human brain is far from clear. During my PhD I have tried to understand how temporal information in the millisecond range is extracted from visual inputs; i.e. how it is encoded/read-out and perceived. Specifically, I have asked “when”, at which stage of temporal information processing (i.e., from sensory drive integration to duration recognition) visual and premotor areas, brain regions known to play a role in temporal computations, are engaged. Focusing on the chronometry of these areas in duration encoding, I tried to better understand the functional role of these areas and at the same time, using stimuli with different sensory load and different durations ranges I have tried to gain insight on the mechanisms underlying duration perception. To test the chronometry of different areas in duration encoding I used Transcranial Magnetic Stimulation (TMS), that I have applied over visual and premotor areas at different timings from the onset (to the offset) of visual stimuli that had to be judged in duration. In other words, I assumed that TMS applied at different timings after visual stimulus onset would affect duration judgments differently depending on the involvement over time of the target area during the processing of temporal information. The combination of high temporal resolution (paired-pulse) TMS and duration discrimination tasks allowed me to test: A. the chronometry of primary visual cortex (V1) and extrastriate area V5/MT during the encoding of visual stimuli with different sensory load i.e., empty intervals and filled durations (chapter 2). B. the chronometry of V1 and Supplementary Motor Area (SMA) in duration encoding and reading out of visual temporal information (chapter 4) and C. to test the existence of a topographic representation of time in SMA (chapter 5). Finally, the experimental data of chapters 2 and 4 were modeled using a recently developed leaky integration model (Toso et al., 2021). This model sees duration perception as a result of the leaking integration of a sensory drive from primary sensory cortex (in Chapter 3 a description of the model with the modelling of chapter 2’s data and in Chapter 4, experimental data and modelling).