Neural Circuitry of the Foveation System

The image-stabilization system (ISS) functions to ensure that the visual image as a whole is stationary on the retina. This suffices for many species, but with the evolution of the fovea came the need to deliberately shift the direction of gaze, independent of the ISS, in order to purposively foveate different stimuli. Because the foveal specialization covers only ~ 1/1O,OOO of the visual field, the eyes must be moved accurately. Furthermore, they must be moved intelligently to selected targets because systematic scanning of the whole visual field (like radar or television) would require hours and mostly be a waste of time. Therefore, the foveation system (FS) orchestrates eye movements that provide the fovea with an endless succession of the most important, informative, and pleasing visual stimuli.

As mentioned earlier, the primate FS uses three distinct eye movements: saccades, smooth pursuit, and fixation. These eye movements are voluntary in that they are controlled by, or made in the service of, the subject's choice to foveate a particular stimulus. In contrast, the ISS reflexively responds to the aggregate of vestibular and visual motion sensations and requires only a state of wakeful alertness.

Although choosing to foveate a particular stimulus is a voluntary action, doing so automatically activates the appropriate foveation subsystem(s). For example, after choosing to fixate a particular stationary target, (i) if the target moves quickly to a new location (e.g., a "step" motion), then a saccadic movement will be made in order to foveate it at its new location; (ii) if the target begins to smoothly move (e.g., "ramp'' motion), then smooth-pursuit movements will match the target velocity and thereby both maintain foveation and reduce retinal blur; and (iii) if the target remains stationary, then fixation continues.

The cerebral cortex has a large role in these foveating eye movements. This seems obvious considering that occipital, temporal, and parietal neocor-tex are all heavily engaged in visual processing. Nor is involvement of the frontal lobe surprising, because the FS concerns voluntary movement. Although saccadic and smooth-pursuit eye movements are represented in separate cortical areas that have different subcortical projections, it is still likely that a common network of cortical areas mediates the target choice decisions for the FS as a whole.

Despite their functional differences, the FS and ISS engage the same basic brain stem neural circuits to effect eye movements. More precisely, the FS engages

OMNs only indirectly via the ISS circuits: Voluntary saccadic eye movements exploit the quick-phase generating circuitry in the midbrain and pontine reticular formation and smooth-pursuit eye movements target the vestibular nuclei where the vestibu-looptokinetic velocity commands are assimilated. By engaging these established motor (as opposed to sensory) aspects of the ISS circuitry, foveating eye movements automatically achieve proper connectivity to the neural integrator in order to overcome elastic forces in the orbit and hold the eye steady wherever it has moved. Note that an important implication of this strategy is that the FS, like the ISS, ultimately formulates both its fast and slow eye movement objectives as eye velocity commands.

Usually the FS and the ISS work together; for example, while walking through a garden, saccades might foveate different flowers, with the VOR holding the image of each flower stationary on the retina despite movements of the head while walking. However, it is easy to pit the systems against each other, and in such situations the FS usually prevails. For example, given a small stationary spot in front of a background of moving stripes, one can elect to fixate the spot and thereby subdue the OKR ordinarily evoked by a large moving pattern. Conversely, given a moving spot on a stationary background of stripes, one can choose to pursue the spot smoothly across the stripes and the ensuing retinal slip of the pattern, caused by the pursuit eye movements, would be ignored. Finally, the FS can even suppress vestibular signals to move the eyes. For example, by looking at an object fixed relative to the head (e.g., the brim of a cap) and then rotating the head (like in Fig. 1O), one can keep the brim fixated and thus keep the eye stationary in its orbit, despite the vestibular signals that indicate head rotation.

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