THE SIZE AND NUMBER OF PLAID BLOBS MEDIATE THE MISPERCEPTION OF TYPE-II PLAID DIRECTION

The misperceived direction of type-II plaids has posed a problem for t he intersection of constraints (IOC) model of two-dimensional motion p erception, Alais et al, (1994, Vision Research, 34, 1823-1834) examine d the perceived direction of type-II plaids and concluded that in addi tion to the direction signalled by the IOC process, a monocular mechan ism signalling the motion of plaid features (blobs) is also involved i n plaid perception. It was shown that the prominence of this monocular signal in plaid direction judgements depended on several variables, a nd the notion of blob ''optimality'' was introduced, This explained th e more veridical direction of ''optimal'' blob plaids in terms of thei r more effectively activating the proposed feature-sensitive motion me chanism, One distinction between ''optimal'' and ''non-optimal'' blob plaids is their different component spatial frequencies, which necessa rily entails a difference in the number and size of the blobs and thus raises potential confounds, since both the nature of the blobs and th e components differ, which might affect the postulated blob mechanism and/or the IOC process, In the present paper, by offsetting changes in spatial frequency with changes in aperture size so that blob number i s held constant, we examine whether differences in sheer blob number o r size can alter perceived type-II plaid direction, The results reveal effects of both blob number and blob size, and their implications for the underlying mechanism are considered, Alternative accounts of the results in terms of the IOC model or revisions of it cannot explain th e data, Comparison of monocular and binocular conditions adds further systematic evidence in support of the monocularity of the feature-sens itive motion mechanism. Copyright (C) 1996 Published by Elsevier Scien ce Ltd