On cochlear encoding: Potentialities and limitations of the reverse-correlation technique

Abstract

This paper presents a description of the interrelation between two major properties of the responses recordable from auditory nerve fibers: frequency selectivity and partial synchrony between stimulus and response. In the course of this work the influence of nonlinearity on the cochlear encoding process can be assessed. The theory of the reverse-correlation technique is derived in a most general way. It is based on a model in which a filter--assumed to be linear--is followed by a stochastic pulse generator--the probability of producing an output pulse being an instantaneous but nonlinear function of its input signal. Insofar as such a model represents stimulus transformations in a primary auditory neuron, the technique can be applied to the responses recorded from an auditory nerve fiber. Several illustrative examples of experimental reverse-correlation functions-abbreviated: revcor functions--are presented and discussed. These functions have the general character of impulse responses of sharp bandpass filters. They show very little phase modulation. For noise stimuli of up to 70 dB per third octave the revcor functions are almost invariant. Above that level some (but not all) of the revcor functions show a loss of frequency selectivity.If a nerve fiber can be contacted for a sufficiently long time, it is possible to compare the response with that of a model filter, in which the revcor function of that fiber is substituted as its impulse response. The output signal of the model filter is shown to be a very good predictor of the firing probability of the fiber under study. This property is demonstrated for noise as well as for tone stimuli. There is surprisingly little evidence of nonlinear filtering in these results. This so-called simulation method can also be applied when the stimulus is switched on and off. The results show, apart from effects due to filtering, clear manifestations of fast adaptation. Again, the filtering appears to be independent of the latter effect. It is concluded that for wide-band noise and single-tone signals the firing probability is predominantly controlled by a linearly filtered version of the acoustical stimulus; this constitutes the principle of specific coding. The conspicuous absence of nonlinear effects in the results can partly be explained in terms of the response properties of a class of networks in which sharp filtering occurs after the generation of nonlinear distortion products. It can then be predicted that this property will hold only for wide-band and tonal stimuli. That our results show so little evidence of cochlear distortion appears to be a property of signal transformations and is not due to linearization tendencies of the experimental method.

Reverse Correlation Analysis of Auditory-Nerve Fiber Responses to Broadband Noise in a Bird, the Barn Owl

“…5B) as reported in mammals (Rhode and Smith 1985;Mc Laughlin et al 2007). Also like in mammals (de Boer and de Jongh 1978;Carney and Yin 1988;Mc Laughlin et al 2007), BW 10dB increased with input level (Fig. 6B).…”

“…2B). At first glance, impulse responses looked similar to what has been reported in the past (de Boer and de Jongh 1978;Carney and Yin 1988;Recio-Spinoso et al 2005;Fischer et al 2011;Fontaine et al 2013): They consisted of a high-frequency oscillation (at around the best frequency of the neuron) modulated in time by an envelope that rose and decayed in a few milliseconds. We fitted analytical functions called gamma chirps to the portion of the STAs where the envelope was higher than 5 % of the maximum, as previously reported (Wagner et al 2009;Fischer et al 2011;Fontaine et al 2013).…”

“…Conventional reverse correlation techniques require neurons to phase lock to the stimulus fine structure (de Boer and de Jongh 1978). While they can be used across the whole hearing range of the barn owl where phase locking is present (Köppl 1997b), for mammals, such techniques are limited to the phase locking range.…”

“…For a broadband noise stimulus, the impulse response h(t) is the average stimulus that elicits spikes (de Boer and de Jongh 1978;Schwartz et al 2006), that is, h t…”

“…However, the highly nonlinear character of auditory transduction restricts the characterization of cochlear mechanics using pure-tone stimulation. Experiments presenting broadband stimuli have been extensively performed in the mammalian auditory system (e.g., de Boer and de Jongh 1978;Carney and Yin 1988;van der Heijden and Joris 2003;Recio-Spinoso et al 2005;Versteegh et al 2011) as they are thought to linearize the system for a fixed input level (Recio-Spinoso et al 2009). …”