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2 Acta Neurobiologiae Experimentalis

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Cortical synchronization suggests neural principles of visual feature grouping

100%

Eckhorn R.

Acta Neurobiologiae Experimentalis

2000

tom 60

nr 2

261-269

Compositions of visual scenes are related here to neural signals in visual cortex and to cortical circuit models to understand neural mechanisms of perceptual feature grouping. Starting from the hypothesis that synchronization and decoupling of cortical gamma-activities (35-90 Hz) define the relations among visual objects, we concentrate on synchronization related to (1) static retinal stimulation during ocular fixation, and (2) transient stimulation by sudden shifts in object position. The synchronization hypothesis has been tested by analyzing signal correlations in visual cortex of monkeys with the following results: Static retinal stimuli induce loosely phase-coupled gamma-activities among neurons of an object's cortical representation. Patches of gamma-synchronization become decoupled across the representation of an object's contour, and therby can code figure-ground segregation. Transient stimuli evoke synchronized volleys of stimulus-locked activities that are typically non-rhythmic and include low frequency components in addition to those in the gamma-range. It is argued that stimulus-induced and stimulus-locked synchronizations may play different roles in perceptual feature grouping.

Neural mechanisms of visual associative processing

38%

Eckhorn R. , Gail A. , Bruns A. , Gabriel A. , Al-Shaikhli B. , Saam M.

Acta Neurobiologiae Experimentalis

2004

tom 64

nr 2

239-252

This is a review of our work on multiple microelectrode recordings from the visual cortex of monkeys and subdural recordings from humans - related to the potential underlying neural mechanisms. The former hypothesis of object representation by synchronization in visual cortex (or more generally: of flexible associative processing) has been supported by our recent experiments in monkeys. They demonstrated local synchrony among rhythmic or stochastic gamma-activities (30-90 Hz) and perceptual modulation, according to the rules of figure-ground segregation. However, gamma-synchrony in primary visual cortex is restricted to few millimeters, challenging the synchronization hypothesis for larger cortical object representations. We found that the spatial restriction is due to gamma -waves, traveling in random directions across the object representations. It will be argued that phase continuity of these waves can support the coding of object continuity. Based on models with spiking neurons, potentially underlying neural mechanisms are proposed: (i) Fast inhibitory feedback loops can generate locally synchronized gamma -activities; (ii) Hebbian learning of lateral and feed forward connections with distance-dependent delays can explain the stabilization of cortical retinotopy, the limited size of synchronization, the occurrence of gamma -waves, and the larger receptive fields at successive levels; (iii) slow inhibitory feedback can support figure-ground segregation; (iv) temporal dispersion in far projections destroys coherence of fast signals but preserves slow amplitude modulations. In conclusion, it is proposed that the hypothesis of flexible associative processing by gamma -synchronization, including coherent representations of visual objects, has to be extended to more general forms of signal coupling.