S.L. Bressler, M. Ding, H. Liang

Center for Complex Systems & Brain Sciences

Florida Atlantic University

The understanding of bottom-up and top-down influences in the visual cortex is of great theoretical importance. A number of elaborate theories of visual cortical function have been devised that posit separate roles for bottom-up and top-down effects. We used Local Field Potentials (LFPs), recorded from different visual cortical areas, as a means to explore these effects experimentally. The Directed Transfer Function (DTF), a statistical measure of directional influences in time series data based on multivariate autoregressive modeling, was used to disambiguate directional influences in the visual cortex. The DTF was computed from LFPs recorded from sites in different parts of the visual cortex of macaque monkeys performing a visuomotor pattern discrimination task. DTF analysis was performed in 50 msec long windows that were stepped point by point over the length of the trial. In each window, a DTF spectrum was computed over the range of 0 to 100 Hz for all pairwise combinations of recording sites, and in both directions for each pair. One site was considered to exert a causal influence on another whenever a significant rise in the DTF occurred as compared to the prestimulus baseline level. Bottom-up causal influences were those that were exerted from striate to extrastriate sites, and top-down influences were those exerted from extrastriate to striate sites. Separate bottom-up and top-down causal influences could be observed in the visual cortex as different DTF time courses. In some cases, the top-down influence preceded the bottom-up, suggesting that a top-down priming influence was exerted in these highly trained monkeys prior to actual stimulus processing. A rapid alternation of predominance between bottom-up and top-down influences was observed during the time of stimulus processing, suggesting that reciprocal influences may play a crucial role in visual pattern discrimination, as proposed by several theories of visual perception.