Response latency of macaque area MT/V5 neurons and its relationship to stimulus parameters

A total of 310 MT/V5 single cells were tested in anesthetized, paralyzed ma caque monkeys with moving random-dot stimuli. At optimum stimulus parameter s, latencies ranged from 35 to 325 ms with a mean of 87 +/- 45 (SD) ms. By examining the relationship between latency and response levels, stimulus pa rameters, and stimulus selectivities, we attempted to isolate the contribut ions of these factors to latency and to identify delays representing interv ening synapses (circuitry) and signal processing (flow of information throu gh that circuitry). First, the relationship between stimulus parameters and latency was investigated by varying stimulus speed and direction for indiv idual cells. Resulting changes in latencies were explainable in terms of re sponse levels corresponding to how closely the actual stimulus matched the preferred stimulus of the cell. Second, the relationship between stimulus s electivity and latency across the population of cells was examined using th e optimum speed and direction of each neuron. A weak tendency for cells tun ed for slow speeds to have longer latencies was explainable by lower respon se rates among slower-tuned neurons. In contrast, sharper direction tuning was significantly associated with short latencies even after taking respons e rate into account, (P = 0.002, ANCOVA). Accordingly, even the first 10 ms of the population response fully demonstrates direction tuning. A third st udy, which examined the relationship between antagonistic surrounds and lat ency, revealed a significant association between the strength of the surrou nd and the latency that was independent of response levels (P < 0.002, ANCO VA). Neurons having strong surrounds exhibited latencies averaging 20 ms lo nger than those with little or no surround influence, suggesting that neuro ns with surrounds represent a later stage in processing with one or more in tervening synapses. The laminar distribution of latencies closely followed the average surround antagonism in each layer, increasing with distance fro m input layer IV but precisely mirroring response levels, which were highes t near the input layer and gradually decreased with distance from input lay er IV. Layer II proved the exception with unexpectedly shorter latencies (P < 0.02, ANOVA) yet showing only modest response levels. The short latency and lack of strong direction tuning in layer II is consistent with input fr om the superior colliculus. Finally, experiments with static stimuli showed that latency does not vary with response rate for such stimuli, suggesting a fundamentally different mode of processing than that for a moving stimul us.