Disorders in haptic perception deprive patients of a powerful instrument providing knowledge of the environment, for they prevent the identification of manipulated objects in the absence of vision. This perceptual modality can present different types of perturbation. Already at the end of the 19th century, Wernike (1895) distinguished between disturbances in tactile recognition of the characteristics of objects from those related to their integration. Delay (1935) distinguished amorphognosia and ahylognosia (the deficits of recognition of the size and shape of objects or of certain of their properties such as weight and material), from tactile asymbolia (or tactile agnosia) which is a disorder in the identification of objects in the absence of the two preceding conditions.
Tactile agnosia must however be distinguished from tactile aphasia (or anomia) which concerns a disorder in the denomination of perceived objects (Geschwind 1965). Tactile aphasia is generally observed in patients having undergone a surgical or accidental sectioning of the callous body ("split-brain" patients), and concerns primarily the left hand; only a few examples of deficits involving both hands have been identified (Beauvois, Saillant, Meininger, & Lhermitte 1978; Endo, Miyasaka, Makishita, Yanagisawa, & Sugishita 1992). According to the generally accepted interpretation, this disorder in the denomination of objects palpated in the left hand is due to the hemispheric disconnection which prevents the relation between the representation of the tactile object and the corresponding name being made. And indeed, the cortical areas involved in the construction of the spatial representation of the objects palpated in the left-hand are situated in the right hemisphere which, subsequent to the sectioning of the callous body, is no longer in contact with the zones of language situated in the left hemisphere. The results of the study by Baynes, Tramo, Reeves and Gazzaniga (1997) - which stressed the representational capacity of the right hemisphere in split-brain patients - support this proposition. During subsequent investigation of the patient described by Baynes et al. (1997), Badan and Caramazza (1997) showed that in the presence of a material which is more difficult to identify than common objects (letters and shapes with or without signification cut out of glass-paper), the patient also presented disturbances of haptic recognition in the right hand. This result underlines the limits of the left hemisphere concerning the processing of haptic information and stresses the necessity for a contribution from the right hemisphere in the construction of haptic spatial representation.
Finally, as a disorder of haptic spatial perception, tactile agnosia should be differentiated from astereoagnosia, for the latter seems to present in addition a deficit of somaesthetic perception. According to Caselli (1997), astereoagnosia results from a lesion of the sensory system, going from the peripheral nerves to the primary somaesthetic area (S1), via the median lemniscus and the thala-
mus. The restrictions imposed by these diverse distinctions pose the question of the reality of tactile agnosia and of its interpretation, for in the majority of clinical observations there are disturbances, sometimes only slight, of somaesthetic sensitivity (Bauer 1993; Caselli 1991, 1997; Endo et al. 1992; Nakamura, Endo, Sumida, & Hasegawa 1998; Platz 1996).
On the basis of a detailed analysis of the literature and of the study of two patients presenting troubles of haptic perception, Endo et al. (1992) concluded that tactile agnosia and tactile aphasia existed independently. According to their model, in the case of tactile agnosia, information on the characteristics (shape, material) analyzed by the short-term memory could not reach the semantic memory, while in the case of tactile aphasia, the contents of the semantic memory could not be put into contact with the lexical memory. The authors retain the idea that tactile aphasia results from a tactile-verbal disconnection, and they show that in their patient, tactile agnosia is the consequence of a sub-cortical lesion including the left angular gyrus, disconnecting the associative sensory areas and the areas of the inferior temporal lobe involved in semantic memory (Endo et al. 1992). One of the authors and his collaborators later described a patient with bilateral tactile agnosia due to a bilateral sub-cortical lesion in the region of the angular gyrus (Nakamura et al. 1998).
Another interpretation is proposed by Platz (1996), who studied a patient suffering from right parietal meningioma and presenting the indications of tactile agnosia despite having conserved his basic motor and sensory ability. The author refutes the idea that perception and recognition constitute successive stages of the processing and memorization of information. On the contrary, he interprets the difficulties of the patient as being due to the disconnection of the different systems of analysis of object characterization. These systems are activated in parallel, but they become disconnected and so prevent the constitution of a functional unit (including characteristics and entity), which is essential to the haptic modality.
In a recent study, Valenza et al. (2001) presented a patient with a right hemisphere infarction who showed disorders of tactile recognition of the left hand due to impaired Lederman and Klatzky's (1987) exploratory procedures (cf. Chapter 5) occurring without tactile agnosia. Functional MRI during sensory stimulation of the left hand showed preserved activation of the spared primary sensory cortex in the right hemisphere. This case of pure tactile apraxia was interpreted as a specific inability to use tactile feedback to generate the exploratory procedures necessary for tactile shape recognition.
Other studies found perturbations in the visual and haptic perception of spatial orientations in the fronto-parallel plane of patients with left visuo-
spatial neglect (VSN). Thus, Kerkhoff (1999) observed lower global performance (independently of the orientation value) in neglects patients. Gentaz, Badan, Luyat and Touil (2002) found also a haptic orientation deficit in the fronto-parallel plane in neglect patients. The precision of response was lower in the neglect patients than in the control (young adults and seniors) subjects. However, the same haptic oblique effect (lower performances in oblique orientations than in vertical-horizontal orientations, cf. Chapter 8) was observed in the three groups. These results were similar in the left and right hemiespaces of the participants, even in the neglect patients. Taken together, this means that, in spite of the global haptic orientation deficit, no specific pattern is observed in the haptic perception of different orientations in these VSN patients as compared to the two other groups. The haptic orientation deficit of VSN patients seems to affect in the same way all values and spatial positions of orientations.
1.3 The role of cerebral hemispheres in the control of cutaneous and haptic perceptions
The question of the neuro-anatomical substrate of cutaneous and haptic perceptions must be considered at the level of inter- and intra-hemispheric control of these perceptual modalities, always remembering the role of the corpus callosum in the transfer and attentional control of tactile information (Gaz-zaniga 1970; Mayer, Koenig, & Panchaud 1988). Contrary to the situation in other functions (such as language which is mainly represented in the left hemisphere), the existence of the functional lateralization of tactile perception is the subject of considerable debate in the literature (see Chapters 6 and 13). It should be noted that the somaesthetic and motor areas are distributed in an almost equivalent manner in the two hemispheres. But let us remember that the organization is crossed, for each hemisphere essentially controls the information relating to the contralateral half of the body. The role devolved to S1 and S2 is different, and recent papers stress the importance of lesions in region S2, in particular the parietal operculum and the posterior part of the insula, in the generation of tactile agnosia (Schnider 1997; Vallar 1997). One can, then, consider that the intra-hemispheric distribution of lesions engendering tactile perception pathology reveals the preponderant role of S1 in the region of the postcentral gyrus, and of S2 in the region of the inferior parietal lobule, the activation of which may be at least partly serial (Bassetti, Bogous-slavsky, & Regli 1993; Caselli 1997; Corkin 1970; Critchley 1953; Hécaen & Albert 1978; Knecht, Kunesch, & Schnitzler 1996; Pause, Kunesch, Binkofsky, & Freund 1989; Schnider 1997; Vallar 1997).
Paradoxically, and contrary to the predictions based on the almost identical distribution of sensory and motor areas in the two hemispheres (noted above), the consequences of left and right cerebral lesions give rise to an asymmetry which is a determining factor in the incidence of somaesthetic and motor disorders. For example, Sterzi et al. (1993) showed that disturbances in the sense of position are more frequent following a right lesion (37%) than a left lesion (25%). Others symptoms, such as the loss of sensitivity to pain, motor deficits and visual-spatial deficits, also present a certain degree of asymmetry. Thus, the higher frequency of symptoms resulting from right lesions indicates the dominance of the right hemisphere in tactile perception, in the broad sense of the term. This dominance is compatible with the idea, exposed by several authors, of a right hemispheric advantage in the recognition of objects by manual exploration (Bottini, Cappa, & Vignolo 1991; Franco & Sperry 1977; Ledoux, Wilson, & Gazzaniga 1977; Milner & Taylor 1972). Franco and Sperry (1977), for example, asked patients with a partial (anterior) or complete section of the corpus callosum (preventing information transfer from one hemisphere to the other) to distinguish geometrical shapes palpated with the right and left hand. They found that the left hand - right hemisphere had an advantage in this type of task. To interpret the divergences, it is possible to consider that the processing of haptic information brings about the intervention of high-level processes, which are represented more in the right hemisphere and, as suggested above, are spatial or attentional in nature (Bauer 1993; Sterzi et al. 1993; Vallar 1997).
This brief consideration of the consequences of cerebral lesions on the recognition of objects and of the body itself shows that the processes operating at several levels of processing are involved in tactile perception. Certain aspects necessary for the construction of spatial representations are undertaken by high-level mechanisms with asymmetrical hemispheric distribution.
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