Evidence exists of asymmetries in human stereoscopic ability depending on the direction sign of the stimulus. This includes the discovery of the phenomenon of stereoanomaly, an independent deficit in either crossed or uncrossed disparity processing in the absence of other binocular issues, thought to be neurological in origin. Despite this, measures of stereopsis are commonly conducted using only crossed disparity stimuli. The main aims of this project were to explore differences in the perception of stereoscopic depth between crossed and uncrossed stimuli, to determine whether significant differences exist and to evaluate whether there may be merit in incorporating assessment of stereopsis in both the crossed and uncrossed direction in routine clinical assessment. Furthermore, it aimed to evaluate how stereoanomaly may relate to any uncovered differences and to standardise how stereoanomaly is defined clinically. The initial experiments of this thesis showed that significant differences in crossed and uncrossed stereoacuity can be detected by clinical stereotests in visually normal adults, with crossed disparity stereoacuity commonly superior to uncrossed, but these differences are dependent on both the choice of stereotest and the effect of repeated measures. A distinction is made, however, between statistical and clinical significance; the typical magnitude of differences seen were generally clinically insignificant and thus unlikely to be representative of the phenomenon of stereoanomaly. Although individual cases with larger, clinically significant differences between crossed and uncrossed stereoacuity were observed, these effects are inconsistent across tests and thus are not diagnostically suggestive of a neurophysiological stereoanomaly. Chapter Five extended the evaluation beyond threshold stereoacuity measures to consider potential differences in the perceived depth magnitude between crossed and uncrossed disparities. Novel findings included the confirmation that greater disparity produces an approximately linear increase to the magnitude of perceived depth with both crossed and uncrossed disparities within the fine range of disparities utilised by clinical stereotests. As in the previous experiment, a modest bias toward crossed disparities was uncovered but, unlike with previous research, without a correlation to threshold stereoacuity measures. Individuals demonstrating asymmetries in perceived depth dependent on the disparity sign were discovered, but like the previous experiment, these asymmetries were not replicated across the tests. Chapter Six then explored whether asymmetries between crossed and uncrossed stereopsis and the phenomenon of stereoanomaly may be more likely to manifest in individuals with compromised binocularity, specifically with monocular defocus and in individuals with anisometropic amblyopia. Monocular optical defocus (both induced and in true anisometropic amblyopes) was shown in some individuals to produce larger, directionally specific deficits in stereopsis, supporting a possible role for developmental factors in stereoanomaly. In conclusion, that significant differences in crossed and uncrossed stereopsis cannot be detected with clinical stereotests was rebutted. However, separate routine clinical measurement of stereoacuity utilising both disparity directions was not supported by the results of this thesis; the typical magnitude of differences reported were insufficiently large to be reliably detected clinically or to suggest an independent neurological deficit in either crossed or uncrossed processing and instead may reflect a form of experiential bias towards crossed disparity. The results of this thesis therefore caution the conclusions of previous research. Specifically, the conflation of small, statistically significant differences in crossed and uncrossed stereoacuity with the phenomenon of stereoanomaly is not supported; these two findings are unrelated and should be considered separately. This project provides an exciting scaffolding for future investigation into asymmetries in crossed and uncrossed stereopsis and their functional implications, confirming that clinical stereotests can evaluate for differences whilst revealing an exciting possibility that compromised binocularity in childhood may provide a cause for stereoanomaly.
| Date of Award | 18 Feb 2026 |
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| Original language | English |
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| Supervisor | Baskar Pitty Theagarayan (Main Supervisor) & John Siderov (Co-Supervisor) |
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