The role of the occipital cortex in capacity limits and precision of visual working memory

摘要

Several lines of evidence suggest a role for the occipital cortex in the storage of information in working memory (WM). For example, single pulse TMS to retinotopic visual cortex reduces shape change detection performance, but only during the time period when performance is sensitive to masking (van de Ven et al., 2012). Furthermore, although BOLD response in the occipital cortex is elevated during encoding but not maintenance, features such as color, orientation, or motion of remembered stimuli can be decoded from the occipital cortex by analyzing patterns of activity during WM maintenance (Emrich et al., 2013; Harrison & Tong, 2009; Serences et al., 2009). Furthermore, pattern classifier sensitivity is correlated with the precision of representations, but not capacity (Emrich et al., 2013). This suggests that the visual cortex may serve a storage function in WM and that visual cortex activity may determine the precision of WM representations. To test this hypothesis, single pulse TMS was applied to visual cortex at 0, 100, or 200ms after the offset of the memory stimulus. TMS-related changes in the capacity and precision of WM representations, as well as the likelihood of mis-binding objects to locations (swap errors), were analyzed. Occipital simulation decreased capacity and swap errors, and produced a small, non-significant increase in precision when applied immediately at stimulus offset, but not when applied 100 or 200ms later. Preliminary analysis of data from a follow-up experiment (n=5) revealed no change in capacity or precision when arrhythmic trains of high-frequency rTMS were applied to the occipital cortex either 125ms or 575ms after stimulus offset. This suggests that the occipital cortex contributes to VWM capacity, but the quality of representations may be specifically dependent on the number of items encoded into VWM; furthermore, occipital-based representations may not be vulnerable to disruption after initial encoding.