Human amblyopia and its perceptual consequences
The presence of ocular defects of an optical or muscular nature during early childhood can cause amblyopia: a reduction in visual acuity of the defective eye. The research reported in this thesis investigated the impact of amblyopia on some aspects of visual perception by evaluating three main perceptual functions: precision of judgement of spatial relationships (in three-dimensional space), ability to detect depth in tests of stereopsis, and contrast sensitivity. In some experiments amblyopic subjects were paired with non-amblyopic subjects who had monocular acuity deficits owing to uncorrected refractive errors, in order to assess the importance of the acuity deficit as a determinant of other perceptual losses suffered by amblyopes. In an alignment task non-amblyopes with monocularly reduced acuity performed significantly better than amblyopes, suggesting that the acuity deficit was not solely responsible for amblyopes' perceptual deficit in this task. However, in another experiment in which a greater variety of spatial cues was provided amblyopes performed as well as non- amblyopes. Thus their perceptual skills would seem to be adequate for efficient functioning in most normal environments where spatial cues are abundant. Previous reports that amblyopes generally lack stereopsis were confirmed in two experiments with a few interesting exceptions, whose cases are discussed. The data obtained in the four experiments on space perception and stereopsis in amblyopia provided support for most current theories in these areas. Experiments on contrast sensitivity showed that the losses suffered by amblyopes, as measured by interocular comparison, varied between individuals, both in depth and in bandwidth (the range of spatial frequencies affected). This variation was not directly related to the extent of acuity deficit, or to the condition which originally gave rise to amblyopia, but did seem connected with the age at which the subject first received treatment for the primary causative ocular defect. A similarity between the contrast sensitivity functions of amblyopic eyes and those of infant eyes is considered as a basis for explaining the nature of contrast sensitivity loss in amblyopia. Some preliminary attempts to measure contrast sensitivity in infancy by methods suitable for screening purposes are described in the final chapter. The thesis includes a historical review of theories of amblyopia derived from clinical and experimental work on human subjects, and a critical evaluation of experimental work in which animals were visually deprived with a view to measuring the contributions of experience to visual development. The claims of some authors that such work may have clinical relevance for preventing or treating amblyopia are refuted, since clinical experience has already furnished sufficient evidence to achieve these ends. The perceptual consequences of human amblyopia, as characterised in the present research have important practical implications for the amblyope, and important theoretical implications for models and mechanisms of visual perception and its development.