On mapping the human somatosensory cortex : fMRI and PET imaging

Abstract: This work is based on experiments and simulations for the purpose of mapping the human somatosensory cortex. By using a combination of structural mapping (cytoarchitecture), connectivity mapping, functional mapping, and somatotopical mapping an overview of the organisation of the human somatosensory cortex at different levels is possible. Based on the structure of the cortex, cytoarchitectonic delineation Provides an unbiased localisation of brain areas. A novel technique is used to transform cytoarchitectonic data into a format compatible with functional image analysis procedures. This allows the study of standard image processing on the size and location of objectively defined areas from 10 postmortem brains. From this data, implications for interpreting neuroimaging results are made and a new map is created to more accurately locate activation results from functional neuroimaging studies. With the exact location of cortical areas in 10 postmortem brains, a method for possibly determining the connections between cortical areas in the human in vivo is investigated. Correlations of blood flow in brain areas during the rest state in humans have been reported to reflect the anatomical connectivity of cortical areas in humans. Using the objectively defined cytoarchitectonic brain areas, a correlation analysis is performed and the results compared with the known anatomical connectivity of the macaque monkey brain. Results from the pattern of correlations partially matched the pattern of connectivity in the macaque brain. Therefore, the correlations of brain regions during the rest state in humans appear to reflect something other than the anatomical connectivity. Somatotopical mapping of the human somatosensory cortex is performed. To examine the somatotopical organisation of known somatosensory areas in the postcentral gyrus, new, and putative somatosensory areas, cytoarchitectonically parcellated areas are examined for activations caused by stimulation of the hand and foot. fmRI is used to measure the BOLD response in 10 subjects whilst passively discriminating various parallelepiped objects presented to their right hand or foot. Separate representations of hand and foot activations should represent somatotopical organisation. Areas 3a, OP2 and OP3 had inconsistent representations of both the hand and foot. The following order, from somatotopy to no somatotopy, was observed: 3a, 3b, 1, 2, IP1, OP4, IP2, and OP1. The effects of attention on the cortical response to somatosensory stimulation were mapped. Attentional modulation of the cortex was investigated using a signal detection task of an air puff on the finger and a shape discrimination task of objects presented on the hand and foot. Attention was observed to enhance the cortical response to somatosensory stimulation in areas 1, 2, and IP1. For a shape discrimination task, attentional effects were isolated in areas 2, IP1, for the hand, and in area OP4 bilaterally for the foot. In this work, the human somatosensory cortex has been mapped on different levels; anatomically, connectivity, somatotopically, and functionally. By integrating information about the organization of the somatosensory cortex on various levels, it appears that the areas are organised from lower to higher cortical areas as; 3a, 3b, 1, 2, IP1, OP4, IP2, OP1.