Analysis of evoked responses in man elicited by sinusoidally modulated noise

Publisher Website
Google Scholar
Abstract
For two subjects responses at the cortex, elicited by sinusoidally modulated white noise, have been measured. The influences of the modulation frequency, in a region around 9 Hz, and of the modulation depth are investigated. For one subject an undistored sine wave was measured at frequencies between 8 and 11 Hz, with a maximum amplitude of 3 μV at about 9 Hz. For the other subject a distorted wave was found. This wave has been analysed into its harmonic components. The spectrum consists mainly of the first and the second harmonic. Plotting the amplitudes of the different spectral components as a function of the modulation frequency gives a characteristic for the first harmonic which is comparable with that for subject 1, although less sharp. For the second harmonic the greatest amplitudes are found at lower frequencies. By varying the modulation depth a saturation effect was found at about 25% modulation depth for both subjects. The characteristics found with evoked response techniques are very different from those found in psychophysical experiments with sinusoidally amplitude-modulated white noise. In this case the transfer function suggests a low-pass filtering effect with cut-off frequencies between 50 and 80 Hz, depending upon the subject. The discrepancy between the results of the psychological and the electrophysiological experiments can possibly be explained by a model which consists of a low-pass filter, a rectifier, a saturation mechanism and a cortical filter.

This publication has 8 references indexed in Scilit: