The relationship between motor and sensory processes in the fusion of disparate retinal images

In these experiments, the relationship between simultaneously measured objective and subjective fixation disparities at many angles of forced vergence was examined.;Eye position measurements were made using the scleral search coil technique. Objective fixation disparities were first determined using a monocular occlusion method. It was discovered that this method was not highly reliable. Therefore, objective fixation disparities were determined using a direct method. In this method, measurements of eye position were made while the subject viewed the nonius lines (subjective measurements) and the fusion targets with both eyes.;When peripheral fusion targets were used, differences between the objective and subjective fixation disparities increased for some subjects as the angle of forced vergence increased. For other subjects, no relationship was found between the differences in objective and subjective fixation disparities and the angle of forced vergence. The results suggested that significant differences between the objective and subjective fixation disparities occurred at forced vergence angles where subjects showed little or no vergence adaptation.;It was concluded that there were two forms of sensory compensation available for the fusion of disparate retinal images. The first was a shift in retinal correspondence. This shift resulted in differences between the objective and subjective fixation disparities. The second sensory process was responsible for the true fusion or melding of the fusion targets into a unified percept. This unification process would occur within Panum's fusional areas.;In other experiments a central fusion target was used to study the area over which the mechanism which resulted in shifts in correspondence could operate. The results suggested that this mechanism operated over a finite area around the fixation point. This finite area was about 3 to 4 degrees of visual angle (vertical) by about 4 to 6 degrees of visual angle (horizontal). The local shift in correspondence was described as a "dimple" in the nonius horopter. Finally, it was concluded that measurements of fixation accuracy made using nonius lines are most valid when the nonius lines are moved away from a central fusion target.