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Why do we have eye movements?. To assist and improve vision What would the world be like if we did not have eye movements? As we move around, images of objects in the world would move on our retinas, just as they do on hand-held video cameras. It would be very hard to see details clearly
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Why do we have eye movements? • To assist and improve vision • What would the world be like if we did not have eye movements? • As we move around, images of objects in the world would move on our retinas, just as they do on hand-held video cameras. • It would be very hard to see details clearly • We would feel that the world is moving constantly
Rapid or relocating Fast phases of VOR and OKN Saccades Miniature Tremor Microsaccades Slow or holding Slow phases of VOR and OKN Smooth pursuit Vergence Drifts during fixation Types of Eye Movements
Why are there different types of eye movements? • Each type solves a different type of problem for the visual system • Saccades let us look quickly from one object to another • Vergence eye movements keep our eyes aligned as we look from close objects to far ones (and vice versa) • Vestibular and optokinetic eye movements stablize our eyes as we move in the world
Vestibular movements (VOR) • As our heads move, our eyes move in the opposite direction and at the same speed • Permits us to see objects clearly while moving • Otherwise, we experience motion smear • People lacking a vestibular system can only see details clearly if they hold their heads still • Preservation of vestibular function in brain injured patients is a positive sign
More on theVOR • Caused by angular motion of the head • Sensed by the semicircular canals of the inner ear • Dynamic. Stabilizes our eyes best if our speed and direction vary over time • Static or otolith-ocular reflexes respond to maintained head tilt and linear motion
Angular motion is rotational motion Nodding or shaking your head Spinning like a top Linear motion is straight ahead Pushing a wheel chair straight down the hall Walking is mostly linear Angular vs. linear motion
Vestibular nystagmus • When we start to move, the vestibular system first engages the slow phase response, in which our eyes move opposite to our head movement and at the same speed • Before our eyes get to the edge of the orbit, it engages the fast phase system to re-center our eyes. This lets us see where we are going. • The alternation of slow and fast movements is called vestibular nystagmus. It continues as long as we continue to move in the same direction
Optokinetic nystagmus • Once called railroad nystagmus, this occurs when most of the visual field moves across the retina in a given direction • We feel like we are moving even though we are not (the Omnimax experience) • Our eyes follow the movements of the objects in the world (slow phase) and then re-center (fast phase)
Clinical testing of reflex eye movements • Because OKN is involuntary, it can be elicited even in patients who do not respond well to instructions • Malingering patients will still show good OKN because they cannot suppress the response • OKN requires a visual stimulus, usually a revolving “drum”, while VOR does not. VOR should be present even in total darkness
Smooth pursuit • These are made to follow or track small moving objects • Unlike the VOR and OKN, they are not reflex eye movements. We have to be interested in pursuing the target. • They cannot be made without a target. We can not voluntarily decide to make them without a moving target. We will make a series of small saccades if there is no moving target.
Clinical testing of smooth pursuit • Patients must be told to follow the target • The target should be interesting and have some detail that patients will want to see clearly • The target should not be moved too rapidly or too slowly--about 10°/s is optimal • Pursuit should be tested in all principal directions
Saccades • Rapid eye movements made to scan the environment • Let us look at successive objects with our foveas, as in reading • The head does not have to move • Highly predictable in speed and duration for a given size
Clinical testing of saccades • Saccades can be made voluntarily, without any visual target • Patients with ocular motor apraxia lack the ability to make voluntary saccades upon request (e.g., please look to your right) unless they are allowed to use their heads • They have intact reflex eye movements • Ability to make voluntary saccades should be checked in every primary care exam
Vergence • Disjunctive eye movements • The eyes move in opposite directions • Necessary to keep the eyes aligned to avoid double vision • Part of the fusion reflex • Also linked to accommodation
Clinical testing of vergence • A large part of a comprehensive eye exam consists of near point testing • The absence of complaints does not justify skipping this important part of the eye exam
Miniature eye movements • These occur even when we try to fixate steadily • Tremor • Drifts • Microsaccades • Somewhat under voluntary control • Vision fades if they are eliminated
