Evoked potentials as indices of adaptation in the somatosensory system in humans: a review and prospectus.

Population-level behavior of large neural aggregates can be efficiently monitored by corresponding population-level indices such as somatosensory evoked potentials (SEPs). The literature reviewed clearly indicates that SEPs undergo systematic and often marked changes under conditions of repetitive stimulation. Similar results have been reported for several mammalian species and with a diversity of stimulation, recording and analysis protocols. The most characteristic finding is a loss of SEP component amplitude as a function of decreasing time between stimulus presentations. The effects become larger and appear at longer ISIs at higher levels of the somatosensory pathway, are more readily evoked by stimulus trains than by stimulus pairs and are most pronounced for response components generated in the upper cortical layers. These findings are consistent with a recently proposed neurophysiological model of short-term plasticity in somatosensory cortex, which incorporates detailed and current information on cortical microcircuitry, receptor and neurotransmitter characteristics, topographical organization and dynamic response to repetitive sensory drive. Recommendations are provided for further research, emphasizing the potential of frequency-domain analysis methods in conjunction with mechanical vibrotactile stimuli as a vehicle for more detailed testing of the proposed neurophysiological model and for closer integration with psychophysical studies of vibrotactile adaptation.