Modality specificity of evoked potential augmenting/reducing

Visual and auditory evoked potentials to 4 stimulus intensities in each modality were recorded from Fz, Cz, and Oz (visual) and Fz and Cz (auditory). Response amplitudes for P1-N1 and N1-P2 components were analyzed. Visual responses at Oz showed no increase in amplitude with brighter stimuli for either component. At both Fz and Cz, response amplitude increased with stimulus intensity at each lead for both modalities. For P1-N1 the slope of the stimulus/response function was similar for both modalities. However, for N1-P2 the slope was significantly higher for auditory than for visual EPs. These results are inconsistent with a view of augmenting/reducing as resulting from sensory modulation by a non-specific mechanism, such as the mesencephalic reticular formation, and instead suggest that a more selective mechanism such as regulation of sensory transmission by the prefrontal cortex may underlie this phenomenon.