The neural mechanisms underlying the maintenance of visual working memory contents

Abstract

Visual working memory (WM) refers to the maintenance and manipulation of visual information that is not in the visual field. Visual WM provides an essential connection between perception and higher cognitive functions. Previous fMRI studies with classification methods showed that early visual cortex could retain information of visual features maintained in WM. However, we still know little about the neural mechanisms of visual WM during maintenance period in higher visual cortex and the influence of memory load on such mechanisms. Therefore, the present study utilized a modified delayed match-to-sample paradigm to investigate the visual WM performance. Participants were required to maintain one or two target items (face or house) for 11s. At the same time, function magnetic resonance imaging (fMRI) was applied to acquire the neural activities in visual cortical areas. Behavioral results showed that memory load but not stimuli significantly affected the accuracy of memory. Next, the amplitude of the neural activity during maintenance in each area was extracted. In order to reveal the areas sensitive to mnemonic stimuli, we compared the activity induced by face and house WM. The results showed that parahippocampal place area (PPA) and intra-parietal sulcus (IPS) were activated differently to different stimuli. Specifically, the neural response was stronger when house relative to face was held in memory. To further investigate whether other areas also retained visual information in WM, activation pattern analysis with decoding method was applied to the neural response in each area. The results demonstrated decoding performance in face selective areas including superior temporal sulcus (STS), FFA and V4. Taken together, these results indicated that higher visual cortex could retain visual information held in WM. Specifically, PPA and IPS showed general stronger responses to mnemonic houses than mnemonic faces, while FFA, STS, and V4 contained different detailed representations of mnemonic faces and mnemonic houses. These findings were consistent with previous findings concerning visual features, and supported the view that the contents in visual working memory are represented in visual cortex. Notably, we did not find any effects of memory load on the neural activity in PPA and IPS. However, the activity pattern analysis about other areas showed significant effects only in high memory load condition, suggesting that high memory load might refine the representations in face selective areas without boosting the average activation level. In order to find the cortical areas of which responses were significantly modulated by memory load, we further performed a voxel-based whole brain analysis. Specifically, a 2 (stimuli: face, house) * 2 (memory load: 1 item, 2 items) repeated measures ANOVA was performed for each voxel on the neural response during the maintenance. No interaction was observed in each voxel. Interestingly, the main effect for memory load was significant in bilateral putamen. In summary, our findings revealed the crucial role of higher visual cortex in maintaining visual information in WM and the important role of putamen in modulating memory load.