NEURAL ACTIVITY IN AREAS V1, V2 AND V4 DURING FREE VIEWING OF NATURALSCENES COMPARED TO CONTROLLED VIEWING

UNDER natural viewing conditions primates make frequent exploratory ey e movements across complex scenes. We recorded neural activity of 62 c ells in visual areas V1, V2 and V4 in an awake behaving monkey that fr eely viewed natural images. About half of the cells studied showed a m odulation in firing rate following some of the eye movements made duri ng free viewing, though the proportions showing a discernible modulati on varied across areas. These cells were also examined under controlle d viewing conditions in which gratings or natural image patches were f lashed in and around the classical receptive field while the animal pe rformed a fixation task. Activity rates were generally highest with fl ashed gratings and lowest during free viewing. Flashed natural image p atches evoked responses between these two extremes, and the responses were higher when the patches were confined to the classical receptive field than when they extended into the non-classical surround. Thus th e reduction of activity during free viewing relative to that obtained with flashed gratings is partly attributable to natural images being l ess effective stimuli and partly to suppressive spatio-temporal neural mechanisms that are important during natural vision.