Functional neuroimaging studies of human somatosensory cortex

Two studies were carried out to assess the applicability of echoplanar fMRI at 3.0 T to the analysis of somatosensory mechanisms in humans. Vibrotactile stimulation of the tips of digits two and five reliably generated significant clusters of activation in primary (SI) and secondary (SII) somatosensory cortex, area 43, the pre-central gyrus, posterior insula, posterior parietal cortex and posterior cingulate. Separation of these responses by digit in SI was possible in all subjects and the activation sites reflected the known lateral position of the representation of digit 2 relative to that of digit 5. A second study employed microneurographic techniques in which individual median-nerve mechanoreceptive afferents were isolated, physiologically characterized, and microstimulated in conjunction with fMRI. Hemodynamic responses, observed in every case, were robust, focal, and physiologically orderly. These techniques will enable more detailed studies of the representation of the body surface in human somatosensory cortex, the relationship of that organization to short-term plasticity in responses to natural tactile stimuli, and effects of stimulus patterning and unimodal/cross-modal attentional manipulations. They also present unique opportunities to investigate the basic physiology of the BOLD effect, and to optimize the operating characteristics of two important human functional neuroimaging modalities—high-field fMRI and high-resolution EEG-in an unusually specific and well-characterized neurophysiological setting.