Meridional Anisotropy in the Discrimination of Parallel and Perpendicular Lines—Effect of Body Tilt

Abstract

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.

Reference frames and haptic perception of orientation: Body and head tilt effects on the oblique effect

“…Thus, head tilt activates mainly the otolith organs, the neck muscles, and the neck joint receptors. Moreover, contrary to what occurs in the whole-body tilt, the trunk remains vertical during head tilt, and as was suggested by Chen and Levi (1996), it could be used as a reference axis in line with a gravitational reference frame.…”

“…Some experiments using orientation-discrimination tasks showed results in favor of a retinal reference frame (Chen & Levi, 1996;Orban et al, 1984). In these cases, the most accurately processed orientations were those aligned with the vertical and horizontal retinal meridians, whatever the position of the head.…”

“…In vision, the effect of head or body tilt on the visual oblique effect has been investigated with different tasks, such as visual search (DeFord, Prather, & Essock, 1998, 1999, orientation-discrimination (Attneave & Olson, 1967;Buchanan-Smith & Heeley, 1993;Chen & Levi, 1996;Ferrante, Gerbino, & Rock, 1995;Orban, Vandenbussche, & Vogels, 1984), contrast sensitivity (Banks & Stolarz, 1975;Lennie, 1974), and vernier acuity (Corwin, Moskowitz-Cook,& Green, 1977;Saarinen & Levi, 1995). Some experiments using orientation-discrimination tasks showed results in favor of a retinal reference frame (Chen & Levi, 1996;Orban et al, 1984).…”

“…The Class 2 oblique effects, observed in paradigms reflecting cognitive processes (such as identifying, remembering, or categorizing the orientations of stimuli), could be tied to another reference frame, such as the gravitational reference frame. Chen and Levi (1996) have pointed out other factors that could explain the apparent inconsistency in the studies using orientation-discrimination tasks: (1) The countertorsion of the eyes (see Bles & de Graaf, 1991;de Graaf, 1990) provoked by body or head tilt, which could lead to an apparent gravitational mapping if it is not taken into account, (2) the length of the stimuli (long stimuli could enhance a retinal reference frame), (3) the tilted segment (tilting the head alone while keeping the trunk aligned with gravity could favor a gravitational mapping), and (4) the presence of an explicit spatial reference (parallel or perpendicular line) during the task, which could enhance a retinal reference frame.…”

Effects of Context on a Visual 3-D Pointing Task

“…It refers to superior performance in tasks involving horizontally or vertically oriented stimuli than in tasks involving obliquely oriented stimuli. Chen and Levi (1996) found oblique effects for retinotopic coordinates with a parallelism discrimination task. Hence, perceptual phenomena like the`oblique effect' are influenced not only by allocentric references, but also by egocentric ones.…”