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.

The receptive fields of inferior temporal cortex neurons in natural scenes.

Inferior temporal cortex neurons have generally been found to have large visual receptive fields that typically include the fovea and extend throughout much of the visual field. However, a problem of such a large receptive field is that it does not easily support object selection by subsequent processing areas, in that all objects within such a large receptive field might activate inferior temporal cortex cells. To clarify this, we recorded from inferior temporal cortex neurons while macaques searched for objects in complex natural scenes or in plain backgrounds, as normally used. Inferior temporal cortex neuron receptive fields were much smaller in natural scenes (mean radius, 11 degrees) than in plain backgrounds (39 degrees). With two objects in a scene, one of which was a target for action (a touch), the firing rates were equally high during foveation of the effective stimulus when it was the target and when it was the distractor in both the plain and the complex scenes. With a plain background and two objects present, the receptive fields were much larger (24 degrees ) for the stimulus when it was the target than when it was the distractor (9 degrees ). This effect of object-based attention was much less evident in the complex scene, when the receptive fields were small both when the stimulus was a distractor and when it was a target. The results show that the temporal visual cortex provides an unambiguous representation in natural scenes by responding to the object shown at or close to the fixation point.