Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Ernst Mach (1897) first observed that bilateral symmetry was most easily observed when the axis of symmetry was vertical, and proposed this occurred because of symmetric connections across the vertical midline of the visual system. This neuroanatomical account has been reasserted by Julesz (1971) and Braitenberg (1984, 1990). Braitenberg suggested that the corpus callosum could serve as a conduit for connections between cells representing symmetric areas in space around the vertical midline. If vertical symmetry in the visual system mediates the vertical advantage there are a number of predictions that follow. One would expect that the tuning of symmetry detection around vertical should be narrow, eccentric presentation of patterns should reduce the vertical advantage, and the vertical advantage should be absent at fixation in individuals without a corpus callosum. Five experiments were conducted to test these predictions. Subjects were tested using a signal detection paradigm. Symmetric and random patterns composed of 72 dots placed within a circular field were presented for brief durations. Vertical symmetry was found to be more detectable at fixation than symmetry at other orientations (from 5{dollar}\sp\circ{dollar} to 90{dollar}\sp\circ{dollar} off vertical, Experiments 1 & 3). No systematic differences were observed for the detection of non-vertical symmetry when patterns were presented at different positions along the horizontal midline (up to 4.8{dollar}\sp\circ{dollar} from fixation to the left or right, Experiments 2, 3, & 4), and the vertical midline (Experiment 4). Vertical symmetry was best detected when presented at fixation (Experiments 2, 3, & 4), and detectability dropped off when stimuli were presented as little as 0.6{dollar}\sp\circ{dollar} off fixation (Experiment 3). Two individuals born without a corpus callosum did not detect vertical symmetry best at fixation, whereas their age, intelligence, and gender matched controls demonstrated a vertical symmetry preference (Experiment 5). These results are in general agreement with the neuroanatomical proposal, and are interpreted in relation to other proposals for how bilateral symmetry may be detected. It is unclear how non-vertical or non-fixated bilateral symmetry is detected, although arguments for a spatial-frequency decomposition of images are discussed.



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