The spatial weighting functions of both retinal and lateral geniculate nucleus (LGN) X-cell receptive fields have been viewed as the difference of two Gaussians (DOG). We focus on a particular shortcoming of the DOG model, that is, suppression of responses of LGN cells at spatial frequencies above those to which the classical receptive field surround is responsive. By simultaneously recording one of the retinal ganglion cell (RGC) inputs (S-potentials) to an LGN cell, we find that half of this suppression at high spatial frequencies arises from the retinal input and that suppression in LGN cells is greater than that in RGCs, regardless of spatial frequency. We also inactivated the ipsilateral visual cortex and show that one quarter of the suppression at high spatial frequencies arises from corticothalamic feedback. We show that this suppression at high spatial frequencies is colocalized with the classical surround, is not dependent on the relative orientation of the center and surround stimuli, and that the cortical component of this suppression is divisive. We propose that the role of this suppression at high spatial frequencies is to restrict the response to large stimuli composed of high spatial frequencies.
I N T R O D U C T I O NSince the classic studies of Kuffler (1953) and Barlow (1953), the spatial weighting functions of both retinal and lateral geniculate nucleus (LGN) X-cell receptive fields have been viewed as the difference of two Gaussians (DOG) (Rodieck 1965). The DOG model consists of a strong and narrow central Gaussian and a weak but broad surround Gaussian. This linear model accounts for many properties of retinal ganglion cells (RGCs) and LGN cells, but numerous authors have described excitatory and/or suppressive effects in LGN cells that are not accounted for by the classic DOG model (Alitto and Usrey 2003;Cudeiro and Sillito 1996;Levick et al. 1972;Murphy and Sillito 1987;Solomon et al. 2002;Webb et al. 2002).Many of these effects are attributed to a "nonclassical" receptive field and have proven to be controversial. Several authors have proposed that these effects are spatially localized to a region outside of the classical receptive field (e.g., Webb et al. 2002), although a recent study suggests that they are colocalized with the classical receptive field (Bonin et al. 2005). Sillito and colleagues described suppression in LGN cells that was dependent on the relative orientation of grating stimuli presented to the center and surround of the receptive field (Cudeiro and Sillito 1996;Sillito et al. 1993). They also showed that the suppression found with isooriented, but not orthogonally oriented, stimuli was partially released on ablation of visual cortex. Others have failed to see these orientation effects in the intact animal (Bonin et al. 2005;Solomon et al. 2002;Webb et al. 2002) and Webb et al. (2002) reported no differential release of suppression after cortical ablation. Some studies have attributed changes in the gain of the classical receptive field to stimuli in the nonclassical...