THE HUMAN BRAIN HAS AN AMAZING ability to direct action to objects in the visual world—a baby reaching for an object, a tennis player hitting a ball, an artist looking at a model. This ability requires that the visual system solve three problems: making a spatially accurate analysis of the visual world, choosing the object of interest from the welter of stimuli in the visual world, and transferring information on the location and details of the object to the motor system.
The Brain Compensates for Eye Movements to Create a Stable Representation of the Visual World
Although the visual system produces vivid representations of our visual world, as described in preceding chapters, a visual image is not like an instantaneous photographic record but is dynamically constructed from information conveyed in several discrete neural pathways from the eyes. When we look at a painting, for example, we explore it with a series of quick eye movements (saccades) that redirect the fovea to different objects of interest in the visual field. The brain must take into account these eye movements in the course of producing an interpretable visual image from the light stimuli in the retina.
As each saccade brings a new object onto the fovea, the image of the entire visual world shifts on the fovea. These shifts occur several times per second, such that after several minutes the record of movement is a jumble (Figure 25–1). With such constant movement, visual images should resemble an amateur video in which the image jerks around because the camera operator is not skilled at holding the camera steady. In fact, however, our vision is so stable that we are ordinarily unaware of the visual effects of saccades. This is so because the brain makes continual adjustments to the images falling on the retina after each saccade.
Eye movements during vision. A subject viewed this painting (An Unexpected Visitor by llya Repin) for several minutes, making saccades to selected fixation points, primarily to faces. Lines indicate saccades, and spots indicate points of fixation. (Reproduced, with permission, from Yarbus 1967).
A simple laboratory experiment, shown in Figure 25–2, illustrates the biological challenge to the brain.
The double-step task illustrates how the brain stabilizes images during saccades.
A. A subject starts by looking at a fixation point (FP) that disappears, after which two saccade targets A and B appear and disappear sequentially before the subject can make the saccade. The first saccade (to target A) is simple. The retinal vector (FP→A) and the saccade vectors are the same. After the first saccade, the subject is looking at A. The retinal vector is A→B′, but the monkey must make a saccade whose vector is A→B. The brain must ...