Detecting collinear dots in noise

We estimated the sensitivity for detecting a row of collinear target elemen ts (usually dots) by measuring the maximum density of: randomly positioned noise elements that allowed 75% correct detection of the orientation of ali gnment (binary choice: horizontal versus vertical) of the target elements. We varied the number of target elements, their mode of generation, and thei r accuracy of positioning. As reported previously (Moulden (1994) Higher-or der processing in the visual system. Ciba Foundation Symposium 184. Chiches ter: Wiley), target detection improved rapidly until the number of target e lements reached about seven, and then improved more slowly beyond this poin t. However, this break was reduced (and often removed entirely) when the ta rget array was formed by repositioning pre-existing noise elements lying cl ose to the target location, rather than by superimposition of additional ta rget elements onto the noise array. This almost linear slope of improvement , coupled with the observation that target detection was disrupted more by random jitter of target elements at right angles to their axis of alignment than by jittering along this axis, argues against a two-stage process of p erceptual grouping (Moulden, 1994) and supports instead an explanation base d on the operation of a single mechanism. This single mechanism explanation is further supported by the observation that intrinsic positional uncertai nty (estimated from the results of jitter experiments) was independent of t arget element number. Additional experiments showed that target detection i s facilitated by aperiodic noise dots that fall close to the target axis. T he results are discussed in relation to alternative explanations of percept ual grouping. (C) 1999 Elsevier Science Ltd. All rights reserved.