Motion in the retinal image may occur either in the form of spatiotemporal variations in luminance (first-order motion) or as spatiotemporal variations in characteristics derived from luminance, such as contrast (second-order motion). Second-order motion patterns were employed in an attempt to establish the principles used for the detection of image motion in the human visual system. In principle, one can detect motion at a high level of visual analysis by identifying features of the image and tracking their positions (correspondence-based detection) or at a low level by analysis of spatiotemporal luminance variations without reference to features (intensity-based detection). Prevailing models favor the latter approach, which has been adapted to account for the visibility of second-order motion by postulation of a stage of rectification that precedes motion energy detection [ J. Opt. Soc. Am. A 5, 1986 ( 1988)]. In two experiments it is shown that second-order motion is indeed detected normally by use of the strategy of transformation plus energy detection but that detection can also be achieved by use of the feature-correspondence strategy when the intensity strategy fails. In the first experiment, a stimulus is employed in which opposite directions of motion perception are predicted by the two strategies. It is shown that normally the direction associated with motion energy in the rectified image is seen but that the direction associated with feature motion is seen when the energy system is disabled by the use of an interstimulus interval. In the second experiment, masking of features is shown to have little effect on motion detection under normal conditions but a marked effect when an interstimulus interval is employed, suggesting that features are used in the latter case but not in the former.
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