Human visual cortex area V2 is necessary for visual awareness
The human visual cortex (that encompasses posterior parts of the occipital lobes in the very back of the brain) is composed of multiple functionally distinct areas. According to classical hierarchical processing models, neurons in the primary visual cortex (also known as V1) are sensitive to fairly simple visual stimulus features (such as lines of certain orientation), and as one progresses from the V1 to the neighboring area V2 (and from V2 to higher order areas) inputs converge so that neurons begin to respond to increasingly complex visual features, such as perceptual objects (e.g., chairs, cows, hats) in the lateral occipital complex. What has puzzled researchers, however, is the point(s) at which visual awareness takes place in this processing chain; while there are studies indicating that removal of/damage to V1 results in lack of visual awareness, it is possible that this is due to V1 distributing information to higher-order areas, rather than V1 generating visual awareness per se.
Salminen-Vaparanta and colleagues, by inducing currents on the cortical surface using transcranial magnetic stimulation (TMS), specifically disturbed the functioning of area V2 in healthy volunteers (N.B. the disturbance of a cortical area using TMS is highly transient and does not produce any longer-lasting adverse effects). During this transient disruption of area V2, the subjects lost awareness of visual stimuli that were presented to them. Specifically, suppressing the V2 (without concomitant suppression of V1) 44-84 ms from the onset of a visual stimulus resulted in lack of conscious percept of the stimulus. The results of Salminen-Vaparanta thus suggest that area V2 is necessary for conscious visual experience. Methodologically, their study nicely demonstrates how TMS can be utilized to probe the role of various cortical areas in higher-order cognitive functions, such as visual awareness.
Reference: Salminen-Vaparanta N, Koivisto M, Noreika V, Vanni S, Revonsuo A. Neuronavigated transcranial magnetic stimulation suggests that area V2 is necessary for visual awareness. Neuropsychologia (2012). http://dx.doi.org/10.1016/j.neuropsychologia.2012.03.015