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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.

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Bromoperoxidase from Ascophyllum nodusum was the first vanadium-containing enzyme to be isolated. X-ray absorption spectra have now been collected in order to investigate the coordination of vanadium in the native, native plus bromide, native plus hydrogen peroxide, and dithionite-reduced forms of the enzyme. The edge and X-ray absorption near-edge structures show that, in the four samples studied, it is only on reduction of the native enzyme that the metal site is substantially altered. In addition, these data are consistent with the presence of vanadium(IV) in the reduced enzyme and vanadium(V) in the other samples. Extended X-ray absorption fine structure data confirm that there are structural changes at the metal site on reduction of the native enzyme, notably a lengthening of the average inner-shell distance, and the presence of terminal oxygen together with histidine and oxygen-donating residues.

Mechanical responses in the basal turn of the guinea-pig cochlea were measured with broad-band noise stimuli and expressed as input-output cross-correlation functions. The experiments were performed over the full range of stimulus intensities in order to try to understand the influence of cochlear nonlinearity on frequency selectivity, tuning, signal compression and the impulse response. The results are interpreted within the framework of a nonlinear, locally active, three-dimensional model of the cochlea. The data have been subjected to inverse analysis in order to recover the basilar-membrane ͑BM͒ impedance, a parameter function that, when inserted into the ͑linearized version of that͒ model, produces a model response that is similar to the measured response. This paper reports details about intensity effects for noise stimulation, in particular, the way the BM impedance varies with stimulus intensity. In terms of the underlying cochlear model, the decrease of the ''activity component'' in the BM impedance with increasing stimulus level is attributed to saturation of transduction in the outer hair cells. In the present paper this property is brought into a quantitative form. According to the theory ͓the EQ-NL theorem, de Boer, Audit. Neurosci. 3, 377-388 ͑1997͔͒, the BM impedance is composed of two components, both intrinsically independent of stimulus level. One is the passive impedance Z pass and the other one is the ''extra'' impedance Z extra . The latter impedance is to be multiplied by a real factor ␥ (0р␥р1) that depends on stimulus level. This concept about the composition of the BM impedance is termed the ''two-component theory of the BM impedance.'' In this work both impedances are entirely derived from experimental data. The dependence of the factor ␥ on stimulus level can be derived by using a unified form of the outer-hair-cell transducer function. From an individual experiment, the two functions Z pass and Z extra are determined, and an approximation (Z pass ϩ␥Z extra ) to the BM impedance constructed. Next, the model response ͑the ''resynthesized'' response͒ corresponding to this ''artificial'' impedance is computed. The same procedure is executed for several stimulus-level values. For all levels, the results show a close correspondence with the original experimental data; this includes correct prediction of the compression of response amplitudes, the reduction of frequency selectivity, the shift in peak frequency and, most importantly, the preservation of timing in the impulse response. All these findings illustrate the predictive power of the underlying model.