Different types of signal coupling in the visual cortex related to neural mechanisms of associative Processing and perception

The hypothesis of object representation by synchronization in the visual cortex has been supported by our recent experiments in monkeys. They demonstrated local synchrony among γ activities (30-90 Hz) and their perceptual modulation, according to the rules of figure-ground segregation. However, γ-synchrony in primary visual cortex is restricted to few mm, challenging the synchronization hypothesis for larger cortical object representations. The restriction is due to randomly changing phase relations among locally synchronized patches which, however, form continuous waves of γ-activity, traveling across object representations. The phase continuity of these waves may support coding of object continuity. Interactions across still larger distances, measured among cortical areas in human data, involve amplitude envelopes of γ signals. Based on models with spiking neurons we discuss potentially underlying mechanisms. Most important for γ synchronization are local facilitatory connections with distance-dependent delays. They also explain the occurrence of γ waves and the restriction of γ-synchrony. Fast local feedback inhibition generates γ oscillations and supports local synchrony, while slow shunting inhibitory feedback supports figure-ground segregation. Finally, dispersion in inter-areal far projections destroys coherence of γ signals, but preserves their amplitude modulations. In conclusion, we propose that the hypothesis of associative processing by γ synchronization be extended to more general forms of signal coupling.