Binocular Vision &

Eye Muscle Surgery Qtrly°

Visual acuity was approximately 20/20 in each eye at the same time and the directional values of each fovea were consistent with the physical space. In other words, the subjective angle was zero. Their subjects had HARC across the nasal retina with an extension at the binocular field, i.e., panoramic viewing. In a subsequent study, Cooper & Dibble (87) using dichoptic anaglyphic stimuli, attempted to identify the depth of the suppression scotomas in the temporal retina previously identified by Jampolsky (84), Pratt-Johnson & Wee (96), and Awaya et al (85). Instead, they found non-suppression of temporal retina with perfect HARC.

The difference in the incidence of ARC/NRC reported is clearly dependent upon the stimulus condition used for testing. Thus, ARC is probably not “wired” in and is modifiable. The stimulus conditions which elicit the greatest number of ARC responses in XT seem to be opposite to that of ET. In XT the more natural the testing conditions, the greater the chance for NRC responses, while the more artificial the environment the greater the chance of an ARC response. Burian (98),

Major Review: Intermittent Exotropia; Basic and Divergence Excess Type

Cooper & Feldman (86) and Cooper & Dibble (87) have all reported an increase in ARC responses with afterimages compared to Bagolini Striated Glasses.

These findings are in conflict with Burian’s adaptation theory of ARC. Burian suggested that ARC is a sensory adaptation to a motor misalignment. ARC is an attempt to reestablish binocular vision. The more natural the environment of testing the greater propensity for ARC responses on testing. Thus, afterimage testing should provide the lowest incidence of ARC.

But X(T)s do not follow Burian’s adaptation theory in that the highest incidence of ARC responses occurs with afterimage testing. Also, immediate post surgical ARC responses are inconsistent with adaptation theories. Further, unlike ET, unharmonious ARC is not common in XT (102).

The best explanation of dual retinal correspondence in X(T) (and possibly ARC in ET) is provided by Morgan’s motoric theory of ARC (103), which is based on Urist’s findings with afterimages (104): If a vertical afterimage is flashed on

Summer of 1993

Volume 8 (No.3): 185-216

one eye while the other eye is occluded, and a version is performed, Urist noted that the afterimages would move with the eye. The lateral movement of the afterimage results in a change in egocentric localization and is called a registered movement. On the other hand, if an accommodative vergence movement is performed instead, the afterimage will not change its egocentric direction. The afterimage (Al) will appear to regress or approach the subject. These movements are known as unregistered movements. (It should be noted in both cases that the eye with the AI was adducted an equal amount).

Urist’s findings show that egocentric localization of an Al is based upon the type of eye movement and that projection is not cortically fixed. Accommodative vergence anomalies such as in an accommodative ET would be expected to have non-registered movements. Thus, changes in the deviations due to vergence induced by accommodation-vergence would not result in an alteration of projection of an AI. The result would be NRC. Version related defects as proposed by Keiner (105) would be associated with registered movements and a change in egocentric localization or

ARC.

Morgan postulates that during manifest XT, registered movements take place resulting in a change in position and egocentric localization of the Al. Morgan’s theory would predict NRC when the XT patient was straight and ARC when deviated. Though Morgan’s theory is the most appealing theory Qf ARC, it fails to account for the discrepancy in findings of retinal correspondence found on various tests, and the rare, but recorded, instantaneous changes in correspondence without an appropriate motor response.

Sperling (106) has shown that retinal disparity and motor information can easily be mathematically combined to give an accurate prediction of relative depth no matter where the eyes are postured.

Boucher (107), using common visual direction, mapped out the horopter of a deviating X(T). Ocular position was monitored with an infrared eye monitor system. The shape of the horopter measured during exodeviation was similar to the horopter obtained during normal alignment except for a larger Panum’s area. This finding suggests dual retinal correspondence with ARC occurring across the entire visual field. Boucher states that the X(T) patient overcompensated for distance localization since the horopter was closer to him than to a normal person.

Figure 9 (Cooper & Medow): Afterimnages are placed on the fovea of the OD and OS of a DEX(T). During alignmnent both after imnages project egocentrically straight ahead. During deviation the after imnage associated with the deviated eye may be projected egocentrically to the right of the position the eye is pointed indicating ARC. Thus, DEX(T) may have dual correspondence NRC when straight; I-L4RC when deviated.

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