Meridional anisotropy in the discrimination of parallel and perpendicular lines--effect of body tilt

It is well documented that orientation discrimination is poorer for stimuli oriented obliquely than for those that are vertical or horizontal. Buchanan-Smith and Heeley recently reported that in the absence of a spatial reference this anisotropy follows gravitational rather than retinal coordinates, suggesting a high-level basis for the anisotropy in unreferenced orientation discrimination tasks. In the present study, unlike the previous one, the effects of body tilt on orientation discrimination have been examined in the presence of explicit simultaneous spatial references. The thresholds for discrimination of two parallel or two perpendicular lines were estimated for the retinally principal and oblique orientations, with the body either erect or tilted 45 degrees with respect to gravity. In agreement with previous studies, meridional anisotropy for both parallelism and perpendicularity discrimination was found when observers were seated upright. When the observer's body was tilted, the anisotropy for the parallelism task was mapped to retinal and not to gravitational coordinates after compensating for countertorsion. Initially, the anisotropy for the perpendicularity task was not mapped to retinal coordinates, but after extensive practice for both the erect and the tilted body conditions it eventually followed retinal coordinates. The results reported here suggest that contrary to orientation discrimination without a spatial reference, the ultimate limits for both parallelism and perpendicularity discriminations are located at orientation-sensitive cortical neurons. However, the effect of perceptual learning in the perpendicularity task suggests that the internal frame of reference (gravity cues and body axis) also plays an important role.