Neural efficiency and processing: understanding visual perceptual processing using transcranial electrical stimulation and fMRI in cortically blind humans

Thesis (Ph. D.)--University of Rochester. Department of Brain and Cognitive Sciences, 2018.This work examines neural efficiency under conditions of noninvasive electrical stimulation, and in patients with damage to primary visual cortex. Transcranial random noise stimulation (tRNS) was used in combination with an equivalent noise paradigm in order to better understand the effect of added low current random noise on visual perception of oriented Gabor stimuli under varying quantities of external white noise. The design implemented allowed us to distinguish online effects of stimulation from offline effects, and the extent to which perception was altered across a variety of stimulus conditions and difficulties. We observe an improvement of neural processing only with online stimulation at high levels of external noise, suggesting that the added noise introduced to the system by tRNS A) can improve perception B) can do so only while actively stimulating cortex during a task and C) improves only stimuli of a specific type, making it critical to consider the effect of noise in tRNS effects moving forward. This work narrows the possible mechanisms by which tRNS may improve perception, and helps to explain another stimulus dimension that may be critical for successfully leveraging the greatest benefit from this type of transcranial stimulation. In patients with damage to primary visual cortex, we present work that considers the neural mechanisms underpinning recovery of visual discriminations in the damaged hemifield of homonymous hemianopic (or cortically blind) patients. Specifically, we examine the role that motion processing and adaptation play in recovery of visual processing following psychophysical training. We find evidence for recovery of motion adaptation that supports the hypothesis that motion adaptation occurs early in the visual hierarchy and is inherited by extrastriate regions (Kohn & Movshon, 2003; Larsson et al, 2016). Previously unreported delays in cortical processing are observed across visual cortex for moving dot stimuli, including the intact hemisphere of cortically blind patients that cannot be accounted for by differences in hemodynamic response. Finally, we observe highly differential functional connectivity that suggests a neural basis for behavioral deficits not only in untrained regions of the damaged hemisphere, but for deficits in visual processing that occur in otherwise intact vision