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Previous studies suggested that SI neurons in well-trained monkeys participated in the haptic-haptic unimodal delayed matching-to-sample (DMS) task.In the present 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 the sample.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 in the situation of working memory as was generally thought.The delay activity observed in the present 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 passive tactile memory.Over 400 SI neurons were also recorded in monkeys performing the haptic-haptic DMS task for comparison of delay firing of SI neurons between working and nonworking memory situations.The similarity observed in those two situations suggests that working memory uses already existing memory apparatus (the passive memory network) by activating it temporarily.Our data also suggest that, through training (repetitive exposure to the stimulus), SI neurons may increase their involvement in short-term retention of the sample stimulus.