Motion-detecting mechanisms: Dual input devices?

Displacement thresholds for luminance step edges were measured for a wide range of contrasts and mean luminances. Thresholds for extended edges (longer than about 0.5°) are determined not by contrast but rather by the amplitude (L_max-L_min) of the luminance change produced by the displacement. Arguing from the standpoint of the Marr-Ullman model of movement detection, we had expected that thresholds might be jointly determined by both contrast and amplitude. Using a range of edges of different lengths, we found that differential effects of luminance and contrast can be revealed: for short edges (less than about 0.5°) thresholds are influenced by both amplitude and contrast, while for more extensive edges only amplitude has an influence. The results are consistent with the properties of a mechanism that has two separate inputs, one from a spatial operator that is contrast-dependent and one from a temporal operator that is amplitude-dependent. The spatial operator is markedly sensitive to changes in edge extent, the temporal operator much less so. The output of the spatial operator saturates early as a function of contrast.     

Modelling visual detection: Luminance response non-linearity and internal noise

Two experiments that investigate the effect of various display factors on the detectability of a thin line signal in random visual noise are described. Three statistical decision models are described, together with their ability to account for the results. The first is an “ideal detector” model, the second an “energy integrator” model, and the third a model based upon the operation of retinal ganglion cells which incorporates a gain control mechanism. The ideal detector model fails to give a good account of human performance, whereas the other two models provide a good fit to the data. The digital Laplacian with gain control model has the slight advantage over the energy integrating model in being able to account for a small superiority in the detection of dark as opposed to bright signals. Finally, both models require the inclusion of an estimate of the internal noise of the human visual system to account for the pattern of performance observed under changing conditions of display contrast.     

The mazes of practicing and the horizons

Teaching must take account of young people's individual needs and talents; it must develop their desire for criticism and effort. The spiritual framework must therefore rest on values of rationality and sensitivity. Morality must rest on the notion of personal dignity. In this way, we shall be able to create a horizon, a goal which can inject meaning into life.     

A normal approximation for testing the equality of two independent Chi-square variables

A simple normal approximation is supplied to test the equality of two independent Chi-square variables. The approximation appears to be valid for degrees of freedom as small as two. Thus no new table of critical values is needed. The standard unit normal distribution tables are sufficient.     

THE INFERIORITY CURRICULUM

Bernard looks back at the last decade and notes that although many things have improved, women at all levels in academia still face more subtle forms of discrimination, which comprise what she calls the "Inferiority Curriculum." Exposure of these subtle discriminatory behaviors is gradually occurring through what Bernard has termed the "Feminist Enlightenment," a discipline-by-discipline process which began in the mid-1960s. Bernard documents many "down-putting" behaviors that continue to undermine women's achievement and argues the necessity of overcoming the damage by building positive self-images among women.