The primary visual cortex as a modality-independent ‘screen’ for working memory

INTRODUCTION: Increasing evidence suggests that the function of the early visual areas is not restricted to sensory processing. Recently, Harrison & Tong (2009) have shown that areas V1-4 play a role in the maintenance of visual information in visual working memory. This result, however, leaves open the question of whether these areas can also support non-visual working memory, and if so what are the implications for the underlying functional architecture. METHODS: We addressed these questions in an fMRI drawing paradigm with blindfolded subjects in a Siemens 3T scanner. Four experimental conditions, separated by 20sec intervals, were run: Tactile Exploration - line-images (faces and objects) were explored with one hand and remembered to guide a future drawing task; Drawing - subjects had to draw the image remembered from a tactile template presented 20 seconds earlier to the non-drawing hand; Scribbling - a control task of pure hand movements with no image structure or memory component; Copying - drawing the image, but with concurrent access to the tactile template, minimizing demands on memory. The drawing trajectory was recorded with a custom MRI-compatible system, incorporating a fiber-optic stylus. RESULTS AND CONCLUSIONS: Of the occipital areas, only V1 was significantly activated in any condition, implying direct top-down feedback from high-level cortex beyond the occipital lobe. Tactile Exploration activated foveal V1 only, as if it were representing the local focus of attention during the exploration. In contrast, tactile-memory-guided Drawing while blindfolded activated full peripheral V1, as if responding to a large-scale visual representation. Frontal, parietal and inferotemporal activations specific to the memory-guided condition provide a basis for extensive top-down involvement in the V1 memory maintenance. The overall pattern of results suggests that V1 can operate as the conscious ‘visualization screen’ for working memory, even when evoked by non-visual sensory and complex memory guided tasks.