Fixation and perception.

Abstract

Boyce and West [3] have recently recorded eye-position while subjects were instructed to fixate successively the apices of the two Miller-Lyer figures. Provided that the two figures were simultaneously present in the visual field, the separation of the mean fixation points during observation of the underestimated figure was found to be significantly less than the corresponding distance recorded during inspection of the overestimated one. Boyce and West conclude that they have demonstrated an effect of perception upon fixation. A very similar experiment was performed by Delabarre in 1897 and is described in his preliminary note " A Method of Recording Eye Movements" [4]. Delabarre's apparatus, though relatively primitive, and his results, though never reported in detail, were very comparable to those of Boyce and West. Initially he employed an optical-lever system, recording directly on to a photographic plate, but difficulties with the plate led him to settle on a mechanical method. Delabarre [4, 5] considered that he had shown an effect of fixation upon perception. Effects of both duration and locus of fixation upon perception, and in particular upon the magnitude of the geometrical illusions, have been reported [9, 12,15, 17], and it is probable that causality is present in both directions, but the persistence of the classical distortions in after-images, and with tachistoscopic or stabilized presentation [10, 13], renders implausible a primary explanation of the geometrical illusions in terms of eye-movement or eye-position. There remains the third possibility that the fixational effect is causally independent of the perceptual distortion. Boyce and West themselves considered the hypothesis that the fixation control mechanism seeks the position at which maximum information may be secured, and attempted by their second experiment to exclude this possibility. They state, without discussion, that the Mifiller-Lyer illusion is destroyed when each figure is observed alone. Repeating their measurements with only the fixated figure present in the visual field they found that there was now no difference in the distances between the mean fixation points. This second finding would seem strong evidence against any hypothesis proposing the independence of the perceptual and fixational anomalies. Yet the suggestion that the perceptual target differs from the geometrical target only when the second figure is present is a curious and unsubstantiated one. How far, spatially and temporally, may the second figure be removed Correspondence before interaction ceases and the illusion is abolished? Must the subject, during fixation, make continual comparison between …