Visual cortex: Supression by depression?

The response of a neuron in the visual cortex to an oriented light bar is strongly reduced by concurrent presentation of a stimulus with a different orientation. New data suggest this ‘cross-orientation suppression’ is caused, not by intracortical inhibition, but by rapid depression of thalamocortical synapses.
The images of the world are decomposed by our retinae and transmitted to the brain as neuronal spike trains. At each successive stage in the visual pathway, neurons display progressively more complex response properties and interactions, ultimately leading to the reconstruction of elaborate visual features in higher areas of the visual cortex. One such response property, which already emerges in primary visual cortex (V1), is orientation selectivity: a nerve cell in V1 fires vigorously when stimulated with a light bar of a certain orientation, but it fails to do so when presented with the same bar rotated by 90°. It is thought that the initial orientation bias is created by the spatial arrangement of non-oriented thalamic inputs from the lateral geniculate nucleus (LGN) onto neurons in V1 [1]. In addition, local cortical circuitry may act to sharpen and fine tune the orientation-selective responses of V1 cells [2].
Since the discovery of orientation selectivity, millions of neurons in V1 have been recorded from, and researchers have learned that many of these cells are even pickier when it comes to the finer details of visual stimuli. For example, when a light bar activating a cell is increased in length so that it extends over the boundaries of the cell's receptive field center – the region of the visual field that evokes responses in a cell – many cells decrease their firing rate, a property known as ‘end-inhibition’ [3,4]. Similarly, some cells fire less when additional bars are placed alongside its receptive field [5]. These are just two prominent examples of a general phenomenon that is commonly referred to as ‘surround inhibition’.
There is yet another type of suppressive effect in V1 which originates from within a cell's receptive field. The response to an optimally oriented bar or grating – the ‘test stimulus’ – is strongly decreased by the simultaneous presentation of a second, orthogonally oriented grating – the ‘mask stimulus’ – superimposed on the first (Figure 1) [6,7]. This effect is termed ‘cross-orientation suppression’ and has been implicated, among other things, in sharpening orientation selectivity of cortical cells. Importantly, however, the suppressive action can be induced not only by mask gratings of the orthogonal orientation, but by gratings of any orientation [8].