Category-sensitive excitatory and inhibitory processes in human extrastriate cortex.

Single-cell recordings from the temporal lobe of monkeys viewing stimuli show that cells may be highly selective, responding for example to particular objects such as faces. However, stimulus-selective cells may be inhibited by nonpreferred stimuli. Can such inhibitory mechanisms be detected in human visual cortex? In previous recordings from the surface of human ventral extrastriate cortex, we found that specific categories of stimuli such as faces and words generate category-specific negative event-related potentials (ERPs) with a peak latency of about 200 ms (N200). Laminar recordings in animal cortex suggest that the human N200 reflects excitatory depolarizing potentials in apical dendrites of pyramidal cells. In this study we found that, at about half of word-specific N200 sites, faces generated a positive ERP (P200); conversely, at about half of face-specific sites, words generated P200s. The electrogenesis of N200 implies that P200 ERPs reflect hyperpolarizing inhibition of apical dendrites. These recordings, together with the prior animal recordings, provide strong circumstantial evidence that in human cortex populations of cells responsive to one stimulus category (such as faces) inhibit cells responsive to another category (such as words), probably by a type of lateral inhibition. Of the stimulus categories studied quantitatively, face-specific cells are maximally inhibited by words and vice versa, but other categories of stimuli may generate smaller P200s, suggesting that inhibition of category-specific cells by nonpreferred stimuli is a general feature of human extrastriate cortex involved in object recognition.