The role of human somatosensory cortex in tactile stimulus processing: fMRI responses to microstimulation of individual tactile afferents

Mounting evidence shows that primary somatosensory cortex, particularly SI, is actively engaged in dynamic processing of sensory information evoked by skin stimulation. SI plays both active and modulating roles in amplitude and frequency discrimination as well as in the processing of noxious mechanical and thermal stimuli. SI is one cortical region in which interactions between cortical activity evoked by low frequency flutter and high frequency vibration are spatially and temporally integrated. In this dissertation, functional magnetic resonance imaging was used to image cortical responses to microstimulation of individual, functionally identified skin mechanoreceptive afferents which are, under natural stimulus conditions, effectively activated by very selective ranges of stimulation frequency. Microstimulation of mechanoreceptive and nociceptive afferents evoked significant activation in multiple cortical areas. Peak magnitudes of hemodynamic responses in SI and SII did not differ significantly when rapidly adapting afferent microstimulation frequency was increased from 30 to 200Hz. Thus, rapidly adapting afferent activation alone cannot account for the suppression of SI cortical activity that is observed when high frequency vibrotactile stimuli are applied to the skin; this leads to the hypothesis that PC microstimulation causes suppression of SI cortical activity regardless of stimulus frequency. Images of activity evoked by microstimulation of PC afferents support this hypothesis. Additionally, microstimulation of nociceptive afferents evoked significant activation in multiple cortical areas consistent with animal studies utilizing invasive data collection methods. Future studies of hemodynamic responses to vibrotactile stimulation and noxious stimulation will clarify human cortical mechanisms responsible for the spatial-temporal integration evoked by different modalities of skin stimulation. The construction and utilization of an MRI-compatible vibrotactile stimulator that can be used in such future studies is also described in this manuscript.