The effect of contrast on vertical motion processing asymmetries in 11-week-old infants

Recent forced-choice preferential looking (FPL) experiments with random-dot patterns [Wattam-Bell, 1998 Investigative Ophthalmology & Visual Science 3 9(4) S885] found evidence for a perceptual asymmetry of vertical-motion pro cessing in young infants: a preference for patterns that move downwards. Th is asymmetry was in the opposite direction to the asymmetry of vertical opt okinetic nystagmus, which was biased towards upwards motion. However, the F PL bias was weak, and the object of present experiments was to explore the possibility that it could be enhanced by reducing stimulus contrast. In exp eriment 1, contrast thresholds for gratings moving upwards and downwards we re compared, and no directional asymmetry at threshold was found. In experi ment 2, the effect of contrast on infants' preference between simultaneousl y displayed upwards-drifting and downwards-drifting gratings was examined. Infants showed no preference at 5% contrast, a marked preference for downwa rds motion at intermediate contrasts (10% and 20%), and a similar but small er preference at 40% contrast. These results suggest that the vertical-moti on asymmetry is a result of differences in the gains of directionally selec tive mechanisms for upwards and downwards motion.