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Ramisha Knight, Gabriele Gratton, Monica Fabiani, Diane M Beck; A TMS-EROS investigation of the role of feedback to early visual cortex in visual awareness.. Journal of Vision 2019;19(10):169a. doi: https://doi.org/10.1167/19.10.169a.
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© ARVO (1962-2015); The Authors (2016-present)
Although it is well established that early visual cortex (EVC) plays a critical role in visual awareness, it is less clear whether this is due purely to its critical role as input to the visual system or whether it is also necessary during continued cortical dynamics. These two possibilities have been difficult to assess, in part because it is difficult to dissociate input from later processing; that is, most visual experiences originate in the eye and enter the cortex via V1. Here, we bypass V1 input functions by using single-pulse transcranial magnetic stimulation (TMS) over left posterior parietal cortex (PPC) to produce visual experiences in the form of a phosphene. We simultaneously recorded activity in bilateral occipital cortex and under the TMS coil using the event-related optical signal (EROS) to ask whether activity in early visual areas or in PPC is predictive of visual awareness. Critically, since TMS to PPC served as the primary input of visual information into the cortex, any downstream activations in EVC that predict awareness would be due to feedback rather than feedforward mechanisms. TMS intensity was set, for each individual participant (N=12), to evoke a phosphene in 50% of the experimental trials. EROS activity predictive of phosphene perception was then identified by comparing phosphene-present versus phosphene-absent trials. Results indicate that visual cortex activity rather than PPC activity predicts the likelihood of perceiving a phosphene. EVC activity predicts awareness in two temporal intervals: 0–24 ms after the TMS pulse, activity in V1–V2 predicted phosphene perception, the timing of which is consistent with the state of EVC at the time of input being predictive of awareness. Additionally, 38–88 ms after the TMS pulse, activity in V3 predicted phosphene perception, consistent with a role of feedback dynamics in visual awareness.
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