Oscillatory and non-oscillatory synchronizations in the visual cortex and their possible roles in associations of visual features.

It was postulated that the perceived association of visual features is based on the synchronization of those neural signals that are activated by a coherent visual object. Two types of synchronized cortical signals were found by us in cat and monkey visual cortex, and were proposed as candidates for feature association: (1) stimulus-locked signals, evoked by transient retinal stimulation, and typically non-rhythmic; (2) oscillatory signals, induced by sustained stimuli, and typically not locked in their oscillation phases to stimulus events. Both types of signals can occur synchronously in those neurons that are activated by a common stimulus. Synchronized activities were found in paired recordings within vertical cortex columns, in separate columns of the same cortical area, and even between different cortical areas or hemispheres. The average phase difference between such common oscillatory events was typically close to zero (< 1 msec mean +/- 2 msec S.D.). For the dependence of synchronization from stimulus and receptive field properties, a preliminary 'rule' can be given: the coherence of fast oscillations in separate cortical assemblies depends inversely on the 'coding distance' between the assemblies' RF properties, but directly on the degree of overlap between the assemblies' respective coding properties and the features of a common stimulus. This means that oscillatory events in any two assemblies, in the same or in different cortical areas or hemispheres, are more closely correlated the more similar are their receptive field properties, and the better a common stimulus activates the assemblies simultaneously. Our results can explain some neural mechanisms of perceptual feature-linking, including mutual enhancement among similar, spatially and temporally dispersed features, definitions of spatial and temporal continuity, scene segmentation, and figure-ground discrimination. We further propose that mutual enhancement and synchronization of cell activities are general principles of temporal coding by assemblies, that are also used within and among other sensory modalities as well as between cortical sensory and motor systems.