A signal achievement: Charles D. Dawson's signal averaging technique and the discovery of brainstem evoked responses

Edward J. FINE1,2, Linda A. LOHR3 and Agnes SUPALA2,4
1Neurology, Department of Veterans' Affairs Medical Center, Buffalo, New York, USA.  efine AT buffalo.edu
2Department of Neurology, School of Medicine and Allied Health, State University of New York, Buffalo, USA.
3Robert L. Brown History of Medicine Collection, Health Sciences Library, State University of New York, Buffalo, USA.  lalohr AT buffalo.edu
4Jacobs Neurological Institute, Buffalo General Hospital, New York, USA.  asupala AT buffalo.edu

Evoked responses are electrical potentials from brain generated by specific stimuli to peripheral or cranial nerves. Richard Caton (1842-1926) is credited as the discoverer of somatosensory evoked responses. In 1875 Caton observed fluctuations of a galvanometer by electrical currents from exposed dog cortex in response to stimulation of its hind paw. He lacked equipment to record these responses.

Evoked potentials from human scalp were difficult to extract from higher voltage, oscillating cortical EEG noise. In 1947 Charles Dawson (1912-1983) developed the superimposition technique that improved signal to noise ratio of somatosensory evoked responses (SER). Dawson time-locked stimulation of median nerves to his oscilloscope’s sweeps. Electrode pairs placed on human scalp, overlying parietal lobes, picked up responses to repetitive electric stimuli of median nerves that were amplified and displayed on a storage oscilloscope screen. Dawson’s photograph of multiple superimposed SERs showed a first positive peak latency at 28 ms.

In 1954 Dawson developed the signal averager (SA). The SA consisted of a rotating switch that fed each individual scalp response to a single median nerve stimulus into one of 62 condensers every .1 seconds. The SA serially discharged each condenser into an amplifier connected to a storage oscilloscope. Dawson noted that the superimposed SER had a positive latency of 28 ms. To prove that the SA could eliminate brain activity noise not related to the stimulus, Dawson fed random electronic noise mixed with square wave signals into the averager, which removed the noise. Dawson’s median SERs resemble modern responses.

Dawson’s averaging technique allowed Jewett and Williston to record the minute auditory response from human scalp electrodes in 1970. Dawson should be considered the founder of human evoked potentials, because his averaging technique extracted time-locked evoked responses from higher amplitude cortical EEG noise.

Supported in part by local funds, Buffalo Veterans' Administration Medical Center.

Session I.  Instrumentation and Laboratory Sciences
Wednesday, 20 June 2007, 11:00 - 11:30 am

12th Annual Meeting of the International Society for the History of the Neurosciences   (ISHN)
Los Angeles, California, USA, 19-23 June 2007