Visual Processing In The Primate Brain

National Eye Institute

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Abstract

As we look at complex visual scenes, we actually look at not the whole scene at once, but instead our eyes move in succession from one part of the scene to another. To do this we shift visual attention from one region of the visual field and then to another until the areas of possible interest have been examined. Accompanying the shifts of attention to different regions of the visual field are rapid or saccadic eye movements that move the eyes quickly from one part of the visual scene to another and bring that part of the scene onto the fovea. As a result of the work in this laboratory and many others throughout the world, the brain mechanisms that generate these eye movements have been studied in a model of the human visual and oculomotor system, the old world monkey, and are now understood, at least in anatomical outline. In contrast, what brain mechanisms underlie the shifts of attention to one part of the field or the other remain unknown. One hypothesis is that the same mechanisms that underlie the saccade generation underlie the shifts of attention to one part of the visual field; a shift of attention occurs as an integral part of the mechanisms that will lead to the saccade. In previous years we have tested the hypothesis that attention is shifted as part of the same mechanism that moves the eyes. Using a task in which the monkey has great difficulty seeing a change in the visual scene unless its attention is drawn to the region of the field by a visual cue, we demonstrated that with this shift of attention the monkey could see the change but without the cue it rarely could. We then substituted a brief stimulation in the saccade pathway for the visual cue by stimulating the superior colliculus (SC), a midbrain saccade related structure, to see if we could direct attention to particular regions of the visual field. We presented multiple fields of moving random dots positioned around a central fixation point. After a random delay while the monkey fixated, the direction of dot motion in just one of the fields changed on about half the trials. The monkey was rewarded for making a saccade to the target if it changed, and for maintaining fixation otherwise. During some trials, there was a 150ms blank at the time of the possible change during which only the fixation point remained, thus masking the change. We previously learned that a visual cue facilitates detection of changes, even in blanked "change-blindness" trials, presumably due to the directed allocation of attention. By overlapping fields of dots and movement fields in the SC, we determined whether sub-threshold stimulation of the SC facilitates detection of changes at a particular location. We found that the monkey could more easily detect changes in the areas of the visual field corresponding to the activation sites in the SC. We observed an increase in correct saccades to changes in both blanked and non-blanked trials - the proportion of incorrect saccades did not increase. This indicates that activating the SC facilitates detection of visual changes in particular locations, much like directing attention with a visual cue. A limitation to this interpretation, however, is that the stimulation in the brain may not have produced a shift of attention but may have instead produced a perceived flash of light and the monkey simply looked at where the flash of light occurred. This is a problem in not only our experiments but in others that use stimulation to shift attention. We therefore performed a series of critical control experiments that used either a real visual cue or the electrical brain stimulation. In all cases we found that the visual cue did not act to shift the attention as did the brain stimulation. In addition, if the stimulation shifted attention by producing a visual flash, then stimulating the superficial layers of the SC, which is devoted to visual processing, should produce the shift of attention. No such shift in visual attention resulted from this superficial stimulation. Taken together, these experiments and the critical controls lend strong support to the hypothesis that the neuronal basis of shifts of attention to different regions of visual space is closely related to the generation of an eventual eye movement to that same region of space. This opens the possibility of understanding the basic mechanisms in the brain that underlie visual spatial attention in both normal vision and in cases of disrupted attentional abilities.