Neurophysiological Studies of Auditory Systems Using TC

de Boer (1967; de Boer and Kuyper, 1968) was the first worker to apply TC to the study of audiofrequency selectivity associated with single units in the cat cochlear nucleus, presumably auditory nerve fibers. Using TC to find x+(t) , de Boer found that many cat auditory neurons appeared to be preceded by a filter whose estimated h(x) denoted a high degree of frequency selectivity, or Q. The results of his TC studies say nothing about the detailed neural mechanism responsible for the high Q, but they do indicate what to look for (e.g., lateral inhibition at the level of the

FIGURE 8.2-6 Three triggered correlograms using a SKL-302 variable electronic filter set to 1-kH bandpass (1 kHz low-pass in series with a 1 kHz high-pass). (a) The conditional expectation, x + (t) , with the filter given 100-kHz bandwidth Gaussian noise. (The noise associated with 2500 spikes was averaged.) (b) The conditional expectation, x + (t) , with the filter given 3-kHz bandwidth Gaussian noise. (The noise associated with 2500 spikes was averaged.) (c) The true cross-correlogram, Rxy (t) , of the analog noise output of the filter with 140-kHz bandwidth limited Gaussian noise. A PAR Model 100 signal correlator was used. This was the gold standard for the TC conditional expectations. Rxy(t) has the shape of the impulse response of the filter. (From Wu, J.M., Ph.D. dissertation, University of Connecticut, Storrs, 1970.)

FIGURE 8.2-6 Three triggered correlograms using a SKL-302 variable electronic filter set to 1-kH bandpass (1 kHz low-pass in series with a 1 kHz high-pass). (a) The conditional expectation, x + (t) , with the filter given 100-kHz bandwidth Gaussian noise. (The noise associated with 2500 spikes was averaged.) (b) The conditional expectation, x + (t) , with the filter given 3-kHz bandwidth Gaussian noise. (The noise associated with 2500 spikes was averaged.) (c) The true cross-correlogram, Rxy (t) , of the analog noise output of the filter with 140-kHz bandwidth limited Gaussian noise. A PAR Model 100 signal correlator was used. This was the gold standard for the TC conditional expectations. Rxy(t) has the shape of the impulse response of the filter. (From Wu, J.M., Ph.D. dissertation, University of Connecticut, Storrs, 1970.)

cochlea). An example of a cat auditory neuron x+(-x)is shown in Figure 8.2-7. (Note that time t increases from right to left, t = 0 is at the right, because of the way the TC conditional averaging was done.) The relatively long settling time of x+(-t) suggests that the filter is of the bandpass type with a high Q. To demonstrate this behavior, note that the transfer function of a simple, high-Q, quadratic bandpass filter can be written:

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