Nonmotor component of fusional response to vertical disparity: a second look using an afterimage method

An afterimage method has been used to investigate the relative magnitudes of the nonmotor and motor components of the fusional response to vertical disparity in a complex visual stimulus of diameter 57 degrees consisting of 50 horizontal lines and a square of side 2.5 degrees in the middle. The largest vertical disparity that evoked a stable fusional response was found to be in the range 3-6 degrees, of which the nonmotor component amounted only to 8-15', i.e., 2-10% of the total. At these fusional amplitudes, binocular single vision was already disrupted in the foveola. When the 50 horizontal lines were omitted from the stimulus so that only the central square of side 2.5 degrees remained, the fusional amplitudes decreased by only 25% while the absolute level of the nonmotor components remained the same. The nonmotor components found here are much smaller than those (amounting to about 2 degrees, or 25-40% of the total response) reported recently in the literature.