The role of neural oscillations in the visual system and their relation to conscious perception

Neural oscillations are intrinsically linked with attention, vigilance and featural sensitivity and therefore often associated with visual perception. However, the neural oscillation literature remains conflicted on several issues. Here, I describe four experiments investigating these conflicts using a variety of experimental and analysis techniques. We first explored the relationship between the inhibitory neurotransmitter GABA and gamma frequency oscillations in the rodent visual cortex. We found no evidence that synaptic and extrasynaptic GABA concentration altered gamma oscillations, suggesting that GABAergic inhibition cannot be linked directly to GABA concentration and instead depend on postsynaptic receptor kinetics. The second chapter examined how spontaneous alpha activity related to performance in an orientation discrimination task. Alpha amplitude was a significant predictor of reaction time but not task accuracy. The results suggested that alpha can modulate visual perception through top-down mechanisms. Interestingly, we also found that the relationship between alpha activity and task accuracy was determined by the subject’s task expertise. The third chapter examined how exogenous rhythms (generated by chromatic gratings) within visual cortex may interact with ongoing endogenous oscillations. Univariate analysis of single EEG channels revealed significantly higher endogenous power during chromatic than achromatic stimulation. An additional multivariate classifier showed distinct patterns of activity at very high frequencies, suggesting phase coupling between exogenous and endogenous signals. This finding was extended in the final chapter, which examined the neural correlates of rapid chromatic stimulation. Robust BOLD responses were found even when stimuli flickered above the consciously perceptible frequency, indicating that the temporal filtering stage limiting perception is later than V4. Additionally, chromatic preference in ‘colour area’ V4 was strongly dependent on stimulus frequency.