The effect of refractive error on central and peripheral motion sensitivity at various exposure durations

Pennsylvania State University, University Park, Pennsylvania 16802 The influence of refractive error on movement sensitivity was determined for a range of stimulus durations in the fovea and periphery. At short durations, threshold is determined by a constant displacement of the stimulus. At longer durations, a constant velocity is required. The correction of peripheral refractive error increases movement sensitivity in the displacement (short duration) component, but does not influence sensitivity in the velocity (long duration) component of the motion threshold. Implications concerning the mechanisms underlying motion perception are discussed.     

Causal explanations of male and female academic performance as a function of sex-role biases

The study was designed to assess whether attributions of causality vary as a function of sex-role attitudes. Thirty-two male and thirty-two female undergraduates were presented with one of four story completions about a medical student in which sex of the student and success versus failure were varied. Subjects were asked to account for the medical student's success (or failure) in terms of ability, motivation, ease of the goal, and luck. Results suggest that attributions do vary as a function of sex-role attitudes, as measured by the Attitudes Toward Women Scale. Subjects with liberal attitudes were less likely to offer sex-biased attributions about lack of motivation. Subjects' attributions with respect to lack of ability suggest, however, that sex-typed notions about competence may still be deeply ingrained despite recent social changes.The author would like to thank Cheryl Sharp and Robin Koenig for their assistance in compiling the data for this project, as well as Robert Heaton, James Gumina, and Louise Bickman for their editoral comments.     

Expectation and autokinesis

The reported direction of autokinesis for naive observers is not biased by gaze deviations of 22.5 deg of arc. However, instructions to expect either “inward” or “outward” movement produce reports of autokinetic movement that are progressively biased in the expected direction. Instructions to expect “more” or “less” movement resulted in a decrease in the reported duration of apparent movement for the “less” group but no change for the “more” group. Response bias is not considered to play a role in these results. The results are discussed in terms of fixational bias and selective attention mechanisms of autokinesis.     

The Fraser illusion: complex figures

The cause of the Fraser illusion, which occurs when a line made up of tilted segments itself appears tilted, is examined further. In this series of experiments, we used figures that resembled the original Fraser illusion; they were more complex than those reported on in our previous paper (Stuart & Day, 1988). The figures were used to explore two theories of the Fraser illusion further: that it is the result of interactions between orientation selective units, and that it is a consequence of the local, distributed processing of orientation. The presence of background elements like those used in the original illusion led to an increase in the strength of the illusion, but the shape of these elements had no differential effect on illusion strength. There was a differential effect of the background on the assimilative and contrast illusions, owing respectively to small and large tilts of the inducing elements. The illusion was markedly reduced at small visual angles when the background was absent, but it was only slightly affected when the background was present. All these findings are difficult to explain in terms of interactions between single units, either at the same or at different scales in the image. The effects of luminance contrast and isoluminance on the illusion were not consistent with either theory, but they indicated that researchers need to consider the role of figure-ground organization in this illusion.     

A faith for all seasons

A FAITH FOR ALL SEASONS:ISLAM AND WESTERN MODERNITY. By Shabbir Akhtar. London: Bellew Publishing, 1990. 251pp. Hb. £25.00. ISBN 0–947792–41–4.     

Psychiatry, religious conversion, and medical ethics.

The interface between religion, psychiatry, and ethics is often a locus for considerable controversy. This article focuses on the response of American psychiatry to religious nonconformism, and to religious conversion generally. At issue is the societal pressure against unpopular religious movements. The author argues for an ethic that conserves the freedom of religious conscience, and that guards against inquisitions in the guise of medical expertise and nosology.     

Poisson signal-detection theory: Link between threshold models and the Gaussian assumption

The Gaussian model of signal detection cannot fit asymmetric data as long as the variances of the distributions are kept equal. It is therefore common practice to assume unequal variances in order to fit these data. But this assumption leads to the well-known crossover problem. The present paper provides new arguments for the abandonment of the Gaussian model with unequal variances. In its stead, this paper reevaluates multiple-parallel-threshold models. In particular, the Poisson model turns out to be very useful: it can handle data with any degree of asymmetry, giving a reasonable interpretation of the two parameters of the receiver-operating characteristic. The three-state-threshold model (Krantz, 1969) is given a new interpretation in light of the Poisson model. The slope of Poisson double-probability plots turns out to be much closer to unity than is predicted by the Gaussian approximation.