Dynamics and specificity of cortical map reorganization after retinal lesions.

Neurons in the mature visual cortex deprived of their normal retinotopic inputs by matched binocular retinal lesions are initially silenced but become reactivated with time when the "blind" cortical lesion projection zone (LPZ) is filled in by new suprathreshold visual responses. In an attempt to gain further insight into the dynamics of this process, we investigated in detail the spatiotemporal pattern of single-cell properties and recording probability during cortical reorganization up to 12 months after retinal lesions. In the early phases of filling in, a transient peak of hyperactivity moves from the border of the normal cortex into the LPZ and forms the leading edge of a functional reconnection process. In the course of this process hyperactive cells inside the LPZ develop ectopic receptive fields that are initially enlarged and regain orientation specificity. During the proceeding recovery, hyperactivity and receptive field size normalize, while the quality of orientation tuning remains reduced at longer distances inside the LPZ at all stages of recovery up to 1 year. Within the adult anatomical framework of cortical connectivity, the maximal lateral distance of reconnection is limited, and the probability to encounter spiking cells decreases with increasing distance inside the LPZ. However, this recording probability was significantly increased after 1 year.

Correlations, feature-binding and population coding in primary visual cortex.

To test the hypothesis that correlated neuronal activity serves as the neuronal code for visual feature binding, we applied information theory techniques to multiunit activity recorded from pairs of V1 recording sites in anaesthetised cats while presenting either single or separate bar stimuli. We quantified the roles of firing rates of individual channels and of cross-correlations between recording sites in encoding of visual information. Between 89 and 96% of the information was carried by firing rates; correlations contributed 4-11% extra information. The distribution across the population of either correlation strength or correlation information did not co-vary systematically with changes in perception predicted by Gestalt psychology. These results suggest that firing rates, rather than correlations, are the main element of the population code for feature binding in primary visual cortex.