Low-frequency Tones Research Articles

The effect of the helicotrema on low-frequency loudness perception.

Below approximately 40 Hz, the cochlear travelling wave reaches the apex, and differential pressure is shunted through the helicotrema, reducing hearing sensitivity. Just above this corner frequency, a resonance feature is often observed in objectively measured middle-ear-transfer functions (METFs). This study inquires whether overall and fine structure characteristics of the METF are also perceptually evident. Equal-loudness-level contours (ELCs) were measured between 20 and 160 Hz for 14 subjects in a purpose-built test chamber. In addition, the inverse shapes of their METFs were obtained by adjusting the intensity of a low-frequency suppressor tone to maintain an equal suppression depth of otoacoustic emissions for various suppressor tone frequencies (20-250 Hz). For 11 subjects, the METFs showed a resonance. Six of them had coinciding features in both ears, and also in their ELC. For two subjects only the right-ear METF was obtainable, and in one case it was consistent with the ELC. One other subject showed a consistent lack of the feature in their ELC and in both METFs. Although three subjects displayed clear inconsistencies between both measures, the similarity between inverse METF and ELC for most subjects shows that the helicotrema has a marked impact on low-frequency sound perception.

Airborne Acoustic Perception by a Jumping Spider

Jumping spiders (Salticidae) are famous for their visually driven behaviors [1]. Here, however, we present behavioral and neurophysiological evidence that these animals also perceive and respond to airborne acoustic stimuli, even when the distance between the animal and the sound source is relatively large (∼3 m) and with stimulus amplitudes at the position of the spider of ∼65 dB sound pressure level (SPL). Behavioral experiments with the jumping spider Phidippus audax reveal that these animals respond tolow-frequency sounds (80Hz; 65 dB SPL) by freezing-a common anti-predatory behavior characteristic of an acoustic startle response. Neurophysiological recordings from auditory-sensitive neural units in the brains of these jumping spiders showed responses to low-frequency tones (80Hz at ∼65 dB SPL)-recordings that also represent the first record of acoustically responsive neural units in the jumping spider brain. Responses persisted even when the distances between spider and stimulus source exceeded 3m and under anechoic conditions. Thus, these spiders appear able to detect airborne sound at distances in the acoustic far-field region, beyond the near-field range often thought to bound acoustic perception in arthropods that lack tympanic ears (e.g., spiders) [2]. Furthermore, direct mechanical stimulation of hairs on the patella of the foreleg was sufficient to generate responses in neural units that also responded to airborne acoustic stimuli-evidence that these hairs likely play a role in the detection of acoustic cues. We suggest that these auditory responses enable the detection of predators and facilitate an acoustic startle response. VIDEO ABSTRACT.

Temporal asynchrony and rhythm perception: Neural bases of perceptual asymmetries

Temporal gap detection has been proposed as a perceptual and neural measure of temporal resolution. This measure has not shown strong frequency dependence but, if anything, high frequencies have been found to produce slightly lower (better) thresholds than very low frequencies. In apparent contradiction to these findings, a recent study claimed superior time perception for lower frequencies and proposed the finding as an explanation for why low-frequency instruments typically carry the rhythm in music [Hove et al. (2014), Proc. Natl. Acad. Sci. U.S.A. 111, 10383-10388]. The present study reviewed those findings and replicated them, but found that the results in fact reflect an asymmetry in asynchrony detection, reported earlier, whereby asynchronies are more readily detected when the lower-frequency tone is leading than when the higher-frequency tone is leading [Wojtczak et al., (2012). J. Acoust. Soc. Am. 131, 363-377]. New results using behavioral and EEG approaches reconcile the data from these differe...

In-vivo intra-cellular characterization of “OFF” responses in the superior para-olivary nucleus

The superior para-olivary nucleus (SPN) is an auditory-brainstem OFF channel. Typically, SPN neurons are inhibited during sound presentation and fire at sound offset. Release from sound-induced inhibitory potentials is the hypothesized mechanism, but this has not previously been directly observed in vivo. We obtained in vivo axonal recordings from 33 SPN neurons in the chinchilla, and patch recordings from 17 SPN cell bodies in the Mongolian gerbil. We retrieved several cells anatomically by labeling with biocytin or neurobiotin. Labeled SPN neurons had large dendritic trees, and axons heading into the lateral lemniscus. Their responses typically showed offset spiking to contralateral, and sometimes to ipsilateral stimulation. Spikes in response to amplitude-modulated tones were highly synchronized, up to hundreds of Hz (vector strength >0.9). Some SPN neurons were ITD sensitive, either to envelope or low-frequency fine-structure components. Patch-clamp recording allowed us to study sub-threshold potentials. Large IPSPs were generated by contralateral, and often also by ipsilateral sounds, with few EPSPs. Individual phase-locked IPSPs were discernable to low-frequency tones. High-frequency tones elicited a large onset IPSP, followed by sustained inhibition. For amplitude-modulated tones, IPSPs were locked to the stimulus envelope, with spikes at the offset of inhibitory events.

The potential use of low-frequency tones to locate regions of outer hair cell loss

Current methods used to diagnose cochlear hearing loss are limited in their ability to determine the location and extent of anatomical damage to various cochlear structures. In previous experiments, we have used the electrical potential recorded at the round window –the cochlear response (CR) –to predict the location of damage to outer hair cells in the gerbil. In a follow-up experiment, we applied 10 mM ouabain to the round window niche to reduce neural activity in order to quantify the neural contribution to the CR. We concluded that a significant proportion of the CR to a 762 Hz tone originated from phase-locking activity of basal auditory nerve fibers, which could have contaminated our conclusions regarding outer hair cell health. However, at such high concentrations, ouabain may have also affected the responses from outer hair cells, exaggerating the effect we attributed to the auditory nerve. In this study, we lowered the concentration of ouabain to 1 mM and determined the physiologic effects on outer hair cells using distortion-product otoacoustic emissions. As well as quantifying the effects of 1 mM ouabain on the auditory nerve and outer hair cells, we attempted to reduce the neural contribution to the CR by using near-infrasonic stimulus frequencies of 45 and 85 Hz, and hypothesized that these low-frequency stimuli would generate a cumulative amplitude function (CAF) that could reflect damage to hair cells in the apex more accurately than the 762 stimuli. One hour after application of 1 mM ouabain, CR amplitudes significantly increased, but remained unchanged in the presence of high-pass filtered noise conditions, suggesting that basal auditory nerve fibers have a limited contribution to the CR at such low frequencies.

Neural Representation of Interaural Time Differences in Humans-an Objective Measure that Matches Behavioural Performance.

Humans, and many other species, exploit small differences in the timing of sounds at the two ears (interaural time difference, ITD) to locate their source and to enhance their detection in background noise. Despite their importance in everyday listening tasks, however, the neural representation of ITDs in human listeners remains poorly understood, and few studies have assessed ITD sensitivity to a similar resolution to that reported perceptually. Here, we report an objective measure of ITD sensitivity in electroencephalography (EEG) signals to abrupt modulations in the interaural phase of amplitude-modulated low-frequency tones. Specifically, we measured following responses to amplitude-modulated sinusoidal signals (520-Hz carrier) in which the stimulus phase at each ear was manipulated to produce discrete interaural phase modulations at minima in the modulation cycle—interaural phase modulation following responses (IPM-FRs). The depth of the interaural phase modulation (IPM) was defined by the sign and the magnitude of the interaural phase difference (IPD) transition which was symmetric around zero. Seven IPM depths were assessed over the range of ±22 ° to ±157 °, corresponding to ITDs largely within the range experienced by human listeners under natural listening conditions (120 to 841 μs). The magnitude of the IPM-FR was maximal for IPM depths in the range of ±67.6 ° to ±112.6 ° and correlated well with performance in a behavioural experiment in which listeners were required to discriminate sounds containing IPMs from those with only static IPDs. The IPM-FR provides a sensitive measure of binaural processing in the human brain and has a potential to assess temporal binaural processing.

Low-frequency bias tone suppression of auditory-nerve responses to low-level clicks and tones

We used low-frequency “bias” tones (BT's) to explore whether click and tone responses are affected in the same way by cochlear active processes. In nonlinear systems the responses to clicks are not always simply related to the responses to tones. Cochlear amplifier gain depends on the incremental slope of the outer-hair-cell (OHC) stereocilia mechano-electric transduction (MET) function. BTs transiently change the operating-point of OHC MET channels and can suppress cochlear-amplifier gain by pushing OHC METs into low-slope saturation regions. BT effects on single auditory-nerve (AN) fibers have been studied on tone responses but not on click responses. We recorded from AN fibers in anesthetized cats and compared tone and click responses using 50 Hz BTs at 70–120 dB SPL to manipulate OHC stereocilia position. BTs can also excite and thereby obscure the BT suppression. We measured AN-fiber response synchrony to BTs alone so that we could exclude suppression measurements when the BT synchrony might obscure the suppression. BT suppression of low-level tone and click responses followed the traditional pattern of twice-a-BT-cycle suppression with more suppression at one phase than the other. The major suppression phases of most fibers were tightly grouped with little difference between click and tone suppressions, which is consistent with low-level click and tone responses being amplified in the same way. The data are also consistent with the operating point of the OHC MET function varying smoothly from symmetric in the base to offset in the apex, and, in contrast, with the IHC MET function being offset throughout the cochlea. As previously reported, bias-tones presented alone excited AN fibers at one or more phases, a phenomena termed “peak splitting” with most BT excitation phases ∼¼ cycle before or after the major suppression phase. We explain peak splitting as being due to distortion in multiple fluid drives to inner-hair-cell stereocilia.

In vivo Whole-Cell Recordings Combined with Electron Microscopy Reveal Unexpected Morphological and Physiological Properties in the Lateral Nucleus of the Trapezoid Body in the Auditory Brainstem.

The lateral nucleus of the trapezoid body (LNTB) is a prominent nucleus in the superior olivary complex in mammals including humans. Its physiology in vivo is poorly understood due to a paucity of recordings. It is thought to provide a glycinergic projection to the medial superior olive (MSO) with an important role in binaural processing and sound localization. We combined in vivo patch clamp recordings with labeling of individual neurons in the Mongolian gerbil. Labeling of the recorded neurons allowed us to relate physiological properties to anatomy at the light and electron microscopic level. We identified a population of quite dorsally located neurons with surprisingly large dendritic trees on which most of the synaptic input impinges. In most neurons, one or more of these dendrites run through and are then medial to the MSO. These neurons were often binaural and could even show sensitivity to interaural time differences (ITDs) of stimulus fine structure or envelope. Moreover, a subpopulation showed enhanced phase-locking to tones delivered in the tuning curve tail. We propose that these neurons constitute the gerbil main LNTB (mLNTB). In contrast, a smaller sample of neurons was identified that was located more ventrally and that we designate to be in posteroventral LNTB (pvLNTB). These cells receive large somatic excitatory terminals from globular bushy cells. We also identified previously undescribed synaptic inputs from the lateral superior olive. pvLNTB neurons are usually monaural, display a primary-like-with-notch response to ipsilateral short tones at CF and can phase-lock to low frequency tones. We conclude that mLNTB contains a population of neurons with extended dendritic trees where most of the synaptic input is found, that can show enhanced phase-locking and sensitivity to ITD. pvLNTB cells, presumed to provide glycinergic input to the MSO, get large somatic globular bushy synaptic inputs and are typically monaural with short tone responses similar to their primary input from the cochlear nucleus.

Spike timing in auditory-nerve fibers during spontaneous activity and phase locking.

In vertebrates, all acoustic information transmitted from the inner ear to the central auditory system is relayed by primary auditory afferents (auditory-nerve fibers; ANFs). These neurons are also the most peripheral elements to use action potentials (spikes) to encode the acoustic information. Here, we review what is known about the spiking of ANFs during spontaneous activity, when spike timing might be regarded as largely random, and during stimulation by low-frequency sounds, when spikes are phase locked to the stimulus waveform, a phenomenon generally considered a hallmark of temporal precision and speed in the auditory system. We focus on mammals, in which each ANF is driven by a single ribbon synapse in a single receptor cell, but also cover relevant research on ANFs of vertebrates from other classes. For spontaneous activity, we highlight several spike-history effects in interspike interval distributions, hazard-rate functions, serial interval correlations, and spike-count statistics. We also review models that have attempted to account for these properties. For phase locking, we focus on the responses to low-frequency tones, rather than to low-frequency components of broadband signals such as noise or clicks. We critically review the measures commonly used to quantify phase locking and urge caution when interpreting such measures with respect to spike-timing precision. We also review the dependence of phase locking on stimulus amplitude and frequency. Finally, we identify some open questions.

Roles for Coincidence Detection in Coding Amplitude-Modulated Sounds.

Many sensory neurons encode temporal information by detecting coincident arrivals of synaptic inputs. In the mammalian auditory brainstem, binaural neurons of the medial superior olive (MSO) are known to act as coincidence detectors, whereas in the lateral superior olive (LSO) roles of coincidence detection have remained unclear. LSO neurons receive excitatory and inhibitory inputs driven by ipsilateral and contralateral acoustic stimuli, respectively, and vary their output spike rates according to interaural level differences. In addition, LSO neurons are also sensitive to binaural phase differences of low-frequency tones and envelopes of amplitude-modulated (AM) sounds. Previous physiological recordings in vivo found considerable variations in monaural AM-tuning across neurons. To investigate the underlying mechanisms of the observed temporal tuning properties of LSO and their sources of variability, we used a simple coincidence counting model and examined how specific parameters of coincidence detection affect monaural and binaural AM coding. Spike rates and phase-locking of evoked excitatory and spontaneous inhibitory inputs had only minor effects on LSO output to monaural AM inputs. In contrast, the coincidence threshold of the model neuron affected both the overall spike rates and the half-peak positions of the AM-tuning curve, whereas the width of the coincidence window merely influenced the output spike rates. The duration of the refractory period affected only the low-frequency portion of the monaural AM-tuning curve. Unlike monaural AM coding, temporal factors, such as the coincidence window and the effective duration of inhibition, played a major role in determining the trough positions of simulated binaural phase-response curves. In addition, empirically-observed level-dependence of binaural phase-coding was reproduced in the framework of our minimalistic coincidence counting model. These modeling results suggest that coincidence detection of excitatory and inhibitory synaptic inputs is essential for LSO neurons to encode both monaural and binaural AM sounds.

Asymmetric hearing loss is common and benign in patients aged 95 years and older.

The objective of our study was to investigate age-specific auditory function in the patient population aged 95 years and older. Retrospective chart review at a tertiary medical center. Medical records of 51 patients older than 95 years (82% female, 18% male) who underwent audiologic testing were reviewed. The following information was collected: age at time of most recent audiogram and prior audiograms; results of pure tone, immittance, and speech audiometry; and findings on radiologic imaging. None of the subjects had hearing in the normal range. For the poorer hearing ear, average low-frequency, high-frequency, and overall pure tone averages (PTA) for the population were 67.9, 82.1, and 74.9 dB hearing level, respectively. Mean word recognition score (WRS) was 57.6% and deteriorated with increasing PTA (P = .0002). Asymmetry, defined by a 10-dB difference at two frequencies, was present in 39.2% of the sample, and WRS asymmetry, defined as a difference of 12% in WRS between ears, was present in 33.0% of the sample. Retrocochlear evaluation did not identify pathology in any of the cases tested. In the poorer hearing ear, average decline in PTA per year was 2.9 dB. In individuals >95 years of age, hearing loss was universal, moderately severe to profound in magnitude, and associated with substantial loss of speech recognition. Hearing loss progresses at a rate greater than for younger cohorts. In this "oldest old" population, asymmetry of loss and WRS was common and is not indicative of retrocochlear pathology. 4 Laryngoscope, 126:1630-1632, 2016.

Investigating the role of the medial olivocochlear reflex in selective attention

Selective attention can dramatically attenuate the cortical responses to unattended stimuli relative to attended stimuli. The mechanisms behind this attenuation and their locus in the auditory pathways are not yet understood. Recent animal work suggests that changes may begin to occur as early as the cochlea itself, potentially through the medial olivocochlear (MOC) reflex. Since measuring MOC activity directly is too invasive to be done in humans, otoacoustic emissions (OAEs) are used to examine changes in cochlear function. Previous investigations of attention-related efferent effects on cochlear processing using OAEs have generally found small and inconsistent effects. The previous lack of effects could be due in part to shortcomings in OAE and MOC-reflex measurement methodology. A new experimental paradigm designed to overcome these shortcomings was used here to examine the effects of selectively attending to visual or auditory stimuli. Participants directed attention to either low-frequency tones, hi...

Modeling dynamic properties of spontaneous otoacoustic emissions: Low-frequency biasing and entrainment

Spontaneous otoacoustic emissions (SOAE) are sounds generated inside the living cochlea and are regarded as by-products of the active mechanism present in the peripheral auditory system. There is still debate whether these emissions are the product of local oscillators located at various positions along the cochlea (local oscillator model) or if they result from standing waves due to a global collective phenomenon that involves different aspects of the cochlea (global standing wave model). This paper uses a global standing wave cochlear model to predict various features of SOAEs. This involves a state-space formulation with a spatially distributed set of nonlinear active micromechanical elements coupled via cochlear fluid coupling. Simulation results have been compared with available experimental data and demonstrate two interesting nonlinear features of the cochlea: first, nonlinear properties of SOAEs modulated by external low-frequency bias tones are easily predicted and can be used to investigate the plausibility of different nonlinear functions incorporated in the micromechanical elements. Second, entrainment patterns can be obtained when the model is stimulated by a swept-tone: results show distinct areas of beating and others of entrainment between the external stimulus and the SOAE, which depend on the level and instantaneous frequency of the sweep.

Transaural experiments and a revised duplex theory for the localization of low-frequency tones.

The roles of interaural time difference (ITD) and interaural level difference (ILD) were studied in free-field source localization experiments for sine tones of low frequency (250-750 Hz). Experiments combined real-source trials with virtual trials created through transaural synthesis based on real-time ear canal measurements. Experiments showed the following: (1) The naturally occurring ILD is physically large enough to exert an influence on sound localization well below 1000 Hz. (2) An ILD having the same sign as the ITD modestly enhances the perceived azimuth of tones for all values of the ITD, and it eliminates left-right confusions that otherwise occur when the interaural phase difference (IPD) passes 180°. (3) Increasing the ILD to large, implausible values can decrease the perceived laterality while also increasing front-back confusions. (4) Tone localization is more directly related to the ITD than to the IPD. (5) An ILD having a sign opposite to the ITD promotes a slipped-cycle ITD, sometimes with dramatic effects on localization. Because the role of the ITD itself is altered by the ILD, the duplex processing of ITD and ILD reflects more than mere trading; the effect of the ITD can be reversed in sign.

Simultaneous Monitoring of CD and PMD Using RF Tone Power

We propose and demonstrate a novel method for simultaneous chromatic dispersion (CD) and polarization-mode-dispersion (PMD) monitoring using RF spectrum analysis. In this method, the clock frequency of single sideband of the optical signal is removed by using a narrow bandwidth fiber Bragg grating (FBG) notch filter. After photon detection of the filtered optical signal, the clock tone power of the radio frequency (RF) signal is changed with differential group delay (DGD), which is insensitive to the CD variation. Once DGD value is known, CD value can be estimated from the low-frequency RF tone power after the DGD induced RF power variation is removed. This method enables simultaneous CD and PMD monitoring using detected RF spectrum in the tap line without affecting the received data stream and can be implemented without major modification of the transmitter.

Decreasing auditory Simon effects across reaction time distributions.

The Simon effect for left-right visual stimuli previously has been shown to decrease across the reaction time (RT) distribution. This decrease has been attributed to automatic activation of the corresponding response, which then dissipates over time. In contrast, for left-right tone stimuli, the Simon effect has not been found to decrease across the RT distribution but instead tends to increase. It has been proposed that automatic activation occurs through visuomotor information transmission, whereas the auditory Simon effect reflects cognitive coding interference and not automatic activation. In 4 experiments, we examined distributions of the auditory Simon effect for RT, percentage error (PE), and an inverse efficiency score [IES = RT/(1 - PE)] as a function of tone frequency and duration to determine whether the activation-dissipation account is also applicable to auditory stimuli. Consistent decreasing functions were found for the RT Simon effect distribution with short-duration tones of low frequency and for the PE and IES Simon effect distributions for all durations and frequency sets. Together, these findings provide robust evidence that left and right auditory stimuli also produce decreasing Simon effect distribution functions suggestive of automatic activation and dissipation of the corresponding response.

Effects of Interaural Differences on the Loudness of Low-Frequency Pure Tones

The loudness related to a sound may vary according to the localization of its source. This phenomenon is described as directional loudness and has been mainly observed for high-frequency sounds and for sources located in the horizontal plane. Because of the acoustic shadow of the head, the left and right ear pressures are modified depending on the source azimuth and the global loudness resulting from a summation process may vary accordingly. But directional loudness has also been reported to occur at 400 Hz, where shadowing effects are usually rather small. It might therefore be suspected that directional loudness effects could be influenced by other parameters involved in the localization process. In a previous study, a small but significant increase of loudness with increasing interaural time difference (ITD) was shown for low-frequency pure tones (200 and 400 Hz) at a low loudness level (40 phon). The present study aimed at getting insight into the potential cause and extent of this effect by assessing the loudness of similar pure tones lateralized with headphones by applying an interaural level difference (ILD) in addition to an ITD and by measuring the effect of ITD at the hearing threshold. It showed significant effects of both ILD and ITD on loudness, and no interaction between these factors. As the effects added even when the factors were contradictory, it supports the hypothesis that the effect is caused by the ITD itself and is not related to the localization process. However, the ITD effect was not significant at the hearing threshold.

An analysis of cochlear response harmonics: Contribution of neural excitation.

In this report an analysis of cochlear response harmonics is developed to derive a mathematical function to estimate the gross mechanics involved in the in vivo transfer of acoustic sound into neural excitation (f(Tr)). In a simulation it is shown that the harmonic distortion from a nonlinear system can be used to estimate the nonlinearity, supporting the next phase of the experiment: Applying the harmonic analysis to physiologic measurements to derive estimates of the unknown, in vivo f(Tr). From gerbil ears, estimates of f(Tr) were derived from cochlear response measurements made with an electrode at the round window niche from 85 Hz tone bursts. Estimates of f(Tr) before and after inducing auditory neuropathy-loss of auditory nerve responses with preserved hair cell responses from neurotoxic treatment with ouabain-showed that the neural excitation from low-frequency tones contributes to the magnitude of f(Tr) but not the sigmoidal, saturating, nonlinear morphology.

Impact of Perioperative Oral Steroid Use on Low-frequency Hearing Preservation After Cochlear Implantation.

To investigate the efficacy of a perioperative oral steroid taper on low-frequency hearing preservation after cochlear implantation. Retrospective case series. Tertiary referral center. Consecutive cochlear implant recipients between January 2012 and December 2014 who were candidates for a hearing preservation approach based on preoperative pure tone thresholds were identified. A subgroup of patients received a 2-week oral corticosteroid taper beginning 3 days before surgery. All patients were implanted using standard length electrodes. The preservation of low-frequency pure tone thresholds on the first postoperative audiogram was assessed. Secondary outcome measures included the respective impacts of age, diabetes, and electrode array type on hearing preservation, the durability of hearing preservation over time, and the development of medical complications related to steroid use. Twenty-seven ears met inclusion criterion, and the mean age at implantation was 49.8 years (median 62 yr, range 2-81 yr). Twenty patients (74.1%, mean age 48.7 yr) received an oral corticosteroid taper, whereas 7 (25.9%, mean age 53.1 yr) did not. The rate (p < 0.01) and degree (p < 0.01) of hearing preservation as measured at implant activation was greater for patients who received an oral steroid taper than those who did not. There were no medical complications related to steroid use. A 2-week oral prednisone taper beginning before surgery may positively affect the incidence and degree of low-frequency hearing preservation after cochlear implantation.

Specificity of the Human Frequency Following Response for Carrier and Modulation Frequency Assessed Using Adaptation.

The frequency following response (FFR) is a scalp-recorded measure of phase-locked brainstem activity to stimulus-related periodicities. Three experiments investigated the specificity of the FFR for carrier and modulation frequency using adaptation. FFR waveforms evoked by alternating-polarity stimuli were averaged for each polarity and added, to enhance envelope, or subtracted, to enhance temporal fine structure information. The first experiment investigated peristimulus adaptation of the FFR for pure and complex tones as a function of stimulus frequency and fundamental frequency (F0). It showed more adaptation of the FFR in response to sounds with higher frequencies or F0s than to sounds with lower frequency or F0s. The second experiment investigated tuning to modulation rate in the FFR. The FFR to a complex tone with a modulation rate of 213 Hz was not reduced more by an adaptor that had the same modulation rate than by an adaptor with a different modulation rate (90 or 504 Hz), thus providing no evidence that the FFR originates mainly from neurons that respond selectively to the modulation rate of the stimulus. The third experiment investigated tuning to audio frequency in the FFR using pure tones. An adaptor that had the same frequency as the target (213 or 504 Hz) did not generally reduce the FFR to the target more than an adaptor that differed in frequency (by 1.24 octaves). Thus, there was no evidence that the FFR originated mainly from neurons tuned to the frequency of the target. Instead, the results are consistent with the suggestion that the FFR for low-frequency pure tones at medium to high levels mainly originates from neurons tuned to higher frequencies. Implications for the use and interpretation of the FFR are discussed.