A Comparison of Certain Gamma Band (40-HZ) Brain Rhythms in Cat and Man

I still recall my astonishment when I learned that if you open your eyes the electroencephalogram (EEG) alpha waves will disappear, and that if then you close them the alpha waves will reappear. The year was 1934, and I have been hoping ever since to learn what causes the lO-Hz rhythm in the first place, and then what brain process turns it off and on. The editors of this book have asked me to compare another human brain rhythm-in the region of 40 Hz this time-with a cat rhythm in the same frequency range that is discussed elsewhere in this volume by Gray and Singer and their colleagues, and by Eckhorn and his colleagues. I do as much as I can to oblige them in what follows, first describing some properties of the human 40-Hz phenomena, then comparing these with the microelectrode data from the cat. After concluding that the two differ in several ways, I branch out to consider the general prob­ lem of brain rhythms briefly, and to speculate on their possible physiological origins and functions. The human steady-state response Examples of human evoked potentials (EPs) elicited by flash, click, and tactile stimuli delivered at a rate of 32 per second appear in Figure 1. Known also as steady-state responses (SSRs), these EPs were recorded using standard EEG procedures from the scalp of an adult male. Each trace averages responses to nearly 6000 stimuli (3 min worth), and replications of each condition are superimposed. The vertical lines show when the stimuli were delivered. Stimu­ lus intensity was adjusted in each case so as to yield a response amplitude of approximately 1 /lV at the electrode located on top of the head (Cz ). The three other channels recorded SSRs obtained near the cortical termination of the visual (Oz), tactile (C3), and auditory (T3) modalities. Several features of these SSRs are notable: 1. Human EPs to stimuli delivered at high stimulus rates are not unique to the visual modality. 2. In the three modalities they all resemble sinusoids and are similar in amplitude.