Effects of stimuli intensity and frequency on auditory p50 and n100 sensory gating

Sensory gating is the brain's ability to adjust its sensitivity to incoming stimuli, i.e., to diminish its response to irrelevant or repetitive stimuli (gating out) and to increase it when a novel stimulus is presented (gating in). Most of the existing studies have investigated the gating out mechanism, giving little attention to the gating in function. Although both the P50 and N100 components of the auditory ERPs (event related potentials) show amplitude reductions to stimuli repetition, it is not clear if both components are part of a common gating system or if their sensory modulation is uncorrelated. In order to respond to these questions and to further characterize the sensory gating functions, we examined to what extent P50 and N100 are influenced by changes in the stimuli parameters and whether the sensory modulation of both components are interrelated. To this end, we obtained ERPs from 23 healthy volunteers using pairs of auditory stimuli which could be identical (S1 = S2), different in frequency (S1 = 1000 Hz; S2 = 2000 Hz) or different in intensity (S1 = 80 dB SPL; S2 = 100 dB SPL). As expected, the amplitudes of P50 and N100 decreased in response to the second stimuli of the identical pairs. With non-identical pairs, amplitude increases of P50 and N100 were observed only in pairs with different intensity, but not frequency. Thus, the results showed that both P50 and N100 are sensory modulated, showing that amplitude decreased to stimuli repetition (gating out) and increased when the two stimuli of a pair differed in intensity (gating in). A correlational analysis of the sensory gating indices (S2/S1 ratio and S1-S2 difference) obtained for P50 and N100 suggested that the sensory gating function of both components may be of a different nature. The reliability of the ratio and the difference indices of sensory gating is also discussed.