Tachistoscopic perception under head tilt

Subjects attempted to identify singly presented letters, flashed briefly at six different locations equidistant from fixation. The locations were at 1, 3, 5, 7, 9, and 11 o’clock on an imaginary circle. The subjects performed both with their heads upright or tilted 60° (i.e., through 2 and 8 o’clock or through 10 and 4 o’clock). In Experiment 1, the letters were projected in a gravitationally fixed rectangular surround, and accuracy of report depended largely on the locations of the letters in gravitational coordinates, although there was clear evidence of a retinal influence as well. In Experiment 2, the surround was circular and accuracy depended principally on retinal location, although there was also a gravitational influence. Analysis of individual data suggested that, when their heads were tilted, subjects generally adopted a reference frame which lay between the retinal and gravitational frames, closer to one than the other but typically coinciding with neither. We conclude that the subjective reference frame does not depend on a poststimulus correction, but rather that stimulus information is incorporated directly into the subjective frame. The data also suggested that lateral asymmetries depend on a perceptual rather than a retinal interpretation of left and right visual fields.