The effect of perceived length on visuomotor localization

Purpose: The purpose of this experiment was to study visuomotor localizatio n in the presence of either a horizontal array of equally spaced dots or a thin horizontal line. Methods Pointing behavior was used to assess directional localization. In e xperiment 1, subjects were made myopic using a contact lens and then correc ted with a spectacle lens. Subjects were tested in the presence and absence of a regularly spaced, horizontal array of dots with and without the conta ct lens/spectacle combination. In experiment 2, subjects wore the contact l ens/spectacle in all cases. Some subjects were tested in the presence and t hen in the absence of a regularly spaced, horizontal array of dots while th e order of conditions was reversed for other subjects. In experiments 3 and 4, subjects were tested without the contact lens/spectacle combination. In experiment 3, subjects were tested in the absence and then in the presence of a regularly spaced, horizontal arrays of dots. In experiment 4, subject s were tested in the absence and then in the presence of a thin horizontal line. Results in experiment 1, in the absence of the array of dots, subjects unde rshot targets with the contact lens/spectacle combination. When the array w as present, pointing with the contact lens/spectacle combination was accura te, in experiment 2, subjects undershot targets in the absence of the array of dots if this condition was performed first. If the array was present in the initial condition, the pointing undershoot in the second condition (ar ray absent) was reduced. In all cases, the pointing undershoot was reduced in the presence of the array. In experiments 3 and 4, a pointing overshoot was found in the presence of an array of dots or a thin line. Conclusions: It is concluded that extraretinal eye position information is not the primary determinant of visuomotor localization in the presence of a horizontal contour. The overshoot produced by a horizontal contour may be related to a length illusion brought about by spatial filtering in the visu al system or inaccurate distance judgments. (C) 2001 The college of Optomet rists. Published by Elsevier Science Ltd. All rights reserved.