The versatility and absolute efficiency of detecting mirror symmetry in random dot displays

Abstract

The detection of mirror symmetry has been investigated by measuring discriminability ( d ') between two populations of dot displays that contain mirror pairs and random dots in different proportions. The difficulty of the task was varied by changing the proportions of paired dots in the two populations, and also by changing the accuracy of positioning the paired dots. Symmetry can be detected in brief exposures, monocularly or binocularly. when the axis is not vertical, and when the axis is not central in the visual field. The mechanism is therefore versatile. Its efficiency can be measured when the pairing is imperfect, for unpaired dots then fall where a pair is expected, thus causing the spurious appearance of a pair. When the pairing has an accuracy of about ± 6′ vertically and horizontally an ideal mechanism would achieve d′ values about double those attained by subjects; they are thus using 25% of the statistical information available, which is a high figure considering the versatility and complexity of the mechanisms required.