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| Paper IPM / Cognitive / 14533 |
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Prefrontal cortex is known to modulate sensory signals in posterior visual cortex, possibly via the direct projections from the frontal eye field (FEF) to multiple visual cortical areas. Upon examining the nature of the FEF signal sent to visual cortex, we found that persistent, working-memory related activity is the predominant feature of FEF neurons projecting to extrastriate cortex. This signal was not sufficient to drive spiking activity in extrastriate areas V4 and the middle temporal area (MT); however, we found that during memory maintenance visually driven responses in both these areas were modulated by the content of spatial working memory. Using linear array electrodes, responses of neurons in MT and V4 were recorded during a memory-guided saccade task. The probe-evoked visual responses and receptive fields (RFs) were compared during the fixation and spatial working memory periods. Although the firing rate of the MT neurons was not altered by the memory location in the absence of any visual stimuli, the visual responses of the same neurons depended upon the content of spatial working memory. The RFs of MT neurons also expanded and shifted toward the remembered location. The net effect of these changes was to increase the number of neurons responding to a probe stimulus near the remembered location, improving their ability to represent the stimulus (as assessed by the performance of an SVM classifier using the population activity). Moreover, we measured the efficacy of visual cortical input into the FEF and found that it was enhanced when remembering a corresponding spatial location. These results suggest persistent signals sent from the FEF as a means to alter representations within posterior visual areas, and imply a positive feedback loop between prefrontal and visual areas to gate the processing of visual signals according to the content of working memory.
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