Small but reproducible fixation disparities occur in normal subjects when they view certain types of dichoptic stimuli. During dichoptic as well as stereoptic stimulation the motor fusion process determines first the average vergence state of the eyes. The subsequent fine tuning of vergence is shown to depend on the spatial distribution of contrast edges both of the same contrast sign (‘stereoptic edges’) and of opposite contrast sign (‘dichoptic edges’). Stereoptic edges tend to induce superposition attempts of the vergence control system and dichoptic edges tend to antagonise this process. If a single low-contrast dichoptic edge is presented with zero disparity and within a stereoptic reference frame, a fixation disparity of several minutes of arc results. This influences depth vision since dichoptic edges are perceived (as monocular edges) at the actual rather than at the intended fixation distance. The findings explain previous paradoxical results of eg Kaufman and Pitblado who reported seeing depth in opposite-contrast stereograms. Their results seemed to contradict the well-established ‘same-sign rule’ (SSR) which states that the stereoptic system only detects disparities of edges with the same contrast sign. It is concluded that (i) the SSR holds; (ii) dichoptic (and monocular) edges are seen at the horopter; (iii) the vergence fine tuning prevents superposition of dichoptic edges even if this causes a fixation disparity.
Periodic letter strokes within a word affect fixation disparity during reading
