OPT 215 Vergence Eye Movements

What falls into Maddox's classifications of vergence?

-tonic
-proximal
-accommodative
-fusional (disparity)

What is disparity (fusional) vergence?

vergence driven by binocular retinal disparity to reduce disparity to a fusible range

What are different ways to characterize the disparity vergence system?

-by the type of disparity on the retina that initiates the movement
-by the direction of movement of the eyes
-by the target's spatial location and the resultant motor response

What is symmetric disparity vergence?

-targets placed on the midline result in retinal disparity symmetrically placed with respect to the fovea
-smooth vergence responses are generally reasonably symmetric

What is asymmetric disparity vergence?

-occurs for targets placed in any other direction
-the overall movement is not symmetric
-the asymmetric vergence is usually a small vergence movement, then a saccade to create a reasonably symmetric disparity then a symmetric vergence movement

When can disparity vergence be initiated?

when the two eyes have dissimilar or uncorrelated imaged with a retinal disparity between the two

What is the fast component?

responds rapidly to changes in retinal disparity

T or F: disparity vergence can only be sustained when the two eyes have quite similar or correlated images.

True (slow component- takes over the maintenance of static eye position alignment)

What are characteristics of disparity vergence?

-disparity vergence will fuse two blurry lines about as well as two sharply focused lines
-changes in contrast and luminance of the stimulus don't have much effect on disparity vergence
-disparity vergence velocity increases as the correlation between the left and right eye images Increases

T or F: under natural viewing conditions, a single object in space produces an image in each eye. If the optics of the left and right eyes are exactly the same, then the images will be exactly the same.

True; they are considered 100% correlated

What is the neurologically based relationship between amplitude and peak velocity?

peak velocity increases as amplitude increases (slope ratio of 4:1)

holds true for total darkness, free space, or confined instrument space

T or F: Once a decision is made to execute a vergence movement, the response is relatively independent of stimulus and environmental conditions

True

What are the two types of vergence adaptation?

-prism adaptation (phoria adaptation)
-asymmetric vergence adaptation in spectacle based anisometropia

What is meant by prism adaptation?

if prism is placed before one eye and the viewer can maintain fusion, an immediate shift occurs in the person's phoria by an amount equal to the prism; if the disparity introduced by prism is large enough, the eye will make a vergence response to the prism

What is the fixation disparity in the situation explained in the previous card?

the difference between the new target position and the new intersection of the visual axes

T or F: With regard to prism adaptation, within a few minutes both the phoria and the fixation disparity measured with prism still in place will revert back to the pre prism values.

True; by re setting back to the habitual phoria, the viewer maintains a reasonably constant range of vergence response under a variety of stimulus conditions and states of the body/oculomotor system

What is asymmetric vergence adaptation in spectacle based anisometropia?

when the spectacle power of the two eyes differs significantly, the amount of prism induced for each eye when gazing away from the optical centers at an object located at a different direction and distance will be unequal

What does eccentric gaze result in?

asymmetric retinal disparity necessitating an asymmetric disparity vergence motor response within 1 second or so to avoid diplopia

What is the clinical relevance of research in vergence eye movements?

diagnosis of binocular vision disorders

What is a tonic controller?

slow adapting component, responds to inputs of phasic controller

What is a range limiter?

limits the enhancement of vergence response received from accommodative systems (accommodative vergence) to a maximum amount

What are controllers?

neurological mechanisms that transform physical stimuli to neurological codes for innervating accommodation and vergence

What is a leaky integrator controller?

neurological storage mechanism that builds up innervation in response to a stimulus and also dissipates its response when the stimulus is removed

T or F: a small error in response must remain (as feedback) in order to keep the stored response from decaying completely

True

What things can affect the steady state error?

-optical biases
-motor biases

refractive error, dark focus and vergence, adaptation effects of prolonged viewing at a given distance

What about the models of the vergence system is clinically relevant?

-small error of accommodation and fixation are necessary to provide continuous signals to accommodation (blur) and vergence (disparity) systems to sustain focus and position of the eyes at the desired distance
-understanding the dynamics of vergence and accommodation can give clues to localizing the deficits in anatomy and to design tx plans
-high AC/A ratio can be balanced by a low CA/C ratio

What are the components of the vergence neural signal?

-average latency is 160 ms
-when motion is predictable, latency decreases
-the bigger the stimulus amplitude, the bigger the pulse (higher velocity) and the bigger the amplitude of the movement

What is the average vergence velocity range?

40-70 deg/sec (but can be less than 2 or as fast as 200)

can be combined with saccades to decrease time to completion of movement

T or F: divergence is generally slower than convergence.

True; but if eyes start in a closely converged position, divergence can be quite fast

What are the types of midbrain neural cells that have been identified in vergence control?

-vergence burst neurons
-vergence tonic neurons
-vergence burst tonic neurons

What are characteristics of vergence burst neurons?

-fire before and during actual vergence response
-have an activity profile similar to that of the instantaneous vergence velocity
-believed to encode peak vergence velocity because the number of spikes generated is correlated with vergence amplitude

What are characteristics of vergence tonic neurons?

-fire just before vergence movement
-firing rate is proportional to the vergence angle
-many more tonic convergence cells than divergence

What are characteristics of vergence burst-tonic neurons?

neuron activity is related to both vergence velocity and vergence angle

What is binocular suppression?

all or part of the ocular image of one eye is prevented from contributing to the binocular percept

T or F: For patients with strabismus, using large stimuli (>40 deg in size) can induce vergence movements but only 2/3 amplitude of that produced in subjects with normal bv

True, suggests reduced gain of disparity vergence