Perceived length in the central visual field: evidence for visual field asymmetries.

Visual performance for judging the length of a simultaneously presented pair of radial lines, reciprocally opposed by 180 degrees at a central fixation point, was assessed for 24 radial positions of test lines, for three viewing conditions (binocular, left and right monocular) and five different standard line sizes (1.43-7.13 degrees ). Generally, the results showed underestimation of the test line. Furthermore, clear visual field asymmetries were observed between the upper versus lower visual fields and the left versus right visual fields with greater underestimation for test lines presented in the lower and right visual fields. Also, asymmetries tended to be strongest along the 30 and 150 degrees radial orientations. Fourier analysis indicated that these asymmetries are mainly described by summing up the f0, f1, f2 and f5 components.