Neural processing of visual attention: modulation of the orientation-induced gamma response

Stimulus-induced gamma oscillations are a general neuronal feature, and are thought to play a functional role in visual processing. If gamma oscillations indeed reflect cortical processing, their degree of synchronisation should be modulated by attention. Using magnetoencephalography, we investigated how oscillatory responses to a stimulus optimal for inducing gamma in visual cortex changes with spatial attention. In separate blocks, subjects traced the orientation of either a parafoveal grating patch or a small line at fixation that each unpredictably and independently rotated up to 40 degrees around one of four angles, but were both always present. We observed a sustained attention-related increase in gamma power (30-70 Hz) in early visual cortex contralateral to the grating, supporting a role for gamma in visual processing, even as early as V1/V2. In addition to gamma, we also investigated modulations in other frequency bands, and found the classic decrease in alpha power (5-15 Hz) with attention, strongly supporting our attentional manipulation. We subsequently investigated how actively inhibiting a stimulus affects the gamma response, by manipulating the behavioural relevance to grating stimuli, providing further insights in the functional significance of gamma oscillations in visual processing.