Shape discrimination and the judgement of perfect symmetry: dissociation of shape from size.

We measured the accuracy with which subjects judged that a square or circle was perfectly symmetrical i.e. that aspect ratio (a/b) was exactly unity (where a and b were, respectively, the vertical and horizontal dimensions). Errors were remarkably small, ranging from 0.7 to 0.4% for the judgement of squareness and from 1.4 to < 0.1% for the judgement of circularity. Precision in judging aspect ratio was measured by requiring subjects to judge whether the aspect ratio (a/b)TEST of a test rectangle was greater or less than the aspect ratio (a/b)REF of a reference rectangle. Similar measurements were made for elliptical targets. To ensure that subjects based judgements on aspect ratio rather than a, b or (a-b), the area of each successive presentation was varied randomly. The just-discriminable percentage change of aspect ratio was as low as 1.6% at (a/b)REF = 1.0 (i.e. for a square or circular reference), and rose progressively as (a/b)REF was made progressively larger or smaller than 1.0. Aspect ratio discrimination threshold was independent of mean area over a sixteen-fold range of 0.25-4.0 deg2. For both rectangles and ellipses, the best value of aspect ratio discrimination threshold corresponded to a precision of encoding a and b of 14 sec arc or better. In further experiments, the method of constant stimuli was used to measure an aspect ratio aftereffect produced by adapting separately to rectangles of (a/b)ADAPT equal to 1.5, 1.0 and (1/1.5). Similar aftereffects were obtained whether the area of the test stimulus was fixed or varied randomly from trial to trial, and whether the test stimulus was rectangular or elliptical. The aftereffect could not be explained in terms of fatigue of neurons sensitive to linear dimension a or b. Nor could the aftereffect be explained in terms of the "contour repulsion" hypothesis, or in terms of orientation discrimination. We conclude (1) that the same neural mechanism determines aspect ratio discrimination threshold for rectangles and ellipses and (2) that this mechanism is sensitive to aspect ratio independently of linear dimensions. We propose that aspect ratio perception is determined by the balance of excitation of two pools of neurons that are selectively sensitive to different, but overlapping ranges of (a/b). One pool prefers aspect ratios > 1.0 and the others prefer aspect ratios < 1.0. We suppose that the two pools respond identically to changes in area (a * b).(ABSTRACT TRUNCATED AT 400 WORDS)     

Periodicity in orientation discrimination and the unconfounding of visual information

Orientation discrimination is a periodic function of mean orientation. Discrimination sensitivity (i.e. threshold-1) was measured at 7.5 deg increments around the clock for an 8 c/deg grating subtending 1.0 deg dia located 1.25 deg from the foveal centre. Discrimination sensitivity was best for horizontal and vertical orientations, but did not fall monotonically to minima at 45 deg and 135 deg. Instead it fell precipitously to minima at angles of only 20 deg to the horizontal and vertical, and there was a weak submaximum near 45 deg. This finding is consistent with the proposal that orientation discrimination is determined by the relative activity of broadly-tuned, orientation-sensitive neural elements, and that only a small number of elements are effective in any small retinal region. This idea can also explain why subjects do not confound a change of orientation with a simultaneous change of contrast or spatial frequency.