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| Paper IPM / Cognitive / 7500 |
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Speed of recognition of behaviorally important faces has critical significance for human and non-human primates. We recorded the activity of inferotemporal neurons (n=396) from two macaque monkeys performing a fixation task. More than 1000 object images including human and non-primate animal faces were presented up to 10 times, each for 105 ms without intervals. The category selectivity of responses was examined by calculating the ratio of the number of category members that evoked significant responses to the number of ineffective members in the category. Cells were considered selective to the category 1) if this ratio was significantly (p<0.01) larger than the corresponding ratio calculated for all the other stimuli and 2) if there was a significant averaged neural response to all of the members in a category (t-test, p<0.05). Seventy cells had selective responses to both human and non-primate animal face categories. The onset and peak latencies of responses to a category were measured in the averaged response histogram. The onset latency was determined as the first bin that exceeded the mean+2.57xSD calculated for the initial 70ms, and the peak latency as the first bin with the highest firing rate after the onset latency. The onset latencies were significantly shorter for responses to human faces than those to animal faces (p<0.0001, paired t-test, mean difference=22ms). The peak latencies showed a similar difference (p<0.0001, mean difference=31ms). One third of the 70 cells showed stronger average responses to animal faces than to human faces. Responses to human faces showed shorter latencies even in these cells (p<0.0001, mean difference=24 ms for onset; p<0.0001, mean difference=31ms for peak). These findings suggest that the latency as well as magnitude of responses are used in coding category of objects in the ventral visual stream.
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