Persistent neuronal firing in primary somatosensory cortex in the absence of working memory of trial-specific features of the sample stimuli in a haptic working memory task.

Previous studies suggested that primary somatosensory (SI) neurons in well-trained monkeys participated in the haptic-haptic unimodal delayed matching-to-sample (DMS) task. In this study, 585 SI neurons were recorded in monkeys performing a task that was identical to that in the previous studies but without requiring discrimination and active memorization of specific features of a tactile or visual memorandum. A substantial number of those cells significantly changed their firing rate in the delay compared with the baseline, and some of them showed differential delay activity. These firing changes are similar to those recorded from monkeys engaged in active (working) memory. We conclude that the delay activity is not necessarily only observed as was generally thought in the situation of active memorization of different features between memoranda after those features have been actively discriminated. The delay activity observed in this study appears to be an intrinsic property of SI neurons and suggests that there exists a neural network in SI (the primary sensory cortex) for haptic working memory no matter whether the difference in features of memoranda needs to be memorized in the task or not. Over 400 SI neurons were also recorded in monkeys well-trained to discriminate two memoranda in the haptic-haptic DMS task for comparison of delay firing of SI neurons between the two different working memory tasks used in this study. The similarity observed in those two situations suggests that working memory uses already-existing memory apparatus by activating it temporarily. Our data also suggest that, through training (repetitive exposure to the stimulus), SI neurons may increase their involvement in the working memory of the memorandum.