Sensory Physiology Matthew D Gearinger Goals To be

Sensory Physiology Matthew D. Gearinger

Goals To be able to use sensory testing to improve management of patients with strabismus and amblyopia

Learning Objectives • Define corresponding points, horopter, and Pannum’s space. • Describe three sensory outcomes to strabismus • Outline clinical testing of anomalous retinal correspondence

Which of the following is a true statement? A. All objects that fall on the empirical horopter are seen singly. B. All objects that do not lie on the horopter are seen doubly.

Which of the following statements is false? A. Fusional amplitudes are less at near than distance. B. Strabismus can cause suppression or diplopia. C. Central fusion depends upon precise retinocortical ordering.

Which of the following is true? A. A “crossed” response with red glass testing signifies esotropia. B. Worth 4 -Dot testing elicits suppression, fusion or diplopia. C. Patients with ARC never develop diplopia following eye muscle surgery. D. Bagolini lenses are the most dissociative sensory test.

Sensory Physiology The unfortunate marriage of geometry and neurophysiology

Sensory Physiology The process of unifying the electrical impulses representing two separate (and different) visual environments from the two eyes into a single perception.

The Geometry • The object of regard falls on both foveae. • Corresponding points are two points in either retina that, when stimulated, are localized in the same visual direction (and seen singly). • Vieth-Muller circle is a theoretical construct localizing all corresponding points for a particular fixation distance. Circle with points on the fixation object and optical centers of both eyes.

More Geometry • The empirical horopter is an experimentally derived surface of corresponding points- flatter than VMC. • Any object on the horopter will stimulate corresponding points and will be seen singly. • Any object off the horopter will fall on disparate points, and should be diplopic.

Stereopsis • Objects slightly in front of or behind the horopter can be perceived singly in depth. The limits of the area describes Pannum’s space. • Any object outside Pannum’s space will be seen as diplopic. • The horopter and Pannum’s space shift with each fixation distance.

More Stereopsis Area near fovea has small receptive fields, i. e. very little separation before diplopia elicited. Periphery has larger receptive fields, i. e. larger degree of separation before diplopia.

Fusion • The cortical unification of visual objects into a single percept. • Images must be of similar size and shape and fall on corresponding areas. • Sensory fusion based on retino-cortical correspondence (corresponding retinal points project to the same cortical locus) • Motor fusion is the process of extraocular muscles maintaining the object of regard on corresponding retinal elements.

Fusion Examples • Viewing stereo disk photos is a process of sensory fusion (two separate images sensed as single percept). • Maintaining single vision in presence of base out prism is a process of motor fusion (convergence causes the object to project to corresponding points. )

Fusional Amplitudes Greater at near than distance. Convergence- 38 D at near. Divergence- 16 D at near. Vertical 2. 6 D at near.

Abnormal Binocularity • Visual Confusion- misaligned eyes have foveae aimed at separate objects. These non-fusable objects appear in the same space in the lack of central suppression. • Diplopia- object of regard falls on one fovea and a peripheral element in fellow eye- appears to be in two places.

Diplopia-perception by fovea and non-foveal element Confusion-simultaneous bifoveal perception

Abnormal Binocularity • Central disruption of fusion (horror fusionis)absence of suppression and lack of motor fusional amplitudes leads to intractable diplopia. • Usually from head trauma/surgery or prolonged monocular occlusion.

Sensory Adaptations

Suppression • Central (foveal) suppression- Two foveae unable to simultaneously perceive disparate objects. Prevents confusion. May lead to amblyopia. • Peripheral suppression- eliminates diplopia by suppressing hemi-retina that perceives object of regard.

No Suppression Central and peripheral suppression Central Suppression

Anomolous Retinal Correspondence • Deviated fovea is suppressed, and non-foveal element takes up foveation. Gives a crude binocularity. • Binocular phenomenon only. • When deviated eye views monocularly, foveates normally. • Can develop “paradoxic diplopia” following surgery - usually transient.

Sensory Tests • Used to determine binocularity, stereoacuity, and ARC. • Worth 4 -Dot, stereo, 4 D Base-out, red glass, Bagolini lenses.

Worth 4 -Dot • Red lens in front of right eye. • Test at near and distance (distance target subtends smaller angle on retina). • Responses can be 2 red (suppressing OS), 3 green (suppressing OD), 4 (fusing), or 5 (diplopic). • Fairly dissociative.

“fusion” “suppr OD” “suppr OS” “diplopia”

4 D Base Out Test • Used to determine small scotomas (monofixation). • In normal state, it will cause a version in direction of apex, followed by a convergence. • If patient doesn’t do version or vergence, there is a scotoma.

Normal version Normal convergence Scotoma OS Version only No version

Red-Glass Testing • Helps determine presence of ARC. • In NRC: If tropia present, red and white targets will be “crossed” in XT and “uncrossed” in ET. • If harmonious ARC present: Red glass testing will show superimposition of red and white lights.

NRC “uncrossed”=ET “crossed”=XT ET ET

Bagolini Lenses No diopteric power, micro striations give Maddox rod-like streaks. Least dissociative sensory test. Allows cover testing.

NRC Sm scotoma Suppr OD

Which of the following is a true statement? A. All objects that fall on the empirical horopter are seen singly. B. All objects that do not lie on the horopter are seen doubly.

Which of the following statements is false? A. Fusional amplitudes are less at near than distance. B. Strabismus can cause suppression or diplopia. C. Central fusion depends upon precise retinocortical ordering.

Which of the following is true? A. A “crossed” response with red glass testing signifies esotropia. B. Worth 4 -Dot testing elicits suppression, fusion or diplopia. C. Patients with ARC never develop diplopia following eye muscle surgery. D. Bagolini lenses are the most dissociative sensory test.