The haptic oblique effect in the perception of rod orientation by blind adults

Edouard Gentaz, Yvette Hatwell
1998 Perception & Psychophysics  
The haptic perception of vertical, horizontal, +45 0 oblique, and + 135 0 oblique orientations was studied in completely blind adults. The purpose was to determine whether the variations of the gravitational cues provided by the arm-hand system during scanning would affect the manifestation of the oblique effect (lower performance in oblique orientations than in vertical-horizontal ones) as they did in blindfolded sighted people (Gentaz & Hatwell, 1996). In blindfolded sighted adults, the
more » ... e effect was reduced or absent when the magnitude of gravitational cues was decreased. If visual experience participated in the haptic oblique effect, we should observe no oblique effect in early blind subjects in the conditions of manual exploration where late blind and blindfolded sighted manifest this effect. The magnitude of gravitational cues was therefore varied by changing gravity constraints, whereas the variability of these cues was varied by changing the plane in which the task was performed: horizontal (low variability) and frontal (high variability). Early and late blind adults were asked to explore haptically a rod and then to reproduce its orientation ipsilaterally in one of two exploratory conditions in each plane. In the horizontal plane, the oblique effect was absent, whatever the gravity constraints, in both groups (early and late blind subjects). In the frontal plane, the oblique effect was present, whatever the gravity constraints, in both groups. Taken together, these results showed that, in blind people, the variability of gravitational cues played a role in the haptic oblique effect; no effect of previous visual experience was observed. The problem studied here is how totally blind adults perceive vertical, horizontal, +45 0 oblique, and + 135 0 oblique orientations of a rod in the haptic (tactualkinesthetic) modality. We were interested in examining this spatial property because previous research had shown that the modes ofcoding orientation may differ according to the perceptual modality. In vision, the vertical and horizontal orientations are always coded more accurately than the oblique orientations. This anisotropy, which Appelle (1972) called the oblique effect, has been found in a wide variety ofperceptual tasks (Bryant
doi:10.3758/bf03211925 pmid:9503919 fatcat:jm6mothagvecxe5srqfyh4ps7y