Análisis y discusión

Monkeys with an ablation of the insular cortex have been shown to be impaired on tactile discrimination tasks (other than size or roughness - Murray et al, 1980) which implies that the insula does play an integral part in the tactile recognition and memory processing pathway although some of its functions remain elusive.

3.4.4 Area 5

The defects produced in monkeys by ablation of the entire posterior parietal cortex (areas 5 and 7) include the impairment of visuo-spatial orientation. Monkeys were deficient in route finding tasks (Bates and Ettlinger, 1960; Petrides and Iverson, 1979; Sugishita et al, 1978), showed a disorder in reaching for objects (e.g. Hart je and Ettlinger, 1973) and misdirected manual searching in the dark (Ratcliff et al, 1977), Pohl (1973) and Ungerleider and Brody (1977) found that parietal lesions disturbed the monkey's ability to perceive spatial relations between objects and Milner et al

(1977) suggest that sensori-motor coordination rather than simply spatial perception is disturbed. Although these studies were not directed to particular cytoarchitectonie areas within the parietal cortex, many of the deficits described related to the sensori-motor functions of area 5 suggested from electrophysiological studies. The specific ablation of area 5 by Peele (1944) and the cooling techniques adopted by Stein (1976, 1978) indicate that area 5 contains mechanisms necessary for the somaesthetic control of reaching.

Somaesthetic discrimination deficits may also be produced by posterior parietal lesions, e.g. difficulties in roughness discrimination (Murray and Mishkin, 1984; Semmes and Turner, 1977; Stein, 1978). Separate ablation of area 5 did not impair tactile discrimination (Murray and Mishkin, 1984; Semmes and Turner, 1977) so although somaesthetic deficits may be produced by lesions in area 5 this area does not appear to play an exclusive role in somatosensory discrimination (Murray and Mishkin, 1984; Semmes and Turner, 1977). This function has already been suggested for SII in the first pathway.

3.4.5 Area 7

The effects of general posterior parietal cortex lesions have been discussed earlier but it is helpful to re-emphasize some results of these studies. With lesions of the posterior parietal cortex sensorimotor coordination is disturbed (Milner et al, 1977) and deficits are found in visually guided somatomotor tasks (Petrides and Iverson, 1979) and in visual search (Latto, 1978). After lesions

limited to the cytoarchitectural region of area 7 Peele (1944) V ? observed that the monkey's contralateral hand groped for targets and

was ataxic. Ablation of area 7b resulted in an impairment in both tactile and visual placing and grasping (Fleming and Crosby, 1955). Stein (1978) observed slowing of eye movements and the neglect of the contralateral visual half-field with the cooling of area 7 and concluded that area 7 contains mechanisms for the visual control of reaching accuracy in contrast to somaesthetic control in area 5. Ablation studies in the monkey suggest that area 7 participates in the localization of stimuli in different sensory systems and the guidance of movements with the aid of visual and somaesthetic stimuli (Leinonen, 1981). The intersensory associations required for such behaviours are revealed at the cellular level.

3.4.6 STS

These deficits, resulting from the ablation of certain areas of somatosensory neocortex, help us to see where these areas of the brain

fit into a general schema of tactile processing for recognition, i discrimination, learning and memory. An obvious question is,

therefore: Do selective impairments of the STS go anyway towards revealing a tactual function for this area? The current literature cannot adequately answer this question as specific tactile tasks have not readily been incorporated into STS ablation studies. No impairments have been reported on tactual to visual cross-modal matching or visual-tactile cross-modal recognition (Ettlinger and Garcha, 1980). The only positive report on any form of tactile

deficit is given by Luh et al (1986) who report severe trimodal neglect with a bilateral STS lesion. It is perhaps a tautological argument to infer that physiological evidence of single neurone function in the STS may be necessary for 'appropriate' tasks to be designed on which impairments could be reflected. What seems clear is the need for further study on all aspects of tactile processing within the STS.

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