Delay-period activities in two subdivisions of monkey inferotemporal cortex during pair association memory task.

The macaque inferotemporal cortex, which is involved in encoding and retrieval of visual long-term memory, consists of two distinct but mutually interconnected areas: area TE (TE) and area 36 (A36). In the present study, we compared delay-period activities of the two subdivisions in terms of their signal contents. We recorded single-unit activities from TE and A36 during a delayed pair association task, in which monkeys were required to choose the paired associate of a cue stimulus after a delay period. The stimulus-selective delay-period activities of single neurons were characterized by using partial correlation coefficients of delay-period activities for each cue stimulus with the cue-period responses to that stimulus (cue-holding index, CHI) and with the cue-period responses to its paired associate (pair-recall index, PRI). The delay-period activities of TE neurons preferentially represented the paired associate (PRI, median = 0.54) rather than the cue stimulus itself (CHI, 0.23) (P < 0.001, n = 70), while the delay-period activities of A36 neurons retained both the cue stimulus and its paired associate equivalently (CHI, 0.44; PRI, 0.46) (P = 0.78, n = 38). These results indicate that the signal contents of delay-period activities differ between the two subdivisions: TE mostly represents a sought target that is retrieved from long-term memory, while A36 in addition retains cue-stimulus that is transmitted from earlier visual areas.