Neural Circuitry of Fixation

When an interesting or important stimulus is foveated, it may be important to maintain its fixation and to temporarily suppress the generation of saccades to other stimuli. For example, during reading (Fig. 2), the average fixation time depends on the overall difficulty of the material, and individual fixation durations depend on the complexity of each word fixated. Thus, controlling fixation duration is an important component of eye movements.

The rostral end of the SC (adjacent to the representation of the smallest saccades) is specialized for fixation. Cells there project to the omnipauser region of the pons, and electrical stimulation at the rostral pole of the SC prevents saccades. Because most visually responsive neocortex projects strongly to the SC, this rostral fixation zone provides a pathway for cortical areas that process foveated stimuli to temporarily suppress saccades. Stimulation at some FEF sites can inhibit saccades as well, and foveally responsive FEF cells resemble cells at the SC rostral pole.

Figure 17 Activity of FEF pursuit neuron and pursuit deficits after FEF deactivation. (A) The pursuit neuron, recorded in the left hemisphere FEF, was active in conjunction with leftward (negative moving on the traces) smooth pursuit and responded throughout the leftward phase in all three cycles of sinusoidal motion (0.5 Hz, 7 20°A). Notice that it also discharged prior to leftward predictive pursuit that occurred at the end of sinusoidal tracking (see also Fig. 4). (B) The neuron responded very little to visual motion that was not pursued and was silent, except for three spikes on the initial cycle, when the monkey fixated a stationary point while viewing the same sinusoidally moving stimulus used in A. (C) Sinusoidal (1.0 Hz, 710°A) smooth pursuit of this monkey immediately before the injection. (D) Deficient smooth pursuit immediately following a small muscimol injection at the site of the pursuit neuron shown in A and B. Leftward smooth eye velocity was much less on every cycle, especially the first, and there was no longer predictive pursuit after the target was extinguished (adapted with permission from D. Shi, H. Friedman, and C. Bruce, J. Neurophysiol. 80, 458-464, 1998).

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