Detection probabilities, determined by the temporal forced-choice method, are presented for targets of several diameters between 2 and 64 minutes, for bin¬ocular and monocular target presentation. In the experiments of Series I, the target was presented 20 degrees peripherally at zero background luminance. For Series II, the background luminance was approximately 10 foot-lamberts and target presentation was foveal. The ratio of binocular to monocular threshold luminances is 1. 61 for Series I. and 1. 46 for Series II, and is unrelated to target area in each case. The extent of binocular summation is greater than that expected, if the eyes are considered to be independent probability mechanisms, and is estimated to approximately its true amount by the assumption that sensory excitation increases as the square of target luminance, and adds algebraically between the two eyes. The extent of binocular summation is independent of target size.
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