A long-standing view of receptive field mechanisms in primary visual cortex (V1) neurons has been challenged recently; a paper in this issue now provides strong evidence to suggest that the textbooks have it right. Hubel and Wiesel were the first to describe neurons in cat V1 with elongated receptive fields, which respond to stimuli such as bars in a particular orientation. In 1962, they also described classes of neurons with distinctive receptive field structures. Simple-cell receptive fields are made up of nonoverlapping subregions; either bright or dark bars in the appropriate location excite a particular cell. For complex cells, these subregions are overlapping. Hubel and Wiesel hypothesized that simple-cell receptive fields are constructed from systematically arranged inputs from the lateral geniculate nucleus of the thalamus, whereas complex-cell receptive fields are constructed by combining inputs from multiple simple cells.

More recent work, however, has led to the suggestion that two distinct classes of neural circuitry are not necessary to account for the different properties of simple and complex cells. Such views have been supported by evidence that whereas simple and complex cells seem to form two classes when evaluated by their spiking behavior, the synaptic input responses of both cell types form a unimodal distribution. It has been proposed that spike threshold differences could lead to this apparent separation of the cell types by their spiking behavior (N. Priebe et al., Nat. Neurosci. 7, 1113–1122, 2004). Now, Martinez and colleagues (p. 372) combine intracellular recording (to characterize the spatial distribution and interaction of responses to bright and dark stimuli; red and blue regions in the figure) with anatomy (to locate neurons within different cortical layers). They found that simple cells were exclusively located in layer 4 and upper layer 6, the region where thalamic inputs enter the cortex. In contrast, complex cells were found in all layers, with cells in different layers having different receptive field structures. These results support the original proposal of Hubel and Wiesel that simple and complex cells belong to distinct neural circuits and that simple cells are an exclusive feature of an earlier stage of visual processing.