Quantitative characteristics of the cortical neuronal population response to vibrotactile stimulation in man: Effects of NMDA receptor block on properties of the driving response

Stimulus conditions that produce the psychophysical phenomenon of improved frequency discrimination with a repetitive vibrotactile stimulus have also been shown to produce dramatic moment-to-moment changes in the response of cortical somatosensory neurons and neuron populations. A recently proposed model of SI functional organization implies that these dynamic changes are highly dependent on the activity of N-methyl-D-aspartate (NMDA) receptors (Whitsel et. al., 1991). Experiments were conducted to study dose-dependent effects of NMDA receptor block on properties of the SI cortical neuronal population response to repetitive vibrotactile stimulation, as represented in scalp EEG.; Precisely controlled mechanical vibrations (33 Hz, 200 μm peak amplitude) were applied repeatedly (175 four second trials) to the right index finger of three subjects. First-order population responses were recorded at the wrist, and EEG was recorded from an array of 25 electrodes centered over the contralateral somatosensory cortex. Subjects blindly ingested 0, 30, or 60 mg dextromethorphan in 300 ml V8® juice, 30, 75, or 120 minutes before the start of a run.; Frequency-following response topographies and amplitudes were measured and evaluated statistically in relationship to the experimental conditions. The topographies were highly reproducible, and three main effects of dose were observed independently in two subjects: (1) an increase in background EEG activity (24–48 Hz) prior to stimulus onset; (2) a decrease in total driving-response amplitude; and (3) a shift toward a more posterior-lateral scalp topography. The third and most experienced subject showed smaller effects of similar types, masked by a previously unrecognized long-term training effect which led to appearance of essentially the same response topography under all experimental conditions. There were no systematic effects of dose or time on the first-order responses.; The reduction in total response amplitude with concurrent changes in response topography is consistent with known physiology and previously observed effects of NMDA receptor block on dynamic processes in SI cortex in response to prolonged or repetitive vibrotactile stimulation. The present investigation shows that the SI cortical population response to vibrotactile stimulation can be safely manipulated in humans using DM, and establishes the feasibility of combined studies of cortical dynamic processes and vibrotactile psychophysics.