Two-tone inhibition in auditory-nerve fibers

Neural correlates of cubic difference tones in the medial geniculate body of the cat

Single unit responses to the cubic difference tone CDT (2f1 - f2 = CF) and the difference tone DT (f2 - f1 = CF) were studied in the medial geniculate body (MGB) of the cat. Out of 66 units tested with CDT stimuli and having characteristic frequencies (CF) below 10 kHz, 77% gave a response to the two-tone combination stimulus. The component tones when presented alone evoked no responses, or in some cases a response pattern that was different from the one observed for the combination tone. The CDT response pattern was always similar to that seen for a pure tone at the CF. The threshold of response for the CDT was 10-70 dB higher than for a pure tone stimulus at the CF. The few units which were phase-locked could be synchronised with the CF, CDT, or DT, depending on the particular stimulus conditions. The index of synchrony was in many cases found to be higher for CDT responses than for a pure tone at CF.

Mechanical and acoustical influences on spontaneous oto-acoustic emissions

Spontaneous, tone-like emissions produced by normal ears and measured in the closed outer ear canal can be affected by mechanical and acoustical events. Such effects can be measured in steady-state conditions as well as for transient stimulation, and are seen in response to the stapedius reflex, to ear canal air pressure changes, and to the presentation of external tones. Frequency and level of the emissions follow certain characteristics which are described and discussed. The emissions seem to react with 2 ms delay and with an exponential rise and decay, the time constant of which is about 13 ms.

Spontaneous and impulsively evoked otoacoustic emissions: indicators of cochlear pathology?

The first author's right ear produces a spontaneous otoacoustic emission (SOAE) at 7529 Hz and 16 dB SPL. An external continuous tone is able to suppress the SOAE. The 3 dB iso-suppression curve is broadly tuned and displaced, relative to the SOAE, toward higher frequencies. An audiogram notch exists at frequencies just below that of the SOAE. We explain the occurrence of both spontaneous and impulsively evoked OAEs in terms of disruption of active feedback mechanisms of the OHCs upon basilar membrane vibration. According to this hypothesis, each segment of the organ of Corti feeds back positively upon its segment of basilar membrane and negatively upon adjacent segments. If a patch of OHC loss exists, adjacent segments of the basilar membrane are released from the negative feedback and respond to an impulsive stimulus with exaggerated oscillations at their resonance frequencies, thus producing OAEs. At particularly sharp transitions between normal and abnormal regions of the organ of Corti SOAEs may be generated.

Lateral inhibition in the anteroventral cochlear nucleus of the cat: a microiontophoretic study

The spontaneous firing rates of non-prepotential (NPP) units of the anteroventral cochlear nucleus are quite low so it has not been possible to determine whether side band tones are inhibitory when presented alone. Microiontophoretically-applied excitatory amino acids can be used to excite non-spontaneous cells directly. Using this technique it can be shown that side band tone bursts 1/2 to 3/4 octave above the characteristic frequency (CF) of a NPP unit inhibit the amino acid-induced firing. Side band tones which inhibited the amino acid-induced firing were beyond the tuning curve. Side band tones within the tuning curve produced excitation. Both, however, usually reduced the activity evoked by a CF tone burst (i.e., two-tone interaction). The data suggests that lateral inhibition and two-tone interactions are separate phenomena in the auditory system and that lateral inhibition may play a critical role in determining the shape of the tuning curve of NPP units.

The neuronal architecture of the anteroventral cochlear nucleus of the cat in the region of the cochlear nerve root: electron microscopy

We have studied the posterior division of the anteroventral cochlear nucleus, where the cochlear nerve root enters the brain, in the cat. In Nissl preparations, this region contains two types of neuronal cell bodies: globular and multipolar. The two types can be identified in the electron-microscope by comparing Nissl substance and rough endoplasmic reticulum. Globular cell bodies receive many synaptic terminals, which cover 85% of the surface. In contrast, multipolar cell bodies are almost entirely wrapped by thin glial sheets--synaptic terminals contact less than 15% of the surface and tend to cluster at the bases of dendrites. Synaptic terminals are of three kinds, types 1, 2, and 3, which contain large round, small round-to-oval, and small flattened synaptic vesicles, respectively. Terminals of all three kinds synapse on both types of cell bodies. However, only globular cell bodies receive the largest type 1 terminals, which correspond to end-bulbs, seen in Golgi impregnations to arise from cochlear nerve axons. Cochlear ablation leads to degeneration of type 1, but not type 2 or 3 terminals. We conclude that neurons with globular cell bodies receive heavy somatic input from the cochlear nerve, as well as from other sources. Neurons with multipolar cell bodies receive very little input to their perikarya--giving their dendrites a more important role in determining their response properties. We suggest a morphological basis for correlating individual kinds of neurons with certain electrophysiological response types.

AP unmasking and AP tuning in normal and pathological human ears

In a group of seven normal and eight abnormal hearing subjects three-tone AP unmasking experiments and/or AP tuning experiments were performed during electrocochleography. In the unmasking experiments the Shannon forward masking paradigm was applied, i.e. two simultaneous tones (the masker and the suppressor) influence a test tone in a forward masking procedure. Frequency and intensity of the suppressor were varied. It appeared that unmasking effects are clearly present, i.e. the suppressor stimulus can reduce masking of the AP. This effect, however, is very variable, in normal ears as well as in pathological ears. AP tuning quality deteriorated with increasing hearing loss but no correlation was found between AP unmasking parameters and hearing loss. It seems that AP suppression areas shift less upward (or not at all) than the AP tuning curve does upon increasing the test tone level. These results raise some questions about the intercorrelation of the triad: hearing loss, quality of tuning and suppression effects, as single fibre experiments and psychophysical investigations suggest.

A comparison of compound action potential and cochlear microphonic two-tone suppression in the guinea pig

Cochlear microphonics (CM) and compound action potentials (AP) were recorded simultaneously with differential electrodes in the basal turn of the guinea pig cochlea. When CM two-tone suppression (2TS) curves were compared to AP simultaneous masking curves, good correspondence was observed between CM and AP suppression effects. The relationship between the 10 dB bandwidths of CM and AP 2TS curves remained constant for each animal despite differences between animals resulting from natural variations. Under pathological conditions (acute cochlear hypoxia) both CM and AP two-tone suppression effects were greatly reduced or disappeared. These results can be taken as evidence that CM suppression and AP suppression are the products of a common underlying mechanism.

Two-tone interactions in the cochlear microphonic

Two-tone interactions are explored for the cochlear microphonic (CM) in the guinea pig. Recordings are made from turns one and three using differential electrodes in the perilymphatic space or pipettes placed in scala media through a fenestra over the stria vascularis. We focus on magnitude changes associated with the introduction of appropriate interference tones and on various types of phase shift concomitant with these magnitude variations that have not received documentation in the literature. Based on extensive parametric data, it is suggested that some features of the gross interference phenomenon may be a consequence of the vectorial summation of outputs from contributing hair cell generators. These spatial effects appear to determine phase behavior and the influence of probe frequency on the frequency of maximal interference. In addition, the apparent interval between out defined best frequency (CF) and the frequency of maximal interference is most likely due to an underestimation of CF resulting from phase cancellation between CM-producing hair cell populations. However, after compensating for these spatial effects, several aspects of the CM interference phenomenon seem to be analogous to two-tone suppression in auditory nerve fibers. A direct one-to-one relationship is not implied since the latter reflect the outputs of inner hair cells while CM interference most likely reflects outer hair cell behavior. As a result, the association between suppression and interference must be sought in the process by which outer hair cell influence inner hair cell transduction.

Boundaries of two-tone rate suppression of cochlear-nerve activity

Two-tone rate suppression was examined in the responses of single cochlear-nerve fibers in Mongolian gerbils. The iso-rate tracking algorithm developed by Kiang and Moxon (Kiang, N.Y.-S. and Moxon, E.C. (1974): J. Acoust Soc. Am. 55, 620-630) for obtaining tuning curves was modified to track iso-rate suppression boundaries as a function of frequency with the excitor tone fixed at the characteristic frequency (CF) of the fiber. Lower threshold boundaries of the areas of suppression flanking the tuning curve above and below CF were outlined for fibers over a large CF range. It was found that the boundaries of rate suppression obtained below CF were very stable in their absolute positions on the intensity-frequency plane. This stability was evident both as a function of fiber CF (0.6-15 kHz) and as a function of the shape of the tuning curve at a given CF. In other words, the suppression boundary obtained below CF was largely independent of the tuning curve. In a second series of experiments tuning curves were taken in the presence of a fixed tone placed in the suppression area located above the fiber CF. The fixed tone by itself was not excitatory. These tuning curves were compared to tuning curves obtained with a single tone. It was found that frequencies around the fiber CF were most affected (suppressed) by the presence of the second tone, and that the low-frequency tail of the tuning curve tended to shift toward the boundary of the suppression area below CF. Because this suppression boundary lies below the threshold of the normal tail of the tuning curve for many mid- and high-CF fibers, these fibers often became hypersensitive at low frequencies in the presence of the second tone above CF.

Reduced temporary and permanent hearing losses with multiple tone exposures

Acoustic overstimulation of the guinea pig cochlea with a 16 kHz pure tone induces a loss in threshold sensitivity that can be either temporary or permanent depending on the duration of the trauma. When a second tone of lower frequency (10, 5 or 2 kHz) is presented to the same cochlea simultaneously with the first tone, then the resultant threshold loss produced by the 16 kHz tone is significantly less. This applies to both temporary and permanent threshold losses. The reduced threshold loss disappears as the intensity of the second tone decreases. This type of nonlinear cochlea behaviour is similar to other acoustically evoked nonlinearities generally grouped under the term, two-tone suppression or inhibition. The results disagree with the "equal energy hypothesis' as a method to establish damage risk criteria in noise-induced hearing loss.

Location-specific components of the gross cochlear action potential: an assessment of the validity of the high-pass masking technique by cochlear nerve fibre recording in the cat

The auditory high-pass masking technique has been used in attempts to define the origin, along the cochlear partition, of the gross cochlear action potential (CAP) and the gross brain stem potential. Theoretically, the high-pass masking paradigm should be frequency and location specific at the cochlear level, and some indirect evidence does point to this specificity. However, this hypothesis has not yet been directly substantiated. In the present experiment, click-evoked cochlear nerve activity was recorded simultaneously from the round window and from single fibres of the cochlear nerve, with and without high-pass maskers spaced in octaves from 0.5 to 16 kHz, at three intensities, in the anaesthetized cat. The "derived' CAPs were computed and compared with the mapping of single cochlear fibre responses under the same conditions. With one main exception, the conclusions drawn on the origin of the frequency components of the "derived' potentials were found to be valid in the normal cat. The exception concerned fibres with characteristic frequencies below 1-2 kHz, where the substantial spread towards the high frequencies of their frequency threshold curves, and the effects of lateral suppression or of other "remote masking' phenomena rendered the high-pass masking less location specific. From these results and certain assumptions, we would predict the high-pass masking technique to be valid in electrophysiological investigations in normal humans for frequencies down to 0.5-1 kHz.

Auditory-nerve fiber responses to tones in a noise masker

The phase-locked responses of single auditory-nerve fibers were measured for a continuous tonal stimulus presented in a noise background. The response amplitude, the primary Fourier component of the period histogram, was found to be dependent on the relative levels of the noise and tone. Different noise-to-tone level ration resulted in quite different response amplitude; changing overall level keeping noise-to-tone constant (constant dB difference) provided little change in response. With transient stimuli, phase-locked response to the tone at noise onset was consistently greater than to a tone presented during the steady-state noise exposure. When responses were normalized to the average rate, the difference between onset and steady-state responses were not clearcut.

Patterns of residual masking

"Residual masking' was measured in a tonal forward masking paradigm. In one experiment, psychophysical tuning curves and masking patterns were obtained at several frequencies and levels for a fixed masker-probe time delay. In a second experiment, tuning curves and masking patterns were measured as masker-probe time delay was varied. Our results indicate that tuning curves and masking patterns are sharpest at low levels, high frequencies and brief masker-probe time delays. In addition, we observed that masked probe threshold returned to the level of unmasked probe threshold at approximately the same post-masker time regardless of masker level or the probe-to-masker frequency relationship. These findings suggest that frequency, level and time delay all affect the degree of frequency selectivity observed with these measures.

Non-linearities in the responses of turtle hair cells

1. Intracellular recordings were made from single cochlear hair cells in the isolated half-head of the turtle. Receptor potentials were recorded while the ear was stimulated with high-intensity tones in order to examine the cochlear non-linearities which shape the hair cell responses.2. The size of a hair cell's voltage response to a tone burst was reduced, abolished and then reversed by steady depolarizing currents of increasing strength. The average current needed to produce reversal was about 0.3 nA, the reversal potential being close to zero with respect to the scala tympani.3. Short current pulses injected on the peaks and dips of the receptor potential showed that the membrane resistance and time constant were decreased on the depolarizing phase of the receptor potential. These changes were not due to non-linearity in the hair cell's current-voltage curve in the absence of acoustic stimulation. The results are consistent with the idea that the transducer causes the cell to depolarize by increasing the membrane conductance to ions with an equilibrium potential close to zero.4. Saturated receptor potentials from poorly tuned cells exhibited a pronounced asymmetry, with the maximum depolarizing excursion being several times the maximum hyperpolarizing excursion. This asymmetry was not seen in sharply tuned cells. It is proposed that the asymmetry is present in the transducer conductance change and in sharply tuned cells is reduced in the receptor potential by subsequent filtering.5. For high sound pressures which produced close to a saturated response, the hair cell voltage wave form displayed a number of non-linear features dependent upon the frequency of stimulation relative to the characteristic frequency (c.f.). The most prominent feature occurred at very low frequencies where the potential exhibited damped oscillations on the depolarizations and hyperpolarizations; these ;ringing frequencies' lay above and below the c.f. of the cell respectively.6. The ;ringing frequencies' varied with the c.f. of the cell but for a given cell were largely independent of the frequency of stimulation. The ;ringing frequencies' could be changed by injecting steady currents into the cell during acoustic stimulation; depolarizing currents increased the ringing frequencies and hyperpolarizing currents decreased the frequencies.7. The hair cell's response to a continuous test tone at the c.f. of the cell could be suppressed by simultaneous addition of a second tone whose sound presure was comparable to, or greater than, the test tone. The degree of suppression varied with the intensity and frequency of the second tone, and was maximal close to the c.f. of the cell. The sound pressure required to produce a constant suppression as a function of frequency was sharply tuned, and the tuning of the suppression showed similarities to the frequency selectivity of two-tone suppression described in the auditory nerve.8. An attempt was made to reconstruct the main features of the receptor potential at high intensities.

Auditory-nerve fiber responses to frequency-modulated tones

Discharge patterns of cat auditory-nerve fibers were obtained in response to frequency-modulated (FM) tones. The rate and direction of frequency change and the sound-pressure level of the sweep tones were systematically varied, and aspects of the discharge patterns were compared to aspects of the discharge patterns elicited by pure tones. Increases in SPL broaden the frequency range over which the fiber responds, as is the case with pure-tone stimuli. Increases in the rate of frequency change have little effect on frequency selectivity for the rates tested. In general, the pure-tone response area is a good predictor of the response area to FM. Although approximately equal numbers of spikes are elicited by ascending and descending sweeps, the discharge patterns differ slightly; for each direction of frequency change, the FM response area is shifted in the direction of the earliest-occurring frequencies. Most of this shift can be accounted for by neural adaptation. This asymmetry is small, relative to those observed in the central nervous system.

Responses of primary auditory fibers to combined noise and tonal stimuli

Responses to tonal stimuli, with and without added noise of different bandwidths, were obtained from anesthetized cat auditory-nerve fibers using glass micropipettes. When low-pass noise with a cut-off frequency at least one octave below best (or characteristic) frequency was used, every fiber tested at high enough intensities showed a suppression of the tonal response. This suppression did not cause a general reduction of neural responsiveness to all sounds, but rather took the general form of a frequency-specific reduction in the effective intensity of the tonal stimuli. The suppression mechanism(s) involved thus adjust the sensitivity of these fibers to cover higher intensity ranges in the presence of noise. The frequency of the most severely affected tones was always at or near best frequency, in confirmation of previous work (Abbas, P.J. and Sachs, M.B. (1976): J. Acoust. Soc. Am. 59, 112-122; Kiang, N.Y.-S. and Moxon, E.C. (1974): J. Acoust. Soc. Am. 55, 620-630). The suppresson is a direct but highly nonlinear function of the intensity and bandwidth of the noise. The effects on tonal response of wide-band noise were more variable, sometimes causing suppression similar to that induced by the low-pass noise and sometimes causing only 'strong-signal capture' effects. A model of noise-induced suppression has been developed whereby each sound produces both an excitatory effect, sharply tuned at best frequency, and a suppressive effect, which also has its lowest threshold at best frequency but is more broadly tuned.

Frequency analysis in normal and hearing-impaired listeners

Recent studies of frequency analysis in listeners with normal hearing and in those with sensorineural hearing losses are reviewed and compared with related physiological data. These studies suggest that it is now possible to obtain detailed audiological or psychoacoustic data for human listeners that closely parallel physiological data obtained in eighth nerve recordings from animals. Implications of these developments for future research with impaired listeners are discussed.

Cochlear models: evidence in support of mechanical nonlinearity and a second filter (a review)

We have studied properties of a nonlinear one-dimensional model for motion of the basilar membrane. The model is able to reproduce a variety of physiological and psychophysical effects, and in some instances predictions of the model have been subssequently confirmed. We regard the agreement between model and nature as evidence that the major features of the model are qualitatively correct. Areas of agreement include generation and propagation of two-toine distortion products, two-tone suppression, and Zwicker's psychophysical masking period patterns. In order to reproduce all of the above effects, the model must contain the following major features: (1) A mechanical nonlinearity. This nonlinearity is asymmetric, with membrane damping increasing during the same phase of membrane motion that elicits response activity in primary auditory fibers. (2) An additional stage of sharpening, a 'second filter', between the physical variable controlling mechanical nonlinearity and the physical variable controlling neural excitation.

Combination tones and unmasking

Some important relationships between two auditory nonlinearities, unmasking and combination tone (CT) production, are described. An example of unmasking is provided by using a forward-masking paradigm where a baseline is first obtained by determining the threshold of a 2000 Hz signal preceded by a 2000 Hz masker. When a second tone (the suppressor) is then added concurrently with the masker, the signal can be easier to hear. At a suppressor level of 80 dB SPL, this unmasking occurs for suppressor frequencies around 2300 Hz. At a suppressor level of 55 dB SPL, the unmasking effect occurs at suppressor frequencies closer to the masker frequency. An example of CT production is provided by presenting two sinusoids (f1 and f2) as maskers in a forward-masking experiment. The threshold of the 2000 Hz signal is shown to decrease as f2/f1 is increased keeping 2f1 - f2 = 2000 Hz. This is consistent with the notion that the magnitude of the CR decreases as f2/f1 increases. We then provide evidence that CTs can produce ummasking effects similar to acoustic tones. Again a baseline was determined with a 2000 Hz signal amd masker (f1). Two higher-frequency sinusoids (f2 and f3) were added simultaneously to the masker, neither of which produced unmasking when presented individually. When 2f2 - f3 or f3 - f2 was approximately 2300 Hz, unmasking was observed. Next was explored 2f2 - f3 unmasking as a function of f3/f2, keeping 2f2 - f3 frequency fixed at 2300 Hz. As f3/f2 increases, the magnitude of the unmasking decreases. These CT unmasking effects suggest that the generation of CTs must be at the same site or peripheral to the site of suppression.

An "inhibitory" influence on brainstem population responses

Forward masking was used to obtain measurements of physiological masking and two-tone unmasking from short-latency evoked potentials and psychophysical responses in human subjects. The physiological results are in qualitative agreement with data on inhibitory phenomena in nonhuman auditory systems. The neural and behavioral data obtained thus far agree well.

The maturation of frequency selectivity in C57BL/6J mice studied with auditory evoked response tuning curves

The present study was designed to examine the ontogeny of frequency selectivity in the neonatal auditory system. Mice were tested between 12 and 65 days of age. At each age two measures of auditory sensitivity were made from cochlear nucleus evoked responses. Tone-burst evoked-response thresholds in the quiet were determined for frequencies between 1.0 and 39.0 kHz. A two-tone simultaneous masking procedure was then used to obtain evoked response tuning curves. The frequency selectivity of the tuning curves was quantified by calculating a Q ratio. The results show that tuning is poor in neonates but rapidly improves to adult-like levels within 5-16 days after the inception of auditory function. The data also indicate that the development of frequency selectivity varies directly with the maturation of threshold sensitivity.

Relations between frequency selectivity and two-tone rate suppression in lizard cochlear-nerve fibers

Cochlear-nerve fibers innervating the apicial region of the alligator lizard basilar papilla show sharp frequency selectivity in response to single tones (measured with the frequency threshold contour, or FTC), and the phenomenon of two-tone rate suppression (TTRS) in response to two simultaneously presented tones (measured with the iso-TTRS contour, or ITC). The gross shapes of the FTCs, as characterized by the slopes of the sides and Q10dB, vary systematically with the fiber's characteristic frequency (CF). 'Fine-structural' features are also found: below CF, notches (frequency regions of relatively high threshold) occur in the FTC at frequencies related to CF. Above CF, a break frequency, which varies with CF, divides the FTC into segments of different slope. Features of the ITC also vary with CF. The detailed shapes of the FTCs and ITCs are related: lobes of the ITC interdigitate with notches in the FTC; the side of the FTC with steepest slope is closely associated with the side of the ITC with steepest slope. The close relation that is observed between sharp frequency selectivity and TTRS suggests that both phenomena arise from a common cochlear mechanism.

Frequency selectivity in bird and man: a comparison among critical ratios, critical bands and psychophysical tuning curves

The relative frequency selectivity of the internal auditory filters, as measured by critical bands or critical ratios, and psychophysical tuning curves, was compared in an avian species (parakeet) and human listeners. The results indicate that the critical band predicts the frequency selectivity of psychophysical tuning curves and vice versa. These findings suggest that there are similar mechanisms which contribute to the frequency selectivity measured by these two procedures and that these mechanisms operate in both the human and avian ear.

Funneling mechanism in hearing

The funneling mechanism in hearing is reviewed and its underlying mechanism clearly elucidated. It is concluded that GABA is a principal candidate among the putative transmitters.

Action potential suppresion, tuning curves and thresholds: comparison with single fiber data

The collection of compound action potential (AP) threshold curves and their use to define the sensitivity of individual animals are described. Forward masking AP tuning curves (APTC) have also been collected in the chinchilla. Characteristics of APTCs are compared with single fiber frequency-threshold curves (FTC) in the same group of animals. The two sets of data are quite similar when the probe frequency used to collect the APTC is equated with a fiber's characteristic frequency (CF). The major difference is that APTCs are usually broader than FTCs. A paradigm utilizing two maskers in a forward masking situation, developed to study psychophysical unmasking [19,37], has been modified for measuring AP suppression. AP suppression areas are described as similar to single fiber two-tone suppression areas when probe frequency and CF are above 3 kHz. Relationships among single fiber, AP and psychophysical thresholds, tuning curves and suppression areas are discussed.

The origin of the waveform of cochlear whole action potential

An explanation is given why the recorded action potentials of the cochlea manifest themselves in the complex N1N2-form. The separate AP of the nerve fibers are not diphasic but triphasic in form. With algebraic addition of the, in different phase, AP present in the auditory nerve the N1N2 complex originates. This is illustrated by the observed form changes in the recorded signals during the cutting experiments of the auditory nerve.

Cochlear frequency sharpening-a new synthesis

Recent evidence indicates a substantial difference in sharpness of tuning between basilar membrane mechanics and primary neuron responses in mammals. This paper describes a new qualitative model for a sharpening mechanism. It is suggested that the inner hair cells are sensitive to d.c. potential changes in scala media that are induced by sound stimuli, and that these d.c. potentials can suppress neuron activity in a frequency-dependent way. The model explains sharpening of both sides of neural tuning curves, the shape of the low-frequency part of the tuning curves and is also compatible with other phenomena such as two-tone inhibition and the effects of electrical polarization of the basilar membrane.

Dynamic properties of excitation and two-tone inhibition in the cochlear nucleus studied using amplitude-modulated tones

The dynamic properties of excitation and two-tone inhibition in the cochlear nucleus were studied from extracellularly recorded unit responses to two simultaneously presented tones. One tone was presented at the unit's characteristic frequency, CF, the other at the unit's best inhibitory frequency, BIF. One or both of the tones were amplitude-modulated with pseudorandom noise. The system under study is in general nonlinear, but can be considered to function as a linear system for small changes in sound intensity around a certain operating point. The dynamic properties are likely to be different at different operating points. A suitable method for the study of dynamic properties of such a system employs tones that are amplitude-modulated with pseudorandom noise. In the present study, the dynamic properties were assessed by cross-correlating the unit discharge rate with the modulation. This was accomplished by computing the cross-covariance function between a period of noise and a period histogram of the discharges, the histogram being locked to the periodicity of the pseudorandom noise. In this way, it has been shown in previous works (Moller, 1973, 1974b), that the cross-covariance function is a valid approximation of the system's impulse response function at a certain sound intensity, provided the modulation is kept at a low value. In the present study the computed cross-covariance function is thus an approximation of the change in discharge rate of the cochlear nucleus units in response to a brief increase in stimulus intensity. As the response of the system under the given circumstances is approximately that of a linear system, the integrated cross-covariance is an approximation of the system's step response function, i.e the change in discharge rate that resulte from a hypothetical step increase in stimulus intensity. The results of the present study can be summarized as follows: 1. The impulse and step response functions computed from the responses to the modulated inhibitory tone of the great majority of units from which recording was made were found to be virtual mirror images of those obtained when the excitatory tone was modulated, the inhibitory response being somewhat smaller in amplitude than the excitatory. 2. When both tones were modulated simultaneously, the step response function was approximately the algebraic sum of the two responses obtained when the tones were modulated singly, further indicating that the system functions as a linear system when the stimulus amplitude is varied slightly around a certain operating point. 3. The shape of the cross-covariance functions is similar for all three stimulus situations, but varies with stimulus intensity and is different in different units. 4. The implication of the results is that the inhibition studied may either originate from the inhibition (suppression) seen in primary fibers or it may be the result of a true neural inhibition in the cochlear nucleus that occurs without any interneurons.

Single unit responses recorded from the first order neuron of the pigeon auditory system

Intracochlear recordings from the first order neurons in the eighth nerve of the pigeon were made with microelectrodes. The basic response characteristics of the single units were similar to those recorded from mammalian first order auditory neurons with the following exceptions: units responding solely to 'sweep frequencies' were found and one unit was found for which the rate of discharge decreased with increased intensity. Suppression of spontaneous activity during tonal stimulation was found within the response curve of the unit and for frequencies beyond the boundaries of the response curve. In addition, suppression of spontaneous activity was found after the termination of the tone. The amount of suppression and the recovery process varied considerably from unit to unit. The controlling variables were intensity and duration of the stimulus.

Normal critical bands in the cat

Critical bands in the cat were measured by a behavioural psychophysical method. Pure tones were masked by noise by variable bandwidth but constant total power, geometrically centred on the test tone; the point at which the masked threshold began to fall as the masker bandwidth was increased estimated the critical bandwidth. At 2 kHz the critical bandwidth was also measured from the wideband masked thresholds of both tones and noise of variable bandwidth: this produced the same result as the first method. The measured critical bandwidth was greater than previously published values of the effective bandwidths of single fibres of the auditory nerve. The results do not fit in with the commonly accepted theory that the critical band represents the resolution of the cochlea.

A study of neurone activity in the spiral ganglion of the cat's basal turn

A small region of the spiral ganglion in the cat was surgically exposed through the round window. Metal microelectrodes were used to record extracellularly the electrical activity of single spiral ganglion cells. The response characteristics of the cells seemed to be, in general, similar to those seen for auditory-nerve fibres recorded with micropipets in the internal auditory meatus. Data are presented on spontaneous activity, tuning curves, responses to clicks, continuous tones, tone bursts and noise bursts. The relation between frequency selectivity of units and location along the basilar membrane is discussed. -Some units differed in behaviour from auditory-nerve fibres with respect to dead times in interspike-interval histograms and shapes of poststimulus-time histograms of responses to tone bursts and noise bursts. The significance of these deviations is unknown.