A SYSTEM TRANSFER FUNCTION FOR VISUAL EVOKED POTENTIALS

The electroencephalogram (EEG) and evoked potentials (EP) have long held the promise of being a way of studying the sensory processing of the brain. If we take the view that the EEG is a continuous output signal, some features of which represent the response to input signals consisting of sensory stimuli, we have an input/output system that seems suitable for application of engineering analysis techniques. Using this approach, Clynes et al. (1964) studied the brain wave responses to step, ramp and sine wave light stimuli. The step stimuli allowed them to obtain the transient response of the “system”, and the sine wave stimuli allowed them to obtain the steady state response. These results, as well as the work of other investigators (Donker, 1975; Montagu, 1967; van der Tweel and Ver- duyn-Lunel, 1965), have demonstrated the nonlinear nature of steady state evoked potentials (SSEP). For stimulation by sine wave modulated light (SML) in the frequency range 5–9 Hz, a persistent second harmonic response is seen even at very low modulation depths. Further evidence of nonlinearity is seen in the poor results in attempting to predict the response to high flash rates based on superposition of responses from low flash rates.

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