A three alternative tracking paradigm to measure vernier acuity of older infants.

Vernier acuity was measured in infants 1 to 14 months of age using a dynamic three alternative tracking paradigm. The location of the vernier offset would move randomly to one of three screen locations. An observer, unaware of the stimulus location, viewed the infant from behind a screen and guessed the position of the vernier offset. The magnitude of the vernier offset was controlled by a staircase and the results were analyzed using a broken line psychometric function. These thresholds were compared to thresholds obtained from the same infants during the same session with a two alternative forced choice preferential looking paradigm. Differences in the results of these two procedures and those reported previously in the literature are discussed in terms of the differences in the nature of the visual stimuli, one containing motion and one without motion.