How do we know where objects are in the environment and how do we use this information to guide our actions? Recovering the three-dimensional (3D) structure of our surroundings from the two-dimensional retinal input received from the eyes is a computationally challenging task and depends on the brain processing and combining ambiguous sources of sensory information (cues) to depth. This thesis combines psychophysical and computational techniques to gain further insight into (i) which cues the brain uses for perceptual judgments of depth and motion-in-depth; and (ii) the processes underlying the combination of the information from these cues into a single percept of depth. The first chapter deals with the question which sources of information the visual system uses to estimate the time remaining until an approaching object will hit us; a problem that is complicated by the fact that the variable of interest (time) is highly correlated to other perceptual variables that may be used (e.g. distance). Despite these high correlations we show that the visual system recovers a temporal estimate, rather than using one or more of its covariates. In the second chapter I ask how extra-retinal signals (changes in the convergence angles of the eyes) contribute to estimates of 3D speed. Traditionally, extra-retinal signals are reputed to be a poor indicator of 3D motion. Using techniques to isolate extra-retinal signals to changes in vergence, we show that judgments of 3D speed are best explained on the basis that the visual system computes a weighted average of retinal and extra-retinal signals. The third and fourth chapters investigate how the visual system combines binocular and monocular cues to depth in judgments of relative depth and the speed of 3D motion. In chapter three I show that differences in retinal size systematically affect the perceived disparityde defined depth between two unfamiliar targets, so that a target with a larger retinal size is seen as closer than a target with a smaller retinal size at the same disparity-defined distance. This perceptual bias increases as the retinal size ratio between the targets is increased but remains constant as the absolute sizes of the targets change concurrently while keeping the retinal size ratio constant. In addition, bias increases as the absolute distance to both targets increases. I propose that these findings can be explained on the basis that the visual system attempts to optimally combine disparity with retinal size cues (or in the case of 3D motion: changing disparity information with looming cues), but assumes that both objects are of equal size while they are not. In chapter 4 these findings are extended to 3D motion: physically larger unfamiliar targets are reported to approach faster than a smaller target moving at the same speed at the same distance. These findings cannot be explained on the basis of observers' use of a biased perceived distance, caused by differences in the retinal size (as found in chapter 3). I conclude that, in line with contemporary theories of visual perception, the brain solves the puzzle of 3D perception by combining all available sources of visual information in an optimal manner, even though this may lead to inaccuracies in the final estimate of depth.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:549200 |
| Date | January 2012 |
| Creators | Lugtigheid, Arthur Jacobus Pieter |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.bham.ac.uk//id/eprint/3249/ |
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