Tactile acuity of fingertips and hand representation size in human Area 3b and Area 1 of the primary somatosensory cortex.

Intracortical mapping in monkeys revealed a full body map in all four cytoarchitectonic subdivisions of the contralateral primary somatosensory cortex (S1), as well as positive associations between spatio-tactile acuity performance of the fingers and their representation field size especially within cytoarchitectonic Area 3b and Area 1. Previous non-invasive investigations on these associations in humans assumed a monotonous decrease of representation field size from index finger to little finger although the field sizes are known to change in response to training or in disease. Recent developments improved noninvasive functional mapping of S1 by a) adding a cognitive task during repetitive stimulation to decrease habituation to the stimuli, b) smaller voxel size of fMRI-sequences, c) surface-based analysis accounting for cortical curvature, and d) increase of spatial specificity for fMRI data analysis by avoidance of smoothing, partial volume effects, and pial vein signals. We here applied repetitive pneumatic stimulation of digit 1 (D1; thumb) and digit 5 (D5; little finger) on both hands to investigate finger/hand representation maps in the complete S1, but also in cytoarchitectonic Areas 1, 2, 3a, and 3b separately, in 21 healthy volunteers using 3T fMRI. The distances between activation maxima of D1 and D5 were evaluated by two independent raters, blinded for performance parameters. The fingertip representations showed a somatotopy and were localized in the transition region between the crown and the anterior wall of the post central gyrus agreeing with Area 1 and 3b. Participants were comprehensively tested for tactile performance using von Freyhair filaments to determine cutaneous sensory thresholds (CST) as well as grating orientation thresholds (GOT) and two-point resolution (TPD) for spatio-tactile acuity testing. Motor performance was evaluated with pinch grip performance (Roeder test). We found bilateral associations of D1-D5 distance for GOT thresholds and partially also for TPD in Area 3b and in Area 1, but not if using the complete S1 mask. In conclusion, we here demonstrate that 3T fMRI is capable to map associations between spatio-tactile acuity and the fingertip representation in Area 3b and Area 1 in healthy participants.

Investigations on maladaptive plasticity in the sensorimotor cortex of unilateral upper limb CRPS I patients.

BACKGROUND: Patients with a complex regional pain syndrome (CRPS) in the upper limb show a sensory and motor impairment of the hand. Decreased intra-cortical-inhibition (ICI) of the motor representation of the affected hand muscle and decreased somatosensory hand representation size were related to maladaptive plasticity. OBJECTIVE: To achieve new insights about CRPS we examined whether these alterations were present in a single cohort. METHODS: We used a multi-modal approach comprising behavioral testing, transcranial magnetic stimulation, and high resolution fMRI combined with a new analysis technique for improved neuronal specificity. RESULTS: We found a decreased pinch-grip performance, two-point discrimination on the fingertips, ICI in the motor cortex, and representation size of the hand in Brodmann Area 3b (BA3b) in the somatosensory cortex. Our analysis further showed that correlations with ICI on the non-affected side were absent on the affected side. CONCLUSIONS: This study is the first to gather behavioral, neurophysiologic and imaging measurements for one patient cohort and it therefore enables a comprehensive view of collapsed associations of function and representation focused on the hemisphere contralateral to the affected hand.

Usage of the middle finger shapes reorganization of the primary somatosensory cortex in patients with index finger amputation.

PURPOSE: The primary somatosensory cortex (S1) is somatotopically reorganized after limb amputation. The duration of the amputation, the intensity of phantom limb pain but also a multifactoral model of altered cerebral input have been discussed to be associated with cortical changes. Patients with finger amputation rarely show phantom limb pain, the deafferented cortical area is small but other fingers might well overtake function. METHOD: We selected a group of index finger amputated patients and performed a high resolution (in plane: 1.5 mm2) S1-mapping during tactile stimulation of finger tips. RESULT: We found an interhemispheric imbalance of the distance between the thumb and middle finger only for the patient-group. When patients used their middle finger more they showed less interhemispheric imbalance, increased spatial tactile discrimination and increased fMRI-activation in response to stimulation. Phantom limb pain was not associated with somatotopic representation parameters in S1. CONCLUSIONS: Overall, our fMRI-data point to a usage dependent plasticity of Brodmann's area 3b in man.