Oculomotor responses to step-ramp targets by young human infants.

Previous research has shown that the smooth pursuit system in early infancy is quite immature. Infants' tracking of a single, small target moving at velocities greater than 10 deg/sec is almost entirely saccadic until the end of the second postnatal month. The emergence of smooth pursuit is characterized by low gain (less than 0.5) and frequent saccadic intrusions. To provide a quantitative description of pursuit to relatively slow target velocities, 10 infants ranging in age from 7 to 11 weeks viewed a 2 deg target that was stepped 5 to 10 deg from screen center and then ramped back to screen center and 10 deg beyond at a constant velocity of 3, 6 or 12 deg/sec. Smooth pursuit was observed even in the youngest infant whose segments of pursuit between saccades were up to 5 sec in duration. At the slowest target velocity, mean pursuit gain across infants was 0.50, while at 6 and 12 deg/sec mean pursuit gain was 0.25 and 0.11. This systematic decrease in pursuit gain with increasing target velocity implies that pursuit velocity was invariant across the three target velocities. These findings suggest that smooth pursuit can be generated consistently by the end of the second postnatal month, but that it is slow and uncalibrated to the velocity of the target.