ORIENTATION IDENTIFICATION AT DIFFERENT BACKGROUND LEVELS - ITS PRECISION AND DISTORTIONS

Orientation identification was studied at high (1000 td) and low (I td) background illuminations. The subject had to estimate in degrees the orientation of a line presented in one of 18 orientations, 10 deg apart. Unlike other spatial discriminations, the precision of orientation judgments was not significantly impaired at low background if a suprathreshold stimulation was used. However, OUT earlier data showed that the identification at threshold was deteriorated at low background level. Different mechanisms of threshold and suprathreshold identification were suggested. The mechanism of suprathreshold identification was thought to contribute to the stability of the orientation perception during adaptation. The deviations of the subjective estimates from real orientations were also studied. The bias of perceived orientation toward the vertical increased at low background. This bias remained the same in egocentric coordinates, when the subjects were in supine position.