Of the many factors which influence vergence movements (tonicity of the extraocular muscles, distance of the fixated object,1 concave lenses and accommodation,2 prisms,3 and disparity,4 among others) target size has perhaps been given the least experimental attention. In an important analysis of this problem Ittelson and Ames found that an increase in both vergence and accommodation was associated with an increase of the image of a playing card projected (with a Clason projector) on a screen 40 cm. in front of the observer's eyes.5 It was soon pointed out, however, that the measured changes in vergence might only be pure accommodative vergence associated with the changes in accommodation.6 Are changes in target size associated with changes in vergence when all other relevant variables (including the accommodation posture of the eyes) are held fixed?
The experiment was essentially a modification of the original Ittelson-Ames
ALPERN M. Vergence and Accommodation: I Can Change in Size Induce Vergence Movements? AMA Arch Ophthalmol. 1958;60(3):355–357. doi:10.1001/archopht.1958.00940080371001
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