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. V1 receives input directly from subcortical structures (retina and lateral geniculate nucleus). Previous studies have demonstrated that most, if not all, neurons in V2 are muted when V1 is lesioned. Hence, V1 assumes a critical role in visual processing as it is the first cortical area reached by visual input. However, neural response to stimuli in other layers of V1 and V2 has been found to be almost simultaneous. This leads to the question of whether visual processing is a serial or a parallel process. If the process is entirely serial, we would expect latencies in V1 to be shorter than those in V2, and likewise the latencies of V2 to be less than V4. Conversely, in a parallel arrangement, their latencies would overlap. Much of the delay in increased neural activity following stimulation has been attributed to two factors: the axonal conduction time and the neural integration time. The axonal delay is a result of the time required for propagation of the action potential along the axon. This potential consequently reaches the terminals of the axon where it faces the integration delay: the time required for the target neuron to reach a minimal membrane potential so that the spike may transfer to it. Our results indicate long response latencies in V4, but relatively short (and similar) latencies in V1 and V2. The V1-V4 and V2-V4 latencies are about 8 ms, which is the delay expected for axonal conduction between those areas. Thus, there is very little time, if any, for serial processing within areas V1 and V2 before their output is sent to V4.
