Stereoscopic depth perception was tested in human infants by a new method based on attracting the infant's attention through movement of a stereoscopic contour formed from a dynamic random-element stereogram. The results reveal that stereopsis emerges at 3 1/2 to 6 months of age, an outcome consistent with evidence for rapid postnatal development of the visual system.

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Infants' perception of the direction and speed of motion could be mediated by several underlying mechanisms, including sensitivity to temporal variations in luminance, the relative position of pattern elements, or time-locked spatial displacements. Velocity thresholds were estimated in 6-and 12-week-old infants who viewed a set of moving stripes surrounded by sets of stationary stripes. The forced-choice preferential looking technique was used to collect data from infants who were presented with high-contrast stripes of 2 widths moving at several different velocities. Velocity thresholds were approximately 9*/s in the 6-week-olds and 4"/s in the 12-week-olds. Thresholds did not vary with stripe width at either age, suggesting that motion perception in young infants is not based on a simple flicker-sensitive mechanism. Rather, infant velocity thresholds are based either on a positionsensitive or a motion-sensitive mechanism, at least for high-contrast spatial patterns.Movement is one of the most effective stimulus variables for attracting and sustaining the attention of infants. Moreover, movement leads to systematic motor responses such as optokinetic nystagmus (

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