PERCEIVED VISUAL SPEED CONSTRAINED BY IMAGE SEGMENTATION

LITTLE is known about how or where the visual system parses the visual scene into objects or surfaces. However, it is generally assumed that the segmentation and grouping of pieces of the image into discrete en tities is due to 'later' processing stages, after the 'early' processi ng of the visual image by local mechanisms selective for attributes su ch as colour, orientation, depth, and motion(1). Speed perception is a lso thought to be mediated by early mechanisms tuned for speed(2-5). H ere we show that manipulating the way in which an image is parsed chan ges the way in which local speed information is processed. Manipulatio ns that cause multiple stimuli to appear as parts of a single patch de grade speed discrimination, whereas manipulations that perceptually di vide a single large stimulus into parts improve discrimination. These results indicate that processes as early as speed perception may be co nstrained by the parsing of the visual image into discrete entities.