Early behavioral studies with fixation tasks showed that the receptive field (RF) of inferior temporal (IT) cortical neurons covered a large area of visual field (Op de Beeck, and Vogels, 2000). On the other hand, a recent report revealed that RF can be as small as 2.6 degree when monkeys performed a task to recognize small visual stimulus presented in a small region of visual field (DiCarlo and Maunsell, 2003). This suggests that shape and size of RF can be dynamically modulated by types of tasks. However, the neural mechanisms that control RFs are not yet known well. Here, we examined effect of spatial attention on shape and size of RF of IT neurons and found that RF was not a fixed property of neurons but dynamically modulated by spatial attention.
We trained a monkey with a spatial attention task, where monkey had to detect a change in luminance of one of the two points presented on the screen. The probability of the luminance change was set to 90% for attended and 10% for unattended points. During the task, various objects were presented at various locations in background. We examined neural responses to these objects with a single dimensional 16-ch electrode array that were penetrated vertically to the cortical surface of anterior IT cortex.
We found that RFs of multiunit responses shifted toward the location of the attended point. This shift was only observed when the attended point appeared in the contralateral side. Furthermore, we identified the granular layer by current source density analysis of local field potentials, and compared differences in RF modulations in infra- and supra-granular layers. We found that 1) RF sizes were larger in infra-granular layers than in supra-granular layers regardless of attended locations, and 2) the attentional modulation of receptive fields was more pronounced in infra-granular layers than supra-granular layers. Temporal dynamics of RFs in infra- and supra-granular layers will be further discussed.