Following from the classical work of Hubel and Wiesel, it has been recognized that the orientation and the on- and off-zones of receptive fields of layer 4 simple cells in the visual cortex are linked to the spatial alignment and properties of the cells in the visual thalamus that relay the retinal input. Here we present evidence showing that the orientation and the on- and off-zones of receptive fields of layer 6 simple cells in cat visual cortex that provide feedback to the thalamus are similarly linked to the alignment and properties of the receptive fields of the thalamic cells they contact. However, the pattern of influence linked to on- and off-zones is phase-reversed. This has important functional implications.

There is a tightly coupled bidirectional interaction between visual cortex and visual thalamus [lateral geniculate nucleus (LGN)]. Using drifting sinusoidal grating stimuli, we compared the response of cells in the LGN with and without feedback from the visual cortex. Raster plots revealed a striking difference in the response pattern of cells with and without feedback. This difference was reflected in the results from computing vector sum plots and the ratio of zero harmonic to the fundamental harmonic of the fast Fourier transform (FFT) for these responses. The variability of responses assessed by using the Fano factor was also different for the two groups, with the cells without feedback showing higher variability. We examined the covariance of these measures between pairs of simultaneously recorded cells with and without feedback, and they were much more strongly positively correlated with feedback. We constructed orientation tuning curves from the central 5 ms in the raw cross-correlograms of the outputs of pairs of LGN cells, and these curves revealed much sharper tuning with feedback. We discuss the significance of these data for cortical function and suggest that the precision in stimulus-linked firing in the LGN appears as an emergent factor from the corticothalamic interaction.corticofugal feedback ͉ lateral geniculate nucleus ͉ synchronization ͉ visual processing T he patterning of activity in the brain, whether by the grouping of action potentials or synchronization of firing across sets of neurons, can have a major impact on the effectiveness with which information is transferred to other brain areas (1, 2). Common to many sensory pathways, both burst firing and tightly timed monoand/or heterosynaptic inputs have been argued to increase the effectiveness with which thalamic input can drive the cortex (3-7). In the visual system heterosynaptic thalamic inputs from the dorsal lateral geniculate nucleus (LGN) that fall within 5 ms of each other exhibit a supralinear enhancement of transmission to visual cortical simple cells (7). The ability of visual stimuli to generate heterosynaptic facilitation from the convergent inputs of LGN cells on cortical cells will depend on the precision in the way the LGN cells respond to visual stimuli. We know that the corticofugal feedback to the thalamus has a strong influence on this patterning via direct connections to the thalamus and thalamic reticular nucleus (8)(9)(10)(11)(12)(13)(14). In many ways the thalamus, thalamic reticular nucleus, and cortex form part of a circuit rather than distinct steps in an ascending system. How does this interaction serve to refine the way the thalamus accesses the cortex?Clearly, the precision in the timing and structure of the firing pattern to visual stimuli is critically important in understanding their neural representation and impact. One can hypothesize that the contrast edges of a complex object moving through visual space could provoke synchronous firing in groups of thalamic cells that optimally drive the representation...

How primates perceive objects along with their detailed features remains a mystery. This ability to make fine visual discriminations depends upon a high-acuity analysis of spatial frequency (SF) along the visual hierarchy from V1 to inferotemporal cortex. By studying the transformation of SF across macaque parafoveal V1, V2, and V4, we discovered SF-selective functional domains in V4 encoding higher SFs up to 12 cycles/°. These intermittent higher-SF-selective domains, surrounded by domains encoding lower SFs, violate the inverse relationship between SF preference and retinal eccentricity. The neural activities of higher- and lower-SF domains correspond to local and global features, respectively, of the same stimuli. Neural response latencies in high-SF domains are around 10 ms later than in low-SF domains, consistent with the coarse-to-fine nature of perception. Thus, our finding of preserved resolution from V1 into V4, separated both spatially and temporally, may serve as a connecting link for detailed object representation.

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.