Is it possible to know something just by touching it? Is it useful, and indeed necessary to do so in certain cases? Touch is a modality whose function, from a cognitive point of view, has given rise to diverse and even contradictory appreciations. Because it is largely redundant with vision for the acquisition of spatial knowledge of the environment and the object properties, it allows blind people to acquire a certain autonomy in their everyday life. Therefore, it is considered as a powerful and effective tool. On the other hand, the dominance of vision seems so complete in normally sighted people that touch appears as a minor sense which usefulness is restricted to some particular domains, such as the control of posture based on corrective insoles or the grasping of objects in order to move or transform them.
The processes at work in the tactile perception have similarly been subjected to diverging evaluations. In the 1960's and 70's, following the work of E. J. Gibson (1969) and of J. J. Gibson (1966), researchers tended to concentrate on those aspects common to all the perceptual modalities, and in particular common to touch and vision. By contrast, more recent studies have emphasized the specific character of this perceptual system. Moreover, progress made in cerebral imagery methods and in neuropsychology today permits a finer approach to how this modality functions and how it is integrated with other sources of information. Some reviews of the works in this field of haptic perception had already been published in the 80's and at the beginning of the 90's (Hatwell 1986; Heller & Schiff 1991; Millar 1994; Streri 1993). But knowledge evolves rapidly and it is our intention to summarize what is currently known about the tactile perceptual processes and the relations that this sense has with the other senses, particularly with vision.
Touch differs from vision and hearing in that it depends on contact and that its receptors are spread over the whole body. This quality of proximal reception has consequences which are at the origin of almost all the problems discussed in the present study. Indeed, because of this property, the tactile perceptual field is limited to the zone of contact with objects. In this passive tactile perception (cutaneous perception) where stimulation is applied to an immobile segment of the body, the perceptual field is drastically reduced and has the exact dimensions of the surface of the skin in contact with the stimulus. While certain discriminations are still possible in this situation, tactile perceptual capacity is limited because of the lack of any exploratory movement (Katz 1925/1989; Gibson 1962,1966; Revesz 1950).
In most cases, in order to apprehend the whole object, voluntary movements must be made in order to compensate for the smallness of the tactile perceptual field. The size of this field thus varies according to the body parts which are mobilized (a finger, the whole hand, both hands associated to movements of the arms, etc.). The kinesthetic perceptions resulting from these movements are necessarily linked to the purely cutaneous perceptions generated by skin contact, and they form an indissociable whole labeled "haptic" (or tactilo-kinesthetic, or active touch) perception. As a result, object perception is initially incomplete, it may lack coherence and it is highly sequential. This latter property increases the load on working memory and requires, at the end of exploration, a mental integration and synthesis in order to obtain a unified representation of the object (Revesz 1950).
The successive nature of touch has long struck observers and has been opposed to the simultaneous nature of vision. This opposition should however be qualified. Although it is true that the vast visual perceptual field allows a global and immediate apprehension of many aspects of the stimulus, exploratory movements are still necessary in vision to identify the invariants which specify the spatial properties of the environment. However, the magnitude of these ocular and head movements is consistently lower than that of manual movements, especially when hand movements are associated to those of the arms. It seems quite justifiable, therefore, to consider touch as being much more sequential than vision.
This does not mean that touch functioning is more similar to audition than to vision. Audition is specialized in the perception of successive information and it is therefore the most efficient modality for the perception of temporal stimuli (duration, rhythms, speech, etc.), whereas vision excels in space perception. But, although touch is highly sequential, it is nevertheless a spatial modality because it does not explore in a linear way and in an imposed order. In audition, the order of the sequence of stimuli cannot be changed since it carries meaning (in speech, music, etc.). By contrast, touch can explore the stimulus in any order and it can contact several times the same part of the object or set of objects, in the same way as the eyes explore a wide scene or a large picture. Therefore, touch provides information about the spatial properties of the environment and it is largely redundant with vision since it allows the perception of physical and spatial properties (texture, localization, direction, distance, shape, size, etc.). However, the quality of the tactile percept depends both on exploratory movements and the mental synthesis achieved at the end of the perceptual process.
Owing to the central role that these movements play, the regions which are most mobile and best equipped with sensory receptors are also the most effective in the tactile domain. These are the region around and inside the mouth, which is much used by infants because of their motor immaturity, and the hands (or more exactly the arm-hand system). In adults, the latter constitutes, from a cognitive point of view, the real haptic perceptual system. The present study is therefore concerned with manual haptic perception.
But the hands are also, and perhaps especially, the motor organs used to in reaching, holding, transporting and transforming objects in our everyday life. More than in all the other modalities, perception and action are closely linked in the haptic modality. That is why the relation between perception and action is particularly important in the haptic mode.
The present study is concerned both with the development of tactile perception in infants and children and with the functioning of this system in adults. Data taken from pathology and especially from totally blind persons will also be extensively discussed. In the five sections composing this book, specialized authors review the recent studies in their field of research. The questions treated in these sections are presented below.
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