OPTICAL BLUR AND THE PERCEPTION OF GLOBAL COHERENT MOTION IN RANDOM-DOT CINEMATOGRAMS

We evaluated the effect of +3.25 dioptres of optical blur on the discr imination of motion direction in random dot cinematograms. Dot displac ement between frames varied from 2.1 to 63' of visual angle while the temporal interval was held constant. Optical blur worsened discriminat ion in three normal subjects at displacements below 16', but improved discrimination at displacements of 21' or more. In a second experiment , two subjects viewed equivalent velocity stimuli constructed with dif ferent combinations of temporal interval and spatial displacement. Res ults showed that the effect of blur was specific to displacement and n ot velocity. Furthermore, varying the dot density of the display showe d that the effect of blur correlated with dot displacement and not the probability of dot mismatches. Since optical blur attenuates high spa tial frequencies, this suggests that high spatial frequencies are impo rtant for motion perception when dot displacements are less than 16' t o 21', but reduce motion perception at larger dot displacements. The u se of random dot cinematograms in populations must take into account s timulus displacement and optical causes of reduced spatial acuity. Cop yright (C) 1996 Elsevier Science Ltd.