Rectangular test fields ½°×2°, with long dimension horizontal and vertical, were presented 10° from fixation on the horizontal and vertical meridian of the visual field. Threshold luminances were determined for each eye singly, and for both eyes when the retinal images were coincident or disparate. In the lateral visual field, binocular thresholds for coincident images were lower than when they were disparate. On the vertical meridian, binocular summation of coincident images is insignificant or totally absent. If, however, in this retinal location the images are disparate and the image in one eye falls to one side of the median line, summation occurs. It is thought that binocular summation is a function of the transmission of impulses from both eyes to the same cortical area of the same cerebral hemisphere.
ERNST WOLF and MICHAEL J. ZIGLER, "Excitation of the Peripheral Retina with Coincident and Disparate Test Fields," J. Opt. Soc. Am. 55, 1517-1519 (1965)