Stereoblind Subjects Research Articles

Binocular interaction studied with visual evoked responses (VER) in humans with normal or impaired binocular vision.

In an attempt to evaluate binocular functions in an objective manner, visual evoked responses (VER) were recorded in sixteen subjects, six with normal binocular functions and ten with various degrees of squint and binocular impairment. The two eyes were stimulated monocularly or dioptically with a polaroid checker board pattern viewed through rotating polaroid discs. The rate of pattern reversal was 7 per sec for one eye and 10 per sec for the other, allowing stimulus locked responses to be recorded from each eye individually, even during dioptic viewing. In normal subjects the VER of each eye to binocular stimulation were reduced to half or less of the monocular value. The reduction was much smaller in humans with impaired binocular vision, at least for the dominant eye. In stereoblind subjects the response of the dominant eye was the same for monocular and binocular stimulation. The VER changes might reflect binocular interaction in visual cortex neurons, which probably are binocular in normal subjects but driven only from one eye in stereoblind subjects.

Stereopsis and the representation of space.

Four stereoblind and four normal subjects were tested on a mental rotation task. It was hypothesized that, if stereopsis is an important input for building up the perceptual system that represents three-dimensional space, then subjects lacking it ought to be deficient at mental rotations in depth. Stereoblind subjects were equally efficient at picture-plane and depth rotations, and were nonsignificantly better than normal subjects at rotations in depth. It was concluded that in the absence of stereopsis other cues are sufficient for the development of the "three-dimensional" perceptual system. A puzzling paradox was raised, however, by the finding that the introspections of the two groups differed markedly.

Interocular transfer of the motion after-effect in normal and stereoblind observers

The extent of interocular transfer of the motion after-effect was measured in 4 stereoblind subjects and in 19 subjects having varying degrees of stereopsis. Stereoblind individuals failed completely to show any interocular transfer of this after-effect, while subjects with good stereopsis exhibited between 55 and 82 percent transfer (mean 73 percent). Furthermore, normal subjects who manifested a clear eye dominance tended to show greater transfer from the dominant to the nondominant eye than vice versa. Individuals who either had a history of a strabismus or possessed some other early impediment to clear binocular vision tended to show less transfer. Overall there was a significant positive correlation of 0.75 between the extent of interocular transfer and the subject's stereoacuity. It is argued that the extent of interocular transfer of this after-effect provides a measure of the proportion of the total number of visual cortical neurons that are binocular. Thus stereoblind humans, who show no transfer whatsoever may, like cats and monkeys deprived of concordant binocular visual input early in life, suffer from a lack of binocular neurons.

Interocular transfer of a visual after-effect in normal and stereoblind humans.

1. Following inspection of a high contrast grating a test grating of a slightly different orientation will briefly appear rotated from its true orientation in a direction opposite to that of the adapting grating.2. The extent of interocular transfer of this phenomenon (the tilt after-effect) was measured in a number of normal subjects and in four subjects (three of whom had a strabismus) who lacked stereopsis.3. In contrast to the normal subjects, none of the four stereoblind subjects showed any interocular transfer of the tilt after-effect. Amongst the normal subjects the extent of transfer of this after-effect was positively correlated with the subject's stereoacuity. Maximum transfer (70%) was found in the subject with the best stereoacuity. In many subjects transfer was greater from the dominant eye to the non-dominant eye than vice versa.4. By analogy with experiments on cats deprived of congruent visual input to the two eyes early in life it is argued that the stereoblind subjects lack any binocularly driven cortical neurones.