Topographically specific effects of TMS over early visual cortex during visual working memory

Abstract

Recent fMRI studies demonstrated that patterns of activity in early visual cortex (V1-V4) are predictive of stimulus properties actively maintained in visual working memory. However, exactly why sensory areas represent such information, and whether they are necessary for successful high-precision maintenance, remains largely unknown. In this study, observers remembered the orientations of 4 briefly presented gratings (200ms), one in each quadrant of the visual field. Triple-pulse TMS was applied either directly at stimulus offset, or midway through a 2-second retention interval, targeting early visual cortex corresponding retinotopically to a sample item in the lower hemifield. After retention, memory for one of the gratings was randomly probed, independent of the location targeted with TMS, and participants reported the remembered orientation. Memory performance, defined as the absolute error between memory target and response, was best when the visual field location targeted by TMS overlapped with that of the cued memory item. This result implies that information about memorized items was stored in a topographic fashion in early visual cortex, and that triple-pulse TMS had a memory-enhancing effect at the targeted location. Memory enhancement was most prevalent when TMS was applied midway through the retention, with memory performance being generally worse for early TMS pulses. Next, empirical fitting of the errors was performed (using a mixture model analysis) to characterize memory precision and forgetting rates. Memory was more precise for items coinciding with the pulse location, irrespective of pulse timing. The probability that items were forgotten was larger when pulses were delivered early, regardless of their proximity to the pulse location. Thus, whereas TMS administered at the offset of the stimulus array might disrupt early-phase consolidation, TMS during the retention phase acts to boost the precise representation of an item in working memory, perhaps by increasing attentional resources at its retinotopic location. Meeting abstract presented at VSS 2015