Control of visual cortical responses by microstimulation of the frontal eye field

Not all of the light that stimulates the retina is used to drive perception, behavior, and memory. Visual attention provides a means of selecting relevant signals from among the barrage of incoming information. Psychophysical and neurophysiological studies have established that visual signals are enhanced at the locus of attention. The ability to covertly enhance signals at a particular location has inspired a significant amount of research into characterizing the manner in which visual cortical responses are modulated during attention; however, comparatively less work has been devoted to revealing the neural mechanisms that produce these effects. Recent evidence suggests that during spatially-directed attention, visual response modulations are driven by saccadic eye movement commands. In this dissertation, I describe experiments that examine the functional interaction of saccade preparation and visual coding by electrically stimulating neural tissue involved in eye movement commands and examining the effect on visual responses. More specifically, we microstimulated sites within the frontal eye field (FEF) of macaque monkeys and measured the impact on the activity of neurons in extrastriate visual cortex. We found that visual signals in area V4 are enhanced following brief, subthreshold stimulation of retinotopically corresponding sites within the FEF. We compared the stimulation-driven enhancement to several known effects of voluntary attention, and in all cases FEF microstimulation produced response modulations akin to those observed during attention. The magnitude of response enhancement depended on the effectiveness of receptive field (RF) stimuli as well as on the presence of competing stimuli inside and outside the RF. In addition, the discriminability of V4 responses to stable visual stimuli was transiently increased following microstimulation. Importantly, as in voluntary attention, the improvement in discriminability resulted only from changes in the magnitude of V4 responses, and not in the relationship between response magnitude and variance. Improved visual signaling began within 40 ms of microstimulation onset. These results demonstrate that subthreshold FEF microstimulation drives visual response modulations that, so far, are indistinguishable from the known effects of spatial attention, suggesting that a common neural circuit underlies both saccade specification and the filtering of visual signals.