The visual cortex of the rhesus monkey can be divided into several functional areas. Areas V1, V2, and V4 each have a crucial role in visual form and color processing. The time it takes different parts of the brain to process information is an important clue about the neuronal mechanisms involved in visual perception. Comparing stimulus-response latencies of individual cells in different cortical areas gives an estimate of that processing time. Previous studies have measured latencies in visual areas V1, V2 or V4 of anesthetized monkeys, and in V1 of awake monkeys performing a simple fixation task. This paper reports, for the first time, latencies of neuronal responses in areas V1, V2, and V4 of awake monkeys performing a difficult visual discrimination task involving colored patterns. Whereas previous studies used different stimuli for each cell, this study used identical stimuli for each cell. Response latencies here were shorter, and more importantly, differences between latencies in different areas were also shorter, than previously reported. Although it has been inferred from previous studies that neuronal processing was too slow to allow feedback to play a role in visual processing, these new results show that there is ample time for feedback mechanisms to contribute to visual perception.
