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A history of the observational era of vision is presented through selected descriptions of phenomena by natural philosophers from Aristotle to Wheatstone. The descriptions are listed under the headings of optics, colour, subjective visual phenomena, motion perception, eye movements, binocular vision, and space perception.
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Recent research has shown that visuomotor adaptation to a lateral displacement of the visual field induces significant perceptual aftereffects in normal observers, and in right hemisphere patients with spatial neglect [Neuroreport 11 (2000) 1899; Nature 395 (1998) 166]. These findings suggest that adaptive realignment following prism exposure induces a bias in visual space perception, even in tasks that require no visually guided motor response. Given recent neurophysiological and behavioural data suggesting independent visual representations for peripersonal and extrapersonal space, here we asked whether adaptive aftereffects extend beyond participants' immediate reaching space to stimuli presented beyond arms' reach (i.e. in extrapersonal space). Thirty-two participants underwent adaptive realignment to 10 degrees left- or right-displacing wedge-prisms. Before and after adaptation participants performed a visual landmark task that required estimation of the midpoint of horizontal line stimuli. There was a significant rightward shift in visual midpoint judgements following adaptation to left-deviating prisms, which was evident in both peripersonal and extrapersonal space. Paradoxically, a significant rightward shift also occurred following adaptation to right-deviating prisms, but only in extrapersonal space. We suggest that the pattern of adaptive aftereffects observed reflects the different reference frames used by participants to perform spatial judgements in peripersonal and extrapersonal space. We also propose that an underlying hemispheric asymmetry in the processing of spatial errors during adaptation may contribute to the direction of aftereffects in both normal observers, and in patients with unilateral lesions.
Patients with unilateral neglect show disorders in horizontal space perception. It has been argued that these disorders may depend on a left-right relaxation of the representational medium that becomes progressively "relaxed" toward the contralesional space and progressively "compressed" toward the ipsilesional space. We tested this hypothesis in 31 right-brain-damaged patients, 17 with neglect and 14 without neglect in two different experiments. Patients were asked to read words in canonical and anisometric letter spacing. Only in neglect patients, the manipulation of letters spacing may ameliorate neglect dyslexia. These results support the idea that the abnormalities observed in typical neglect tests are due to a distorted internal representation of the outside world. In addition, the space distortion seems to depend on the degree of horizontal relaxation of the representational medium and it is unrelated to hemianopia.
Velo-cardio-facial syndrome (VCFS) is associated with deletions on the long arm of chromosome 22, mild intellectual disability, poor social interaction and a high prevalence of psychosis. However, to date there have been no studies investigating the neuropsychological functioning of adults with VCFS. We compared 19 adults with VCFS with 19 age, gender and IQ matched controls using a comprehensive neuropsychological battery. Compared to controls, adults with VCFS had significant impairments in visuoperceptual ability (Visual Object and Space Perception Battery), problem solving and planning (Tower of London) and abstract and social thinking (Comprehension WAIS-R). It is likely that haploinsufficiency (reduced gene dosage) of a neurodevelopmental gene or genes mapping to chromosome 22q11 underlies the cognitive deficits observed in individuals with VCFS.
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Ophthalmic lenses are produced in a variety of forms each having unique effects upon visual space. The foundation for evaluating visual space distortion associated with ophthalmic lenses is an understanding of the optical input to the visual system. We developed a photographic technique to evaluate the effects of ophthalmic lenses on optical imagery. This technique uses a pinhole camera and an associated lens holder to simulate the eye-lens relation and provide an eye-referenced view of optical space. A rectilinear grid was photographed through Varilux progressive addition lenses (PAL's) and with no lenses in place. Comparison of these photographs provides a measure of relative optical space distortion that should closely approximate relative visual distortion under similar viewing conditions. Our technique provides an optical baseline with which more in depth studies of space perception and adaptation may be compared.
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To examine the role of extraretinal eye position information (EEPI) in visual perception of target location in normal room illumination, subjects participated in experiments in which EEPI was manipulated using the eye press maneuver with either monocular or binocular viewing. The viewing condition and eye press caused EEPI and retinal information about target location to conflict. Pointing responses in eye press trials were all in the direction of EEPI showing that EEPI is the dominant source of information in egocentric visual space perception. In binocular viewing, version and vergence occur in response to the eye press to maintain fusion and EEPI based on these movements also determine perceived location. An unanticipated finding was that the eye press was variable in its effectiveness in rotating the eye, which contributed to large variability in pointing errors and suggested the method would be a poor choice for future work.
We studied spatial orientation across saccadic eye movements by adding a disturbance to the system that achieves space constancy. A small target was flickered between 33 and 960 Hz, and it was displaced during saccadic eye movements. The threshold for detecting its displacement was measured. Sensitivity to displacement was almost twice as great when the target was jumped in the direction opposite the eye movement as when it was jumped in the same direction. This would be expected from a partial breakdown of space constancy: The world should seem to move in the direction opposite to an eye movement (when the eyes move to the right, for example, the world would appear to move to the left). Sensitivity to displacement was greater at lower flicker rates. The results imply that both masking and extraretinal signals are important in suppressing the detectability of target displacements during saccades, and that flicker on video display terminal may distort space perception. These and other results lead to a recommendation that video terminals be refreshed at least 120 times/s.
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