Saccades (Guest Lecture)

The video of the normal eye movement examination was made with the assistance of Dr. Terrence Millette, a neurologist and former Fellow with me in 1985-1986. Introduction to the Saccadic System Saccades are fast eye movements that bring the image of an object of interest onto the fovea. They consist of a hierarchy of rapid eye movements from the most rudimentary form quick phases of vestibulo-optokinetic nystagmus, to reflexive saccades made in response to the sudden appearance of a novel visual stimulus, to high-level volitional saccades. To test Saccades: Ask the patient to look alternately at two targets held apart horizontally or vertically and note velocity, latency, accuracy, trajectory and conjugacy. 1.Velocity normal range 30-700 degrees/sec 2.Duration 30-100 msec 3.Accuracy 4.Latency (initiation time) 160-250 msec Clinical Syndromes Abnormal velocity - slow saccade syndrome Increased latency - ocular motor apraxia Inaccuracy - saccadic dysmetria hypermetria = overshoot hypometria = undershoot Inappropriate saccades - saccadic intrusions and oscillations e.g. opsoclonus To localize a saccadic abnormality test the hierarchy of saccades: Visually guided (reflexive) saccades Present the patient with a suddenly appearing visual or auditory target. Voluntary saccades to command Ask the patient to make saccades rapidly between two stationary targets. (Loss of voluntary saccades with preservation of quick phases and visually guided (reflexive) saccades is characteristic of acquired ocular motor apraxia) Predictive, anticipatory saccades Hold both hands up and ask the patient to make a saccade when one of your fingers move. With predictable timing, move first a finger on one hand and then a finger on the other, and repeat this cycle several times, occasionally not moving one finger to determine if the patient makes a predictive saccade. (Defect of predictive saccadic control occurs in Parkinson's disease) Memory guided saccades Saccades generated to a location in which a target has been previously present Antisaccades Hold both hands up and move a finger on one hand suddenly. Ask the patient to look away from the moving finger i.e. look to the finger that does not move. To do this correctly the patient needs to suppress reflexive saccades (Errors on antisaccade task are common in dementia with lesions of the prefrontal cortex) Quick phases Visual ocular reflex Spin the patient in a swivel chair with the patient fixing on the finger held in front of her or use an optokinetic drum to elicit optokinetic nystagmus (Loss of quick phases is usually due to lesion of premotor burst neurons in the brainstem) The Pursuit System Pathway: The smooth pursuit pathway originates in the M (large) ganglion cells in the retina. Signals from these cells are relayed through the magnocellular layers of the lateral geniculate nucleus to the striate cortex (area V1), from there to areas V2 and V3, and then to MT (middle temporal, area V5). Signals from MT are relayed to MST (medial superior temporal, area V5a) and to the visual motor area in the frontal (the smooth pursuit subregion of the frontal eye field and supplemental eye field) and posterior parietal cortex (lateral intraparietal area). In V1, the information about motion is transformed by neurons that respond to particular directions of motion. These signals are further elaborated in MT, where the firing pattern of the population of neurons encodes the speed and direction of motion. Information about motion from MT is then extracted in MST. Both MT and MST, as well as the frontal eye field, project directly to the dorsolateral pontine nuclei (DLPN) in the ipsilateral basal pons. There is a double decussation of the horizntal pursuit pathway. The first decussation is the pontocerebellar projections from the DLPN, through the contralateral middle cerebellar peduncle to the contralateral flocculus/ventral paralocculus and the dorsal vermis. The second decussation is from second-order vestibular neurons in the MVN to the contralateral abducens nucleus. In addition, the nucleus of the optic tract (NOT) receives projections from MT and MST and projects to the DLPN. The NOT participates in horizontal pursuit only. Unilateral or bilateral lesions of NOT do not affect vertical pursuit. Stimului for smooth pursuit: •Target velocity (i.e., retinal slip velocity; a velocity error) is the primary stimulus •Position of target (a position error) •Motor command to the eye (efference coty) •Proprioception (afferent input) e.g. tracking one's outstretched finger in the dark; also uses knowledge of motor command to the limb •Perception of motion (required high-level integration of many motion cues; e.g. stroboscopic motion in which one infers motion of an object from a series of flashes, even though no actual motion occurs) To test pursuit: To test pursuit the patient needs to be attentive. Ask the patient to follow (track) a moving target (examiner's finger, patient's own finger, red headed pin or flash light) Vary the speed of the target to test accuracy of smooth tracking on acceleration and deacceleration. See also: http://content.lib.utah.edu/cdm/ref/collection/ehsl-shw/id/340