Disparity tuning of binocular facilitation and suppression after normal versus abnormal visual development

Abstract

Purpose: To study the pattern of facilitatory and suppressive binocular interactions in stereodeficient patients with strabismus and in healthy controls.

Methods: Visual evoked potentials were recorded in response to a Vernier onset/offset pattern presented to one eye, either monocularly or paired dichoptically with a straight vertical square-wave grating, which, when fused with the target in the other eye, gave rise to a percept of a series of bands appearing in depth from an otherwise uniform plane or with a grating that contained offsets that produced a standing disparity and the appearance of a constantly segmented image, portions of which moved in depth.

Results: Participants with normal stereopsis showed facilitative and suppressive binocular interactions that depended on which dichoptic target was presented. Patients with longstanding, constant strabismus lacked normal facilitative binocular interactions. The response to a normally facilitative stimulus was reduced below the monocular level when it was presented to the dominant eye of patients without anisometropia, consistent with classical strabismic suppression of the nondominant eye. The dominant eye of strabismic patients without anisometropia retained suppressive input from crossed but not uncrossed disparity stimuli presented to the nondominant eye.

Conclusions: Abnormal disparity processing can be detected with the dichoptic VEP method we describe. Our results suggest that suppression in stereoblind, nonamblyopic observers is determined by a binocular mechanism responsive to disparity. In some cases, the sign of the disparity is important, and this suggests a mechanism that can explain diplopia in patients made exotropic after surgery for esotropia.

FIG 1

Stimulus schematic. A vernier onset/offset target was presented to one eye while the other eye viewed either a blank screen (Monocular Lateral Motion), a collinear bar pattern (Disparity Range 0.5–10’) or an offset bar pattern (Disparity Range 5.5–15’). The test stimulus is in the left eye in this illustration, but the tests were presented in both eyes and a range of both crossed and uncrossed disparities was presented in separate conditions. The lower row illustrates the perceived surfaces and motions. In the Monocular lateral motion condition, the dynamic offsets appear to move laterally in the fixation plane over a range of displacements between 0.5 and 10’. In the Disparity Range 0.5–10’ condition, lateral motion and motion in depth are both seen and the stimulus alternates between collinear flat plane and a segmented set of panels, illustrated as two depth planes with arrows between them. In the Disparity Offset 5.5–15’ condition, the offsets in the right eye create a standing crossed disparity and the dynamic panels alternate between two planes that lie in front of the fixation plane.

FIG 2

Disparity tuning functions for the first harmonic (1F) components for the normal vision participants (A,B), patients with strabismus without anisometropia (C,D) and strabismus patients with anisometropia (E,F). Data collected when the test was in the dominant eyes are plotted in the first column (Dominant Eye). Data collected when the test was in the non-dominant eye are plotted in the right column (Non-Dominant Eye). Gray filled circles plot the monocular data (Mon), black filled squares plot the 0 disparity pedestal data (Bin0) and black open squares plot the 5 arc min disparity pedestal data (Bin5). In the normal vision observers (A,B), the first harmonic response of the 0 disparity pedestal data lies above the monocular data but the 5 arc min disparity pedestal data lies below the monocular data. In each of the patient groups, the facilitation in the Bin0 condition is reduced. The error bars were calculated by summing the errors on the sine and cosine coefficients computed across observers in quadrature. See text for details.

FIG 3

Two-dimensional plots of the complex-values underlying response amplitude and phase. The x-axis plots the real/cosine component and the y-axis the imaginary/sine component in units of microvolts. Data from the dominant eyes are presented in the left two columns separately for uncrossed and crossed disparities and data from the non-dominant eyes are presented in the right two columns. Data from each observer group are presented as rows. Within each panel, the monocular data are plotted in blue, the 0 disparity pedestal data in magenta and the disparate pedestal data in orange. As amplitude increases, the response phase shifts towards the phase origin (in the clockwise direction, especially in the monocular and zero disparity pedestal conditions. The ellipses plot the dispersion at a nominal 1 s.e.m.. See text for details.

FIG 4

Response amplitude and phase in the dominant eyes of normal vision participants (black; Normal); patients with strabismus with (dark gray; Aniso) and without (light gray; Strab) anisometropia. Data are from the monocular viewing condition. The phase of the response in the strabismus patients without anisometropia is shifted to increased lags/time delays relative to that of the normal vision observers. The phase of the response in the group with strabismus and anisometropia is shifted towards decreased lag/time delay, relative to that of the normal vision participants.