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This paper surveys studies on the anisotropic perception of orientation in the tactual (cutaneous/passive touch or haptic/active touch) system. In the visual system, the vertical and horizontal orientations are known to be perceived more accurately than oblique orientations. This anisotropy is called the oblique effect. The interest of examining the oblique effect in the tactual system stems, in part, from the fact that even its existence is debated in the literature. The aim of the present work was to show that : 1 I the study of factors that induce a tactual (cutaneous or haptic) oblique effect makes it possible to understand more easily the nature of the tactual underlying processes ; 2 I the study of the cutaneous, haptic and visual oblique effects allows comparisons in the three perceptual systems of the nature of the respective processes that underlie the processing of orientation. The first part of this article presents the data on the oblique effect in the cutaneous system. The second part concerns the data on the oblique effect in the haptic system. We examine the hypothesis that the nature of task would influence the oblique effect in the different perceptual systems. When the task induces low-level processing, the presence of an anisotropy and its forms seems to depend on the properties of orientation- selective neurons existing at an early stage in the perceptual hierarchy. Thus, an oblique effect is present in the visual system, an effect of proximo-distal orientation (i.e. along the length of the finger) is present in the cutaneous system and no oblique effect is observed in the haptic system. In contrast, when the task induces high-level processing, an oblique effect seems to be present in the cutaneous, haptic and visual systems.
