Characteristic Frequency Tones Research Articles

The Spectral Extent of Phasic Suppression of Loudness and Distortion-Product Otoacoustic Emissions by Infrasound and Low-Frequency Tones

We investigated the effect of a biasing tone close to 5, 15, or 30 Hz on the response to higher-frequency probe tones, behaviorally, and by measuring distortion-product otoacoustic emissions (DPOAEs). The amplitude of the biasing tone was adjusted for criterion suppression of cubic DPOAE elicited by probe tones presented between 0.7 and 8 kHz, or criterion loudness suppression of a train of tone-pip probes in the range 0.125–8 kHz. For DPOAEs, the biasing-tone level for criterion suppression increased with probe-tone frequency by 8–9 dB/octave, consistent with an apex-to-base gradient of biasing-tone-induced basilar membrane displacement, as we verified by computational simulation. In contrast, the biasing-tone level for criterion loudness suppression increased with probe frequency by only 1–3 dB/octave, reminiscent of previously published data on low-side suppression of auditory nerve responses to characteristic frequency tones. These slopes were independent of biasing-tone frequency, but the biasing-tone sensation level required for criterion suppression was ~ 10 dB lower for the two infrasound biasing tones than for the 30-Hz biasing tone. On average, biasing-tone sensation levels as low as 5 dB were sufficient to modulate the perception of higher frequency sounds. Our results are relevant for recent debates on perceptual effects of environmental noise with very low-frequency content and might offer insight into the mechanism underlying low-side suppression.

Количественная оценка латентных периодов ответов в рецептивных полях нейронов слухового центра среднего мозга и слуховой коры мыши

The variability of response latencies in the excitatory frequency receptive fields of single neurons of the mouse auditory midbrain and cortex was assessed in present study. Our results showed that there are fundamental differences in neuronal response latencies dynamics between the brainstem and cortical levels of the auditory system. The latencies of responses of the inferior colliculus neurons to the tones of non-characteristic frequencies varied in different units from 2 up to 42 ms. In the central nucleus of inferior colliculus neurons, the shift of the neuronal response latency from one assessed while the characteristic frequency tone was presented, as a rule, occurred as a latency increase. In the primary auditory cortex neurons, the response latencies in their excitatory frequency receptive fields were more variable than those of the inferior colliculi neurons, and their variation range reached 85 ms. The obtained data suggest the changes in contribution of inhibition to the sound analysis along the ascending auditory pathway from the auditory brainstem centers to the auditory cortex.

Non-tip auditory-nerve responses that are suppressed by low-frequency bias tones originate from reticular lamina motion

Recent cochlear mechanical measurements show that active processes increase the motion response of the reticular lamina (RL) at frequencies more than an octave below the local characteristic frequency (CF) for CFs above 5 kHz. A possible correlate is that in high-CF (>5 kHz) auditory-nerve (AN) fibers, responses to frequencies 1-3 octaves below CF (“tail” frequencies) can be inhibited by medial olivocochlear (MOC) efferents. These results indicate that active processes enhance the sensitivity of tail-frequency RL and AN responses. Perhaps related is that some apical low-CF AN fibers have tuning-curve (TC) “side-lobe” response areas at frequencies above and below the TC-tip that are MOC inhibited. We hypothesized that the tail and side-lobe responses are enhanced by the same active mechanisms as CF cochlear amplification. If responses to CF, tail-frequency, and TC-side-lobe tones are all enhanced by prestin motility controlled by outer-hair-cell (OHC) transmembrane voltage, then they should depend on OHC stereocilia position in the same way. To test this, we cyclically changed the OHC-stereocilia mechano-electric-transduction (MET) operating point with low-frequency “bias” tones (BTs) and increased the BT level until the BT caused quasi-static OHC MET saturation that reduced or “suppressed” the gain of OHC active processes. While measuring cat AN-fiber responses, 50 Hz BT level series, 70–120 dB SPL, were run alone and with CF tones, or 2.5 kHz tail-frequency tones, or side-lobe tones. BT-tone-alone responses were used to exclude BT sound levels that produced AN responses that might obscure BT suppression. Data were analyzed to show the BT phase that suppressed the tone responses at the lowest sound level. We found that AN responses to CF, tail-frequency, and side-lobe tones were suppressed at the same BT phase in almost all cases. The data are consistent with the enhancement of responses to CF, tail-frequency, and side-lobe tones all being due to the same OHC-stereocilia MET-dependent active process. Thus, OHC active processes enhance AN responses at frequencies outside of the cochlear-amplified TC-tip region in both high- and low-frequency cochlear regions. The data are consistent with the AN response enhancements being due to enhanced RL motion that drives IHC-stereocilia deflection by traditional RL-TM shear and/or by changing the RL-TM gap. Since tail-frequency basilar membrane (BM) motion is not actively enhanced, the tail-frequency IHC drive is from a vibrational mode little present on the BM, not a “second filter” of BM motion.

Systemic Nicotine Increases Gain and Narrows Receptive Fields in A1 via Integrated Cortical and Subcortical Actions.

Nicotine enhances sensory and cognitive processing via actions at nicotinic acetylcholine receptors (nAChRs), yet the precise circuit- and systems-level mechanisms remain unclear. In sensory cortex, nicotinic modulation of receptive fields (RFs) provides a model to probe mechanisms by which nAChRs regulate cortical circuits. Here, we examine RF modulation in mouse primary auditory cortex (A1) using a novel electrophysiological approach: current-source density (CSD) analysis of responses to tone-in-notched-noise (TINN) acoustic stimuli. TINN stimuli consist of a tone at the characteristic frequency (CF) of the recording site embedded within a white noise stimulus filtered to create a spectral “notch” of variable width centered on CF. Systemic nicotine (2.1 mg/kg) enhanced responses to the CF tone and to narrow-notch stimuli, yet reduced the response to wider-notch stimuli, indicating increased response gain within a narrowed RF. Subsequent manipulations showed that modulation of cortical RFs by systemic nicotine reflected effects at several levels in the auditory pathway: nicotine suppressed responses in the auditory midbrain and thalamus, with suppression increasing with spectral distance from CF so that RFs became narrower, and facilitated responses in the thalamocortical pathway, while nicotinic actions within A1 further contributed to both suppression and facilitation. Thus, multiple effects of systemic nicotine integrate along the ascending auditory pathway. These actions at nAChRs in cortical and subcortical circuits, which mimic effects of auditory attention, likely contribute to nicotinic enhancement of sensory and cognitive processing.

Mechanisms underlying azimuth selectivity in the auditory cortex of the pallid bat

This study focused on mechanisms underlying azimuth selectivity in the primary auditory cortex (A1) of pallid bats. The pallid bat listens to prey-generated noise (5–35 kHz) to localize and hunt terrestrial prey. The region of A1 tuned between 5 and 35 kHz consists of two clusters of neurons distinguished by interaural intensity difference (IID) selectivity: binaurally inhibited (EI) and peaked. The first aim of this study was to use sequential dichotic/free-field stimulation to test the hypothesis that IID is the primary cue underlying azimuth selectivity in neurons tuned in the prey-generated noise frequency band. IID selectivity and ear directionality at the neuron's characteristic frequency (CF) were used to predict azimuth selectivity functions. The predicted azimuth selectivity was compared with the actual azimuth selectivity from the same neurons. Prediction accuracy was similarly high for EI neurons and peaked neurons with low CF, whereas predictions were increasingly inaccurate with increasing CF among the peaked neurons. The second aim of this study was to compare azimuth selectivity obtained with noise and CF tones to determine the extent to which stimulus bandwidth influences azimuth selectivity in neurons with different binaural properties. The azimuth selectivity functions were similar for the two stimuli in the majority of EI neurons. A greater percentage of peaked neurons showed differences in their azimuth selectivity for noise and tones. This included neurons with multiple peaks when tested with tones and a single peak when tested with noise. Taken together, data from the two aims suggest that azimuth tuning of EI neurons is primarily dictated by IID sensitivity at CF. Peaked neurons, particularly those with high CF, may integrate IID sensitivity across frequency to generate azimuth selectivity for broadband sound. The data are consistent with those found in cat and ferret A1 in that binaurally facilitated neurons depend to a greater extent (compared to EI neurons) on spectral integration of binaural properties to generate azimuth selectivity for broadband stimuli.

Analysis of single unit activity evoked by tones amplitude-modulated with low-frequency noise in frog medulla

A method for the construction of shuffled autocorrelation functions was used to characterize single units located in the cochlear nucleus of grassy frog (Rana temporaria). A continuous characteristic frequency tone modulated by repeating pieces of low-frequency noises was used as a stimulus. This function was calculated as the correlation between the firing discharges evoked by different repeating pieces of the low-frequency noise. This approach allows one to exclude the influence of refractoriness and also considerably increases the repres. A comparison of the shiffled autocorrelation function with the conventional autocorrelation function allows one to estimate the temporal dynamics of changes in the postspike excitability of the neuron. An analysis of several examples demonstrates the possibility of facilitation just after the period of absolute refractoriness in some tonic units. Neurons demonstrating only a phasic response to the onset of unmodulated voice-frequency pieces were capable to selectively respond to special moments of noise amplitude modulation. These cells demonstrate an extremely high synchronism of reaction. In the cochlear nucleus of the frog, we described a neuron characterized by the internal periodicity of firing, which is not connected with the dynamics of arriving stimuli.

Detection of cavitation in situ operation of kinetic pumps: Effect of cavitation on the characteristic discrete frequency component

Cavitation within a pump causes structural vibration with noise, among other by-products. Experiments have shown that there is a discrete frequency or broadband peak within the audible noise spectra, which is in strong correlation with the development of the cavitation process in the pump. Furthermore, the peak of the discrete frequency, or broadband peak, coincides with the net positive suction head (NPSH) critical value, which corresponds to a 3% drop in the total delivery head. Therefore, the discrete frequency tone can be used to detect the incipience of cavitation and its development as well as to determine the NPSH required or critical value with in situ operation of a pump. In this study, we wanted to clarify the mechanism of noise generation, which is responsible for the discrete frequency component and on which cavitation has an important effect. For this purpose, three different measurement methods were used: the first is based on measurement of the sound pressure level (SPL) in the surrounding air, the second is based on measurement of the underwater acoustics and the third is based on measurement of the structural vibration. Experiments have shown that the characteristic discrete frequency tone, which is in close correlation with the cavitation process, is a result of structural vibrations (modes) or resonances caused by implosion of bubbles and bombardment of the inner surfaces of the pump.

Comment on “Measuring power production in the mammalian cochlea” [Curr. Biol. 17, 1340 (2007)

Recently, a paper by Lakashkin et al. (2007) ("Power amplification in the mammalian cochlea," Curr. Biol. 17, 1340-1344) was published on how power can be measured in the mammalian cochlea. The general subject is of current widespread interest, so the question of whether the method used by Lakashkin et al. is valid may be of interest to the readers of this journal. Power generation in the cochlea can account for the extraordinary sensitivity of hearing. Lukashkin et al. claimed to provide a direct proof of cochlear power generation. A first-order spring-dashpot system was used to model the organ of Corti. The power flux direction can be derived from the sign of the phase difference between the force and displacement, which can be presented as a "hysteresis plot." Basilar membrane (BM) vibration near the characteristic frequency (CF) was measured while applying a low-frequency modulation tone together with the CF tone. A force was derived from the modulation profile of the BM CF vibration and when plotted versus the displacement at the modulation frequency, the function had a counterclockwise direction of hysteresis, suggesting power generation. In this letter, we present comments on the analysis in the report: (1) that it is not appropriate to analyze at the modulation frequency to derive the power generation at CF; (2) that the derivation of a force from just the displacement profile is not justified, followed by an alternative interpretation of the experimental data.

Use of audible sound for safe operation of kinetic pumps

Safe operation of kinetic pumps, as liquid movers, can be threatened by cavitation phenomenon, among others. Cavitation is the Achilles heel of kinetic pumps. It can deteriorate hydraulic performance, damage the pump by pitting and material erosion, and structure vibration and resulting noise. Cavitation can appear within the entire range of operating conditions; therefore it must, by all means, be prevented. To prevent cavitation in a pump, we have to know the beginning and development of cavitation in the pump. For this purpose, the emitted noise in the audible range can be used, among other possibilities. Experiments have shown that there is a discrete frequency tone within the audible noise spectra, which is in strong correlation with the development of the cavitation process in the pump, and so with the net positive suction head ( NPSH) critical value, which corresponds to the 3% drop in the total delivery head. The discrete frequency tone can thus be used to detect the incipient of cavitation and its development as well as to prevent the onset of the cavitation process in the pump and also in situ operation. Two different measurement methods were used to clarify the noise-generating mechanism, which is responsible for the discrete frequency component: by using a microphone and an accelerometer. Experiments have shown that the characteristic discrete frequency tone, which is in close correlation with the cavitation process, is a result of structural vibrations (modes) or resonances caused by implosion of bubbles and bombardment of the inner surfaces of the pump.

Physiology and circuitry of frequency tuning in primary auditory cortex

Event Abstract Back to Event Physiology and circuitry of frequency tuning in primary auditory cortex Monica N. O’Connell1, 2*, P. Lakatos1, A. Falchier1, A. Barczak1, 3, D. Javitt1, 3 and C.E Schroeder1, 4 1 Cognitive Neuroscience and Schizophrenia Prog., Nathan Kline Institute, United States 2 Prog.Cognitive Neuroscience, City College CUNY, United States 3 Department of Psychiatry, New York University School of Medicine, United States 4 Columbia University College of Physicians and Surgeons, Dep. of Psychiatry, United States In the present study, we investigated the contrasting physiology and laminar activity profiles for characteristic frequency (CF) pure tones vs. non-CF pure tones at sites in primary auditory cortex (A1). We analyzed laminar current source density (CSD) and multiunit activity (MUA) profiles obtained with linear array multicontact electrodes in primary auditory cortex of awake macaques. CSD and MUA index synaptic input patterns (transmembrane current flow) and related neuronal firing patterns respectively. Based on the properties of responses to CF and non-CF tones, we differentiated 2 types of input profiles: “driving” and “modulatory.” The response to driving inputs was characterized by excitation (current sink and MUA increase) beginning in Layer 4, then spreading to the supra- and infragranular layers. This classic “feedforward” activation pattern is predictable from the laminar pattern of ventral MGN neuron projections to A1, with subsequent relay projections to extragranular layers. “Modulatory” CSD responses associated with non-CF tones began in Layers 2/3, followed by the activation of lower layers, accompanied by slight MUA changes noticeable mainly in the granular layer. This type of laminar response profile combined with its early onset in the supragranular layers could be indicative of a “non-specific” or “higher-order” thalamic input. In single trial, time frequency analysis, CF responses fit the criteria of “evoked” processes (i.e., substantial post-stimulus amplitude increase), while non-CF pure tone related activity had the characteristics of “phase-reset” processes (i.e., pre- to post-stimulus phase concentration without an increase in post-stimulus amplitude). In several cases the phase of reset oscillations was frequency specific. We speculate that resetting to low excitability phases by non-characteristic frequency pure tones aids in sharpening/changing the frequency tuning properties of the local neuronal population. Conference: 10th International Conference on Cognitive Neuroscience, Bodrum, Turkey, 1 Sep - 5 Sep, 2008. Presentation Type: Poster Presentation Topic: Perceptual Processing and Recognition Citation: O’Connell MN, Lakatos P, Falchier A, Barczak A, Javitt D and Schroeder C (2008). Physiology and circuitry of frequency tuning in primary auditory cortex. Conference Abstract: 10th International Conference on Cognitive Neuroscience. doi: 10.3389/conf.neuro.09.2009.01.331 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 15 Dec 2008; Published Online: 15 Dec 2008. * Correspondence: Monica N O’Connell, Cognitive Neuroscience and Schizophrenia Prog., Nathan Kline Institute, Orangeburg, United States, oconnell@nki.rfmh.org Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Monica N O’Connell P. Lakatos A. Falchier A. Barczak D. Javitt C.E Schroeder Google Monica N O’Connell P. Lakatos A. Falchier A. Barczak D. Javitt C.E Schroeder Google Scholar Monica N O’Connell P. Lakatos A. Falchier A. Barczak D. Javitt C.E Schroeder PubMed Monica N O’Connell P. Lakatos A. Falchier A. Barczak D. Javitt C.E Schroeder Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Reorganization of receptive fields following hearing loss in inferior colliculus neurons

We explored frequency and intensity encoding in the inferior colliculus (IC) of the C57 mouse model of sensorineural hearing loss. Consistent with plasticity reported in the IC of other models of hearing loss, frequency response areas (FRAs) in hearing-impaired (HI) mice were broader with fewer high-frequency units than normal-hearing (NH) mice. The broad FRAs recorded from HI mice had lower cutoffs on the low frequency edge of the FRA. Characteristic frequency (CF) and sharpness of tuning (Q10) calculated from the FRA were used to divide the sample into four categories: low-CF sharp-FRA, low-CF broad-FRA, high-CF sharp-FRA, and high-CF broad-FRA units. Rate-intensity functions (RIFs) for CF tones and noise were used to determine the minimum and maximum response counts as well as the sound pressure levels resulting in 10%, 50%, and 90% of the maximum spike count. Tone RIFs of broad FRA units were shifted to the right of tone RIFs of sharp FRA units in both NH and HI mouse IC, regardless of the unit CF. The main effects of hearing loss were seen in the noise RIFs. The low-CF broad-FRA units in HI mice had elevated responses to noise, and the high-CF sharp-FRA units in HI mice had lower maximum rates, as compared with the units recorded from NH mice. These results suggest that, as the IC responds to peripheral hearing loss with changes in the representation of frequency, an altered balance between inhibitory and excitatory inputs to the neurons recorded from the HI mice alters aspects of the units’ intensity encoding. This altered balance likely occurs, at least in part, outside of the IC.

Contributions of Intrinsic Neural and Stimulus Variance to Binaural Sensitivity

The discrimination of a change in a stimulus is determined both by the magnitude of that change and by the variability in the neural response to the stimulus. When the stimulus is itself noisy, then the relative contributions of the neural (intrinsic) and stimulus induced variability becomes a critical question. We measured the contribution of intrinsic neural noise and interstimulus variability to the discrimination of interaural time differences (ITDs) and interaural correlation (IAC). We measured discharge rate versus characteristic frequency (CF) tone ITD functions, and CF-centered narrowband noise ITD and IAC functions in interleaved blocks in the same units in the inferior colliculus of urethane-anesthetized guinea pigs. Ten "frozen" tokens of noise were synthesized and the responses to each token were separately analyzed to allow the relative contributions of intrinsic and stimulus variability to be assessed. ITD and IAC discrimination thresholds were determined for a simulated two-interval forced-choice experiment, based on the firing rate distributions, using receiver operating characteristic analysis. On average, between stimulus variability contributed 19% (range, 1.5-30%) of the variance in noise ITD discrimination and 27% (range, 3-50%) in IAC discrimination. Noise ITD thresholds were slightly higher than tone ITD thresholds. Taking the mean of the thresholds for individual noise tokens gave a similar result to pooling across all noise tokens. This implies that although the stimulus induced variability is measurable, it is insignificant in relation to the intrinsic noise in ITD and IAC discrimination.

Estimating the rate of response growth using above characteristic-frequency maskers

Growth of forward masking was measured to determine the relative rate of basilar-membrane (BM) response growth at the characteristic-frequency (CF) place that was stimulated with a higher-frequency tone. Two signal frequencies (fs) were used, 1 and 4 kHz. The masker frequency was 1.1fs or 1.2fs. A lower-frequency masker (0.6fs) was also used to replicate previously published data. Previous data obtained using a method for deriving the BM input-output function based on temporal masking curves suggested a linear growth of the BM response to a tone with a frequency equal or higher than 1.1fs and a compressive growth for the CF tone. This study demonstrates that the growth-of-masking method for deriving the BM response provides a similar result (indicating a compressive growth for the CF tone) when a low-frequency masker is used but leads to a fundamentally different result (indicating an expansive response growth) when a higher-frequency masker is used. The expansive growth is observed even when a low-pass noise is used to limit potential off-frequency listening. The data will be related to mechanical BM data and the equivalence of the two psychophysical methods for deriving BM compression will be discussed. [Supported by NIH Grants DC00683 and DC006804.]

Neurons in the Medial Nucleus of the Trapezoid Body and Superior Paraolivary Nucleus of the Rat May Play a Role in Sound Duration Coding

We describe neurons in two nuclei of the superior olivary complex that display differential sensitivities to sound duration. Single units in the medial nucleus of the trapezoid body (MNTB) and superior paraolivary nucleus (SPON) of anesthetized rats were studied. MNTB neurons produced primary-like responses to pure tones and displayed a period of suppressed spontaneous activity after stimulus offset. In contrast, neurons of the SPON, which receive a strong glycinergic input from MNTB, showed very little or no spontaneous activity and responded with short bursts of action potentials after the stimulus offset. Because SPON spikes were restricted to the same time window during which suppressed spontaneous activity occurs in the MNTB, we presume that SPON offset activity represents a form of postinhibitory rebound. Using characteristic frequency tones of 2- to 1,000-ms duration presented 20 dB above threshold, we show that the profundity and duration of the suppression of spontaneous activity in MNTB as well as the magnitude and first spike latency of the SPON offset response depend on stimulus duration as well as on stimulus intensity, showing a tradeoff between intensity and duration. Pairwise comparisons of the responses to stimuli of various durations revealed that the duration sensitivity in both nuclei is sharpest for stimuli <50 ms.

Age-Related Changes in the Inhibitory Response Properties of Dorsal Cochlear Nucleus Output Neurons: Role of Inhibitory Inputs

Age-related hearing loss frequently results in a loss in the ability to discriminate speech signals, especially in noise. This is attributable, in part, to a loss in temporal resolving power and ability to adjust dynamic range. Circuits in the adult dorsal cochlear nucleus (DCN) have been shown to preserve signal in background noise. Fusiform cells, major DCN output neurons, receive focused glycinergic inputs from tonotopically aligned vertical cells that also project to the ventral cochlear nucleus. Glycine-mediated inhibition onto fusiform cells results in decreased tone-evoked activity as intensity is increased at frequencies adjacent to characteristic frequency (CF). DCN output is thus shaped by glycinergic inhibition, which can be readily assessed in recordings from fusiform cells. Previous DCN studies suggest an age-related loss of markers for glycinergic neurotransmission. The present study postulated that response properties of aged fusiform cells would show a loss of inhibition, resembling conditions observed with glycine receptor blockade. The functional impact of aging was examined by comparing response properties from units meeting fusiform-cell criteria in young and aged rats. Fusiform cells in aged animals displayed significantly higher maximum discharge rates to CF tones than those recorded from young-adult animals. Fusiform cells of aged rats displayed significantly fewer nonmonotonic CF rate-level functions and an age-related change in temporal response properties. These findings are consistent with an age-related loss of glycinergic input, likely from vertical cells, and with findings from other sensory aging studies suggesting a selective age-related decrement in inhibitory amino acid neurotransmitter function.

Timing of pure tone and noise-evoked responses in macaque auditory cortex

We compared onset latencies for characteristic frequency pure tone and broadband noise responses in AI and posterior belt regions of the auditory cortex in awake macaques. We found that (1) in AI, responses to characteristic frequency tones and broadband noise have similar latencies, (2) in belt regions, characteristic frequency tone and broadband noise latencies differ significantly; broadband noise latencies are shorter, while characteristic frequency tone latencies are longer than corresponding values in AI, (3) for both pure tone and broadband noise responses in AI, latency decreases with increasing characteristic frequency and (4) despite a similar inverse relationship of tone latency and local characteristic frequency in belt areas, broadband noise latencies are uniformly short, and appear unrelated to local characteristic frequency. Dissociation of broadband noise and pure tone latencies may reflect the use of parallel anatomical routes into belt regions.

Balanced inhibition and excitation underlies spike firing regularity in ventral cochlear nucleus chopper neurons

Ventral cochlear nucleus stellate cells respond to characteristic frequency (CF) tones with sustained (C(S)), transient (C(T)) or onset chopping (O(C)) activity. The mechanisms underlying these different response patterns are not fully understood, and the present study used in vivo intracellular recordings (n = 42) in urethane-anaesthetized rats to examine the possible influence of inhibition on action potential regularity. Hyperpolarization following the offset of a CF tone burst was used as a measure of on-CF inhibition. A cluster analysis based on several membrane potential features, including on-CF inhibition, discriminated three groups in addition to the C(S) response type - two types of C(T) responses and the O(C) type. The different patterns of firing regularity exhibited by C(S/T) neurons reflected different thresholds or degrees of overlap between these cells' narrowly tuned excitatory and inhibitory inputs. C(T) cells with closely matched inhibitory and excitatory response areas showed substantial on-CF inhibition and the greatest decline in firing regularity during a CF tone, whereas those with a mismatch between their response areas showed lateral inhibition and a less marked decline in firing regularity. The presence of inhibition in C(S) neurons did not alter their firing regularity, possibly because of the lower threshold for excitation compared with inhibition. The latency, duration and frequency extent of sustained hyperpolarization in C(S/T) cells is inconsistent with the response properties of O(C) neurons, suggesting that another source(s) of inhibition influences firing regularity, and presumably response magnitude, in these neurons.

Primitive auditory stream segregation: a neurophysiological study in the songbird forebrain.

Auditory stream segregation refers to the perceptual grouping of sounds, to form coherent representations of objects in the acoustic scene, and is a fundamental aspect of hearing and speech perception. The perceptual segregation of simple interleaved tone sequences has been studied in humans and European starlings (Sturnus vulgaris) using sequences of 2 alternating tones differing in frequency (ABA-ABA-ABA-...). The segregation of A and B tones into separate auditory streams is believed to be promoted by preattentive auditory processes that increase the separation of excitation patterns along a tonotopic gradient. We tested the hypothesis that frequency selectivity and forward masking operate as 2 preattentive processes in sequential stream segregation by recording neural responses in the auditory forebrain of awake starlings to repeated ABA- sequences in which we varied the frequency separation (DeltaF) between the A and B tones and the tone repetition time (TRT). The A tones were presented at the neurons' characteristic frequency (CF), and B tones differed from the CF over a one-octave range. Larger DeltaF values and shorter TRTs promote the perceptual segregation of alternating tone sequences in humans and also resulted in larger differences in neural responses to alternating CF (A) and non-CF (B) tones. Our results are consistent with the hypothesis that preattentive auditory processes, such as frequency selectivity and forward masking, contribute to the perceptual segregation of sequential acoustic events having different frequencies into separate auditory streams, but also suggest that additional processes may be required to account for all known perceptual effects related to sequential auditory stream segregation.

Response Patterns Along an Isofrequency Contour in Cat Primary Auditory Cortex (AI) to Stimuli Varying in Average and Interaural Levels

The topographical response of a portion of an isofrequency contour in primary cat auditory cortex (AI) to a series of monaural and binaural stimuli was studied. Responses of single neurons to monaural and a matrix of binaural characteristic frequency tones, varying in average binaural level (ABL) and interaural level differences (ILD), were recorded. The topography of responses to monaural and binaural stimuli was appreciably different. Patches of cells that responded monotonically to increments in ABL alternated with patches that responded nonmonotonically to ABL. The patches were between 0.4 and 1 mm in length along an isofrequency contour. Differences were found among monotonic patches and among nonmonotonic patches. Topographically, activated and silent populations of neurons varied with both changes in ILD and changes in ABL, suggesting that the area of responsive units may underlie the coding of sound level and sound location.

High-Frequency Sensitivity of the Mature Gerbil Cochlea and Its Development

The thresholds of compound action potentials evoked by tone pips were measured in the cochleae of anesthetized gerbils, both in adults and in neonates aged 14, 16, 18, 20 and 30 days, using round-window electrodes. Stapes vibrations were also measured, using a laser velocimeter, in many of the same ears of adults and neonates aged 14, 16, 18 and 20 days to assess cochlear sensitivity in isolation from middle ear effects and to circumvent problems associated with calibration of acoustic stimuli at high frequencies. Whether referenced to sound pressure level in the ear canal or stapes vibration velocity, thresholds in adults were roughly uniform in the entire range of tested frequencies, 1.25–38.5 kHz. In neonates, thresholds decreased systematically as a function of age, with the largest reductions occurring at the highest frequencies. Thresholds remained slightly immature at all frequencies 30 days after birth. The results for adult gerbils are consistent with the recent finding that basilar-membrane responses to characteristic frequency tones normalized to stapes vibrations are as sensitive at sites near the round window as at more apical sites. The results for neonates confirm that the extreme basal region of the cochlea is the last to approach maturity, with substantial development occurring between 20 and 30 days after birth.