The effect of the posture of an observer on the pattern of meridional anisotropy for orientation acuity was examined. Orientation acuity was estimated for the two principal and for one oblique axis with three bodily postures: normal vertical sitting, vertical sitting with the head rotated to 45° from vertical, and prone. The test stimuli were sine-wave gratings covering a range of two octaves of spatial frequency, thin lines, and step edges. Acuity estimates were obtained by two different psychophysical procedures: a single-interval two-alternative forced choice, and a two-interval two-alternative forced choice. Both experimental procedures revealed a strong interaction between the shape of the function that describes the meridional variation of orientation acuity and the posture of the observer. The data are incompatible with models of the oblique effect in orientation perception that are based on a meridional anisotropy in neural properties early in the visual hierarchy.
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