These properties, which we will address below, include (1) cross-orientation

suppression, (2) contrast invariance of orientation tuning width, (3) contrast-dependent changes in response timing and in temporal frequency preference, and (4) the mismatch between measured orientation tuning and the tuning predicted by a simple cell’s receptive field organization. Uncovering the origin of these properties has proven to be one of the keys to selleck compound understanding the nature of the cortical computation. One comprehensive solution to the origin of simple cell nonlinearities was suggested by psychophysics: in the tilt aftereffect illusion, the perceived orientation of a vertical stimulus is shifted away from vertical after prolonged viewing of a slightly oblique stimulus. This result was interpreted to mean that intracortical inhibition, specifically inhibition between cortical neurons of different preferred orientations, sharpened orientation tuning

or even created it de novo (Blakemore and Tobin, 1972). This proposal was strengthened by pharmacological experiments: cortical application of GABAA antagonists cause a broadening of orientation tuning (Sillito, 1975). Cross-orientation inhibition, a form of lateral inhibition (Hartline, 1949), but in the orientation domain rather than the spatial domain, is considered a natural extension of similar mechanisms either observed or proposed to operate throughout the brain. Because of the columnar organization of orientation preference in the cortex, the orientation domain translates into the spatial domain on the cortical surface. http://www.selleckchem.com/products/ON-01910.html Cross-orientation inhibition can then emerge from simple, spatially defined rules of cortical connectivity. Cross-orientation inhibition has been proposed to operate in several distinct modes, depending on the orientation dependence and amplitude of inhibitory interconnections.

In attractor models, those feedback inhibition forms a set of multistable attractors (Ben-Yishai et al., 1995 and Somers et al., 1995), in which the width of orientation tuning of cortical cells is determined by the lateral extent of cortico-cortical connections. In recurrent models, recurrent excitatory connections amplify feedforward inputs in a way that is sculpted by lateral inhibitory connections (Douglas et al., 1995). Here again, the width of tuning and other aspects of cortical responses are set by intracortical rather than thalamocortical interconnections. In balanced models, strong recurrent excitation and inhibition are thought to balance one another tightly (van Vreeswijk and Sompolinsky, 1998). In addition to explaining many aspects of simple cell behavior, this balance can explain the large variability of cortical spiking responses (Shadlen and Newsome, 1998). In push-pull models, cross-orientation inhibition arises from feedforward inhibition from simple cell-like inhibitory interneurons (Troyer et al., 1998 and Troyer et al.