Yvette Hatwell

Role of gravitational cues in the haptic perception of orientation

The haptic perception of vertical, horizontal, +45°-oblique, and +135°-oblique orientations was studied in adults. The purpose was to establish whether the gravitational cues provided by the scanning arm—hand system were involved in the haptic oblique effect (lower performances in oblique orientations than in vertical—horizontal ones) and more generally in the haptic coding of orientation. The magnitude of these cues was manipulated by changing gravity constraints, and their variability was manipulated by changing the planes in which the task was performed (horizontal, frontal, and sagittal). In Experiment 1, only the horizontal plane was tested, either with the forearm resting on the disk supporting the rod (“supported forearm” condition) or with the forearm unsupported in the air. In the latter case, antigravitational forces were elicited during scanning. The oblique effect was present in the “unsupported” condition and was absent in the “supported” condition. In Experiment 2, the three planes were tested, either in a “natural” or in a “lightened forearm” condition in which the gravitational cues were reduced by lightening the subject’s forearm. The magnitude of the oblique effect was lower in the “lightened” condition than in the “natural” one, and there was no plane effect. In Experiment 3, the subject’s forearm was loaded with either a 500- or a 1,000-g bracelet, or it was not loaded. The oblique effect was the same in the three conditions, and the plane effect (lower performances in the horizontal plane than in the frontal and sagittal ones) was present only when the forearm was loaded. Taken together, these results suggested that gravitational cues may play a role in haptic coding of orientation, although the effects of decreasing or increasing these cues are not symmetrical.

Geometrical haptic illusions: The role of exploration in the Müller-Lyer, vertical-horizontal, and Delboeuf illusions

This article surveys studies of the occurrence, in the haptic modality, of three geometrical illusions well known in vision, and it discusses the nature of the processes underlying these haptic illusions.We argue that the apparently contradictory results found in the literature concerning them may be explained, at least partially, by the characteristics of manual exploratory movements. The Müller-Lyer illusion is present in vision and in haptics and seems to be the result of similar processes in the two modalities. The vertical-horizontal illusion also exists in vision and haptics but is due partly to similar processes (bisection) and partly to processes specific to each modality (anisotropy of the visual field and overestimation of radial vs. tangential manual exploratory movements). The Delboeuf illusion seems to occur only in vision, probably because exploration by the index finger may exclude the misleading context from tactile perception. The role of these haptic exploratory movements may explain why haptics is as sensitive as vision to certain illusions and less sensitive to others.

The Haptic ‘Oblique Effect’ in Children's and Adults' Perception of Orientation:

The haptic perception of vertical, horizontal, and diagonal orientations was studied in children (aged 7 and 9 years) and in adults. The purpose was to test the hypothesis that the haptic oblique effect results from the different scanning movements at work when one hand explores an oblique standard and the other hand sets the response rod. In experiment 1, blindfolded subjects reproduced the orientation of a standard rod presented in either the frontal, the horizontal, or the sagittal plane, and this task was achieved either ipsilaterally (the same hand explored the standard and set the response rod) or contralaterally (one hand explored the standard and the other hand set the response rod). Since, in the sagittal plane, scanning movements are analogous when the left and right hands explore oblique orientations, no oblique effect should be observed in this condition if the hypothesis is valid. Moreover, a development effect should be observed, since young children generally rely more on movement coding than do older children and adults. Results did not support these predictions: the same oblique effect appeared in the frontal and the sagittal planes both in the ipsilateral and in the contralateral condition, and the effect of age was not in the direction predicted by the hypothesis. The results were consistent with the hypothesis in the horizontal plane only. Experiments 2 and 3 provided further tests of this hypothesis but both failed to support it. Taken together, the results of these three experiments did not support the assumption and it is suggested that the haptic oblique effect may be linked to the gravitational cues provided by the arm—hand system when it acts in the three spatial planes.

The reproduction of vertical and oblique orientations in the visual, haptic, and somato- vestibular systems

This study investigates whether the vertical orientation may be predominantly used as an amodal reference norm by the visual, haptic, and somato-vestibular perceptual systems to define oblique orientations. We examined this question by asking the same sighted adult subjects to reproduce, in the frontal (roll) plane, the vertical (0°) and six oblique orientations in three tasks involving different perceptual systems. In the visual task, the subjects adjusted a moveable rod so that it reproduced the orientation of a visual rod seen previously in a dark room. In the haptic task, the blindfolded sighted subjects scanned an oriented rod with one hand and reproduced its orientation, with the same hand, on a moveable response rod. In the somato-vestibular task, the blindfolded sighted subjects, sitting in a rotating chair, adjusted this chair in order to reproduce the tested orientation of their own body. The results showed that similar oblique effects (unsigned angular error difference between six oblique orientations and vertical orientation) were observed across the three tasks. However, there were no positive correlations between the visual, haptic, and somato-vestibular oblique effects. Moreover, in some oblique orientations, there was a tendency to overestimate the angle between the oblique orientation and the vertical orientation. This effect varied according to the orientation value and themodality. Taken together, these findings suggest that although vertical orientation is used as a reference normin the visual, haptic, and somato-vestibular systems to define oblique orientations, specific processing mechanisms seem to be at work in each perceptual system.

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

The haptic perception of vertical, horizontal, +45° oblique, and +135° 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 oblique 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 ipsilateraUy 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.

Role of memorization conditions in the haptic processing of orientations and the 'oblique effect'.

The haptic processing of vertical, horizontal, 45 degrees and 135 degrees oblique orientations was studied in blindfolded sighted adults in an exploration-reproduction task. The purpose was to determine whether the variations of the memorization conditions between the exploration and reproduction phases would influence the global performance and the oblique effect (lower performance in oblique orientations than in vertical-horizontal orientations). If orientation coding depended on attentional resources, the increase in memory constraints would affect the haptic processing of orientations and the oblique effect. Memory constraints were therefore varied by changing the length and the nature of the delay in two tasks in which previous research has shown that the availability of gravitational cues affected orientation coding. Blindfolded adults were asked to explore haptically a rod with minimal (Expt 1) or natural (Expt 2) gravitational cues and then to reproduce the orientation of this rod ipsilaterally after one of four memorization conditions: with 5 s or 30 s unfilled delays, and 30 s delays filled with verbal or haptic interpolated tasks. When the delay was unfilled, whatever its length (5 s or 30 s), the performance depended on the conditions of manual exploration: the oblique effect was absent when the gravitational cues were minimal (Expt 1) and was present when these cues were natural (Expt 2). By contrast, when the delay was filled with interpolated tasks, the haptic oblique effect was present whatever the conditions of manual exploration. Taken together, these results showed that memorization conditions played a role in the haptic processing of orientations and in the oblique effect when the gravitational cues were minimal during manual exploration.

Form Perception and Related Issues in Blind Humans

Experimental research with the blind is relatively scarce. While philosophers and psychologists have often speculated on the nature of the world of the congenitally blind, few systematic studies are based upon a coherent theoretic framework. Furthermore, it is often difficult to compare one study with another, due to the heterogeneity of the population samples studied. Blindness is a rare disorder in our geographic area. Consequently, the available subjects may vary in their chronological age, age at onset of blindness, duration of blindness, etiology, intellectual and socioeconomic status, education, etc. Even the degree of residual vision is variable, since “legal blindness” permits a level of visual functioning that allows some form perception (1/10th or 1/20th).

Effects of peripheral and central visual impairment on mental imagery capacity

This paper reviews a number of behavioral, neuropsychological and neuroimaging studies that bear on the question of whether and how visual disorders of peripheral or central origin lead to disorders of mental imagery capacity. The review of the literature suggests that in cases of blindness of peripheral origin lack of vision can progressively lead to representational disorders. However, in patients suffering from peripheral visual deficits, representational disorders can partially or completely be compensated by other sensory modalities as well as by cortical reorganization. Interestingly, in brain-damaged patients, neurovisual disorders following occipital or parietal lesions are not systematically associated with representational deficits, thus demonstrating that visual perception and visual imagery may not rely on the same cortical structures as previously hypothesized. Impairments seen on mental imagery tasks among brain-damaged patients with visual and/or spatial deficits might be due to an often co-existing attentional deficit. We discuss this possible dissociation between visual perception and visual mental imagery and its implications for theoretical models of mental representation.

Dimensional and overall similarity classifications in haptics: A developmental study

The developmental change from global towards dimensional classifications, usually observed in vision, was investigated in haptics with stimuli varying according to their size and roughness. Children aged 5 and 9 years old and adults were presented with a free classification task allowing either an overall similarity sorting or a dimensional sorting. In two experiments, the discriminability of the stimuli along one or both dimensions was varied. Results showed that although more overall similarity classifications were observed in children than in adults, this kind of classification was never dominant (i.e., it was nerve chosen more frequently than would be predicted by chance). In addition to these developmental effects, effects due to the magnitude of stimulus difference were observed: A stimulus tended to be matched with the standard especially if it was slightly different from it or if the other comparison objects were much more different from this standard. This appeared in the first experiment testing dimensional versus overall similarity matching, and in the second experiment testing dimensional size versus dimensional roughness matching. These results were discussed with reference to the characteristics of haptic exploratory procedures.

The haptic recognition of geometrical shapes in congenitally blind and blindfolded adolescents: is there a haptic prototype effect?

Background It has been shown that visual geometrical shape categories (rectangle and triangle) are graded structures organized around a prototype as demonstrated by perception and production tasks in adults as well as in children. The visual prototypical shapes are better recognized than other exemplars of the categories. Their existence could emerge from early exposure to these prototypical shapes that are present in our visual environment. The present study examined the role of visual experience in the existence of prototypical shapes by comparing the haptic recognition of geometrical shapes in congenitally blind and blindfolded adolescents. Methodology/Principal Findings To determine whether the existence of a prototype effect (higher recognition of prototypical shapes than non prototypical shapes) depended on visual experience, congenitally blind and blindfolded sighted adolescents were asked to recognize in the haptic modality three categories of correct shapes (square, rectangle, triangle) varying in orientation (prototypical/canonical orientation vs. non prototypical/canonical orientation rotated by 45°) among a set of other shapes. A haptic prototype effect was found in the blindfolded sighted whereas no difference between prototypical and non prototypical correct shapes was observed in the congenitally blind. A control experiment using a similar visual recognition task confirmed the existence of a visual prototype effect in a group of sighted adolescents. Conclusion/Significance These findings show that the prototype effect is not intrinsic to the haptic modality but depends on visual experience. This suggests that the occurrence of visual and haptic prototypical shapes in the recognition of geometrical shape seems to depend on visual exposure to these prototypical shapes existing in our environment.