(A) study on the nature of working memory representations based on the comparison of tactile and visual working memory in the human brain

Abstract

however, direct evidence of this has yet to be revealed. In this dissertation, to elucidate the nature of working memory representations in the human brain, the tactile and visual working memory processes were compared through functional magnetic resonance imaging (fMRI). First, a newly developed tactile stimuli delivery system made of MR-compatible materials was validated by demonstrating that desirable neural responses can be measured in the somatosensory cortex or visual cortex while participants conduct tactile and visual object perception tasks. Next, using the verified system, working memory representations of tactile and visual braille patterns of corresponding shapes were directly compared. While tactile and visual item information was stored in each responsible sensory cortex during within-modal working memory, the superior parietal cortex retained shared representations of tactile and visual braille during within-modal and cross-modal working memory. In addition, cross-modal working memory engaged additional cortical regions, such as the prefrontal cortex and the inferior parietal cortex, and the cross-task similarity of neural activation patterns in the superior parietal cortex was higher than that during within-modal working memory. These results suggest a supramodal nature of working memory representations of shape information in the superior parietal cortex even when the task goal was to maintain the information within each modality. In addition, transferring from one modality to another would require additional bridging neural processing in the high-level brain regions. In addition, whether value information, which is presumably independent of modality, associated with either tactile or visual objects is also encoded in the superior parietal cortex was assessed. However, the value representations were not commonly evident in the parietal cortex. The associated value information for both tactile and visual objects was represented in the putamen within the striatum. Together, these findings suggest that supramodal representations observed in the superior parietal cortex are specific to working memory.

Our actions are guided by multiple sensory information from the external world in combination with our behavioral goals. These sensory inputs should be temporarily stored to plan future actions to achieve a goal. Thus, working memory is an essential cognitive process for guiding goal-directed behavior. There has been extensive debate regarding the neural nature of working memory representations, whether it is high-level amodal information or sensory features that are dependent on modalities. A recent perspective suggested that working memory representations of different abstraction levels are distributed from the prefrontal cortex to the sensory cortices