DIFFERENTIAL NEURONAL ENCODING OF NOVELTY, FAMILIARITY AND RECENCY INREGIONS OF THE ANTERIOR TEMPORAL-LOBE

Activity of 2072 neurones was recorded in the anterior temporal lobe-i n area TE, perirhinal cortex, entorhinal cortex and hippocampus-during performance of a visual recognition task by monkeys. In area TE, peri rhinal cortex and entorhinal cortex, 454 neurones (38% of the 1162 vis ually responsive neurones) responded differentially on the basis of th e relative familiarity or recency of presentation of the stimuli; in t he hippocampus only one (3% of its 40 visually responsive neurones) di d so. The differentially responsive neurones were classified into thos e signalling information concerning the recency (19%), familiarity (37 %) or novelty (38%) of stimuli. For 98% of these neurones a decreased response signalled that stimuli had occurred previously: no large resp onse increments were observed. The mean differential latency of each o f these types of neurone was shorter (similar to 75 ms) in area TE tha n in the other areas. Examples of each of these types of neurone with memory spans of similar to 24 h were found in each region. The mean me mory span of recency neurones was significantly longer in perirhinal c ortex than area TE. For familiarity neurones a significant mean respon se decrement took 4-8 min to develop, indicating a slow underlying pla stic change, in contrast to the rapid change seen for recency and nove lty neurones. The implications of these results are discussed in relat ion to the neuronal basis of recognition memory. (C) 1998 Elsevier Sci ence Ltd. All rights reserved.