SPATIALLY DISTRIBUTED RESPONSES INDUCED BY CONTRAST REVERSAL IN CAT VISUAL-CORTEX

Field potentials were recorded from cat striate cortex, either between an epidural screw electrode and a cannula-electrode inserted deep in the gray matter (transcortical recording) or with a pair of metal micr oelectrodes. Electrodes were placed bilaterally near the cortical proj ection of the area centralis. The horizontal separation of the recordi ng tips was similar to 2 mm and similar to 300 mu m, respectively. The area of the visual field providing input to the recording site (recep tive field) was determined by measuring the field potentials generated by contrast reversal of high-contrast, achromatic bar gratings. Five- degree-diameter grating patches were presented individually over a lar ge area of the visual field. The gratings were contrast-reversed at 4, 6 or 10 Hz, while also being swept in spatial frequency between 0.56 and 5.24 c/deg. The receptive fields were similar to 20 deg across or more, substantially larger than expected on the basis of cortical reti notopy. Responses were also elicited by stimulation of the hemi-field contralateral to that contributing to the classical receptive field, i mplicating the presence of a callosal projection. The large, spatially distributed receptive fields consisted of patches of high and low sen sitivity. Continuous cortical infusion of either 100 mu M tetrodotoxin or 10 mM muscimol at the recording site totally suppressed the transc ortically recorded field potentials, proving that the local field pote ntials were generated postsynaptically. The present findings suggest t hat a cluster of cortical cells near the projection site of the area c entralis receives input from remote cortical regions to an extent that is comparable with that of anatomically demonstrated long-range later al connections.