Lab 7 - Visual System

Supranuclear Motor Pathways - Saccades

Saccades are rapid (ballistic) conjugate eye movements that are under both voluntary and reflex control. The eyes execute a series of very rapid movements from one point to another, stopping briefly at each point, the fixation point, to check the visual image. Normally we are unaware of these sudden stops as the visual association cortex provides visual constancy and blends each brief image into a smoothly changing view of the visual world. During a saccade, the control signal is retinal position error. Examples of voluntary saccades are self-directed eye movements and those in response to command (e.g., andquot;look to the rightandquot;). The sudden appearance of a peripheral object or an eccentric sound may evoke a reflex saccade in the direction of the stimulus.

Voluntary saccades: The neural commands generating voluntary saccades appear to originate mainly in a region of the frontal lobe referred to as the frontal eye field (area 8 of Brodmann) in conjunction with input from the supplementary eye field (also in frontal cortex) and from the parietal cortex. The frontal eye field is located in the caudal portion of the middle frontal gyrus and extends into contiguous areas of the inferior frontal gyrus. The frontal eye field of one hemisphere controls voluntary saccadic eye movements that are directed toward the contralateral visual hemifield, i.e., the right frontal eye field directs the eyes to the left. Thus, a lesion of the frontal eye field in the right hemisphere could produce an abnormality in the generation of leftward moving saccades. The corticofugal projections from the frontal eye fields travel in the anterior limb of the internal capsule and decussate near their site of termination. They terminate in the superior colliculus, which sends its axons to the midbrain vertical gaze center and the pons horizontal gaze center. There are corticofugal projections that travel directly to and end in the pons horizontal gaze center. The direct cortical projections to the horizontal gaze centers control voluntary lateral (horizontal) saccades. There appears to be no direct frontal eye field projection to the vertical gaze centers.

Gaze centers: The midbrain vertical gaze center sends its axons bilaterally to the trochlear nuclei, which control the superior obliques, and to the oculomotor neurons controlling the inferior obliques and the inferior and superior rectus muscles. During vertical saccades, the midbrain vertical gaze centers insure that the superior and inferior recti act in synergy to produce conjugate movements of the eyes. The pons horizontal gaze center sends its axons to the ipsilateral abducens nucleus, which contains interneurons that send their axons in the contralateral medial longitudinal fasciculus back up to oculomotor neurons controlling the medial rectus muscle. During horizontal saccades, the pons horizontal gaze centers insure that the medial and lateral recti act in concert to produce conjugate horizontal eye movements.

Reflex saccades: Reflex saccadic eye movements appear to be initiated in the superior colliculus, which receives direct input from the retina, inferior colliculus, and cortex. From the superior colliculus, the control of reflex saccades is identical to that described for the voluntary saccades. That is, the superior colliculus sends axons to the midbrain vertical gaze centers and the pontine horizontal gaze centers.