The implications of abnormal stereopsis in typical and atypical development

A remarkable feature of the human visual system is that it is possible to extrapolate a large amount of information about the three-dimensional structure of the environment simply from the pattern of light that falls on the retinae. This information is derived from a number of different cues to depth. The mechanisms by which these are encoded in the brain, combined into an overall percept, and subsequently interpreted are reasonably well understood. However, individuals who participate in studies of depth perception tend to have acute sensitivity to certain depth cues, meaning that the consequences of individual differences in depth perception have been largely ignored. In this thesis I investigate how individual differences in the ability to utilise a single cue, binocular disparity, affects overall perception of depth and then go on to explore the wider function significance of such a deficit. I also examine whether an underlying deficit in stereopsis may account for some of the perceptual differences observed in autism spectrum disorder (ASD).\ud \ud The first set of experiments explored the consequences of individual differences in stereopsis upon perception. The initial study of this thesis used a shape constancy paradigm to identify how individual cues to depth are utilised and combined in typical children and adults. I report that while children are more sensitive to certain depth cues compared to adults, they still show some degree of cue combination (though only for higher- level information). In addition, I observed that an inability to use binocular information appears to cause re-weighting to occur in favour of monocular cues, regardless of age. In the second study, I used the same paradigm to explore depth cue sensitivity and combination in typically-developing (TD) and ASD teenagers. The results from this experiment indicated that contextual and binocular information interact when creating an overall percept of depth. A main effect of ASD diagnosis was found, with this group reporting perception that was less biased and closer to the raw sensory input. Although participants with ASD exhibited poorer stereoacuity than their TD counterparts, this did not explain the differences between the groups. I propose this indicates that perceptual differences in autism likely stem from underlying neurological differences specific to the disorder as opposed to a more general stereopsis deficit. The third study assessed the combination of ordinal and metric depth cues in TD and ASD adults. Cue integration did not depend on sensitivity to disparity or autism diagnosis. Unlike previous research, and inconsistent with perceptual theories of autism, I found that individuals with ASD automatically integrated depth cues, even when it was not advantageous to do so. Additionally, I found that the processing of uncrossed disparities was particularly difficult for those with an ASD.\ud \ud The second part of the thesis aimed to characterise the functional significance of impaired stereopsis. For the fourth study, I wanted to establish whether the functional significance of stereopsis followed a developmental trajectory. I was also interested if the motor deficits observed in those with poor stereopsis were limited to hand-eye coordination tasks. Using three tasks derived from a standardised test of motor proficiency – catching a ball, balancing on one leg, and bead-threading – I measured the effect of binocular vision and stereoacuity on motor ability. Stereoacuity affected performance across a range of tasks involving the use of fine and gross motor skill, and – importantly – the effect of stereopsis did not change with age. In the final study, I enquired as to the further-reaching consequences of poor stereopsis. Using a quantitative survey I aimed to establish how stereopsis, motor skills, and social skills related to one another. While motor ability mediated the relationship between stereopsis and social skill, stereopsis also directly contributed to social skill, causing me to suggest that the functional significance of stereopsis is not limited to motor ability.\ud \ud It is concluded that while individual differences in stereoacuity may affect the amount of depth experienced, they do not affect the ability to combine different cues to depth. While those with ASD experience differences in perception, these cannot be attributed to the increased prevalence of stereopsis impairment. It does, however, seem that individual differences in stereoacuity impact upon the development of motor proficiency and social skill, which are typically compromised in those with ASD