ROLE OF CHROMATIC AND LUMINANCE CONTRAST IN INFERRING STRUCTURE FROM MOTION

We measured the ability to infer structure from motion (SFM) in severa l directions in three-dimensional color space. Only motion cues are us eful to subjects in performing this three-dimensional shape-identifica tion task. We report the following results: (1) SFM performance is at chance for equiluminant stimuli that isolate short-wavelength-sensitiv e cones. Hence the short-wavelength-sensitive-cone input to SFM is neg ligible. (2) SFM performance increases with the \DELTAL - DELTAM\ sign al when DELTAL + DELTAM = 0 (i.e., only chromatic and no luminance con trast is available). We reject the hypothesis that SFM obtains input f rom a single chromatic mechanism combining the long- and medium-wavele ngth-sensitive cones linearly. Our data are compatible with SFM that u ses the output of two mechanisms, one taking the difference between th e long- and medium-wavelength-sensitive-cone signals and the other tak ing the respective sum. We reject the particular hypothesis that SFM u tilizes only the magnitude and not the sign of the long-and medium-wav elength-sensitive-cone signal. (3) We compare SFM performance with thr eshold performance for velocity and motion discrimination. Stimuli wit h luminance contrast yield SFM performance that is superior to stimuli without luminance contrast when they are expressed as multiples of ve locity discrimination threshold. This superiority is even greater when SFM performance is compared with motion-direction discrimination thre sholds.