Binocular perception was tested in subjects with a history since early childhood of exotropia, or outwards deviation of the eyes. All had 20/20 visual acuity in each eye, could alternate ocular fixation freely, and denied diplopia. The simplest explanation for the absence of diplopia would be that perception was suppressed entirely in the deviated eye. To test this idea, the visual fields were examined manually using a Goldmann perimeter. Subjects’ task was to detect the appearance of a small light spot moving from the periphery towards the center of a hemispheric bowl. compares the monocular and binocular visual fields in a 9-year-old girl (Subject 1) with a 16° exotropia since age 8 months. The monocular visual field of each eye extended nasally 55° along the horizontal meridian. After the deviated eye was uncovered, targets were detected out to 90° (the maximum coverage of the hemispheric bowl). All subjects (n = 5) who were tested with this instrument showed an expansion of the visual fields to a full horizontal range of at least 180° under binocular conditions. The increased size of the binocular visual fields, compared to the monocular visual fields, indicated that the deviated eye was not suppressed completely, but rather, that images falling on its peripheral nasal retina were perceived while viewing with both eyes open.
To delineate perceiving versus suppressed retina in the deviated eye, the visual fields were tested under dichoptic conditions (). A 1° purple spot composed of isoluminant blue and red was presented briefly at a peripheral location. The subject’s task was to identify the color of the spot. If the right eye was suppressed locally in the visual field where the spot was presented, the subject responded “blue”, and vice-versa. Occasional red, blue, or blank “catch” trials were interleaved randomly to assess the subject’s reliability on unambiguous trials.
Dichoptic visual field maps in Subject 1 showed a vertical border between the center of gaze for each eye, splitting the visual field into regions where perception was mediated by either the right eye or the left eye (). In regions where one eye was perceptually dominant, the other eye was suppressed. The suppression scotomas were relatively stable on the retinas, shifting location on the tangent screen with switches in fixation. Notably, the fovea of the deviated eye was not suppressed.
On catch trials, Subject 1 identified red or blue targets accurately, even at locations where they were not seen when purple stimuli were presented (). For example, when fixating with the right eye, a purple stimulus 20° to the left of the vertical meridian was reported as blue, because the temporal retina of the right eye was suppressed. However, a red stimulus shown at the same location was identified as red. This result indicated that only stimuli presented to both eyes simultaneously evoked suppression.
In this subject, exotropic deviation of the eyes occurred on an intermittent basis. When her eyes were aligned, she fused and had normal stereopsis (40 arc-sec). During dichoptic visual field mapping, the eye trackers detected occasional epochs of normal foveal alignment. These trials were analyzed separately (). They were characterized by scattered red or blue responses, forming an inconsistent map that differed markedly from the results obtained in the exotropic state. The map generated while the eyes were aligned resembled maps compiled from normal subjects (n = 6), who responded red or blue in an unpredictable fashion throughout the visual field. Their responses reflected the piecemeal, variable suppression that occurs from binocular rivalry.
Figure 4 Responses during binocular fusion in Subject 1. Plot of responses to purple stimuli (white circles) delivered while the eye trackers detected fixation of both (B) eyes at the origin. Responses of either “blue” or “red” (more ...)
When fusion was disrupted in normal subjects (n = 6) using prisms, a different sensation was experienced. On purple trials, subjects saw simultaneous red and blue spots, separated by the angular deviation caused by the prisms. Simultaneous detection of the red and blue components of the purple target occurred because there was no visual suppression. The same result was obtained in subjects (n = 5) with diplopia caused by ocular misalignment acquired in adulthood. shows the dichoptic visual fields in a man with diplopia for one year from a partial oculomotor nerve palsy. Purple stimuli were perceived as separate red and blue spots on the majority of trials.
Figure 5 Dichoptic visual field testing in a 30-year-old man with exotropia from a traumatic partial oculomotor nerve palsy. Testing was conducted about a year after the onset of double vision. Plots show responses to trials with the (a) left or (b) right eye (more ...)
Dichoptic maps compiled from 12 additional subjects with childhood exotropia showed a consistent organization of suppression scotomas (). Each eye was dominant in its temporal visual field, regardless of which eye was fixating. In the nasal fields, the transition between perception and suppression occurred approximately midway between the center of gaze for each eye. In every subject a suppression scotoma was present in both eyes, not just in the deviated eye. The suppression scotomas in each eye fit together in a complementary fashion to eliminate diplopia throughout the binocular visual fields (). The most striking finding was that the deviated eye’s fovea was perceptually active in every subject.
Figure 6 Dichoptic perimetry in 12 subjects (a–l) with exotropia. For each set of plots, left panel shows responses with the left (L) eye fixating and the right panel shows responses with the right (R) eye fixating. Fields subtend 60° horizontally (more ...)
Figure 7 Schematic illustration of the portions of the visual field perceived with the left eye (blue) and right eye (red), while an exotropic subject fixates centrally with the left eye. There is suppression of part of the temporal retina (gray) in each eye, (more ...)
With both foveas engaged simultaneously in perception, visual confusion might occur in a person with misaligned eyes because different images project onto each fovea. In addition, objects whose images fall on the deviated eye’s fovea could be localized erroneously in space. shows the results of dichoptic visual field testing in a 50-year-old woman (Subject 2) with a right exotropia since age 2. There was a characteristic pattern of suppression in each eye. An afterimage test was used to assess how she localized corresponding retinal points in space (Hillis and Banks, 2001
). An electronic flash was used to illuminate the left retina with a horizontal bar of light, centered by a small gap on the fovea. In the same manner, a vertical bar was projected immediately afterwards onto the right retina. The subject then drew the relative positions of the retinal afterimages. Looking straight ahead with the left eye, the foveal afterimage in the right eye was displaced horizontally to the right by 33° (). This separation was close to the magnitude of the exotropia, which averaged 29.2°. Her percept signified anomalous retinal correspondence, that is, a shift in the visual direction of images seen by the right eye relative to the left eye (von Noorden and Campos, 2002
Figure 8 Dichoptic visual field mapping in Subject 2. (a) Color-coded responses to a purple spot presented with either the left or right eye fixating at the origin. Testing was extended to ±40° because the exotropia was large, averaging nearly (more ...)
Afterimage testing was performed in 5 exotropic subjects; all showed anomalous retinal correspondence with a mean difference between afterimage separation and ocular deviation of only 2.5° ± 1.9°. Control subjects with normal eye alignment (n = 6) drew intersecting horizontal and vertical afterimages, corresponding to the location of the two foveas. The cross formed by the afterimages denoted normal retinal correspondence. Even if the eyes were deviated with prisms or displaced mechanically by pressure on the globe, normal subjects continued to perceive a cross.
All subjects showed variability in the size of their exotropia, as shown by scatter in the dots representing the position of the deviated eye during dichoptic visual field testing (). In Subject 2, the exotropia ranged between 25–35° on individual trials. Despite this variability in ocular deviation, she never reported seeing two targets – red and blue – on purple stimulus presentation. The absence of diplopia implies that the border between the suppression scotomas was labile, shifting over a range of 10° as the ocular deviation changed from one moment to the next.
After suppression scotoma mapping, Subject 2 underwent surgery on the horizontal rectus muscles to improve her eye alignment. The surgery resulted in an over-correction of the exotropia. The subject noted constant diplopia immediately after the operation. Measurement with eye trackers revealed an esotropia that varied between 15–32°. Suppression scotoma mapping was repeated 4 days after surgery (). The subject reported seeing red and blue targets simultaneously on most trials. Afterimage testing showed that when she fixated centrally with the left fovea, the afterimage on the right fovea was still perceived on the right side. However, the fovea of the right eye now projected optically to the left side. The discrepancy between her perceived retinal correspondence and actual retinal alignment presumably accounted for her report of diplopia. At most locations, the purple spot now fell on portions of each eye’s retina that were not suppressed.
Another surgical procedure was performed to correct the esotropic position of the eyes. The lateral rectus muscle was advanced, and its position was adjusted after the operation while the patient was awake to eliminate the esotropic deviation. Several weeks later the subject reported that her double vision had improved. There was an exotropia measuring 5°. The subject could not fuse, even with prism correction, owing to the early onset of her strabismus. Visual field testing revealed the same layout of suppression scotomas recorded before the initial surgery, but the foveas were separated by only 5° (). The border where perception of the scene shifted from one eye to the other passed between the foveas. There was inconsistency in the identification of purple stimuli, especially centrally, reflecting the subject’s report of occasional, persistent double vision. The afterimage test showed an anomalous retinal correspondence of 5°, equal to the physical deviation of the foveas. Several months later the patient reported complete resolution of double vision.