PATTERN ADAPTATION AND CROSS-ORIENTATION INTERACTIONS IN THE PRIMARY VISUAL-CORTEX

The responsiveness of neurons in the primary visual cortex (V1) is sub stantially reduced after a few seconds of visual stimulation with an e ffective pattern. This phenomenon, called pattern adaptation, is uniqu ely cortical and is the likely substrate of a variety of perceptual af ter-effects. While adaptation to a given pattern reduces the responses of V1 neurons to all subsequently viewed test patterns, this reductio n shows some specificity, being strongest when the adapting and test p atterns are identical. This specificity may indicate that adaptation a ffects the interaction between groups of neurons that are jointly acti vated by the adapting stimulus. We investigated this possibility by st udying the effects of adaptation to visual patterns containing one or both of two orientations-the preferred orientation for a cell, and the orientation orthogonal to it. Because neurons in the primary visual c ortex are sharply tuned for orientation, stimulation with orthogonal o rientations excites two largely distinct populations of neurons. With intracellular recordings of the membrane potential of cat V1 neurons, we found that adaptation to the orthogonal orientation alone does not evoke the hyperpolarization that is typical of adaptation to the prefe rred orientation. With extracellular recordings of the firing rate of macaque V1 neurons, we found that the responses were not reduced by ad aptation to the orthogonal orientation alone nearly as much as by adap tation to the preferred orientation. In the macaque we also studied th e effects of adaptation to plaids containing both the preferred and th e orthogonal orientations. We found that adaptation to these stimuli c ould modify the interactions between orientations. It increased the am ount of cross-orientation suppression displayed by some cells, even tu rning some cells that showed cross-orientation facilitation when adapt ed to a blank stimulus into cells that show cross-orientation suppress ion. This result suggests that pattern adaptation can affect the inter -action between the groups of neurons tuned to the orthogonal orientat ions, either by increasing their mutual inhibition or by decreasing th eir mutual excitation. (C) 1998 Elsevier Science Ltd. All rights reser ved.