'SII/PV' or 'SIIp/SIIa'
Originally defined chronologically by Adrina in 1941 in the cat (well, if this area was SI, then this one has to be SII), then found to be present in other species by Woolsey (Woolsey and Fairman, 1946). Both were electrophysiological studies (albeit rather crude ones). Later anatomical studies (mainly injections into SI) were split between a single map (Friedman et al., 1980) or multiple maps (Manzoni et al., 1986 - although their large injections led many to believe that the multiple regions labeled could be due to spread of tracer past SI.)
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The two most recent/relevant papers to focus on the lateral sulcal region
in macaques have been Krubitzer et al., 1995 and Burton et al., 1995. Both
claim a redefinition of the cortical fields/areas contained within the
lateral sulcus.
The Burton et al. paper primarily relies on tract tracing (standard techniques) with injections in 3B/1. The authors used electrophysiology to define injection sites in SI -- not in 'SII'. Concluded with an 'alternative' scheme showing two separate representations (SIIa and SIIp) of the body surface where previous studies had assumed only one, covering an A-P distance of roughly 7mm. They found no reliable cytoarchitectural distinctions to separate the two maps -- however, this is not the same as saying that one does not exist! The Krubitzer et al paper used electrophysiological (multiunit) mapping and reconstructions of electrode penetrations, using both Nissl (cell-body) and Myelin (white matter tract) stains. Found evidence for two separate representations in the lateral sulcus, similar to studies performed in other species. They named these fields SII and PV (Parietal Ventral). Combined maps cover roughly 15mm2 in Macaques. Function:
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B: The somatotopic organization of the second somatosensory area (SII) and the parietal ventral area (PV) on the upper bank and parietal operculum of the lateral sulcus of the macaque monkey, as described by Krubitzer et al., [1995]. The lateralmost portion of the sulcus is adjacent to anterior parietal fields 3b, 1, and 2; and the medialmost portion is adjacent to the insula. In these regions, the face is represented most laterally (green) followed by the representation of the hand (blue) and the foot (red) more medially. The representations of distal body parts in SII and PV are adjacent to one another, whereas the proximal portions of the body, such as the shoulder (purple) and hip (yellow), are represented rostrally in SII and caudally in PV. These fields have mirror-symmetric representations of the body surface. Ri, retroinsular area; VS, ventral somatosensory area; 1, 2, and 3b, somatosensory areas of the anterior parietal cortex; 7b, somatosensory area of the lateral sulcus. In A and B, rostral is to the left. In the magnetic resonance image (MRI; top right), rostral is to the bottom, and lateral is to the right and left of the midsagittal sulcus. (Disbrow et al., 2000 from Krubitzer et al., 1995) |
* Larger, Bilateral receptive fields (Whitsel et al., 1969; Robinson and Burton, 1980a/b).
* Responses modulated by attention (Hsiao et al., 1993; Burton et al., 1997).
* An involvement in texture/roughness coding (Sinclair and Burton, 1993; Jiang et al., 1997; Pruett et al., 2000).
* Tactile Learning/Memory/'Higher Tactile Function'/Simultaneous spatial encoding of position/Pain/Furious Handwaving (everyone else)
More is known about the connectivity of these cortical areas. There have been several studies of the connections from anterior parietal areas (i.e. SI) to the SII region (e.g., Friedman et al.; Pons and Kaas, 1986; Cusick et al., 1989; Krubitzer and Kaas, 1990; Burton et al., 1995). However, as with the single unit studies listed above, most of these were carried out before the complexity of this region was fully appreciated. Only a few studies have directly examined the connections of SII in primate (Friedman et al., 1986; Krubitzer and Kaas, 1990; Krubitzer and Kaas, 1992; Disbrow et al., 1998).
Disbrow et al., 1998: SII is densely interconnected with anterior parietal cortex (mainly 3b, some 1). Of the other lateral sulcal areas, SII projects to PV and 7b.
Krubitzer and Kaas, 1990; Disbrow et al, 1998: Less known about PV. Seems to have greater connectivity with 'motor areas' i.e. premotor, and perhaps also BA5. Also projects to a region rostral to PV (identity unknown).
Lastly, while the exact nature of the connections are still contentious, SII receives direct thalamic projections.
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There is evidence for the existence of somesthetic regions in the parietal operculum of humans (cortex superior to the lateral sulcus). However, again, few studies have attempted to treat this area as consisting of separate, possibly functionally distinct, areas. The first metabolic-based mapping (PET) of SII was carried out by Burton and collegaues in 1993 using vibrotactile stimulation. They located SII in the region of the parietal operculum on the upper bank of the sylvian fissure. Similar imaging studies demonstrate that an area of the parietal operculum (SII/PV/?) is active:
i) 'simple' somatosensory stimulation (Coghill et al, 1994; Burton et al., 1997a). ii) sophisticated somatosensory tasks including micro and macrogeometric discrimination (Ledberg et al, 1995; Roland et al, 1998), tactile attention (Burton et al, 1999; Mima et al, 1998) and tactile memory (Bonda et al., 1996). iii) sensorimotor integration (Huttunen et al., 1996). iv) human bimanual coordination (Simoes and Hari 1999; Disbrow et al, 2001)
One of the few studies to attempt to extrapolate from non-human primate work (Disbrow et al., 2000) suggests evidence for SII/PV in humans, i.e. the results are from mapping studies in both species - fMRI in humans during simple somatosensory stimulation.
To right: Summary of the somatotopic organization of SII and PV in the human (bottom) and macaque monkey (top). The schematic drawings of SII and PV have been superimposed on three-dimensional steady precession gradient-recalled images. The hand representation is shown in blue, the foot is in red, the face is in green, the shoulder is in purple, and the hip is in yellow. These digital images demonstrate that, in both humans and nonhuman primates, homologous cortical fields are present. (Disbrow, et. al., 2000) |
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Less obvious than in SI. The structure of the somatosensory areas of the lateral sulcus suggest at least two separate mappings of the body surface in a number of species, including man. The functional significance of these two different fields is more contentious, partly due to the reasonably recent nature of their discovery. Few would dispute their role in 'higher tactile functions' - even fewer would actually give you a good definition of this term.
