Figure-ground modulation in awake primate thalamus.

Figure-ground discrimination refers to the perception of an object, the figure, against a nondescript background. Neural mechanisms of figure-ground detection have been associated with feedback interactions between higher centers and primary visual cortex and have been held to index the effect of global analysis on local feature encoding. Here, in recordings from visual thalamus of alert primates, we demonstrate a robust enhancement of neuronal firing when the figure, as opposed to the ground, component of a motion-defined figure-ground stimulus is located over the receptive field. In this paradigm, visual stimulation of the receptive field and its near environs is identical across both conditions, suggesting the response enhancement reflects higher integrative mechanisms. It thus appears that cortical activity generating the higher-order percept of the figure is simultaneously reentered into the lowest level that is anatomically possible (the thalamus), so that the signature of the evolving representation of the figure is imprinted on the input driving it in an iterative process.

Differential feedback modulation of center and surround mechanisms in parvocellular cells in the visual thalamus.

Many cells in both the central visual system and other sensory systems exhibit a center surround organization in their receptive field, where the response to a centrally placed stimulus is modified when a surrounding area is also stimulated. This can follow from laterally directed connections in the local circuit at the level of the cell in question but could also involve more complex interactions. In the lateral geniculate nucleus (LGN), the cells relaying the retinal input display a concentric, center surround organization that in part follows from the similar organization characterizing the retinal cells providing their input. However, local thalamic inhibitory interneurons also play a role, and as we examine here, feedback from the visual cortex too. Here, we show in the primate (macaque) that spatially organized cortical feedback provides a clear and differential influence serving to enhance both responses to stimulation within the center of the receptive field and the ability of the nonclassical surround mechanism to attenuate this. In short, both center and surround mechanisms are influenced by the feedback. This dynamically sharpens the spatial focus of the receptive field and introduces nonlinearities from the cortical mechanism into the LGN.