This work was supported by NIH (R01 EY10115, R01 NS075436, and RC2 NS069407). “
“When Hubel and Wiesel published their first landmark papers on the primary visual cortex of the cat,

they revealed that its neurons are exquisitely selective for stimulus attributes and that this selectivity defines orderly maps of functional architecture (Hubel and Wiesel, 1959, 1962). These discoveries echoed those made a few years earlier in somatosensory cortex (Mountcastle, 1957) and cemented a view of sensory cortex in which sharply tuned neurons arranged in vertical columns signal substantially different attributes from their neighbors displaced along the horizontal dimension. This view has been extremely fruitful in the subsequent 50 years and was further supported by advances in two-photon imaging. These revealed that maps high throughput screening of functional selleck screening library architecture are organized with crystalline precision down to the resolution of single cells (Ohki et al., 2005, 2006). Soon after these features were discovered, however, an apparently contradictory aspect of the responses began to emerge, suggesting that focal visual stimuli cause cortical activity that spreads over time to a large region of cortex, appearing earlier in the retinotopically appropriate cortical locations and progressively later in more distal

locations. This horizontal spread of neural activity constitutes a traveling wave. Traveling waves are evident in subthreshold potentials and are thus poised to influence the spike responses and thereby the output of area V1. They appear to work against the precise selectivity and orderly arrangement of V1 neurons along the cortical surface. Here we review data that point to traveling much waves as a prominent feature of area V1, both in the presence and in the absence of visual stimuli. We speculate briefly on the possible roles of these traveling waves in sensory

processing and on the possible circuits underlying their propagation, and we discuss how the traveling waves can coexist with the crystalline precision of the cortex. The traveling waves constitute a mode of operation that is mostly engaged when visual stimuli are weak or absent. When a sufficiently high contrast is presented sufficiently often over a sufficiently large region, the waves disappear. In those conditions, primary visual cortex does operate in the highly selective and orderly fashion that had been described by Hubel and Wiesel. We focus on traveling waves that propagate in the mammalian visual cortex, in the horizontal dimension, and at fairly high speeds (about 0.1–0.4 m/s). Other waves travel much slower, for instance, in binocular rivalry (∼0.018 m/s; Lee et al., 2005) or in spreading depression (∼0.00007 m/s; Lauritzen, 2001). We do not review the large literature on traveling waves in turtle cortex (Nenadic et al., 2003) or in nonvisual sensory or motor cortices of mammals (Ferezou et al., 2007; Fukunishi et al., 1992; London et al.