Models of visual processing derived from cortical microelectrode recordings

Abstract

A single object generally activates neurons in many visual cortical areas corresponding to a distributed representation of its features. While single neurons in the “lower” cortical areas, including V1–V5, represent a variety of local features it is still under debate how the distributed representation of an object is bound into a coherent whole and how unrelated features are separated. Synchronization of neural signals has been proposed to code spatial feature binding, supported by the discovery of synchronized neural assemblies in cat and monkey visual cortex which occurred stimulus dependent — either as oscillatory (30–100 Hz) events due to internal processes of self-organization, or as non-rhythmical stimulus-locked responses. Spatial coherence of synchronized oscillations covered generally larger areas in visual cortical representations than the classical receptive field of single neurons. However, coherence was laterally confined to a few millimeters of cortical surface which means that synchronized cortical regions do only span the representational range of small visual objects or parts of larger ones. To relate such restricted segments to perceptual processes we introduced the concept of the “linking or association field” of local neural assemblies in accordance with receptive fields of single neurons. The linking field is defined by the aggregate receptive fields of a neural assembly engaged in a common synchronized state. We further argue that spatial continuity of an object might either be coded 1.) by a continuum of overlapping linking fields, i.e. by overlapping synchronized regions spanning the entire object representation or (and) 2.) by a hierarchy of linking fields in which the assemblies engage in phase locked oscillations at different frequencies — where large objects or larger parts are represented by larger linking fields defined by synchronization at lower frequencies and smaller subparts are represented by smaller linking fields at higher frequencies. Our cortical recordings showed the presence of both types of synchronized states, including phase-locking among different frequencies.