To assess how target size and eccentricity affect binocular summation (BS) of reaction time (RT) at suprathreshold level, we measured RT using targets of 0.108 degrees and 0.216 degrees at four eccentricities (0 degrees, 5 degrees, 15 degrees, 25 degrees) in six normal volunteers. The difference between the monocular/binocular RT differentials for both sizes significantly increased in the periphery (P < 0.05). The smaller target required significantly longer monocular RT at 25 degrees (P < 0.01) and generated greater neural summation than the larger target (P < 0.01). This suggests that when monocular function has reached its limit in visual processing in the periphery, BS increases, facilitates visual processing, and shortens binocular RT. (C) 2010 Elsevier Ltd. All rights reserved.