A statistical model of binocular disparity

Binocular disparity provides important information about the three-dimensional structure of the environment. The current study sought to complement our geometrical understanding of binocular vision by considering the distributions of horizontal and vertical disparities that might be expected in images of the natural environment, using a simple environmental model. It was observed that the distribution of disparities depends critically on fixation, and varies greatly from one image location to another. The results were considered in relation to computational models of binocular stereopsis, and compared to two known properties of the visual system—the small disparity preference in disparity matching, and the influence of eccentricity on Panum's fusional limit. Overall, the study characterizes the binocular disparities that are likely to be encountered in the real world scenes, and discusses the implications of these for our understanding of binocular visual systems.     

Assessing the remedy: the case for contracts in clinical trials

Current orthodoxy in research ethics assumes that subjects of clinical trials reserve rights to withdraw at any time and without giving any reason. This view sees the right to withdraw as a simple extension of the right to refuse to participate all together. In this paper, however, I suggest that subjects should assume some responsibilities for the internal validity of the trial at consent and that these responsibilities should be captured by contract. This would allow the researcher to impose a penalty on the subject if he were to withdraw without good reason and on a whim. This proposal still leaves open the possibility of withdrawing without penalty when it is in the subject's best interests to do so. Giving researchers recourse to legal remedy may now be necessary to protect the science, as existing methods used to increase retention are inadequate for one reason or another.     

Evidence for disparity change as the primary stimulus for stereoscopic processing

Subjects were able to respond to a lens-induced stereoscopic slant more quickly and more accurately when it was imposed on only part of a surface rather than on the whole surface. This shows that the presence of a stereoscopic boundary, where disparity is discontinuous, increases the efficiency of stereoscopic processing. This finding is not consistent with many current models of stereopsis.     

Comparison of black-white differences on the WISC-R and the K-ABC: Spearman's hypothesis

The Kaufman Assessment Battery for Children (K-ABC) and the Wechsler Intelligence Scale for Children-Revise (WISC-R) are compared with respect to the magnitudes of the average white-black differences in standardized scaled scores and in raw scores. The two test batteries were administered to a sample of 172 fourth- and fifth-grade children comprising 86 black-white pairs matched on age, sex, school, and socioeconomic status. The K-ABC and WISC-R are highly correlated, and the general factor, or g, of one battery is virtually identical to the g of the other. The high positive correlation between the size of the white-black difference on the various subtests of both batteries and the subtests' loadings on the g factor bears out Spearman's hypothesis that a test's white-black dicriminability is a direct function of the test's g loading. The lesser white-black discriminability of the K-ABC relative to the WISC-R is attributable to (1) the smaller g loadings of the K-ABC subsets and (2) the presence of other factors, particularly sequential short-term memory which, to some degree, offsets the white-black difference in g.     

A reexamination of black-white mean differences in work performance: more data, more moderators

This study is the largest meta-analysis to date of Black-White mean differences in work performance. The authors examined several moderators not addressed in previous research. Findings indicate that mean racial differences in performance favor Whites (d = 0.27). Effect sizes were most strongly moderated by criterion type and the cognitive loading of criteria, whereas data source and measurement level were influential moderators to a lesser extent. Greater mean differences were found for highly cognitively loaded criteria, data reported in unpublished sources, and for performance measures consisting of multiple item scales. On the basis of these findings, the authors hypothesize several potential determinants of mean racial differences in job performance.     

Effects of home nest odors on black-white preference in the developing rat: implications for developmental learning research

Developing animals 17 and 30 days of age were tested for black-white preference in the presence of either clean shavings or soiled bedding material from the home cage. Home nest shavings markedly reduced dark preference in 17-day-old rats but had no effect on dark preference in 30-day-old rats. Because many developmental studies have used two-compartment, black and white chambers to study the effects of familiar home nest shavings on learning and memory, it may be that differential preference for black influenced the results obtained. The apparent alleviation of learning and memory deficits produced by the presence of home nest shavings may have been the result of changes in black-white preference rather than differences in learning and memory per se. Similar influences may underlie the effects of home nest stimuli on other learning tasks such as spatial alternation.     

Binocular unmasking with vertical disparity

We recently found (Schneider, Moraglia, & Jepson, 1989) that the contrast threshold for the detection of a visual signal in a noisy background can be considerably lower when binocular cues are available then when monocular cues only are present. Here, we investigated the occurrence of binocular unmasking with vertical interocular disparities. Subjects reported about the presence of Gabor signals in fields of two-dimensional broadband Gaussian noise surrounded by a frame of uniform noise. They saw these stimuli through a stereoscope; in all cases, the right-eye noise field was vertically displaced relative to the left one in either an upward or a downward direction, by up to 67.6'. In one condition, the right-eye signal was displaced by an amount equal to that of the noise, so that no opportunities for binocular unmasking existed; in the other, it appeared in exactly corresponding locations in the two fields--here, binocular disparities could be used to unmask the signal. Enhanced signal detectability, by up to 12.7 dB, was observed in the latter case for both directions of displacement, but only for displacements of 13.52' and only when the signal's orientation was horizontal. We argue that these effects result from the summation of monocular inputs carried out by linear binocular mechanisms.     

Stereovision: beyond disparity computations

One of the most powerful sources of information about three-dimensional (3-D) structure is provided by stereovision (or stereopsis). For over a century, theoretical and empirical investigations into this ability have focused on the role of binocular disparity in generating percepts of 3-D structure. Recent work in image segmentation demonstrates that stereovision can cause large changes in perceptual organization that cannot be understood on the basis of binocular disparity alone. It is argued that these phenomena reveal the need for theoretical tools beyond those that have dominated the study of visual perception over the past three decades.     

Binocular disparity as a discriminable stimulus parameter for young infants

The ability of 8-week-old infants to discriminate between projected stereograms with and without retinal disparity was tested with an habituation-dishabituation paradigm. Infants in two experimental groups received six trials with either the disparity or the nondisparity stimulus and then were given two trials with the other display. Infants in two control groups viewed the same stimulus, either disparity or nondisparity, on all eight trials. There was a suggestion of some response decrement over time in both cardiac deceleration and sucking suppression, although this effect was not significant. However, significant increment was obtained on the dishabituation trials for heart rate in the group that was shifted from the nondisparity to the disparity stimulus. These results were interpreted as indicating that the infants could discriminate between stimuli when the only difference between them was binocular disparity.     

Two forms of retinal disparity

The retinal disparities in stereograms where the vertical alignment of pairs of homologous points in one eye differs from that in the other eye were found to be more effective than disparities that do not involve that kind of binocular difference. The presence of such “transverse disparities” was found to shorten the time elapsed until perceived depth was reported in four instances, in two simple stereogram pairs and in two different pairs of random dot pattern stereograms. In an experiment where binocular parallax was in conflict with an effect of past experience, the presence of transverse disparities caused binocular parallax to prevail. The presumption that the amount of perceived depth depends only on the amount of disparity (provided distances from the eyes are unchanged) and not on the configuration in which it manifests itself was found not to hold in stereograms containing transverse disparities.     

Suppressing permutations or rigid planar rotations: A remedy against nonoptimal varimax rotations

Varimax rotation consists of iteratively rotating pairs of columns of a matrix to a maximal sum (over columns) of variances of squared elements of the matrix. Without loss of optimality, the two rotated columns can be permuted and/or reflected. Although permutations and reflections are harmless for each planar rotation per se, they can be harmful in Varimax rotation. Specifically, they often give rise to the phenomenon that certain pairs of columns are consistently skipped in the iterative process, whence Varimax will be terminated at a nonstationary point. The skipping phenomenon is demonstrated, and it is shown how to prevent it.The author is obliged to Henk Kiers for commenting on a previous draft.     

Asymmetries and errors in perception of depth from disparity suggest a multicomponent model of disparity processing

In three experiments, asymmetries between the processing of crossed and uncrossed disparities were investigated. The target was a luminance-defined circle concentric to a fixation mark, viewed stereoscopically on a computer monitor for 105 msec. Fifteen disparities were presented according to the method of constant stimuli. Observers indicated the apparent direction of target depth relative to fixation. All experiments measured both the accuracy and latency of this response. Experiment 1 showed fewer errors and shorter reaction times for identifying crossed disparities. Experiments 2 and 3 replicated Experiment 1 and also showed that observers may often perceive a target in the direction opposite that prescribed by the disparity information. We propose that the asymmetries and reversals result from differences in computation of sign, not of magnitude. This notion is consistent with a scheme of continuous disparity tuning and accounts for such asymmetries and errors without positing disparity pooling mechanisms.