Voltage-sensitive dye imaging of collinear patterns in the visual cortex of a behaving monkey

Abstract

Accumulating psychophysical and physiological evidence suggest the involvement of early visual areas in the process of visual integration and specifically in local facilitation of proximal and collinear stimuli. However, the physiological evidence is primarily based on single cell recording and much less is known about the population level processing. To investigate the early integration mechanisms at the population level, we performed voltage-sensitive dye imaging that is highly sensitive to subthreshold population activity, and imaged from the primary visual cortex (V1) and extrastriate cortex (V2) of a behaving monkey. The animal was trained on a simple fixation task while presented with collinear or non-collinear patterns of small gratings, Gabors or short oriented bars. Following the presentation of a single small visual stimulus (target), cortical response showed increased local activation at the corresponding retinotopic site over V1 area, as expected. The evoked response spread laterally over several mm within area V1. When the animal was presented with an additional high-contrast flanking visual stimulus, activation spread also to the gap between the corresponding patches of activation. Facilitation in terms of increased activity at the corresponding retinotopic site of the target as well as reduced latency of its response were observed for low contrast target bars, while almost no facilitation was observed for high contrast targets. The facilitation effect and its time course depended on the target flanker separation distance, suggesting the role of lateral spread of activity. Finally, the observed facilitation was smaller then the expected linear sum of the separate responses to the target and flanker at the target location. These results suggest that neuronal population activity in area V1 is involved in local visual integration processes, and specifically in the increased sensitivity for low-contrast visual stimuli surrounded by high contrast flankers.