Visual selectivity and top-down modulation of neurons in monkey V2 during free viewing.

Neurons in early visual cortex are selective for local visual features but their responses can also be influenced by factors like border ownership and selective attention. Most of what we know about the function of these neurons is based on neurophysiological studies in monkeys that hold their direction of gaze fixed while isolated visual stimuli are presented (controlled viewing). However, during natural behavior, primates visually explore cluttered environments by changing gaze direction several times each second (free viewing). How does the visual cortex work under these conditions? We explored neuronal responses using a foraging task in which the monkey chooses where to look and when. Geometrical figures were displayed and the monkey was rewarded for fixating the center of a figure for at least 200ms. In each trial an array of 10 figures (5 squares and 5 triangles) was presented, one of which was randomly associated with reward, and a cue informed the monkey whether it was a square or a triangle. The monkey sequentially fixated the cued shapes (and sometimes also the other shapes) in search for reward. Single neurons were recorded from area V2. After mapping the receptive field of a cell, the figure array was constructed so that fixating the center of a figure would, in most cases, bring an edge of another figure into the receptive field. We show that neural feature selectivity, border ownership coding and attentional modulation can be measured reliably with this method despite the fast pace of fixations (mean time between saccades about 350ms). Some neurons showed enhancement when the figure at the receptive field was the target of the next saccade, but only when that figure was on the preferred border-ownership side of the neuron, indicating a link between saccade planning and mechanisms of contour grouping. Meeting abstract presented at VSS 2015.