Stimulus-Specific Synchronization in Cat Visual Cortex and Its Possible Role in Visual Pattern Recognition

Correlations of neural signals have repeatedly been proposed to play a central role in sensory processing (e.g./1–6/). These hypotheses were recently supported by the discovery of stimulus-induced synchronizations of oscillatory neural activities in the primary visual cortex of monkey /7/ and cat /8,9/. The experimental results from cat visual cortex show that stimulus-induced oscillations in local assemblies, representing local visual features, are synchronized over large cortical distances if the stimulus contains common coding parameters of the assemblies /9–14/. We found synchronizations between all combinations of single-cell signals and mass-input and -output activities to occur with almost zero phase difference. Stimulus-induced synchronizations were found among assemblies in the same cortex area, and between spatially separated cell groups in two different visual cortex areas /9–13/. Our results show that synchronization between remote assemblies requires simultaneous sustained visual activation via feeding connections (generating receptive field properties) that cause the local assembly to engage in a common in-phase oscillation. Oscillations in remote assemblies begin uncorrelated but they can become phase-locked via special linking connections. Based on our experimental results, we propose that two types of neural synchronization play a role for the definition of feature relations in a current visual situation: Stimulus-forced (single-shot) synchronizations that are strongly phase-locked to transients in the visual input signals, and stimulus-induced (repetitive) synchronizations that are not phase-locked to the stimulus. Stimulus-forced synchronizations are most likely involved in region- and feature-linking based on primary object-domain parameters, whereas the undefined initial phase of stimulusinduced oscillations makes them particularly suited for linkings based on criteria derived at higher processing stages. We believe that cooperative synchronizations like those we have seen in and between primary visual cortex areas are a general principle underlying cognitive integration.