CF Tones Research Articles

Interfield differences in intensity and frequency representation of evoked potentials in rat auditory cortex

Existing studies have demonstrated interfield differences in functional organizations and neuronal responsive properties at a single neuron level in the auditory cortex, suggesting complicated encoding of sound frequency and intensity. The objective of present work is, by characterizing cortical auditory evoked potentials (AEPs), to bridge neural characteristics between a single neuron and field levels and to identify the interfield differences in the auditory cortex specifically in terms of spatial representation, which will be useful in guiding future unit studies. The AEP mapping found that each of auditory fields, which could be identified by a different tonotopic representation, showed interfield differences in an intensity-dependent spatial change, amplitude, latency, and amplitude-SPL (sound pressure level) function. These results also showed that many aspects of cortical representation were based on the cochlear properties, yet some were inconsistent. The intensity-dependent shift of activation in AI paralleled the tonotopic axis, which was similar to the place code in cochlea, while the shift in AAF and VAF did not parallel. Nevertheless, the amplitude-SPL function suggested that an underlying mechanism of all these shifts can be a compressive nonlinearity to CF tone, which is possibly formed in the cochlea and still preserved in the cortex. These results suggest that each field modifies the representation to handle a different aspect of sound information, which can be better analyzed than the cochlear representation.

Facilitatory and inhibitory frequency tuning of combination-sensitive neurons in the primary auditory cortex of mustached bats.

Mustached bats, Pteronotus parnellii parnellii, emit echolocation pulses that consist of four harmonics with a fundamental consisting of a constant frequency (CF(1-4)) component followed by a short, frequency-modulated (FM(1-4)) component. During flight, the pulse fundamental frequency is systematically lowered by an amount proportional to the velocity of the bat relative to the background so that the Doppler-shifted echo CF(2) is maintained within a narrowband centered at approximately 61 kHz. In the primary auditory cortex, there is an expanded representation of 60.6- to 63. 0-kHz frequencies in the "Doppler-shifted CF processing" (DSCF) area where neurons show sharp, level-tolerant frequency tuning. More than 80% of DSCF neurons are facilitated by specific frequency combinations of approximately 25 kHz (BF(low)) and approximately 61 kHz (BF(high)). To examine the role of these neurons for fine frequency discrimination during echolocation, we measured the basic response parameters for facilitation to synthesized echolocation signals varied in frequency, intensity, and in their temporal structure. Excitatory response areas were determined by presenting single CF tones, facilitative curves were obtained by presenting paired CF tones. All neurons showing facilitation exhibit at least two facilitative response areas, one of broad spectral tuning to frequencies centered at BF(low) corresponding to a frequency in the lower half of the echolocation pulse FM(1) sweep and another of sharp tuning to frequencies centered at BF(high) corresponding to the CF(2) in the echo. Facilitative response areas for BF(high) are broadened by approximately 0.38 kHz at both the best amplitude and 50 dB above threshold response and show lower thresholds compared with the single-tone excitatory BF(high) response areas. An increase in the sensitivity of DSCF neurons would lead to target detection from farther away and/or for smaller targets than previously estimated on the basis of single-tone responses to BF(high). About 15% of DSCF neurons show oblique excitatory and facilitatory response areas at BF(high) so that the center frequency of the frequency-response function at any amplitude decreases with increasing stimulus amplitudes. DSCF neurons also have inhibitory response areas that either skirt or overlap both the excitatory and facilitatory response areas for BF(high) and sometimes for BF(low). Inhibition by a broad range of frequencies contributes to the observed sharpness of frequency tuning in these neurons. Recordings from orthogonal penetrations show that the best frequencies for facilitation as well as excitation do not change within a cortical column. There does not appear to be any systematic representation of facilitation ratios across the cortical surface of the DSCF area.

A unified theory of two-tone suppression and upward-spread of masking

Low-frequency two-tone suppressors (TTS) [Sachs and Abbas], and the upward spread of masking, have traditionally been treated as separate phenomena. We postulate that both these well-known response measures are due to nonlinear (NL) cochlear compression. Furthermore, we attribute this NL compression to a changing basilar membrane (BM) stiffness, which varies inversely with hair cell stimulus. A 65 dB SL low-frequency tonal masker/suppressor excites a broad basal region of the cochlea uniformly, raising the threshold of high-frequency probes throughout this portion of the cochlea. Sufficiently intense low-frequency maskers/suppressors cause a reduction in the stiffness in the base of the BM, resulting in the probe characteristic frequency shifting towards the base. This basal NL shift of the BM response, combined with high-pass filtering by the tectorial membrane, results in an attenuated basal hair-cell response. This change in gain accounts for the expansive slope of the upward spread of masking, having a maximum slope of 2.4 dB/dB. This slope implies a compression proportional to pressure to the 0.8≊2/2.4 power. The same model explains TTS if we treat the CF tone as the probe and the suppressing tone as the masker. The rate of growth of suppression, in this case, is also known to reach a maximum of 2.4 dB/dB [Delgutte].

Level-dependent representation of stimulus frequency in cat primary auditory cortex.

The tonotopicity of the cat's primary auditory cortex (AI) is thought to provide the framework for frequency-specific processing in that field. This study was designed to assess this postulate by examining the spatial distribution of neurons within AI that are activated by a single tonal frequency delivered to the contralateral ear. Distributions obtained at each of several stimulus levels were then compared to assess the influence of stimulus amplitude on the spatial representation of a given stimulus frequency in AI. Data were obtained from 308 single units in AI of four adult, barbiturate-anesthetized cats, using extracellular recording methods. Stimuli were 40-ms tone pulses presented through calibrated, sealed stimulating systems. In each animal, the CF (stimulus frequency to which the unit is most sensitive), threshold at CF, response/level function at CF, and binaural interactions were determined for isolated neurons (usually one per track) in 60-90 electrode tracks. For each unit, regardless of its CF, responses to 40 repetitions of contralateral tones of a single frequency, presented at each of four or five sound pressure levels (SPLs) in the range from 10 to 80 dB were obtained. Different test frequencies were used in each of four cats (1.6, 8.0, 11.0, and 16.0 kHz). For tones of each SPL, we generated maps of the response rates across the cortical surface. These maps were then superimposed on the more traditional maps of threshold CF. All units whose CF was equal to the test frequency could be driven at some SPL, given an appropriate monaural or binaural configuration of the stimulus. There was a clear spatial segregation of neurons according to the shapes of their CF tone response/level functions. Patches of cortex, often occupying more than 2 mm2, seemed to contain only monotonic or only nonmonotonic units. In three cortices, a patch of nonmonotonic cells was bounded ventrally by a patch of monotonic cells, and in one of these cases, a second patch of monotonic cells was found dorsal to the nonmonotonic patch. Contralateral tones of any given SPL evoked excitatory responses in discontinuous cortical territories. At low SPLs (10, 20 dB), small foci of activity occurred along the isofrequency line representing the test frequency. Many of these cells had nonmonotonic response/level functions. (ABSTRACT TRUNCATED AT 400 WORDS)

Neural processes in the dorsal cochlear nucleus of the anaesthetised cat investigated from unit responses to electrical stimulation of the auditory nerve

Extracellular responses of dorsal cochlear nucleus single units were recorded in response to biphasic, bipolar electrical stimulation of spiral ganglion cells and their peripheral processes using a banded electrode array in the scala tympani of the barbiturate anaesthetised cat. The DCN responses to this stimulus were the result of excitatory and suppressive (including inhibitory) processes. The excitatory responses from DCN units were usually within a range of 1.8–2.8 ms and these responses were probably the result of monosynaptic input from the auditory nerve. Latencies > 2.8 ms were most likely due to activation of di- and poly-synaptic pathways from auditory nerve fibres, except that latencies between 3.5–4.75 in hearing animals could have arisen from electrophonic mechanisms. Suppression of spontaneous activity was usually long acting, lasting > 70 ms following each pulse of the pulse train, but short acting suppression with a latency of 3.5–4.75 ms and a duration of < 10 ms was occasionally observed. These suppressive responses probably resulted from synaptic inhibitory input, but neural membrane properties may have contributed. In hearing animals, excitatory latencies within the range 1.8–5.2 ms were similar for units with different response area types or different PSTH patterns in response to acoustic CF tones or noise.

The responses of neurons in subdivisions of the inferior colliculus of cats to tonal, noise and vocal stimuli.

The aim of this study was to gain information from anesthetized cats about the differential coding properties of neurons in the three major subdivisions of the inferior colliculus: the central (CNIC) and external (EN) nuclei and dorsal cortex (DC). Stimuli were presented in the free field from a speaker facing the contralateral pinna. For each unit, the characteristic frequency (CF, where threshold was lowest) was determined, and impulse rates to CF tone bursts, noise bursts and four feline vocal stimuli were measured as a function of increasing sound pressure level (rate/level functions). Peristimulus-time histograms were computed for responses to all stimuli. Sustained firing patterns to CF stimuli were observed for 81% of units in CNIC, for 50% of units in EN and 27% of units in DC. Sustained discharges were evoked by noise in 78-100% of units in all regions, and by at least one vocal stimulus in 86% of units in CNIC, 82% in EN and 55% in DC. In the CNIC, non-monotonic rate/level functions to CF stimuli were more common (41%) than either monotonic or plateau functions, whereas the reverse was the case with noise and vocal stimuli. Non-monotonic functions were uncommon to any stimulus in EN and DC (21-24%). Vocal stimuli were more effective in terms of higher firing rates than noise or CF stimuli in 27% of units in CNIC, 82% in EN and 72% in DC. There were no units that responded exclusively to one vocal stimulus, but a high proportion of units in EN responded strongly to broad band stimuli, and some of these showed clear preferences for one vocal stimulus over others.

Inactivation of the DSCF area of the auditory cortex with muscimol disrupts frequency discrimination in the mustached bat.

1. The Jamaican mustached bat uses a biosonar signal (pulse) with eight major components: four harmonics each consisting of a long constant frequency (CF1-4) component followed by a short frequency-modulated (FM1-4) component. While flying, the bat adjusts the frequency of its pulse so as to maintain the CF2 of the Doppler-shifted echo at a frequency to which its cochlea is very sharply tuned. This Doppler shift (DS) compensation likely is mediated or influenced by the Doppler-shifted CF (DSCF) processing area of the primary auditory cortex, which only represents frequencies in the range of echo CF2s (60.6 to 62.3 kHz when the "resting" frequency of the CF2 is 61.0 kHz). 2. We trained four bats to discriminate between different trains of paired tone bursts that mimicked a bat's pulse CF2 and the accompanying echo CF2. The frequency of these CF2s ranged between 61.0 and 64.0 kHz. A discriminated shock avoidance procedure response was employed using a leg flexion. For one stimulus, the S+, the pulse and echo CF2s were the same frequency (delta f = 0, i.e., no Doppler shift). A leg flexion during the S+ turned off both the S+ and the scheduled shock. For a second stimulus, the S-, the echo CF2 was 0.05, 0.1, 0.3, 0.5, or 2.0 kHz higher than the pulse CF2. A delta f of 0.05 kHz was a frequency difference of 0.08%. No shock followed the S-, and leg flexions had no consequences. Correct responses consisted of a leg flexion during the S+ and no flexion during the S-; these responses were added together to compute the percentage of correct responses. When a bat correctly responded at better than 75% for all the delta f s, muscimol, a potent agonist of gamma-aminobutyric acid, was bilaterally applied to inactivate the DSCF area. Performance on each delta f discrimination was then measured. 3. Initial attempts to condition the bats to flex their legs to the CF tones mimicking part of the natural pulses and echoes failed. When broad-band noise bursts were substituted, however, the conditioned response was rapidly established. The noise band-width was gradually reduced and then replaced with the CF tones. Discrimination training with the tone burst trains then commenced. Throughout this procedure, the bats maintained their responding to the stimuli. The bats typically required approximately 20-30 sessions to perform consistently (> or = 75% correct responses) a discrimination involving a 2 kHz delta f.(ABSTRACT TRUNCATED AT 400 WORDS)

Rate-level functions of neurons in the inferior colliculus of cats measured with the use of free-field sound stimuli

1. The responses as a function of stimulus level of 125 single units in the inferior colliculus of anesthetized cats were studied with the use of free-field acoustic stimuli. 2. The characteristic frequency (CF; frequency at which threshold was lowest) of each unit was determined, and stimuli were presented from one of three speaker positions: 45 degrees contralateral to the midline, midline, and 45 degrees ipsilateral to the midline. 3. For each unit a variety of stimulus levels was presented at CF, and the total spike count was summed for 20 stimuli at each level. If time permitted, a similar series of levels of noise was presented. 4. Four classes of rate-level (RL) functions were observed. Monotonic increases in firing rate were observed in 10% of units stimulated with CF stimuli and 57% of units studied with noise. Nonmonotonic RL functions, for which firing first increased and then declined to less than 50% of the peak level, were observed in 61% of units responding to CF tones and in 10% responding to noise. Plateau functions, with shapes lying between these, accounted for 19% of CF responses and the remaining units excited by noise. Some very complex shapes that could not be categorized into the above groups were seen in the remaining 10% of the units responding to CF stimuli. 5. The RL functions of units studied with both noise and CF tones could belong to different classes; commonly, nonmonotonic RL functions to tones were associated with monotonic RL functions to noise. The noise thresholds averaged 10 dB, some 10-20 dB less sensitive than those to CF stimuli. 6. For the vast majority of both noise and tone responses, stimuli from the contralateral location were more effective than those from the other two positions in terms of a lower threshold, higher peak discharge rate, and, for nonmonotonic units, a lower sound level at which the function became nonmonotonic (turnover point). 7. The turnover points of nonmonotonic functions at any given CF could be spread broadly but, overall, tended to be concentrated between -6 and 44 dB. 8. The dynamic ranges (range of levels over which firing rate increased) were larger for monotonic and plateau functions than for nonmonotonic functions, which had dynamic ranges less than 45 dB. The median dynamic range for units stimulated with CF tones was 20 dB and for noise stimuli, 40 dB.(ABSTRACT TRUNCATED AT 400 WORDS)

Azimuthal sensitivity of neurons in primary auditory cortex of cats. I. Types of sensitivity and the effects of variations in stimulus parameters

1. Preliminary to studying the organization of azimuthal sensitivity of neurons along frequency-band strips in the primary auditory cortex (AI) of cat (see companion paper), this study examined the sensitivity of 251 units in cat AI to variations in the azimuthal location of sound sources in the frontal hemifield. Most units (231) were tested with tones at the characteristic frequency (CF; frequency to which the unit had the lowest threshold). Unit CFs ranged from 5 to 36 kHz. A large number of units (91) were tested with broadband noise stimuli, and a few units were also tested at other frequencies within the cell's tuning response area. 2. When tested at stimulus intensities 20-30 dB above CF or noise threshold, the different forms of azimuthal sensitivity exhibited by AI neurons could be divided into (1) contra-field azimuth functions; (2) ipsi-field functions; (3) central-field functions; (4) omnidirectional functions, and (5) multipeaked functions. Contra-field azimuth functions were the most prevalent, with 45.9% of units tested with CF tones and 42.9% of units tested with noise exhibiting this type of azimuthal sensitivity. Ipsi-field azimuthal sensitivity was found in 16.9% of units tested with CF tones and 19.8% of units tested with noise. Central-field azimuthal sensitivity was seen in 10.8% of units tested with CF tones and 17.6% of units tested with noise. Omnidirectional azimuthal sensitivity was seen in 19.9% of units tested with CF tones and 17.6% of units tested with noise, whereas multipeaked azimuthal sensitivity was found in 6.5% of units tested with CF tones and 5.5% of units tested with noise. 3. The effects of increasing stimulus intensity on azimuthal sensitivity were examined in 185 units tested with CF tones and 67 units tested with noise. For four major classes of azimuthal sensitivity (contra-field, ipsi-field, central-field and omnidirectional), the most common effect (approximately 60% of each class) was for the azimuth function to remain constant in form by the defining criteria for these classes. The next most common effect for all classes except omnidirectional azimuth functions was for an expansion of the azimuthal range eliciting responses. (The definition of omnidirectionality precluded any expansion of the response range in this class of azimuth function). A smaller number of units in some classes showed a compression of the azimuth function to a smaller response range, and others showed more complex expansive and compressive effects with increasing stimulus intensity.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple looks and temporal integration

Recent research on temporal resolution indicates that a person can listen using a temporal window with a time constant of less than 10 ms. A basic question is how information from these brief “looks” is combined and processed. The initial focus is on the old problem of temporal integration and data are reported from an experiment similar to Zwislocki's two-click experiment [J. Zwislocki, J. Acoust. Soc. Am. 32, 1046–1059 (1960)]. The data are well described by assuming true integration for brief click separations and by assuming two independent looks for separations longer than approximately 10 ms. Thus the data are more consistent with multiple looks than with true, long-duration temporal integration. To account for more general “integration” phenomena, a simple multiple-look strategy is compared with neural summation using spike data recorded from the auditory nerve of chinchilla for CF tones. The decision statistic is either an optimally weighted combination of the spike count from contiguous short bins (looks) or the total spike count summed over the duration of the tone. The time-intensity trading functions (threshold for d′ = I versus duration) for these schemes are very similar. This demonstrates, consistent with intuition, that a multiple-look scheme can show integration-like behavior and that such behavior does not imply true integration. [Work supported by NS12125.]

Coding of tone-pulse amplitude by single neurons in auditory cortex of albino rats (Rattus norvegicus)

We examined the neural representation of tone pulse amplitude in the auditory cortex of anesthetized albino rats. Rate-level functions for monaural, contralateral CF tones were obtained from single neurons. Most of these functions were saturating and monotonic in shape. The dynamic ranges of these functions were typically 5 to 35 dB in breadth, although the tail of this distribution extended to beyond 60 dB. The neurons with the widest dynamic ranges were usually those with the lowest CF thresholds. Nonmonotonic neurons were uncommon, and the nonmonotonicity was not as well developed as that seen in the cortex of cats and monkeys. For two individual rats, data are presented for neurons tuned to the same tone frequency in a single cerebral hemisphere. The CF thresholds of these neurons varied over a 50 dB range, and their collective dynamic ranges spanned at least 70 dB. These data provide a minimum estimate of the dynamic range of the cortical code for tone pulse amplitude in the rat. They suggest that there may be no serious mismatch between the neural and behavioral amplitude dynamic ranges.

Distribution of combination-sensitive neurons in the ventral fringe area of the auditory cortex of the mustached bat

1. The orientation sound (pulse) of the mustached bat, Pteronotus parnellii parnellii, consists of long constant-frequency components (CF1-4) and short frequency-modulated components (FM1-4). The auditory cortex of this bat contains several combination-sensitive areas: FM-FM, DF, VA, VF, and CF/CF. The FM-FM area consists of neurons tuned to a combination of the pulse FM1 and the echo FMn (n = 2, 3, or 4) and has an echo-delay (target-range) axis. Our preliminary anatomical studies with tritiated amino acids suggest that the FM-FM area projects to the dorsal fringe (DF) area, which in turn projects to the ventral fringe (VF) area. The aim of our study was to characterize the response properties of VF neurons and to explore the functional organization of the VF area. Acoustic stimuli delivered to the bats were CF tones, FM sounds, and their combinations mimicking the pulse emitted by the mustached bat and the echo. 2. Like the FM-FM and DF areas, the VF area is composed of three types of FM-FM combination-sensitive neurons: FM1-FM2, FM1-FM3, and FM1-FM4. These neurons show little or no response to a pulse alone, echo alone, single CF tone or single FM sound. They do, however, show a strong facilitative response to a pulse-echo pair with a particular echo delay. The essential components in the pulse-echo pair for facilitation are the FM1 of the pulse and the FMn of the echo.(ABSTRACT TRUNCATED AT 250 WORDS)

Frequency selectivity of single cochlear-nerve fibers based on the temporal response pattern to two-tone signals.

The physiological basis of auditory frequency selectivity was investigated by recording the temporal response patterns of single cochlear-nerve fibers in the cat. The characteristic frequency and sharpness of tuning was determined for low-frequency cochlear-nerve fibers with two-tone signals whose frequency components were of equal amplitude and starting phase. The measures were compared with those obtained with sinusoidal signals. The two-tone characteristic frequency (2TCF) is defined as the arithmetic-center frequency at which the fiber is synchronized to both signal frequencies in equal measure. The 2TCF closely corresponds to the characteristic frequency as determined by the frequency threshold curve. Moreover, the 2TCF changes relatively little (2%-12%) over a 60-dB intensity range. The 2TCF generally shifts upward with increasing intensity for cochlear-nerve fibers tuned to frequencies below 1 kHz and shifts downward as a function of intensity for units with characteristic frequencies (CF's) above 1 kHz. The shifts in the 2TCF are considerably smaller than those observed with sinusoidal signals. Filter functions were derived from the synchronization pattern to the two-tone signal by varying the frequency of one of the components over the fiber's response area while maintaining the other component at the 2TCF. The frequency selectivity of the two-tone filter function was determined by dividing the vector strength to the variable frequency signal by the vector strength to the CF tone. The filter function was measured 10 dB down from the peak (2T Q 10 dB) and compared with the Q 10 dB of the frequency threshold curve. The correlation between the two measures of frequency selectivity was 0.72. The 2T Q 10 dB does change as a function of intensity. The magnitude and direction of the change is dependent on the sharpness of tuning at low and moderate sound-pressure levels (SPL's). The selectivity of the more sharply tuned fibers (2T Q 10 dB greater than 3) diminishes at intensities above 60 dB SPL. However, the broadening of selectivity is relatively small in comparison to discharge rate-based measures of selectivity. The selectivity of the more broadly tuned units remains unchanged or improves slightly at similar intensity levels. The present data indicate that the frequency selectivity and tuning of low-frequency cochlear-nerve fibers are relatively stable over a 60-dB range of SPL's when measured in terms of their temporal discharge properties.

Point process model of LSO unit discharges

A point process model for the responses of single auditory‐nerve fibers to time‐varying stimuli was extended to describe lateral superior olive (LSO) unit responses to monaural cf tones. In contrast to auditory‐nerve fiber discharges, successive interspike‐intervals of LSO unit discharges exhibit serial dependence. This dependence was found to be well described by a first‐order Markov model of the underlying point process. The characteristics of this model were measured from the conditional interval histogram: the usual interspike interval histogram restricted to those intervals preceded by an interval of a specified duration. From this quantity, the conditional hazard function was derived. This computation revealed that the influence of the pentultimate interval was to shift the hazard function. This detailed description of the serial dependence cannot be derived from measurements of serial correlation coefficients. The cf tone burst responses of LSO units are characterized as “chopper”‐type. Within the context of the point process model outlined above, the parameters of the model were measured from the sustained portion of the tone burst response. Using the model thus derived, the initial chopping portion of the response was predicted. [Work supported by NINCDS and NSF.]

Onset responses in single auditory-nerve fibers

The maximum response at the onset of a CF tone burst was obtained as a function of sound intensity level and prior adaptation. A stimulus rise time of 1 ms was used to maximize the onset response as well as to produce a simultaneously recorded whole-nerve action potential (AP). As intensity increased, the onset response increased and in some units reached a maximum value of 1 spike per stimulus presentation within a 1-ms interval. The absolute refractory period of the fiber prevented a further increase in the number of spikes. Nevertheless, further increases in intensity affected the onset response by reducing the latency to the onset spike and the variability in its time of occurrence. Consequently, the maximum response in spikes per second continued to increase when it was measured using a sufficiently small time interval, e.g., 55 μs. Prior adaptation reduced the magnitude and increased the latency and variability of the onset response. These effects could account for some of the concurrent changes in the AP.

Effects of cholinergic agents on single‐unit responses in the rat inferior colliculus

Cholinergic agents were injected iontophoretically during unit recordings from the inferior colliculi of rats anesthetized with urethane. Prior to drug administration, neurons typically showed little spontaneous activity and possessed nonmonotonic rate‐versus‐intensity functions for pure‐tone stimuli. This was especially true near CF. Maximum driven rate was usually greater for tones below CF than for tones at or above CF. Application of the cholinergic agonist carbamylcholine usually produced dose‐dependent decreases in driven rates to CF tones but did not alter response threshold. When injected alone, cholinergic antagonists usually produced dose‐dependent increases in driven activity and, at higher doses, generation of “bursty” spontaneous activity. Interactions between agonists and antagonists were common. More units were sensitive to atrophine, a muscarinic antagonist, than to curare, a nicotinic antagonist. Some neurons were sensitive only to curate, and a few neurons were sensitive to both curate and atropine. The data suggest that acetylcholine is not the afferent neurotransmitter in the rat inferior colliculus, but that it mediates a tonically active inhibition which has both muscarinic and nicotinic properties. [Work supported by NINCDS grant NS‐14880 and NSF grant BNS‐8006643.]

Encoding of steady-state vowels in the auditory nerve: representation in terms of discharge rate.

Responses of large populations of auditory-nerve fibers to synthesized steady-state vowels were recorded in anesthetized cats. Driven discharge rate to vowels, normalized by dividing by saturation rate (estimated from the driven rate to CF tones 50 dB above threshold), was plotted versus fiber CF for a number of vowel levels. For the vowels /I/ and /e/, such rate profiles showed a peak in the region of the first formant and another in the region of the second and third formants, for sound levels below about 70 dB SPL. For /a/ at levels below about 40 dB SPL there are peaks in the region of the first and second formants. At higher levels these peaks disappear for all the vowels because of a combination of rate saturation and two-tone suppression. This must be qualified by saying that rate profiles plotted separately for units with spontaneous rates less than one spike per second may retain peaks at higher levels. Rate versus level functions for units with CFs above the first formant can saturate at rates less than the saturation rate to CF to-es or they can be nonmonotonic; these effects are most likely produced by the same mechanism as that involved in two-tone suppression.

Effects of time on decremental responses of single auditory-nerve fibers

Stimuli consisted of CF tone bursts with superimposed increments or decrements in sound intensity. The intensity changes were applied at various time delays from tone onset and the resulting changes in response firing rate were computed from PST histograms. Consistent with previous results [R. L. Smith, J. Acoust. Soc. Am. 65, 166–178 (1979)], the change in response was independent of the time delay to the change in intensity, as long as firing rates were averaged over time windows exceeding 10 ms. When the same data was analyzed using windows of less than 2 ms, the initial increment in response caused by an increment in intensity remained independent of time delay. In contrast, the initial decrement in response caused by a decrement in intensity decreased with increasing time delay and appeared to be closely related to the amount of adaptation preceding the decrement. Similar decreases were observed in the response modulation produced by amplitude-modulated stimuli. Thus, while incremental and decremental responses are basically additive, the initial portion of the decremental response appears to reflect a qualitatively different process.

Rate versus level functions for auditory‐nerve fibers in cats: Bandlimited noise bursts

We will generalize our previous description of the responses to two‐tone stimuli to allow the prediction of responses to continuous spectrum stimuli. For noise bursts of one bandwidth and center frequency, discharge rate was measured as a function of spectral level. Such rate‐level functions were measured for a number of noise bandwidths; either the low‐ or high‐cutoff frequencies were set at fiber characteristic frequency. Narrow bands of noise produce rate‐level functions similar to those for CF tones. For noise bands centered above CF, rate‐level functions become less steep as bandwidth is increased and approach the same saturation rate as that produced by CF tones. For wide bands of noise centered below CF, rate‐level functions can be nonmonotonic or appear to plateau at rates less than the saturation rate to CF tones. Thus, bands centered either above or below CF can produce lower discharge rates than do narrow bands of the same spectral level. This rate reduction has properties similar to those of two‐tone suppression. [Supported by NINCDS and NINGMS.]

Statistical analysis and interpretation of the initial response of cochlear nucleus neurons to tone bursts.

1. Subject of investigation is the initial response of cochlear nucleus neurons and units presumed to be auditory nerve fibres to CF tone burst stimulation. 2. The initial response is characterized by computing the distribution of the latency of the first spike and of the duration of the first interval in the ensemble of responses to a large number of stimuli. 3. In many of the neurons the properties of both distributions appear to be related. The presumed auditory nerve fibres and spontaneously active cochlear nucleus neurons showing only activation responses to tonal stimuli (A type) exhibit irregularity in both response onset and intervals. Minimum latency and minimum first intervals are short. On the other hand, spontaneously active neurons with both activation and suppression in the response area (AS type) and silent neurons showing only activation (A(S) type) often show a more precisely timed onset of response and narrow interval distributions. In many neurons this leads to oscillations in the PSTH (chopping). In these neurons minimum latency and minimum first interval have higher values. The longer minimum latency cannot be attribute-d to longer pure time delays in these neurons. 4. The results are interpreted as speaking in favour of temporal integration as an important mechanism in many of the AS and A(S) neurons, particularly those in the DCN. The firing patterns of A neurons are thought to indicate virtual absence of this mechanism. 5. Using pure time delay estimates derived from cross-correlation functions, computed from the responses to stationary noise, an attempt is made to estimate the integration time in the cochlear and in the cochlear nucleus neurons.