An examination of factors involved in single binocular vision and stereopsis

Abstract

Humans view the world via two laterally separated, forward facing eyes with a large binocular field of view. This thesis aims to contribute to a better understanding of the basic mechanisms that underlie our perception of single vision with two eyes and depth perception from binocular disparities. Specifically, this thesis will examine two factors impacting the range binocular disparities for which single vision is experienced, eccentricity (Study 1) and the disparity gradient (Study 2) as well as the relationship between this range and vergence eye movements (Study 3 and 4). Chapter 1 provides an overview of the process of binocular vision, stereopsis and the range of single vision, chapters 2 through 4 report four empirical studies and chapter 5 summarises the contributions of these findings to our current understanding of single stereoscopic vision.In study 1, I systematically and separately examined the contributions of eccentricity and stimulus separation on the horizontal extent (experiment 1) and vertical extent (experiment 2) of Panum’s fusional range. Critically, I controlled for a long standing confound, present in all previous studies, between eccentricity and separation between the test stimulus and fixation stimulus. The data are consistent with previous conclusions that the increase in Panum’s fusional range in the periphery is driven by increasing eccentricity and not increasing stimulus separation. These findings suggest that the increase in Panum’s fusional range in the periphery is due to increasing receptive field size in binocular neurons in the peripheral visual field.In study 2, I examined three factors influencing the critical disparity gradient, a metric that captures the relative disparity and separation between adjacent objects at the threshold between single and double vision in cluttered visual scenes. In experiment one I investigated the effects of stimulus size and stimulus duration. The results indicate that the critical disparity gradient increases with increasing stimulus size. Stimulus duration did not impact the critical disparity gradient. In experiment two, I investigated the effect of eccentricity on the critical disparity gradient and re-examined the effects of stimulus duration. The results indicate an increase in the critical disparity gradient with increasing eccentricity. There was no impact of stimulus duration. I conclude that the critical disparity gradient is impacted by eccentricity and stimulus size and suggest that duration may impact the disparity gradient dependent on instructions given to participant.Study 3 provides the first measurements of a horopter defined by the criterion of zero vergence movements (the ‘vergence horopter’) and its relationship to Panum’s fusional range. The results demonstrate that there is a stable range of disparities that do not elicit horizontal vergence movements and that these do not increase with eccentricity unlike the corresponding fusional ranges. Secondly, the results of the study demonstrate that the horizontal vergence horopter sits centrally within Panum’s fusional range and the vertical vergence horopter also sits within the corresponding fusional range but is vertical in orientation rather than the top away tilt of the fusional range. I conclude that the range of disparities that do not elicit a vergence movement are different to other aspects of vergence movements which show differences between central and peripheral stimulation.Study 4 examines the elicitation of vertical vergence movements and their relationship to fusion. In this study I investigated the maximum vertical disparities that would elicit a vergence movement for disparities in discrete locations and disparities applied to the full visual field. The results demonstrated no significant impact of the location (horizontal or vertical) of discrete stimuli in eliciting vertical vergence movements, however vergence movements were smaller in than those elicited by full field disparities. Further, centrally presented stimuli drove larger corrective vergence movements than the same stimuli presented at further eccentricities. Regarding the relationship between vergence eye movements and fusion, I found that vergence eye movements can be driven by disparities three times the fusional range. I conclude that the smaller maximum range of vertical disparities (in comparison to previously reported horizontal disparities) may act to maintain alignment of the eyes in stereoscopic depth perception.In summary, the empirical studies in this thesis have clarified the relationship between eccentricity and fusion (chapter 2), examined the effects of surrounding stimuli on the fusional range (chapter 3) and examined the relationship between the fusional range and vergence eye movements (chapter 4), providing the first measurements of the vergence horopter. Taken together, these findings add to our understanding of the stimulus conditions necessary for single vision and precise depth perception.