Time and intensity as determiners of perceived shape.

It has been demonstrated that a round object viewed obliquely is matched with ellipses which are more circular than would be predicted from geometrical optics (16). Such data are examples of the tendency toward "shape constancy," a theoretical condition in which perceived circularity is constant independent of the angle of regard and the corresponding retinal image pattern. Various factors have been shown to affect the extent to which shape constancy occurs. (For a review, see Graham, 7.) The duration of exposure of the test object is particularly critical; an exposure duration of .01 sec. destroys the tendency towards constancy and produces matches which are in good agreement with the shape of the retinal image (13). The purpose of the present study is to analyze the role of exposure duration and luminance as determiners of the extent to which constancy occurs. To this end, the functional relation between the matched shape of an obliquely viewed disc and the exposure duration is determined. The effect of luminance is investigated in view of the reciprocal relation between exposure time and luminance below the critical duration of about .1 sec. (3,4, 5, 6, 8, 10, 11, 15) and the general importance of luminance as a variable in vision. The results indicate that an exposure longer than the critical duration is required for