FPL and VEP measures of fusion, stereopsis and stereoacuity in normal infants

Dynamic random dot fusion, stereopsis and stereoacuity were evaluated in 149 healthy, fullterm infants, using both forced-choice preferential looking (FPL) and steady-state visual evoked potential (VEP) protocols. Few infants aged 2-3 months demonstrated fusion or stereopsis in either the FPL or VEP protocol; most infants aged 5 months and older demonstrated fusion and stereopsis in both protocols. Both FPL and VEP stereoacuity approached adult-level (<60 sec) by 6-7 months of age, Both infants and adults exhibited non-monotonic VEP amplitude vs disparity functions with a step change in phase at an intermediate disparity, consistent with separate fine and coarse disparity mechanisms.