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A mathematical model for assimilation and contrast in the perception of extent is presented, and predictions generated from the model are empirically tested. Implications of the model for the Müller-Lyer illusion are dealt with explicitly, and implications of the model for the Delboeuf, Ebbinghaus, and other illusions of extent are discussed in general terms.
It has been shown that human test subjects have the ability, under controlled laboratory conditions, to use echoes to detect the presence or absence of targets placed before them. In addition, blind and sighted persons have been able to detect a target monaurally, to make simple shape discriminations, and to locate a target in space. Signal, environmental, and individual variability affect performance in a measurable fashion. This research is an initial step in measuring the limits of a human being's ability to use echoes as a source of information about his physical surroundings. At this point it seems unlikely that the unaided human ear can rival the bat's auditory system for echo perception. It may be, however, that modern technology can partially bridge the evolutionary gap and bring more useful echoes to man's ear than those it now receives. Such an accomplishment would allow us to examine the extent to which man might benefit from this means of sensing his environment.
56 obese and 103 normal-weight college students were asked to make line-length estimations of lines on either a light or dark background. Analysis of the results showed significant interactions between weight and contrast and sex and contrast, with the line estimations by women and normal weight individuals being more dependent on the contrast.
It is argued that an unwarranted dichotomization of geometrical illusions has produced theories whose explanatory scope is limited to either assimilation or contrast. Since recent attempts at integration lack precision, a 1977 unified model of the perception of extent by Brigner was revised to predict accurately both assimilative and contrastive phases of the parallel lines illusion. Application of the revised model to other geometrical illusions was discussed and a means of parsimoniously accounting for variations in assimilative illusions occurring with age and with lightness of the stimulus figure was suggested.
Two experiments used Müller-Lyer stimuli to test the predictions of the planning-control model (S. Glover, 2002) for aiming movements. In Experiment 1, participants aimed to stimuli that either remained the same or changed upon movement initiation. Experiment 2 was identical except that the duration of visual feedback for online control was manipulated. The authors found that the figures visible during movement planning and online control had additive effects on endpoint bias, even when participants had ample time to use visual feedback to modify their movements (Experiment 2). These findings are problematic not only for the planning-control model but also for A. D. Milner and M. A. Goodale's (1995) two visual system explanation of illusory bias. Although our results are consistent with the idea that a single representation is used for perception, movement planning, and online control (e.g., V. H. Franz, 2001), other work from our laboratory and elsewhere suggests that the manner in which space is coded depends on constraints associated with the specific task, such as the visual cues available to the performer.
Assessment of visuo-spatial perception (VS) in brain-damaged patients. So far, no comprehensive methods for reliable assessment of visuo-spatial deficits after brain damage have been described. The tests usually applied are subtests of intelligence tests, paper-pencil tests or visual-constructive tests which also require motor and planning abilities. Almost all these methods are unsuitable for measuring therapeutic changes and do not reveal specific impairments. The computational method presented here (VS) is suitable for the quantitative assessment of visuo-spatial functions, e.g. subjective visual vertical and horizontal; orientation, length, distance, form and position discrimination as well as line bisection. It can easily be installed on an IBM compatible PC. A clinical standard examination can be performed within 20-30 minutes including numerical and graphical analysis of the results. The method is sensitive for measuring therapeutic changes. Moreover, it offers a wide range of experimental task manipulations for more sophisticated experimental tests.
Stewart et al (1993, Perception 22 1227-1224) attempted to demonstrate that the ratio of angular speed to angular acceleration, [symbol see text]/[symbol see text], of the approaching object is preferred as the source of time-to-collision information to the ratio of visual angle to angular speed, [symbol see text]/[symbol see text]. It is demonstrated that this is not justified.
A postal/telephone survey of 400 general dental practitioners working in the Greater Glasgow and Lanarkshire Health Board areas was carried out between May and September 1990. The aims were to monitor any changes in the use of sealant restoration techniques in the last year, to investigate the practitioners' perception of the effects of a distance learning programme on their techniques and to provide data on the use of the techniques in the final months of the 'old contract'. A response rate of 66.7% was achieved. Eighty-eight per cent of the group were placing sealant restorations. Dentists' perceptions of the impact of the distance learning programme were encouraging. In key areas of practice, 16-44% of the group reported changes in their clinical techniques as a result of reading the programme.
The perception of linear extent in haptic touch appears to be anisotropic, in that haptically perceived extents can depend on the spatial orientation and location of the object and, thus, on the direction of exploratory motion. Experiments 1 and 2 quantified how the haptic perception of linear extent depended on the type of motion (radial or tangential to the body) when subjects explored different stimulus objects (raised lines or solid blocks) varying in length and in relative spatial location. Relatively narrow, shallow, raised lines were judged to be longer, by magnitude estimation, than solid blocks. Consistent with earlier reports, stimuli explored with radial arm motions were judged to be longer than identical stimuli explored with tangential motions; this difference did not depend consistently on the lateral position of the stimulus object, the direction of movement (toward or away from the body), or the distance of the hand from the body but did depend slightly on the angular position of the shoulder. Experiment 3 showed that the radial-tangential effect could be explained by temporal differences in exploratory movements, implying that the apparent anisotropy is not intrinsic to the structure of haptic space.
Many sports involve aligning a hitting implement with a ball trajectory such that contact is made at the implement's center of percussion or 'sweet spot'. This spot is not visibly distinct; its perception must be haptic. Although it is functionally defined with respect to contact--it is the point of impact that produces the least vibration in the hand holding the implement--hitting success requires appreciating the location of the sweet spot prior to contact. Two experiments verified that perceivers (novices as well as expert tennis players) distinguished perception of length from perception of the position of the sweet spot simply on the basis of wielding, both for tennis rackets and for bats contrived from wooden rods with attached masses. Results conformed to previous research on dynamic touch in showing that perceiving the lengths of wielded objects, including selectively perceiving partial lengths, is constrained by inertial properties of the object.
Despite decades of studies of human infants, a still open question concerns the role of visual experience in the development of the ability to perceive complete shapes over partial occlusion. Previous studies show that newborns fail to manifest this ability, either because they lack the visual experience required for perceptual completion or because they fail to detect the pattern of motion. To distinguish these possibilities, newborns' perception of a center-occluded object was tested, using stroboscopic motion. Infants (mean age of 72 hr) perceived the object as a connected unit, providing the first evidence that the newborn is capable of filling in gaps in the visible surface layout when the relevant visual information can be detected by his or her immature visual system.
Four studies are reported that demonstrate effects of attention on early perceptual processes. These studies show that directing attention to stimulus location influences perceived brightness, perceived length, and the overall organisation of the Müller-Lyer figure and ambiguous figures. Since in all four studies the dependent measure was dissociated from the attentional instructions, the results are not open to traditional criticism of late-selection advocates, and therefore provide strong support for early-selection views of attention.
People coordinate the force and direction of skilled actions with target locations and adjust the calibrations to compensate for changing circumstances. Are the adjustments globally organized (adjusting a particular action to fit a particular circumstance would generalize to all actions in the same circumstance); anatomically specific (every effector is adjusted independently of others); of functional (adjustments would generalize to all actions serving the same goal and generating the same perceptible consequences)? Across 10 experiments, changes in the calibration of walking, throwing, and turning-in-place were induced, and generalization of changes in calibration to functionally related and unrelated actions were tested. The experiments demonstrate that humans rapidly adjust the calibration of their walking, turning, and throwing to changing circumstances, and a functional model of perceptual-motor organization is suggested.
The Müller-Lyer patterns formed by separate dots have been used as stimuli in an experiment on visual perception to assess the influence of the number of dots composing the figures on the magnitude of the illusion. As predicted by our model, based on the Image Function theory, an increase was noted in the magnitude of illusion when the number of dots composing the arrowheads was increased. It follows from the model that filtering of the input image plays a central role in the formation of the illusion.
The purpose of this research was to verify the effect of age on the exponent of the power function in Perceptive, Memory, and Inference experimental conditions. In the Memory condition the intervals of 2 min., 8, 24, and 48 hr. and 1 wk. were used between acquisition of information and remembering. For each experimental condition the ages of observers ranged between 17 and 35 years (Group I), 40-55 years (Group II), and 60-77 years (Group III), and education ranged from high school to graduate school. The observers estimated the areas of the Brazilian satires using the psychophysical method of magnitude estimation. No significant differences were obtained for Groups I, II, and III for each experimental condition, except in the Memory Condition with the 24-hr. interval. Analysis for experimental conditions and ages showed a significant difference between the Perceptive Condition and each of the others, but no difference between the Inference and Memory Conditions. These results indicated that in the remembering processes there is no loss of information as a function of age. From the small variability in the power function exponents for the three ages, we may assume that age could be related to amount of education of the observers, which suggests study is important.
Some highlights of the ongoing study of visuospatial cognition from Descartes to the advent of functional neuroimaging are reviewed. We emphasize that parietal cortex contains multiple representations of space, appropriate to the demands of perception and action in near and far space. Converging evidence from the behavioral effects of relatively focal brain lesions on different aspects of spatial cognition and from the locus of maximal physiological activation when normal volunteers perform spatial tasks is described. Clinical pathologies of spatial attention, including visual extinction, simultanagnosia, and unilateral neglect, are examined for the light they cast on the basic functions of brain circuits involving the parietal lobes.
Two experiments tested whether bias in cued line bisection resulted from perceptual grouping or attentional factors. Before trying to bisect a line, subjects had to report two lateralized letters. These cues were either equally spaced (Experiment 1) or asymmetrically spaced (Experiment 2) with respect to the line. No effect of order of cue-report on performance was found, arguing against the role of attentional factors. Bisection was systematically biased toward the closer-spaced cue. These findings in normal subjects suggest that cuing effects are due to perceptual rather than attentional factors.
The correspondence problem arises in motion perception when more than one motion path is possible for discontinuously presented visual elements. Ullman's (1979) "minimal mapping" solution to the correspondence problem, for which costs are assigned to competing motion paths on the basis of element affinities (e.g., greater affinity for elements that are closer together), is distinguished from a solution based on the differential activation of directionally selective motion detectors. The differential activation account was supported by evidence that path length affects detector activation in a paradignm for which motion correspondence is not a factor. Effects on detector activation in this paradigm also were the basis for the successful prediction of path luminance effects on solutions to the motion correspondence problem. Finally, the differential activation account was distinguished from minimal mapping theory by an experiment showing that the perception of an element moving simultaneously in two directions does not depend on whether the two motions are matched in path-length determined affinity; it is sufficient that the activation of detectors responding to each of the two motion directions is above the threshold level required for the motions to be perceived. Implications of the differential activation solution are discussed for the stability of perceived motions once they are established, and the adaptation of perceived and unperceived motions.