Monocular size and distance scaling is modulated by the presence of a fixation point

Abstract

Fixation points are widely used in psychophysical research to stabilise gaze, yet their influence on perceptual outcomes remains underexplored. This study investigated the effects of the presence versus absence of a fixation point on monocular size–distance scaling, examining whether it functions as a fixation stabiliser, a spatial anchor (i.e., as a direct depth or location cue), or a competing visual reference. Using a widely employed fixation marker, we compared size and distance judgments for real objects and afterimages (stimuli that differ in their reliance on depth cues). Fixation points modestly improved size constancy for both stimuli, supporting their role in retinal stabilization. However, their effects on distance perception varied: real objects maintained stable distance judgments regardless of fixation, whereas afterimages exhibited greater inconsistencies when a fixation point was introduced. Therefore, fixation points appear to interfere with distance scaling for depth-ambiguous stimuli like afterimages. Additionally, correlations between accommodative abilities and errors in size perception for real objects suggest fixation points may reduce reliance on oculomotor cues by stabilizing retinal input. These findings challenge the assumption that fixation points are neutral methodological tools, highlighting their active role in shaping perceptual outcomes. Future research should critically evaluate fixation point designs and their broader impact on size and distance perception.