Investigation of long-term recognition and association memory in unit responses from inferotemporal cortex

We investigated recognition and association memory in the responses of single units isolated in the inferior temporal cortex of a macaque while it performed a visual discrimination task. The unit responses showed significant recognition memory (a decreased response upon image repetition). Furthermore, a recognition memory appeared to be a permanent feature in these units. Such memory was evident in responses recorded at least 1 h after the most recent presentations of the more familiar images and may have been built up over the months of training. For these cells, the shorter-term recognition memory (seconds) and the longer-term recognition memory (hour plus) were significantly correlated (0.68). In these same cells associative memory was investigated with ten abstract images which had been randomly and permanently paired. The monkey had been taught to discriminate these five pairs from other similar pairs of images. Neither the spike count nor temporal response shape (as determined by a principal-components analysis) showed increased similarity for the images that had been paired. The cells that had both short-term and long-term recognition memory had responses to previously paired stimuli that were no more similar than expected by chance.