Christian Grefkes

Human Somatosensory Area 2: Observer-Independent Cytoarchitectonic Mapping, Interindividual Variability, and Population Map

We analyzed the topographical variability of human somatosensory area 2 in 10 postmortem brains. The brains were serially sectioned at 20 microm, and sections were stained for cell bodies. Area 2 was delineated with an observer-independent technique based on significant differences in the laminar densities of cell bodies. The sections were corrected with an MR scan of the same brain obtained before histological processing. Each brains histological volume and representation of area 2 was subsequently reconstructed in 3-D. We found that the borders of area 2 are topographically variable. The rostral border lies between the convexity of the postcentral gyrus and some millimeters deep in the rostral wall of the postcentral sulcus. The caudal border lies between the fundus of the postcentral sulcus and some millimeters above it in the rostral wall. In contrast to Brodmanns map, area 2 does not extend onto the mesial cortical surface or into the intraparietal sulcus. When the postcentral sulcus is interrupted by a gyral bridge, area 2 crosses this bridge and is not separated into two segments. After cytoarchitectonic analysis, the histological volumes were warped to the reference brain of a computerized atlas and superimposed. A population map was generated in 3-D space, which describes how many brains have a representation of area 2 in a particular voxel. This microstructurally defined population map can be used to demonstrate activations of area 2 in functional imaging studies and therefore help to further understand the role of area 2 in somatosensory processing.

Crossmodal Processing of Object Features in Human Anterior Intraparietal Cortex: An fMRI Study Implies Equivalencies between Humans and Monkeys

The organization of macaque posterior parietal cortex (PPC) reflects its functional specialization in integrating polymodal sensory information for object recognition and manipulation. Neuropsychological and recent human imaging studies imply equivalencies between human and macaque PPC, and in particular, the cortex buried in the intraparietal sulcus (IPS). Using functional MRI, we tested the hypothesis that an area in human anterior intraparietal cortex is activated when healthy subjects perform a crossmodal visuo-tactile delayed matching-to-sample task with objects. Tactile or visual object presentation (encoding and recognition) both significantly activated anterior intraparietal cortex. As hypothesized, neural activity in this area was further enhanced when subjects transferred object information between modalities (crossmodal matching). Based on both the observed functional properties and the anatomical location, we suggest that this area in anterior IPS is the human equivalent of macaque area AIP.

Hierarchical processing of tactile shape in the human brain.

It is not known exactly which cortical areas compute somatosensory representations of shape. This was investigated using positron emission tomography and cytoarchitectonic mapping. Volunteers discriminated shapes by passive or active touch, brush velocity, edge length, curvature, and roughness. Discrimination of shape by active touch, as opposed to passive touch, activated the right anterior lobe of cerebellum only. Areas 3b and 1 were activated by all stimuli. Area 2 was activated with preference for surface curvature changes and shape stimuli. The anterior part of the supramarginal gyrus (ASM) and the cortex lining the intraparietal sulcus (IPA) were activated by active and passive shape discrimination, but not by other mechanical stimuli. We suggest, based on these findings, that somatosensory representations of shape are computed by areas 3b, 1, 2, IPA, and ASM in this hierarchical fashion.

Basic Principles of rTMS in Motor Recovery After Stroke

Repetitive transcranial magnetic stimulation (rTMS) can be used to promote recovery of motor function after stroke. We are only beginning to understand the neural underpinnings of stimulation after-effects on motor function. In this chapter, we summarize scientific evidence that motivates the rationale behind the two major rTMS approaches used in the rehabilitation of stroke patients. Finally, we present promising novel developments and future prospects that might help to pave the way to clinical applications of rTMS in stroke.