(a) SI
Classically SI was described as a single and largely continuous representation of the body surface spanning the four distinct architectonic regions 3a, 3b, 1 and 2 anteriorly to posteriorly (Jones and Powell, 1969, 1970 a,b; Powell and Mountcastle, 1959 a,b; Woolsey, 1958). More recent studies have revealed two complete representations of the superficial body surface and structures within SI, coincident with cytoarchitectonie areas 3b and 1 (Kaas et al, 1979, 1981; Merzenich et al, 1978, 1981; Paul et al, 1975) and two representations of deeper body tissues in areas 3a and 2. Iwamura et al (1981) dispute an independent representation of body parts in the posterior part of SI (area 2) and instead propose that this region associates or integrates complex somatosensory information from several areas across several submodalities. Without further clarifying these independent body maps it is difficult to establish which view is correct. Merzenich et al (1981), however, did stress that these body representations could not be accurately depicted by a continuous body figure or 'homunculus'.
The receptive field size and the complexity of the functional properties of the neurones has been observed to increase posteriorly, through the cytoarchitectural subdivisions of the postcentral gyrus, from area 3a to 3b, to areas 1 and 2 (Hyvarinen and Poranen, 1978b; Iwamura and Tanaka, 1978 a,b,c; Iwamura et al, 1985 a,b). Hyvarinen and Poranen (1978b) categorized the receptive fields in SI into three groups: Small (one finger or one palmar whorl on the skin), medium (2-3 fingers on one hand, or an area of half the palm) and large (several fingers, the whole palm or an even larger area). They observed that the percentage of small receptive fields decreased and the large receptive fields increased progressively from area 3b to areas 1 and 2.
In parallel with this increase in receptive field size, the convergence onto single neurones of information from different somaesthetic submodalities is more common in the posterior part of the
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gyrus (Hyvarinen and Poranen, 1978 a,b). In area 3a most neurones | were activated by joint or muscle manipulation, while in area 3b the
majority were driven by light contact to the skin and hairs (Hyvarinen
and Poranen, 1978 a,b; Iwamura et al, 1981; Powell and Mountcastle,
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1959 a,b). In areas 1 and 2 neurones exhibited submodality I
convergence such that skin and hair or skin and joint manipulation | units were found. The occurrence of movement sensitive and direction |
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and orientation-selective (complex) cutaneous neurones increased from anterior to posterior within the postcentral gyrus; with the majority of these neurones being found in area 2 (Constanzo and Gardner, 1980; Hyvarinen, 1982; Hyvarinen and Poranen, 1978 a,b; Iwamura et al, 1981; Mountcastle et al, 1969; Schwarz and Fredrickson, 1971; Whitsel et al.
1972). A number of units were activated only at the moment of voluntary movements or by specific modes of stimuli and these were also more commonly found in area 2 (Iwamura et al, 1981; Sakata and
Iwamura, 1978).
Receptive field integration in SI (more specifically areas 1 and 2) is thought to be useful for discriminating different types of contact of the body to the surroundings (Sakata and Iwamura, 1978). As such it would provide essential information for discriminating between objects of differing size, texture or weight in the tactile modality. The convergence of different submodalities (Iwamura et al, 1981; Sakata and Iwamura, 1978) and the prevalence of neurones in area 2 sensitive to the edges of objects passing across cutaneous receptive fields (Iwamura et al, 1982) indicate the presence in SI of systematic mechanisms for feature extraction and the discrimination of objects grasped by the hand.
The changes that take place in the functional properties of cells when moving across these different cytoarchitectural areas in SI suggest a degree of intracortical information processing (coding larger body regions and more complex combinations of information) within the posterior part of the postcentral gyrus (Hyvarinen and Poranen, 1978 a,b).
(b) SII
The next stage in this cortico-limbic tactile pathway is the second somatosensory cortex (SII) which occupies the superior bank of the sylvian fissure, posterior to the level of the central sulcus. The
topographic organization of SII has been extensively examined and a representation of the whole body surface has been mapped (Burton and Robinson, 1981; Friedman, 1981; Juliano et al, 1983; Whitsel et al, 1969); this representation is limited to the cutaneous body surface (Whitsel et al, 1969). Burton and Robinson (1981) illustrate clearly the somatotopic organization of SII. Briefly, the map of the body is organized in anterolateral to posteromedial slanted strips that align in an anterior to posterior sequence across the parietal operculum (Burton and Robinson, 1981; Friedman, 1981; Whitsel et al, 1969) with the face, jaw, neck, chest, preaxial forelimbs and palmar aspects of the hands represented sequentially in a medial to lateral direction (Whitsel et al, 1969).
A clear delineation of SII into two portions has been reported (Burton and Jones, 1976; Poranen and Hyvarinen, 1982; Whitsel et al, 1969). These regions have been described as: (1) a caudal portion extending several millimetres posterior to the interaural plane, and (2) a rostral portion (termed Sllr) extending anteriorly to the level of the central sulcus (Whitsel et al, 1969). The receptive fields and sensory properties of the neurones in these two anatomically segregated regions of SII are not identical. The caudal portion of SII consists of neurones with wide, cutaneous, often discontinuous and asymmetrical receptive fields (Burton and Jones, 1976; Burton and Robinson, 1981; Poranen and Hyvarinen, 1982; Whitsel et al, 1969). Notably, a large number of contralateral receptive fields were observed in this region of SII (Burton and Robinson, 1981; Poranen and Hyvarinen, 1982).
In contrast, in the rostral (or anterior) portion of SII two types of receptive fields were encountered: Large bilateral receptive fields continuing across the body midline; and smaller, disjointed receptive fields positioned toward the apices of the extremities (Burton and Jones, 1976; Burton and Robinson, 1981; Poranen and Hyvarinen, 1982; Whitsel et al, 1969). The large receptive fields cover both arms, shoulders, chest and back, or the legs and the lower half of the body i.e. the upper and lower torsos (Poranen and Hyvarinen, 1982; Whitsel et al, 1969). Interestingly the body midline appears in the topographic map four times, each time in the context of a different body region (Whitsel et al, 1969).
Similarly, a dichotomy between the sensory properties of the neurones in the rostral and caudal regions of SII has been noted. The caudal (posterior) portion of SII consists of neurones sensitive at a high stimulus threshold to heavy pressing or squeezing, i.e. nociceptive mechanical stimuli (Burton and Jones, 1976; Whitsel et al, 1969). In direct contrast, neurones in the rostral (anterior) portion of SII respond to gentle tactile stimuli, i.e. the onset or offset of light cutaneous stimulation (Burton and Jones, 1976; Friedman, 1981; Poranen and Hyvarinen, 1982; Whitsel et al, 1969), with only few cells responding to deep touch. For the majority of these cells a movement of the stimulus (in any direction) across the receptive field elicited a greater response than a static stimulus (Friedman, 1981; Whitsel et al, 1969).
(c) Insular cortex
postulated as the final component in the somatosensory link to the limbic system (Murray and Mishkin, 1984; Murray et al, 1980). Little electrophysiological investigation of this area of cortex has been made; indeed Robinson and Burton (1980a) stated of their own work that their sample of neurones was too small to permit any extensive comment. However, of the few studies that have been made, all the results seem to be in accordance. There is no topographical organization of the body in the insular cortex; the receptive fields are large (upper or lower torso, or the whole body); and the majority of neurones in the insula respond to passive, innocuous (non-nociceptive) somatic stimuli, although occasionally active movement by the monkey is necessary to obtain a response (Burton and Robinson, 1981; Juliano et al, 1983; Robinson and Burton, 1980 a,b). Robinson and Burton (1980a) have reported a few cells in the granular insular cortex with multimodal properties.