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.