Abstract
A growing body of psychophysical research suggests that perceptual sampling of complex environments might occur at a theta rhythm (3-8 Hz). Electrophysiological recordings point to a neural origin of this theta-rhythmic sampling mechanism in higher level cortical areas, often associated with exerting top-down attentional influences on perception. Theta oscillations can also be found in extrastriate visual areas when animals see multiple stimuli. However it remains unknown whether theta oscillations can be observed in primary visual cortex (V1) and to what extent their emergence might depend on stimulus properties. To address these questions, we recorded multi-unit activity (MUA) and single unit activity (SUA) from the V1 of two macaque monkeys passively viewing a visual stimulus with variable properties. Analysis of the MUA showed that among the visually responsive electrode sites (n = 107 in Monkey 1 and n = 78 in Monkey 2), more than 50 % showed a statistically significant theta oscillation when the stimulus appeared compared to a baseline period without a stimulus. Doing the same analysis for single units (n = 38 in Monkey 1 and Monkey 2), we found that more than 80 % of the sampled visually responsive units showed a statistically significant theta oscillation. Theta power varied depending on size, contrast, and orientation of the stimulus. Within each of these stimulus property domains (e.g. size), there was usually a single stimulus value that induced the strongest theta. The present study shows that a highly stimulus dependent neuronal theta oscillation can be elicited in V1 at the earliest level of visual cortical processing. In contrast to extrastriate areas, theta can be observed in V1 in the presence of a single stimulus. Stimulus driven theta oscillations in visual cortex might be an additional mechanism for perceptual sampling that occurs at the same frequency range.