Figure 1: A. Photograph of the mechano-electrical averaging device designed by Dawson.3 With reduction gears, the motor rotated the spindle carrying the distributors at 10 Hz; the distributors passed the electrical activity from the cortex to a series of 62 capacitors every 10th revolution, following a stimulus. After the procedure had been repeated the requisite number of times, the charge stored on each of the capacitors during successive 100 ms epochs was read out, giving the averaged response. An example, following ulnar nerve stimulation, is shown for the contralateral and ipsilateral somatosensory areas in C (upper and lower traces respectively). B (upper) shows an earlier method of recording evoked potentials, in which photographic superimpositions of faint traces were made, following median nerve stimulation at the wrist. The centripetal impulse volley is recorded from the median nerve at the elbow (lower trace).
From Dawson2,3 with permission.

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