The Development of Visual Motion Processing in Human Infants

The experiments of this thesis have used apparent motion in random-dot patterns to explore the development of motion processing in infants. Most of the experiments involved discrimination of a segregated pattern, in which different regions moved in different ways (eg opposite directions), from a uniform pattern containing just one kind of motion. Maximum displacement limits (dmax) for discrimination of coherent from incoherent motion, and for discrimination of opposite directions of coherent motion, increased between 8 and 15 weeks. The higher threshold of adults indicated that this increase continues beyond 15 weeks. The effect of changing the interval between displacements indicated two processes underlying the increase in direction discrimination dmax: a maturation of the temporal properties of motion detectors (eg improving sensitivity to high temporal frequencies), which is largely complete by about 12 weeks; and a more prolonged development of their spatial properties which dominates the change in dmax after 12 weeks, and may also be involved before this. Measurements of coherence thresholds for direction discrimination showed that, in addition to the rise in dmax with age, there is a substantial improvement in motion sensitivity at displacements below dmax. Hence a uniform increase in sensitivity across all displacements is likely to be an important factor behind the development of dmax. However there may be additional specific improvements in sensitivity to large displacements, perhaps reflecting the emergence of low spatial frequency channels. A series of habituation and preferential looking experiments failed to find evidence for direction discrimination before 6 weeks, though positive evidence was obtained at 6-8 weeks. The results suggest that directionality emerges at about 7 weeks of age. Interestingly, despite their success at discriminating direction in a segregated stimulus, 6-8-week-olds were insensitive to the absolute direction of uniform motion. This suggests that they have not yet learnt to combine measurements of retinal image motion with information about eye movements