Some consequences of stereoscopic depth constancy

Retinal disparity decreases in proportion to the square of the distance of the corresponding objective depth interval from the eyes. Up to a distance of 2 m, stereoscopic depth perception compensates well for this decrease in disparity with observation distance; for a given disparity, experienced depth increases approximately in proportion to the square of the observation distance. When disparities are artificially produced, by anaglyphs or vectograms, or by spectacles, they decrease only in proportion to the first power of the observation distance. The same is true when the Pulfrich effect gives rise to equivalents of disparity. Depth perception, however, compensates for the normal disparity loss. As a result, there should be a net gain in perceived depth approximately in proportion to the first power of observation distance. When perceived depth caused by horizontal magnification in one eye or by the Pulfrich effect was measured, it was found to increase approximately in proportion to observation distance.     

The development of object-permanence: Some studies of existence constancy

Object permanence or existence constancy was one of the phenomena which greatly exercised the philosophical fathers of experimental psychology. The topic has been little studied since. Michotte has described the psychophysics of existence constancy, while Piaget has studied its development. Michotte’s work was restricted to adults while Piaget’s notably lacked careful psychophysical control. The aim of the present study was to bridge the gap between the two, to study the psychophysics of existence constancy in infants. Results showed that there is remarkably little difference between infant and adult as far as psychophysical control is concerned. The major difference seems to be a rate of processing difference. However, changes in processing rate will not account for the development of the concept of object permanence. What seems to be learned is a rule which can override perceptual constancy and nonconstancy in some situations.     

Simultaneity constancy

Sound and light take different amounts of time to reach their respective receptors, to be transduced, and to be transmitted to the cortex. Their processing times also vary with factors such as intensity and retinal eccentricity. We assessed the capability of subjects to perceive simultaneity correctly despite these variations. Temporal asynchronies of up to 200 ms were introduced between the components of sound/light pairs. Using the method of constant stimuli, seven subjects judged which came first. Distance, and hence the times of arrival of paired visual and auditory targets, was varied from 1 to 32 m. Visual intensity was varied by viewing the target through 1.8 dB attenuating glasses, and a retinal eccentricity of 20 degrees was compared to central presentation. Despite large differences in reaction times, which varied in a predictable way with the stimulus parameters, the timing of sound/light pairings judged as simultaneous corresponded to when the light and sound left the source simultaneously. Almost complete compensation was found in all conditions tested, showing that these substantial but predictable variations in timing can be taken into account in creating simultaneity constancy.     

Speed constancy as a function of size constancy

If perceived velocity were to be a function of phenomenal extent traversed per unit time, speed constancy would be derivable from size constancy. Experiments were performed to test this possibility in which visible frames of reference were eliminated. With cues to distance, judgments ofspeed approached constancy; without such cues, judgments of speed were in terms of rate ofimage displacement.     

Direction constancy

A series of experiments investigating the constancy of perceived direction was conducted. Experiments 1, 2, and 3 indicated that the perception of direction, while close to constancy, exhibited a consistent departure from constancy of approximately 2.5 deg. Experiments 4 and 5 investigated the departure from constancy in terms of the information of the position of the eyes within the head and the information of the retinal area (local sign) being stimulated. The results of these experiments indicated that the departure from constancy was a function of the underestimation of the position of the eyes within the head.     

Transparent layer constancy

Perceived transparency was studied as a constancy problem. In the episcotister (E-) model of scission, luminances are partitioned into layer and background components; four luminances determine values of two layer parameters that specify constancy of a transparent layer on different backgrounds. The E-model was tested in an experiment in which 12 Ss matched 24 pairs of four-luminance patterns by adjusting two luminances of the comparison pattern. Both the standard and the comparison were perceived as a transparent layer on a checkerboard. The E-model predicts matches when layer values are identical in the two patterns. One parameter was constant, constraining the adjustment along the second dimension. Obtained values corresponded well with E-predictions. Alternative models based on local luminance or average contrast ratios accounted for less variability. Results indicate that transparency models should utilize luminance, not reflectance, as the independent variable.     

Some Myths about Ethnocentrism

Ethnocentrism, it is said, involves believing certain things to be true: that one's culture is superior to others, more deserving of respect, or at the ‘centre’ of things. On the alternative view defended in this article, ethnocentrism is a type of bias, not a set of beliefs. If this is correct, it challenges conventional wisdom about the scope, danger, and avoidance of ethnocentrism.     

Routes to object constancy: Implications from neurological impairments of object constancy

Previous studies have established the existence of neurological impairments of object constancy: the ability to recognize that an object has the same structure across changes in its retinal projection. Five case studies of brain-damaged patients with deficits in achieving object constancy are reported. To test object constancy, patients discriminated two photographs of a target object, taken from different views, from a photograph of a visually similar distractor object. Four patients showed impaired matching only when the principal axis of the target object in one photograph was foreshortened. The fifth patient showed impaired matching only when the saliency of the target object's primary distinctive feature was reduced. This double dissociation suggests that normally there may be two independent means of achieving object constancy: one by processing an object's local distinctive features, the other by describing the object's structure relative to the frame of its principal axis. Neurological damage can selectively impair either process. Further, this impairment can be independent of deficits in processing visual form, since two patients with a selective deficit in the foreshortened matching task showed relatively normal form discrimination. The patient dependent on local distinctive feature information showed a deficit in size discrimination. It is suggested that this patient fails to utilize global properties of form. This failure may underlie both his impairment in achieving object constancy and in processing certain dimensions of form.     

Some uncertainties about agnosticism

International Journal for Philosophy of Religion -     

Color constancy and dispositionalism

This article attempts to do two things. The first is to make it plausible that any adequate dispositional view of color will have to associate colors with complex functions from a wide range of normal circumstances to a wide range of (simultaneously) incompatible color appearances, so that there will be no uniquely veridical appearance of any given color. The second is to show that once this move is made, dispositionalism is in a position to provide interesting answers to some of the most challenging objections that have been pressed against it. It explains why colors do not seem to “come on” when the lights come on, and why the right thing to say seems to be that the light causes the objects to reveal their colors. It explains why the content of visual experience need not be problematically circular. It eliminates the problem of intrapersonal variation in color appearance. And it even provides resources for dealing with the problem of interpersonal variation. It is true that in moving to the more complex view, the dispositionalist becomes subject to a new objection, which has been called “the problem of unity.” The paper offers a solution to this problem as well.