Visual System Architecture The visual cortex then

Visual System Architecture

The visual cortex, then, is organized functionally into two sets of intersecting connections, one vertical, consisting of functional columns spanning the different cortical layers, and the other horizontal, connecting functional columns with the same response properties.

Organization of V 1 and V 2

The Binding Problem in the Visual System A related view of the effect of attention on the binding problem recently was advanced by John Reynolds and Robert Desimone. They based their interpretation on two observations already described in this chapter: neurons have larger and larger receptive fields at higher levels in the cortical visual pathways and attention to one of several stimuli falling in one of these large receptive fields increases the response to that stimulus. They assume that attention acts to increase the competitive advantage of the attended stimulus so that the effect of attention is to shrink the effective size of the field around the attended stimulus. Now instead of many stimuli with different characteristics such as color and form, only the one stimulus is functionally present in the receptive field. Because the effective receptive field now just includes that one stimulus, all the characteristics of the stimulus are effectively bound together. Another approach to the binding problem has been emphasized by Charles Gray and Wolfgang Singer and Reinhold Eckhorn and their colleagues. They found that when an object activates a population of neurons in the visual cortex, the neurons tend to oscillate and fire in unison. They suggest that these oscillations are indicative of a synchrony among cells and that this synchrony of firing would bind together the activity of cells responding to different features of the same object. To combine the visual features (color, form, motion) of the same object, the synchrony between neurons would, according to this view, extend across neurons in different cortical areas. Quite a different solution to the binding problem was proposed by Lance Optican, Barry Richmond, and their colleagues. They found that neurons extending from the lateral geniculate nucleus to the inferior temporal cortex convey more information if the temporal pattern of their discharge isconsidered. Instead of measuring the total number of spikes in a time period, they measured the distribution of the spikes in that time period and found that different stimulus features (eg, form, contrast, color) tended to be represented by different response patterns of the same cell. They propose that the pattern of discharge in each cell carries information about different features so that the problem of binding across cells, each representing a different feature, is eliminated. Cells in different areas would all convey some information about a number of stimulus features, but different cells would carry comparatively more or less about each feature.

Inferior temporal