Flexible cortical gamma-band correlations suggest neural principles of visual processing

We summarize recent studies of our group from the primary visual cortex V1 of behaving monkeys referring to the hypothesis of spatial feature binding by gamma -synchronization (30-90 Hz). In agreement with this hypothesis the data demonstrates decoupling of gamma -activities among neural groups representing figure and ground. As gamma -synchronization in V1 is restricted to cortical ranges of few millimeters, feature binding may equivalently be restricted in visual space. Closer inspection shows that the restriction in synchrony is due to far-reaching travelling gamma -waves with changing phase coupling. Based on this observation we extend the initial binding-by-synchronization hypothesis and suggest object continuity to be coded by phase continuity. It is further argued that the spatial phase changes of the V1 gamma -waves in general will also limit lateral phase coupling to higher levels of processing. Instead of phase-locked gamma -coupling, corticocortical cooperation among gamma -processes may be mediated by mutual amplitude modulations that are more reliable than phase synchrony over larger distances. The relevance of this concept of corticocortical binding is demonstrated with subdural recordings from human subjects performing cognitive tasks. The experimental results are discussed on the basis of network models with spiking neurons.