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OCULOMOTOR SYSTEM The function of the eye system is to acquire visual targets rapidly and, once acquired, to stabilize the image on the retina in spite of relative movements between the target and the observer. Movements of the eye can be classified as follows: (Kandel, Schwartz, Jessel) Movements that stabilize image when head moves Vestibulo-ocular - Uses vestibular input to hold images during brief or rapid head rotation Optokinetic Uses visual input to hold images stable on the retina during sustained or slow head rotation Movements that keep the fovea on a visual target Saccade Brings new objects of interest onto the fovea Smooth pursuit Holds the image of a moving target on the fovea Vergence Adjusts the eyes for different viewing distances in depth OCULOMOTOR SYSTEM II Vestibulo-ocular reflex (VOR) As we have seen, the semi-circular canals signal the speed of rotation in any direction and the oculomotor system responds by rotating the eyes at an equal but opposite SPEED so that an object of interest can be foveated. (Give example) So if you were to rotate (in the dark) at a constant rate to the left, your eyes would be ‘pinned’ to the right corner of your eyes (cannthus) due to signals from the semicircular canales. This does not occur due to quick reset motions of the eyes (nystagmus). NOTE: movement of head is to the left Right Eye Position fast phase slow phase Left Time OCULOMOTOR SYSTEM III Optokinetic Reflex (OKR) As the eyes move in the environment, the image of fixed objects move across the retina in a direction opposite to the that of the head. The optokinetic system drives the eyes in the direction of visual field motion (to try to foveate fixed objects) which is opposite the head movements inducing that motion. NOTE: movement of striped drum is to the right while the subject is still Right Eye Position fast phase slow phase Left Time Here the OKR interprets visual motion as head movement. OCULOMOTOR SYSTEM IV SO, the VOR and the OKR work in a complementary fashion to keep an image foveated as the head moves in space. The OKR is particularly effective at very low frequency whereas the VOR is not very effective at extremely low frequency. The effectiveness of reflexes are usually measured by the engineering technique of ‘gain’ measurement. The reflexes must be modifiable because sometimes they are counter-functional. (e.g., when an object of interest is moving with you). NOTE: gains of the reflexes can be modified in interesting ways OCULOMOTOR SYSTEM V Smooth Pursuit The Optokinetic System tries to stabilize the eyes in space when head movements occur (involuntary). The Smooth Pursuit System moves the eyes in space to keep a single target on the fovea (voluntary). is present. Smooth pursuits CANNOT be done voluntarily unless a target OCULOMOTOR SYSTEM VI Saccadic System When an image of interest which is on the fovea suddenly moves to another part of the visual field, the eyes remain in the original position for about 200ms and then move quickly to re-acquire the image. The speed of the re-acquiring movement (saccade) is dependent on the length of the eye movement required, andthe initial eye position. Corrections are made (mini-saccades) after the initial saccade gets the eyes near the visual target. OCULOMOTOR SYSTEM VI Vergence When an object of interest moves toward or away from us, our eyes must adjust to keep the image foveated. This requires disconjugate movement of the eyes,i.e., movements in opposite directions. NOTE: All previously described eye movements were conjugate. The mechanism for vergence depends upon the ‘blurring’ of the image as the target moves toward (or away from) you. The ciliary muscles contract to change the shape, and therefore, the focal length of the lens to focus the image. Modifiability of VOR The job of the Vestibulo-ocular Reflex (VOR) is to allow a visual image to remain fixated on the fovea during movements of the head in 3-space. This takes place through the activity of the ocular motor neurons which reflect the velocity and position of the eyes. The VOR can be modified both functionally and experimentally Functional modification - fixation on an object moving through space with you Experimental modification - goggles or training CONTROL OF HORIZONTAL EYE MOVEMENTS lateral rectus medial rectus medial rectus oculomotor nucleus abducens nucleus semicircular canal Vestibular Nucleus lateral rectus GAZE CONTROL Because an animal may be moving with respect to the environment AND the head may be moving with respect to the body AND the eyes may be moving with respect to the head, the problem of finding the visual fixation point relative to the environment is complex. Luckily, the brain (cerebellum) is able to handle the necessary coordinate transformations in terms of GAZE, i.e., where the 12 muscles of the eyes set the point of visual fixation. The signals needed to perform this complex function come from - vestibular system - senses body movement wrt the Earth coordinates - neck receptors - sense head rotations wrt the body coordinates - optokinetic and retinal slip systems - sense eye movement in head coordinates COR / VOR / OKR Rotations of the head on the body are sensed by mechanoreceptors in the first 3 cervical joints. Interactions of the vestibular, cervical and visual control of eye movements are below: Cerebellum Vestibular Oculomotor Nuclei Nucleus Semicircular Canals Cervical Vertebrae Eye Inferior Olive COR VOR OKR OCULARMOTOR SYSTEM SUMMARY More is known about the ocularmotor system than about any other motor system. But Several important things are still not known: For example, ocular muscles are rich in muscle spindles but ocular muscles do not exhibit stretch reflexes so their function is not known. Also, despite the fact that the signal of each type of neuron is known wrt eye position and velocity, the control of recruitment to drive the eye muscles is unknown. So there is lots of science and engineering left to do!!!