Topography of the 40 Hertz evoked potential in the auditory and visual modalities

This study reports on a comparative examination of the scalp topography of the 40Hz evoked potential to auditory and visual stimulation in a preliminary effort to identify the neural substrates involved in the generation of the response. The amplitude and resolution of the response was examined at four scalp locations distributed over frontal, central, temporal, and occipital cortices at stimulation rates of 20 and 40 cycles per second in fifteen healthy subjects aged 18 to 45. Optimal amplitude and resolution to auditory stimulation were found centrally, frontally, and temporally, with diminishing amplitude and resolution occipitally. Conversely, visual stimulation demonstrated maximal amplitude and resolution occipitally and centrally with diminution anteriorly and laterally. Results of this study demonstrate that the 40Hz response is a viable, replicable, cross- modality phenomenon. Examination of amplitude and resolution supports the involvement of sensory specific thalamic relay nuclei coherently interacting with primary cortical areas in the generation of the 40Hz response.