Interocular transfer of a visual aftereffect in early-onset esotropia

Excitatory binocular interactions in two cases of alternating strabismus

PURPOSE

Individuals with alternating fixation due to strabismus have often been considered prime examples of monocular visual function. A growing body of evidence suggests, however, that, at least in the case of a fixed-angle strabismus, excitatory binocular function is possible in the strabismic visual cortex if interocular suppression is taken into account. We investigated whether excitatory binocular function might also be possible for patients with alternating strabismus.

METHODS

Suprathreshold binocular interaction was tested in two individuals with alternating fixation and no amblyopia using a dichoptic motion coherence paradigm that can measure and account for interocular suppression.

RESULTS

Both participants exhibited strong interocular suppression when stimuli of the same contrast were presented to each eye, whereas no such suppressive interactions were present for controls; however, in significantly reducing the contrast of the stimuli presented to the fixing eye, excitatory binocular interactions were demonstrated in both participants similar to those measured in controls without the contrast imbalance.

CONCLUSIONS

The cortical mechanisms necessary for combining information from the two eyes seem to have been present but suppressed in our 2 participants with alternating fixation, just as they have been shown to be present in patients with fixed-angle strabismus.

Interocular transfer of orientation-specific fMRI adaptation reveals amblyopia-related deficits in humans

We devised an experimental strategy for assessing the cortical cross-talk between ocular subsystems. For this purpose we measured the interocular transfer of adaptation (IOTA) at different levels in the human brain, using orientation-selective fMRI adaptation. We tested 10 normally sighted and 10 stereoblind or stereodeficient amblyopic observers by adapting monocularly to phase-reversing, oblique sinusoidal gratings. Following monocular adaptation, cortical activations evoked by the same (monoptic) or the other eye (interocular) were measured for the same and for the orthogonal orientation in a two by two factorial design. In both experimental groups, we obtained significant orientation-selective monocular adaptation in area V1 and in extrastriate regions on the dorsal and ventral visual pathways. In the normally-sighted subjects we found in addition interocular adaptation in V1 and extrastriate visual areas. This interocular adaptation indicates that fMRI adaptation transfers from the adapted ocular subsystem to the non-adapted ocular subsystem, and thus provides a measure of binocular interaction in normally-sighted subjects. In the amblyopic subjects, no interocular adaptation was seen at any of the investigated cortical levels, regardless of which eye was adapted. We suggest that the abnormal pattern of interocular transfer of fMRI adaptation is related to the disturbed integration of binocular signals in amblyopia.

On the relation of stereoacuity to interocular transfer of the motion and the tilt aftereffects

Interocular transfer of both the motion and the tilt aftereffect were tested in 43 subjects with varying degrees of stereopsis, ranging from normal to stereoblind. Although there was an overall tendency for transfer to diminish with decreasing stereovision, stereoblind subjects always showed some transfer of the tilt aftereffect, and sometimes also of the motion aftereffect, while some subjects with normal stereothresholds had greatly reduced or no interocular transfer. No quantitative correlation between stereothresholds and amount of transfer could be found. The results indicate that there is no simple relationship between interocular transfer, stereopsis and cortical binocularity, as had been suggested previously.