'Anterior Parietal Cortex'
The first attempts at systematically mapping all of cortex due to structural criteria were carried out by Brodmann (1909/1994), Vogt and Vogt (1926), and von Economo (1929). They defined the following:
Brodmann - 3 areas - BA1, BA2, BA3
Vogt and Vogt - 3 areas - 3a, 3b, 1 + 2 (advocated sharp boundaries)
Von Economo - 4 areas - PA(3a), PB(3b), PC(1), PD(2) - this system of letters was transcribed to the rhesus monkey by Bonin and Bailey in 1947, and has survived ((to annoy us)) to this day.
Most authors now agree that there are four cytoarchitectonically distinct areas extending (approximately) from the fundus of the central sulcus to the anterior bank of the postcentral sulcus.
'SI'
Initially SI was defined functionally, using electrophysiological mapping, as a single area. (Woolsey and Fairman, 1946; Woolsey, 1958) Marshall and colleagues in 1937 noted that the four cytoarchitectonic zones defined above lay in the same area as 'S1'. For our purposes we will treat BA1/2/3a/3b as the SI complex.
In general, the SI complex records information and relays information about sensory qualities (surprise, surprise).
Initially, it was thought that the SI complex was a single are with a single systematic somatotopic representation of the body surface. However, a later hypothesis suggested that each of the four cytoarchitectonic regions contains a functionally distinct map of the periphery. Now a widely accepted theory, this has been demonstrated in Owl Monkeys (Merzenich et al., 1978 - 3b and 1); squirrel monkeys (Sur et al., 1982); macaque monkeys (Kaas et al., 1979); etc.
---insert example of somatotopy in BA3b/BA1 here---
More specific functions by area below:
|
|
Area 3a |
Area 3b |
Area 1 |
Area 2 |
|
Cytoarchitectonics |
Layer V: Pyramidal cells Layer IV: less granular than 3b |
Granular IV and III |
III, IV and Vi less dense than 3b |
IV more dense than 1 |
|
Thalamic Connections |
VPO, VPLO |
VP (VPL), VPLC |
VP (VPL) |
VP/Pa --> VPS |
|
Known Cortical Connections (BAs) |
1, 2, 3a, 5, 'SII' |
|||
|
Physiological Properties |
Muscle spindles Responses during movement Some cutaneous sensitivity |
Mainly cutaneous input from RA-I and SA afferents |
Cutaneous - RAI and RAII receptors |
Deep receptors - i.e. muscle/joints etc. Some cutaneous sensibility Direction sensitive More complex receptive fields, multidigit, etc. |
*most of the above from Kaas, 1990*
Note - In monkeys, the discovery of two distinct patterns of cytoarchitecture within area 3 prompted its division into areas 3a and 3b: 3a has an extended layer IV when compared to areas 4 (primary motor cortex) and 3b, and larger pyramidal cells in its supra - and intragranular layers (Jones et al., 1978; Jones et al. 1980). See: Area 3a: Is I Is or Is I Ain't an Area. In addition, areas 3a and 3b don't project forward to motor areas such that areas 1, 2 and 5 have to integrate/pass this info along.
Data from modern cyto/chemoarchetectonic studies suggest an overlap of structure and function in the human brain i.e. Zilles et al. 1997; Geyer et al, 1999, 2000 show that there is good agreement between receptor-based mappings (that may relate to gross functionality) and cell-based measures (modern, observer-independent versions of Brodmann et al.). Two important points come out of these studies:
i) There appears to be NO relationship between macroanatomy (i.e. sulcal and gyral patterns) and microanatomy (i.e. cyto/chemoarch. patterns)
ii) Patterns of microstructure vary across brains
This is a problem for functional neuroimaging which can only see macrostructure. The authors suggest probability-based mappings in standard anatomical co-ord. systems as a way to begin to address this problem (see Geyer et al., 1999 and 2000 for details for SI).
Is it possible to parcellate the human SI complex according to the physiological distinctions above? Or rather, have any studies applied the stimuli suggested by monkey physiology?
'Homuncular pattern' of somatopy first was found by invasive stimulation by Penfield (1937). Is there evidence for differential physiological properties i.e. kinesthetic processing in BA3a, light touch in 3b/1? (see Moore et al., 2000.)
Recently, evidence of distinct maps in the different regions of human SI has been unveiled using fMRI techniques. They found the representation pattern of area 1 to be a mirror image compared to area 3b. (Blankenberg et al., 2003)
Most species examined (from bats to flying foxes to monkeys) appear to possess an 'SI,' usually defined by cutaneous sensitivity and a complete isomorphic map of the body surface. Higher primates may have as many as four subdivisions/cytoarchitectonic zones within SI i.e. BA1/2/3a/3b. In monkeys, functional specialization parallels cytoarchitectonics. At present, no definitive conclusion has been reached on this point in humans, though there is every reason to suggest that this relationship is preserved.