Influence of visual perception on spatial coding of saccadic eye movements and fixation

The Müller-Lyer illusion produces differences in visual length perception when lines of equal length are presented as double-arrows with inward-pointing and outward-pointing arrowheads, respectively. The aim of this study was to investigate the influence of length perception on the amplitude of saccadic eye movements and on postsaccadic fixation positions. Video recordings of eye movements were obtained from 35 healthy volunteers (age: 20-30 years) while they scanned the endpoints of Müller-Lyer targets. The targets were presented in horizontal and vertical orientations with an arrow length of 15°. Fixation periods between the saccades were adjusted at 1 s and 4 s. The final saccadic amplitudes between the endpoints of Müller-Lyer figures reflect the visually perceived length differences: lines with outward-pointing arrowheads were scanned with smaller saccades than lines with inward-pointing arrowheads. This bias in final saccadic amplitude was equal in both the horizontal and vertical target orientations as well as for 1-s and 4-s fixation periods. Postsaccadic drift after initial saccadic movements reduced the effect of the Müller-Lyer illusion depending on arrowhead and target orientation: outward-pointing horizontal arrows caused hypometric pulses, while inward-pointing vertical arrows caused hypermetric pulses. The results of this study demonstrate that spatial coding of both saccadic eye movements and fixation is strongly influenced by visual perception. While the influence of the Müller-Lyer illusion on final saccadic amplitudes did not depend on target orientation or fixation period, an additional pulse-step mismatch appeared in the horizontal (hypometria) and vertical (hypermetria) direction.