Early development of sensitivity to radial motion at different speeds.

We examined the sensitivity of 2- and 3-month-old infants to radial expansion/contraction at various speeds. The stimuli comprised one radial motion pattern (expansion or contraction) and one translational motion pattern (up, down, left or right; counterbalanced across infants) placed side by side. The two patterns in each stimulus had the same speed. Three-month-old infants could discriminate between radiation and translation, even under relatively low speeds (5.31 and 2.66 degrees /s), whereas discrimination between the two patterns by 2-month-old infants was very limited. Thus, the range of speeds at which infants can detect radial expansion/contraction changes extensively between 2 and 3 months of age. This change in radial motion sensitivity may reflect the development of cortical motion mechanisms in the dorsal pathway, which is specialised to detect radial motion.