The Detection of Cortical Spike Activity at the Human Scalp

Spike activity has a much wider energy spectrum than does postsynaptic activity. Under the assumption that the frequency spectrum of spike activity ranges from dc to 1000 Hz, the prediction was made that high-frequency electrical activity could be modulated by sensory stimulation and might well be detectable at the human scalp. This prediction was borne out in experiments that were successful in detecting the modulation of activity between 300 and 1000 Hz. This band was selected to suppress the major effects of the low-frequency classical brain waves. A control experiment was conducted in which any surviving harmonics of the classical evoked potential were markedly reduced. The high-frequency activity was essentially unaffected, thereby substantiating the hypothesis that this kind of activity is different from brain waves. It was also shown that the effect is related to the specific frequency of the stimulus and it is not attributable to either a general increase or a redistribution of brain activity. It is suggested that the high-frequency or spike activity might furnish the basis for a better correlation between electrophysiological data and sensory experience than does the classical evoked response.