Previous studies have indicated that crossmodal visual predictions are instrumental in controlling early visual cortex activity. The exact time course and spatial precision of such crossmodal top-down influences on visual cortex have been unknown. In the present study, participants were exposed to audio-visual combinations comprising one of two sounds and a Gabor patch either in the top left or in the bottom right visual field. Event related potentials (ERP) were recorded to these frequent crossmodal combinations (Standards) as well as to trials in which the visual stimulus was omitted (Omissions) or the visual and auditory stimuli were recombined (Deviants). Standards and Deviants elicited an ERP between 50 and 100 ms of opposite polarity known as a C1 effect commonly associated with retinotopic processing in early visual cortex. In contrast, a C1 effect was not observed in Omission trials. Spatially specific Omission and Mismatch effects (Deviants minus Standards) started only later with a latency of 230 ms and 170 ms, respectively. These results suggest that crossmodal visual predictions control visual cortex activity in a spatially specific manner. However, visual predictions do not elicit neural responses that mimic stimulus-driven activity but rather seem to affect early visual cortex via distinct neural mechanisms.
Persistent visual impairments after congenital blindness due to dense bilateral cataracts have been attributed to altered visual cortex development within a sensitive period. Occipital alpha (8-14 Hz) oscillations were found to be reduced after congenital cataract reversal during visual motion tasks. However, it has been unclear whether reduced alpha oscillations were task-specific, or linked to impaired visual behavior in cataract-reversed individuals. Here, we compared resting-state and stimulus-evoked alpha activity between individuals who had been treated for dense bilateral congenital cataracts (CC, n = 13, mean duration of blindness = 11.0 years) and age-matched, normally sighted individuals (SC, n = 13). We employed the visual impulse response function, adapted from VanRullen and MacDonald (2012), to test for the characteristic alpha response to visual white noise. Participants observed white noise stimuli changing in luminance with equal power at frequencies between 0-30 Hz. Compared to SC individuals, CC individuals demonstrated a reduced likelihood of exhibiting an evoked alpha response. Moreover, stimulus-evoked alpha power was reduced and correlated with a corresponding reduction of resting-state alpha power in the same CC individuals. Finally, CC individuals with an above-threshold evoked alpha peak had better visual acuity than CC individual without an evoked alpha peak. Since alpha oscillations have been linked to feedback communication, we suggest that the concurrent impairment in resting-state and stimulus-evoked alpha oscillations indicates an altered interaction of top-down and bottom-up processing in the visual hierarchy, which likely contributes to incomplete behavioral recovery in individuals who had experienced transient congenital blindness.
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