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 39(4) S885] found evidence for a perceptual asymmetry of vertical-motion processing in young infants: a preference for patterns that move downwards. This asymmetry was in the opposite direction to the asymmetry of vertical optokinetic nystagmus, which was biased towards upwards motion. However, the FPL bias was weak, and the object of present experiments was to explore the possibility that it could be enhanced by reducing stimulus contrast. In experiment 1, contrast thresholds for gratings moving upwards and downwards were compared, and no directional asymmetry at threshold was found. In experiment 2, the effect of contrast on infants' preference between simultaneously displayed upwards-drifting and downwards-drifting gratings was examined. Infants showed no preference at 5% contrast, a marked preference for downwards motion at intermediate contrasts (10% and 20%), and a similar but smaller preference at 40% contrast. These results suggest that the vertical-motion asymmetry is a result of differences in the gains of directionally selective mechanisms for upwards and downwards motion.