A strong constraint to the joint processing of pairs of cortical signals.

An important question in neuroscience is how the activity from spatially distributed cortical representations is integrated and processed together. In this study, we used a new approach to investigate the integration of distributed cortical activity. We used microstimulation to directly activate pairs of sites in primary visual cortex of rhesus monkeys. The sites were activated either singly or jointly, and the monkeys were trained to behaviorally report detection of the activation of either cortical site. We compared the detection performance with predictions from two different mathematical models of signal combination. Our data show that, at cortical separations <1 mm, signal integration is well described as a linear combination (d' summation) of individual site activity. At larger separations, signal integration is better described as a maximum operation on the site signals. We compare our neurophysiological findings to existing psychophysical data and suggest the intriguing possibility that cortical activity originating at spatial separations greater than ∼1 mm is processed as if by parallel, independent circuits whose signals can be compared against each other but not summed. This in turn implies that there is a strong constraint to the kinds of computations the brain can perform with spatially distributed cortical activity.