Oscillations and Synchrony in the Visual Cortex: Evidence for Their Functional Relevance

The past 20 years of research on the visual cortex have revealed an organizational complexity that was largely unforeseen (Livingstone and Hubel, 1988; Felleman and Van Essen, 1991; Zeki, 1993). Electrophysiological studies on receptive field properties and visual maps as well as research on the pattern of afferent and efferent projections have inspired investigators to subdivide the visual cortex in ever smaller compartments each of which is characterized by distinct response properties and connections to other brain structures. As a result, more than 30 distinct visual areas have now been classified in the monkey visual system (Felleman and Van Essen, 1991), and presumably such areas are equally numerous in humans and other higher mammals. This parcellation is assumed to reflect some kind of functional specialization since neurons in each of these visual areas are, at least to some degree, selective for a characteristic subset of stimulus features. Thus, for instance, some areas contain predominantly colour-selective cells, while others primarily process information about the form of an object or its direction of motion in the visual field. As a consequence of this functional specialization, any object present in a visual scene will activate neurons in many cortical areas simultaneously. Accordingly, object representations in the visual system are likely to correspond to large and highly distributed assemblies of feature-detecting neurons (Hebb, 1949).