Cortical Neural Activity In Haptic Working Memory

Working memory provides the temporal and spatial continuity between our past experience and present actions. Working memory has been invoked in all forms of cognitive and linguistic processing and is fundamental to both the comprehension and construction of sentences. It is essential to the operations of mental arithmetic, to playing music, to fantasizing and planning ahead.To date, most research on neural substrate of working memory has been studied by either visual or auditory modality. In our daily activities, however, haptic stimulus processing and visual-haptic or auditory-haptic stimulus interactions are ordinary in the skillful and sensitive interaction with our surroundings. The features of brain structure and activity that regulate the access and storage capacity of haptic-haptic working memory and visual-haptic or auditory-haptic working memory are still unknown. The aim of my doctoral work is to find such features by using in-vivo extra cellular recording in monkeys that had been trained in the various working memory tasks.In our experiments, we trained two monkeys to learn haptic-haptic unimodal and visuo-haptic crossmodal delayed matching-to-sample working memory tasks. In addition, we. utilized two naive monkeys that were not trained to retain sensory information of the sample stimulus in tasks. We carried out cell recording in the primary somatosensory cortex (SI) and the prefrontal cortex (PFC) in both the well-trained and untrained monkeys.Our previous results showed that neurons in SI changed their firing rate in the delay period of a haptic working memory task. In the current study, SI neurons of the untrained monkeys also displayed the delay activity in a task that was identical to that in the previous studies but without requiring discrimination and active memorization of the sample. We, therefore, conclude that the delay activity is not necessarily only observed in the situation of working memory as was generally thought. The delay activity appears to be an intrinsic property of SI neurons, and suggests that there exists a neural network in SI for passive tactile memory. Moreover, in the well-trained monkeys, we discovered that in the haptic-haptic task, responses of a subset of neurons in SI were "choice" differential. These choice differential responses suggest a neural correlate of the decision-making process.In PFC, the activity of a substantial number of cells in the untrained monkeys was found to be significantly elevated or declined in the delay after the sample touch of a stimulus. In addition, the delay activity in a certain percentage of neurons could be selective for stimulus features. The existence of the delay activity implies that the tactile passive memory network also exists in PFC, which is activated temporarily in the neural process for active maintenance of tactile sensory information., Additionally, the result that "execution-related" differential activity was only observed in PFC of well-trained monkeys but not in that of untrained monkeys indicates that this neural activity for preparation of execution of the task is the outcome of the active memory network in working memory..This dissertation describes the neural mechanisms of haptic working memory and decision making in the primary somatosensory cortex and the execution function of working memory in the prefrontal cortex.