On the localization of troxler's effect in the visual pathway

The paper describes an attempt to establish the possible seat of Troxler's Effect in the visual pathway. Fully dark-adapted observers viewed a 4.1 × 10−5 cd/m2 stimulus at 20° eccentricity under conditions of steady fixation, the mean time of subjective disappearance being 6.4 sec. A consideration of quantum statistics and the kinetics of local adaptation at this low level shows that the effect cannot originate in the primary receptors. An experiment involving saccadic displacements of a faded image in the light-adapted condition yielded a frequency-of-seeing curve of Poissonian form, with n = 3. From this it is deduced that at least three of the functional units involved need to respond to enable a faded image to reappear, and that the mean diameter of the unit at 20° eccentricity is 10′ of arc under these conditions. A study of extra-foveal acuity under equivalent conditions yielded a value of 8′ of arc, indicating that the functional units involved are probably the same, the ganglionic units. As at least three units at a time seem to be concerned with Troxler's Effect, this indicates a post-retinal origin. A further experiment showed that there was no binocular interaction of Troxler's Effect, which suggests a pre-cortical origin. The lateral geniculate body is suggested as a probable seat of Troxler's Effect.