Visual response latencies in temporal lobe structures as a function of stimulus information load.

In a monkey performing a visual delayed matching-to-sample task, units and visual evoked potentials (VEPs) were sampled from the inferior bank of the superior temporal sulcus (STS; Areas TEa and IPa), the hippocampus, and the presubiculum. VEP latencies indicated that flash information--signaling the imminent presentation of a color sample to be retained--reached the presubiculum and the hippocampus substantially earlier than the STS. In contrast, color sample VEP latencies did not differ between sites, arriving at all sites appreciably later than flash VEPs. Unit data indicated generally excitatory responses to both stimuli at all sites and net inhibition during the interstimulus interval separating flash from sample. As with VEPs, unit latencies to flash were shorter than to sample stimuli. The alerting flash data imply activation of the hippocampus occurring before activation of the STS cortex, whereas the coincident arrival of color sample information suggests temporal synchronization between these structures.