Development of AMPA-selective glutamate receptors in the auditory brainstem of the barn owl

AMPA receptor specific antibodies were used to study the distribution and development of glutamate receptor subtypes (GluR1-4) in nucleus magnocellularis, angularis, laminaris, and the superior olive of the barn owl. Each nucleus in the adult barn owl expresses characteristic levels of AMPA receptor subtypes, and all are enriched in the subunits associated with rapid desensitization (GluR2 and 4). In the auditory hindbrain of the barn owl, the levels of expression of all AMPA receptors were very low at the time of hatching. In all nuclei, the level of GluR1 immunoreactivity was low to undetectable at all ages studied. In the cochlear nuclei, angularis and magnocellularis, levels of GluR2/3 and GluR4 immunoreactivity increased over the first 2 weeks after hatching, coinciding with the morphological maturation of auditory nerve terminals in NM. In the nucleus laminaris and in the superior olive, GluR2/3 and GluR4 immunoreactivity reached adult-like patterns by 3 weeks after hatching. Thus, adult-like patterns of immunoreactivity appeared at least 1 month before the end of the sensitive period in all nuclei studied.     

General characteristics and suppression tuning properties of the distortion-product otoacoustic emission 2f1-f2 in the barn owl

The distortion-product otoacoustic emission (DPOAE) 2f1-f2 was measured in the ear canal of the barn owl. DPOAE were elicited by primary tones in 11 frequency regions from 1 to 9 kHz. The highest DPOAE output levels and best thresholds were found for f1 frequencies of 4 to 7 kHz and additionally at the lowest f1 frequency investigated. In some cases, the DPOAE sound pressures were only 37 dB below the primary-tone levels (PTL). The optimal primary-tone frequency ratios ranged from 1.05 to 1.45 and varied strongly among the different frequency regions investigated. The largest optimal ratios were measured in the middle frequency range for f1. At lower and higher f1, the optimal ratios decreased. DPOAE levels could be suppressed in a frequency-selective way by adding a third tone. As in other non-mammals, the best suppressive frequencies were near f1, suggesting DPOAE generation near the frequency place of this primary tone. This is in contrast to what is known for mammalian species, where the DPOAE is thought to be generated near f2. To obtain 6 dB of suppression of the DPOAE level, suppressor-tone levels ranging from 13 dB below to 4 dB above the primary-tone level were necessary. The Q10dB-values of suppression tuning curves increased as a function of frequency up to a value of 15.8. This tendency resembled the increase in frequency selectivity of auditory nerve fibers in this species.     

Low-frequency suppression of auditory nerve responses to characteristic frequency tones

The effects of low-frequency (50, 100, 200 and 400 Hz) 'suppressor' tones on responses to moderate-level characteristic frequency (CF) tones were measured in chinchilla auditory nerve fibers. Two-tone interactions were evident at suppressor intensities of 70-100 dB SPL. In this range, the average response rate decreased as a function of increasing suppressor level and the instantaneous response rate was modulated periodically. At suppression threshold, the phase of suppression typically coincided with basilar membrane displacement toward scala tympani, regardless of CF. At higher suppressor levels, two suppression maxima coexisted, synchronous with peak basilar membrane displacement toward scala tympani and scala vestibuli. Modulation and rate-suppression thresholds did not vary as a function of spontaneous activity and were only minimally correlated with fiber sensitivity. Except for fibers with CF < 1 kHz, modulation and rate-suppression thresholds were lower than rate and phase-locking thresholds for the suppressor tones presented alone. In the case of high-CF fibers with low spontaneous activity, excitation thresholds could exceed suppression thresholds by more than 30 dB. The strength of modulation decreased systematically with increasing suppressor frequency. For a given suppressor frequency, modulation was strongest in high-CF fibers and weakest in low-CF fibers. The present findings strongly support the notion that low-frequency suppression in auditory nerve fibers largely reflects an underlying basilar membrane phenomenon closely related to compressive non-linearity.     

Head-related transfer functions of the barn owl: measurement and neural responses

Sounds arriving at the eardrum are filtered by the external ear and associated structures in a frequency and direction specific manner. When convolved with the appropriate filters and presented to human listeners through headphones, broadband noises can be precisely localized to the corresponding position outside of the head (reviewed in Blauert, 1997). Such a 'virtual auditory space' can be a potentially powerful tool for neurophysiological and behavioral work in other species as well. We are developing a virtual auditory space for the barn owl, Tyto alba, a highly successful auditory predator that has become a well-established model for hearing research. We recorded catalogues of head-related transfer functions (HRTFs) from the frontal hemisphere of 12 barn owls and compared virtual and free sound fields acoustically and by their evoked neuronal responses. The inner ca. 1 cm of the ear canal was found to contribute little to the directionality of the HRTFs. HRTFs were recorded by inserting probetube microphones to within about 1 or 2 mm of the eardrum. We recorded HRTFs at frequencies between 2 and 11 kHz, which includes the frequencies most useful to the owl for sound localization (3-9 kHz; Konishi, 1973). Spectra of virtual sounds were within +/- 1 dB of amplitude and +/- 10 degrees of phase of the spectra of free field sounds measured near to the eardrum. The spatial pattern of responses obtained from neurons in the inferior colliculus were almost indistinguishable in response to virtual and to free field stimulation.     

Effects of endolymphatic and perilymphatic application of salicylate in the pigeon. I: Single fiber activity and cochlear potentials

The effects of salicylate on the mammalian cochlea function are well documented. However, there is a lack of reports on salicylate effects on the avian auditory periphery and it might well be that salicylate is not ototoxic at all in submammalian vertebrates. We therefore recorded single fiber activities, compound action potential (CAP) and endocochlear potential (EP) during application of salicylate (calculated final concentration of about 2-18 mmol/l) into the scala media of pigeons. We furthermore recorded CAP and EP during perilymphatic perfusion of salicylate (2-20 mmol/l). Salicylate applied into the scala media led to an elevation of tip threshold in single fibers ranging from 5 to 35 dB. The characteristic frequencies of the fibers were not changed. This effect on auditory nerve fibers was reflected in an elevation of CAP thresholds. The mean spontaneous discharge rate was either slightly increased or remained unchanged in the majority of fibers. Perilymphatic salicylate perfusion also led to an elevation of CAP thresholds that was reversible following subsequent perfusion with artificial perilymph. The EP remained unchanged in both application modes. The effects of salicylate were dose dependent and more pronounced in the mid- to high-frequency range. These results are consistent with an action of salicylate on the process (electrical or mechanical, or both) responsible for the sensitivity and frequency selectivity in the avian peripheral hearing organ.     

Demonstration and organization of duct-associated lymphoid tissue (DALT) of the main excretory duct in the monkey parotid gland

We studied the directionality of spike rate responses of auditory nerve fibers of the grassfrog, Rana temporaria, to pure tone stimuli. All auditory fibers showed spike rate directionality. The strongest directionality was seen at low frequencies (200-400 Hz), where the spike rate could change by up to nearly 200 spikes s-1, with sound direction. At higher frequencies the directional spike rate changes were mostly below 100 spikes s-1. In equivalent dB SPL terms (calculated using the fibers' rate-intensity curves) the maximum directionalities were up to 15 dB at low frequencies and below 10 dB at higher frequencies. Two types of directional patterns were observed. At frequencies below 500 Hz relatively strong responses were evoked by stimuli from the ipsilateral (+90 degrees) and contralateral (-90 degrees) directions while the weakest responses were evoked by stimuli from frontal (0 degree or +30 degrees) or posterior (-135 degrees) directions. At frequencies above 800 Hz the strongest responses were evoked by stimuli from the ipsilateral direction while gradually weaker responses were seen as the sound direction shifted towards the contralateral side. At frequencies between 500 and 800 Hz both directional patterns were seen. The directionality was highly intensity dependent. No special adaptations for localization of conspecific calls were found.     

Auditory brainstem response forward-masking recovery functions in older humans with normal hearing

We investigated the auditory brainstem response (ABR) recovery from forward masking using toneburst maskers and probes. Two subject groups matched for hearing thresholds were evaluated: normal-hearing young adults (21-40 years) and older subjects (63-77 years) with normal audiometric thresholds. Stimuli consisted of 1, 4 and 8 kHz tonebursts, with 2-4 cycle rise/fall time and no plateau. Forward maskers were tonebursts of the same frequency, with a 5 ms rise/fall time and a 20 ms plateau time. Probes were presented at 40 dB above threshold, and the forward masker was adjusted to a level that just eliminated the ABR to the 40 dB sensation level toneburst when the probe onset occurred at masker offset. Forward-masker intervals varied from 2 to 64 ms. ABR wave V latencies were similar for the young and old age groups regardless of toneburst frequency. Under forward-masking conditions, wave V latency was prolonged for the shorter intervals, and recovered to baseline latency by 64 ms. The forward-masker recovery functions were nearly identical for the two age groups for the 1 kHz toneburst. In contrast, there were clear differences in the recovery functions for the two age groups for the 4 and 8 kHz tonebursts. Specifically, the mean latency shift was greater for the aged group for forward-masker intervals of 16 ms or less. The two age groups showed identical latency shifts for longer forward-masker intervals. These data demonstrate prolonged recovery from forward masking in older human subjects. As these subjects had audiometric thresholds within normal limits, one plausible interpretation of this finding is that the prolonged recovery time is a manifestation of an aging effect on the central auditory nervous system rather than the periphery.     

Registration of neural maps through value-dependent learning: modeling the alignment of auditory and visual maps in the barn owl's optic tectum

In the optic tectum (OT) of the barn owl, visual and auditory maps of space are found in close alignment with each other. Experiments in which such alignment has been disrupted have shown a considerable degree of plasticity in the auditory map. The external nucleus of the inferior colliculus (ICx), an auditory center that projects massively to the tectum, is the main site of plasticity; however, it is unclear by what mechanisms the alignment between the auditory map in the ICx and the visual map in the tectum is established and maintained. In this paper, we propose that such map alignment occurs through a process of value-dependent learning. According to this paradigm, value systems, identifiable with neuromodulatory systems having diffuse projections, respond to innate or acquired salient cues and modulate changes in synaptic efficacy in many brain regions. To test the self-consistency of this proposal, we have developed a computer model of the principal neural structures involved in the process of auditory localization in the barn owl. This is complemented by simulations of aspects of the barn owl phenotype and of the experimental environment. In the model, a value system is activated whenever the owl carries out a foveation toward an auditory stimulus. A term representing the diffuse release of a neuromodulator interacts with local pre- and postsynaptic events to determine synaptic changes in the ICx. Through large-scale simulations, we have replicated a number of experimental observations on the development of spatial alignment between the auditory and visual maps during normal visual experience, after the retinal image is shifted through prismatic goggles, and after the reestablishment of normal visual input. The results suggest that value-dependent learning is sufficient to account for the registration of auditory and visual maps of space in the OT of the barn owl, and they lead to a number of experimental predictions.     

Effects of click intensity and frequency on the brain-stem auditory evoked potentials in the common marmoset (Callithrix jacchus)

Brain-stem auditory evoked potentials (BAEPs) were recorded in 20 common marmosets (Callithrix jacchus) to investigate the effects of click frequency up to 99 kHz, in consideration of the higher hearing range of the marmoset, and intensity on wave forms and peak latencies. According to the results of BAEP recordings at frequencies of 4, 32, and 99 kHz, the number of components recorded was affected by the stimulus intensity and the clicks at an intensity of 80 dB peak equivalent sound pressure level (pe SPL) had the maximum number of clear components. Therefore, it was indicated that click stimulations at an intensity of 80 dB pe SPL over a broad range of frequencies appears to be useful for recording the maximum number of components in marmosets and may increase the information obtainable from BAEPs. BAEP latencies were prolonged as the stimulus intensity decreased from 100 to 50 dB pe SPL. The effects of stimulus frequency on the wave latencies and amplitudes in response to 80 dB pe SPL at frequencies between 0.5 and 99 kHz revealed various changes: the amplitude of wave I increased at 16 and 32 kHz, but that of waves III and V increased at 4-8 and 64-99 kHz. These increases in amplitudes of the waves may correlate with higher synchronous activity of the peripheral or central auditory pathways.     

Auditory sensitivity in the great tit: perception of signals in the presence and absence of noise

Absolute and masked auditory thresholds (critical masking ratios) were determined behaviourally in the great tit, Parus major, using a GO/NOGO-procedure. Absolute sensitivity was measured between 0.25 and 10 kHz. In the absence of noise, great tits were most sensitive to frequencies between 2 and 4 kHz. In background noise, however, the sensitivity was only a function of the noise level and was independent of frequency. Critical masking ratios determined for signals between 0.25 and 8 kHz were almost constant (median values varied between 23.8 and 25.9 dB) irrespective of signal frequency. Therefore, in contrast to the majority of bird species, great tits have unusually low critical masking ratios at high frequencies. This means that great tits can use high-frequency vocalizations to communicate efficiently in noisy (i.e. natural) environments. Copyright 1998 The Association for the Study of Animal Behaviour.     

Circadian modulation of the rat acoustic startle response.

The acoustic startle response (ASR) of male rats was measured during several sessions over a 24-hr period in both a light–dark cycle and a constant-dark condition. Each session consisted of 10 trials each at 80, 90, 100, 110, and 120 dB white noise. The results indicate robust daily and circadian modulation of ASR amplitude that consist of an approximately 2-fold nocturnal increase at eliciting-stimuli intensities above 80 dB. Similar results were observed in female rats in constant-dark conditions. To determine whether daily changes in auditory thresholds were responsible for the observed modulation, ASR reflex modification procedures were used. These procedures were designed to measure auditory thresholds at frequencies of 10 and 40 kHz at several times of day. The results suggest a lack of significant circadian differences in auditory thresholds at these frequencies. This study demonstrates a novel role of the rat circadian system in the modulation of ASR amplitude. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Thyroxine affects physiological and morphological development of the ear

The onset and development of distortion product otoacoustic emissions (DPE) representing cochlear amplifier activity were studied in neonatal hyperthyroid (n = 10) and control (n = 10) rat pups. These were compared to the onset and development of auditory nerve-brainstem evoked responses (ABR) representing overall cochlear function, and to morphological development of the ear. DPEs were recorded at an earlier postnatal age to high (8 kHz) frequencies and progressed to lower (3 kHz) frequencies with age. ABRs to high-intensity clicks were recorded at least 2 days before DPEs, although DPE onset at 8 kHz preceded adult-like ABR thresholds. Both ABR and DPEs appeared earlier in the hyperthyroid rats. Histological evidence showed earlier morphological development of the ear in these animals. ABR thresholds and DPE amplitudes matured at a slower rate in the experimental group despite their earlier onset. There was no difference in ABR and DPE thresholds between adult hyperthyroid and control rats. However, in the experimental group, DPEs had smaller amplitudes to high (70 dB SPL) and to low (50 dB SPL) stimulus intensities at low frequencies. Hence, despite thyroxine-injected rat pups having earlier onset of auditory structure and function (lower ABR thresholds and earlier functioning active cochlear amplifier), it appeared that neonatal hyperthyroidism affected the later state of the cochlea, such that DPEs, especially to low-frequency stimuli, were depressed during and after maturation.     

Middle-ear development VII: umbo velocity in the neonatal rat

Laser interferometry was used to measure umbo velocity in the developing rat. The tympanic membrane was stimulated with pure tones between 0.4 and 40.0 kHz, at intensity levels between 50 and 130 dB SPL. The corresponding umbo velocity response was measured. Umbo velocity responded linearly with respect to sound pressure throughout development. When the stimulus level was held constant at 100 dB SPL, all animals displayed a velocity response that increased with frequency until a peak response was reached at about 20.0 kHz. Above this frequency the response decreased in all age groups. Umbo velocity increased with age at all frequencies, and at 1.0, 2.0, 4.0, 8.0, 16.0, and 32.0 kHz the velocity reached 90% of its mature value by 68, 24, 24, 15, 19, and 50 days after birth, respectively. These age-related increases in tympanic membrane velocity coincided with improvements in compound action potential (CAP) thresholds (as measured by other investigators) at similar frequencies. Both umbo velocity and CAP thresholds showed substantial growth after 10 days of age. The role of middle-ear functional development with respect to overall auditory sensitivity is discussed.     

Effects of the severity and duration of lesions on the primary and anamnestic humoral responses of sheep to Dichelobacter nodosus and observations of natural resistance to footrot

Choice of outcome measure in reporting hearing results following otologic surgery, including the frequencies used and use of pre- or postoperative bone thresholds, varies from author to author. In this study, data from 550 ossicular reconstruction and pediatric tympanoplasty surgery patients were used to generate a variety of outcome measures, including pure-tone thresholds for frequencies from 0.5 kHz to 8 kHz and different frequency combination pure-tone averages (PTAs) and air-bone gaps. There were no significant differences between mean pre- and postoperative bone conduction thresholds for any of the frequencies from 0.5 to 4 kHz nor for a PTA of 1, 2, and 4 kHz. Mean postoperative air-bone gap differed by no more than 2 dB across six different frequency combination PTAs. If "success" is defined as a postoperative air-bone gap of less than 20 dB, the largest difference in success rate across the six frequency combinations was 5%. There was also little difference in mean postoperative air conduction PTAs for any of the combinations that include frequencies through 4 kHz. Choice of a more conservative or more liberal definition of success was more important than whether air-bone gap or air conduction PTA was used. The authors recommend that a standard reporting procedure be adopted that ensures presentation of the results in a format such that more direct comparisons can be made within the published literature.     

Early monaural occlusion alters the neural map of interaural level differences in the inferior colliculus of the barn owl

Monaural occlusion during early life causes adaptive changes in the tuning of units in the owl's optic tectum to interaural level differences (ILD) that tend to align the auditory with the visual map of space. We investigated whether these changes could be due to experience-dependent plasticity occurring in the auditory pathway prior to the optic tectum. Units were recorded in the external nucleus of the inferior colliculus (ICx), which is a major source of auditory input to the optic tectum. The tuning of ICx units to ILD was measured in normal barn owls and in barn owls raised with one ear occluded. ILD tuning at each recording site was measured with dichotic noise bursts, presented at a constant average binaural level, 20 dB above threshold. The best ILD at each site was defined as the midpoint of the range of ILD values which elicited more than 50% of the maximum response. A physiological map of ILD was found in the ICx of normal owls: best ILDs changed systematically from right-ear-greater to left-ear-greater as the electrode progressed from dorsal to ventral. Best ILDs ranged from 13 dB right-ear-greater to 15 dB left-ear-greater and progressed at an average rate of 12 dB/mm. The representations of ILD were similar on both sides of the brain. In the ICx of owls raised with one ear occluded, the map of ILD was shifted in the adaptive direction: ILD tuning was shifted towards values favoring the non-occluded ear (the direction that would restore a normal space map). The average magnitude of the shift was on the order of 8-10 dB in each of 4 owls. In one owl, the mean shift in ILD tuning was almost identical on both sides of the brain. In another owl, the mean shift was much larger on the side ipsilateral to the occlusion than on the contralateral side. In both cases, the mean shifts measured in each ICx were comparable to the mean shifts measured in the optic tectum on the same sides of the brain. Thus, the adjustments in ILD tuning that have been observed in the optic tectum in response to monaural occlusion are almost entirely due to adaptive mechanisms that operate at or before the level of the ICx.     

Anatomy and physiology of principal cells of the medial nucleus of the trapezoid body (MNTB) of the cat

We have recorded from principal cells of the medial nucleus of the trapezoid body (MNTB) in the cat's superior olivary complex using either glass micropipettes filled with Neurobiotin or horseradish peroxidase for intracellular recording and subsequent labeling or extracellular metal microelectrodes relying on prepotentials and electrode location. Labeled principal cells had cell bodies that usually gave rise to one or two primary dendrites, which branched profusely in the vicinity of the cell. At the electron microscopic (EM) level, there was a dense synaptic terminal distribution on the cell body and proximal dendrites. Up to half the measured cell surface could be covered with excitatory terminals, whereas inhibitory terminals consistently covered about one-fifth. The distal dendrites were very sparsely innervated. The thick myelinated axon originated from the cell body and innervated nuclei exclusively in the ipsilateral auditory brain stem. These include the lateral superior olive (LSO), ventral nucleus of the lateral lemniscus, medial superior olive, dorsomedial and ventromedial periolivary nuclei, and the MNTB itself. At the EM level the myelinated collaterals gave rise to terminals that contained nonround vesicles and, in the LSO, were seen terminating on cell bodies and primary dendrites. Responses of MNTB cells were similar to their primary excitatory input, the globular bushy cell (GBC), in a number of ways. The spontaneous spike rate of MNTB cells with low characteristic frequencies (CFs) was low, whereas it tended to be higher for higher CF units. In response to short tones, a low frequency MNTB cell showed enhanced phase-locking abilities, relative to auditory nerve fibers. For cells with CFs >1 kHz, the short tone response often resembled the primary-like with notch response seen in many globular bushy cells, with a well-timed onset component. Exceptions to and variations of this standard response were also noted. When compared with GBCs with comparable CFs, the latency of the MNTB cell response was delayed slightly, as would be expected given the synapse interposed between the two cell types. Our data thus confirm that, in the cat, the MNTB receives and converts synaptic inputs from globular bushy cells into a reasonably accurate reproduction of the bushy cell spike response. This MNTB cell output then becomes an important inhibitory input to a number of ipsilateral auditory brain stem nuclei.     

Single-unit activity in the lateral nucleus of the amygdala and overlying areas of the striatum in freely behaving rats: Rates, discharge patterns, and responses to acoustic stimuli.

Examined acoustic responses of single units in awake, freely behaving rats in the lateral nucleus of the amygdala (AL). Recordings were made from a movable bundle of 9 microwires. Most cells had very low rates of spontaneous activity (about 3 spikes/sec on average). Firing rates increased during sleep states. Short-latency auditory responses (12–25 msec) were found in the dorsal subnucleus (ALd) of the AL. Cells in the ALd most typically responded in a sustained fashion. Some of the cells in the ALd showed preferences for high frequencies, tone bursts, or frequency-modulated stimuli with center frequencies above 12 kHz. Response latencies were considerably longer in other areas of the AL. Results corroborate the main findings of a previous study (F. Bordi and J. E. LeDoux, 1992) that examined the acoustic response properties of single cells in the AL in anesthetized rats. Together the findings from awake and anesthetized rats provide the most precise information about sensory processing in AL neurons available to date. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hearing in the owl monkey (Aotus trivirgatus): I. Auditory sensitivity.

Determined pure-tone absolute thresholds for 2 male owl monkeys using a tracking procedure. The owl monkey audibility curve obtained had best sensitivity at 8-10 kHz and +40-db sound pressure level (SPL) cutoffs at 125 Hz and 42 kHz. The high-frequency slope was very sharp, the +70-db SPL cutoff occurring at 46 kHz. A comparison of the nocturnal owl monkey with the closely related but diurnal squirrel monkey revealed that their audibility functions are remarkably similar. Comparison of audibility functions of nocturnal and diurnal primates suggests that there is no inevitable specialization of auditory sensitivity in nocturnal species. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dissecting the frog inner ear with Gaussian noise. II. Temperature dependence of inner ear function

Wiener kernel analysis was used to characterize the auditory pathway from tympanic membrane to single primary auditory nerve fibers in the European edible frog, Rana esculenta. Nerve fiber signals were recorded in response to white Gaussian noise. By cross-correlating the noise stimulus and the nerve fiber response, we computed (1) the full second-order Wiener kernel, and (2) the diagonals of the zeroth- to fourth-order Wiener kernels. These diagonals are usually referred to as polynomial correlation functions. The measured Wiener kernels were fitted with a 'sandwich' model. A new fitting procedure was used to compute the response characteristics of (1) the first filter, (2) the static nonlinearity, and (3) the second filter, which form the functional components of the model. The first filter is a bandpass filter. In the majority of low frequency fibers, with best excitatory frequency (BEF) < 800 Hz, this filter was tuned to two frequencies. This dual tuning mechanism gives rise to 'off-diagonal' components in the second-order Wiener kernel. The static nonlinearity resembles a rectifier, and is dominated by second-order (quadratic) nonlinearity. As a function of BEF, the shape of the nonlinearity changes systematically. Finally, the last filter in the model was a low pass filter. Across fibers, its cutoff frequency f-3dB ranged from 106 to 434 Hz.     

Suppression and the upward spread of masking

The purpose of this study is to clarify the role of suppression in the growth of masking when a signal is well above the masker in frequency (upward spread of masking). Classical psychophysical models assume that masking is primarily due to the spread of masker excitation, and that the nonlinear upward spread of masking reflects a differential growth in excitation between the masker and the signal at the signal frequency. In contrast, recent physiological studies have indicated that upward spread of masking in the auditory nerve is due to the increasing effect of suppression with increasing masker level. This study compares thresholds for signals between 2.4 and 5.6 kHz in simultaneous and nonsimultaneous masking for conditions in which the masker is either at or well below the signal frequency. Maximum differences between simultaneous and nonsimultaneous masking were small (< 6 dB) for the on-frequency conditions but larger for the off-frequency conditions (15-32 dB). The results suggest that suppression plays a major role in determining thresholds at high masker levels, when the masker is well below the signal in frequency. This is consistent with the conclusions of physiological studies. However, for signal levels higher than about 40 dB SPL, the growth of masking for signals above the masker frequency is nonlinear even in the nonsimultaneous-masking conditions, where suppression is not expected. This is consistent with an explanation based on the compressive response of the basilar membrane, and confirms that suppression is not necessary for nonlinear upward spread of masking.