An electrophysiological response to different pitch contours in words

A spoken word with more than one syllable contains a specific stress pattern found to be processed during spoken word recognition. The present study investigated the word's pitch contour as a single auditory parameter that marks stress. Event-related brain potentials (ERPs) were recorded while subjects made decisions to artificially pitch manipulated words. ERPs revealed that pitch contours are discriminated already within the first syllable of a word. Furthermore, behavioral responses for words with incorrect pitch contours were longer than for words with correct pitch contours. The results suggest that the pitch contour is an auditory feature of the spoken word that a listener automatically processes during spoken word recognition.

Evaluating auditory performance limits: i. one-parameter discrimination using a computational model for the auditory nerve

A method for calculating psychophysical performance limits based on stochastic neural responses is introduced and compared to previous analytical methods for evaluating auditory discrimination of tone frequency and level. The method uses signal detection theory and a computational model for a population of auditory nerve (AN) fiber responses. The use of computational models allows predictions to be made over a wider parameter range and with more complete descriptions of AN responses than in analytical models. Performance based on AN discharge times (all-information) is compared to performance based only on discharge counts (rate-place). After the method is verified over the range of parameters for which previous analytical models are applicable, the parameter space is then extended. For example, a computational model of AN activity that extends to high frequencies is used to explore the common belief that rate-place information is responsible for frequency encoding at high frequencies due to the rolloff in AN phase locking above 2 kHz. This rolloff is thought to eliminate temporal information at high frequencies. Contrary to this belief, results of this analysis show that rate-place predictions for frequency discrimination are inconsistent with human performance in the dependence on frequency for high frequencies and that there is significant temporal information in the AN up to at least 10 kHz. In fact, the all-information predictions match the functional dependence of human performance on frequency, although optimal performance is much better than human performance. The use of computational AN models in this study provides new constraints on hypotheses of neural encoding of frequency in the auditory system; however, the method is limited to simple tasks with deterministic stimuli. A companion article in this issue ("Evaluating Auditory Performance Limits: II") describes an extension of this approach to more complex tasks that include random variation of one parameter, for example, random-level variation, which is often used in psychophysics to test neural encoding hypotheses.

Auditory brainstem response recovery in the dolphin as revealed by double sound pulses of different frequencies

Recovery of auditory brainstem responses (ABR) in a bottlenose dolphin was studied in conditions of double-pip stimulation when two stimuli in a pair differed in frequency and intensity. When the conditioning and test stimuli were of equal frequencies, the test response was markedly suppressed at short interstimulus intervals; complete recovery appeared at intervals from about 2 ms (when two stimuli were of equal intensity) to 10-20 ms (when the conditioning stimulus exceeded the test by up to 40 dB). When the two stimuli were of different frequencies, the suppression diminished and was almost absent at a half-octave difference even if the conditioning stimulus exceeded the test one by 40 dB. Frequency-dependence curves (ABR amplitude dependence on frequency difference between the two stimuli) had equivalent rectangular bandwidth from +/-0.2 oct at test stimuli of 20 dB above threshold to +/-0.5 oct at test stimuli of 50 dB above threshold.

A new procedure for measuring peripheral compression in normal-hearing and hearing-impaired listeners

Forward-masking growth functions for on-frequency (6-kHz) and off-frequency (3-kHz) sinusoidal maskers were measured in quiet and in a high-pass noise just above the 6-kHz probe frequency. The data show that estimates of response-growth rates obtained from those functions in quiet, which have been used to infer cochlear compression, are strongly dependent on the spread of probe excitation toward higher frequency regions. Therefore, an alternative procedure for measuring response-growth rates was proposed, one that employs a fixed low-level probe and avoids level-dependent spread of probe excitation. Fixed-probe-level temporal masking curves (TMCs) were obtained from normal-hearing listeners at a test frequency of 1 kHz, where the short 1-kHz probe was fixed in level at about 10 dB SL. The level of the preceding forward masker was adjusted to obtain masked threshold as a function of the time delay between masker and probe. The TMCs were obtained for an on-frequency masker (1 kHz) and for other maskers with frequencies both below and above the probe frequency. From these measurements, input/output response-growth curves were derived for individual ears. Response-growth slopes varied from >1.0 at low masker levels to <0.2 at mid masker levels. In three subjects, response growth increased again at high masker levels (>80 dB SPL). For the fixed-level probe, the TMC slopes changed very little in the presence of a high-pass noise masking upward spread of probe excitation. A greater effect on the TMCs was observed when a high-frequency cueing tone was used with the masking tone. In both cases, however, the net effects on the estimated rate of response growth were minimal.

A negative component superimposed on event-related potentials during light drowsiness

The present study examined the nature of the negative shift of event-related potential (ERP) recorded during the fully awake state, wakefulness with minor awareness deficit (light drowsiness) and stage 1 of NREM sleep. The cortical responses evoked by two types of auditory stimuli were recorded in nine subjects at the different levels of consciousness described above. A negative component with peak latency of 250-350 msec, N300, was identified in ERP during light drowsiness but not in the fully awake state. In stage 1a (stage 1 without vertex sharp waves), the amplitude of N300 was higher than that in light drowsiness, and it was higher in stage 1b than in stage 1a. The scalp distribution of N300 was predominantly on the vertex. It also confirmed that the vertex sharp wave evoked during stage 1 is maximal on the vertex and its peak latency is approximately 300 msec. Considering the similarity in scalp distribution and peak latency between N300 and vertex sharp wave, it is possible that these electroencephalogram phenomena are generated by an identical synchronizing mechanism in the brain. We assumed that N300 observed during light drowsiness may be an incomplete product of vertex sharp wave.

Rate and timing cues associated with the cochlear amplifier: level discrimination based on monaural cross-frequency coincidence detection

The perceptual significance of the cochlear amplifier was evaluated by predicting level-discrimination performance based on stochastic auditory-nerve (AN) activity. Performance was calculated for three models of processing: the optimal all-information processor (based on discharge times), the optimal rate-place processor (based on discharge counts), and a monaural coincidence-based processor that uses a non-optimal combination of rate and temporal information. An analytical AN model included compressive magnitude and level-dependent-phase responses associated with the cochlear amplifier, and high-, medium-, and low-spontaneous-rate (SR) fibers with characteristic frequencies (CFs) spanning the AN population. The relative contributions of nonlinear magnitude and nonlinear phase responses to level encoding were compared by using four versions of the model, which included and excluded the nonlinear gain and phase responses in all possible combinations. Nonlinear basilar-membrane (BM) phase responses are robustly encoded in near-CF AN fibers at low frequencies. Strongly compressive BM responses at high frequencies near CF interact with the high thresholds of low-SR AN fibers to produce large dynamic ranges. Coincidence performance based on a narrow range of AN CFs was robust across a wide dynamic range at both low and high frequencies, and matched human performance levels. Coincidence performance based on all CFs demonstrated the "near-miss" to Weber's law at low frequencies and the high-frequency "mid-level bump." Monaural coincidence detection is a physiologically realistic mechanism that is extremely general in that it can utilize AN information (average-rate, synchrony, and nonlinear-phase cues) from all SR groups.

Evaluating auditory performance limits: II. One-parameter discrimination with random-level variation

Previous studies have combined analytical models of stochastic neural responses with signal detection theory (SDT) to predict psychophysical performance limits; however, these studies have typically been limited to simple models and simple psychophysical tasks. A companion article in this issue ("Evaluating Auditory Performance Limits: I") describes an extension of the SDT approach to allow the use of computational models that provide more accurate descriptions of neural responses. This article describes an extension to more complex psychophysical tasks. A general method is presented for evaluating psychophysical performance limits for discrimination tasks in which one stimulus parameter is randomly varied. Psychophysical experiments often randomly vary a single parameter in order to restrict the cues that are available to the subject. The method is demonstrated for the auditory task of random-level frequency discrimination using a computational auditory nerve (AN) model. Performance limits based on AN discharge times (all-information) are compared to performance limits based only on discharge counts (rate place). Both decision models are successful in predicting that random-level variation has no effect on performance in quiet, which is the typical result in psychophysical tasks with random-level variation. The distribution of information across the AN population provides insight into how different types of AN information can be used to avoid the influence of random-level variation. The rate-place model relies on comparisons between fibers above and below the tone frequency (i.e., the population response), while the all-information model does not require such across-fiber comparisons. Frequency discrimination with random-level variation in the presence of high-frequency noise is also simulated. No effect is predicted for all-information, consistent with the small effect in human performance; however, a large effect is predicted for rate-place in noise with random-level variation.

Functional asymmetry in schizophrenic patients during auditory speech processing

In two experiments, functional laterality and interhemispheric transfer was investigated in schizophrenic patients (n=14) and healthy controls (n=17). In Experiment 1, words and pseudowords were presented either to the left or right ear (monaural condition) or simultaneously to both ears (binaural condition). In Experiment 2, subjects had to discriminate two tones differing in frequency during monaural and binaural stimulation. Healthy controls showed a right ear advantage (REA) for word stimuli, indicating left-hemispheric superiority for word processing. The same lateralization pattern was found in schizophrenic patients, indicating unimpaired functional lateralization of auditory language processing. In both groups, no REA was found for pseudowords resulting in significant WordnessxEar interactions. When presented binaurally, auditory processing of words and pseudowords did not differ significantly from any of the two monaural conditions. Tone discrimination did not lead to any ear asymmetry. The results show normal patterns of functional asymmetry during auditory language processing and tone discrimination in schizophrenic patients.

Psychophysical suppression effects for tonal and speech signals

This experiment assessed the benefits of suppression and the impact of reduced or absent suppression on speech recognition in noise. Psychophysical suppression was measured in forward masking using tonal maskers and suppressors and band limited noise maskers and suppressors. Subjects were 10 younger and 10 older adults with normal hearing, and 10 older adults with cochlear hearing loss. For younger subjects with normal hearing, suppression measured with noise maskers increased with masker level and was larger at 2.0 kHz than at 0.8 kHz. Less suppression was observed for older than younger subjects with normal hearing. There was little evidence of suppression for older subjects with cochlear hearing loss. Suppression measured with noise maskers and suppressors was larger in magnitude and more prevalent than suppression measured with tonal maskers and suppressors. The benefit of suppression to speech recognition in noise was assessed by obtaining scores for filtered consonant-vowel syllables as a function of the bandwidth of a forward masker. Speech-recognition scores in forward maskers should be higher than those in simultaneous maskers given that forward maskers are less effective than simultaneous maskers. If suppression also mitigated the effects of the forward masker and resulted in an improved signal-to-noise ratio, scores should decrease less in forward masking as forward-masker bandwidth increased, and differences between scores in forward and simultaneous maskers should increase, as was observed for younger subjects with normal hearing. Less or no benefit of suppression to speech recognition in noise was observed for older subjects with normal hearing or hearing loss. In general, as suppression measured with tonal signals increased, the combined benefit of forward masking and suppression to speech recognition in noise also increased.

Long-lasting memory of sounds combined with reward in rats

We investigated the effects of sound stimuli combined with reward on the subsequent sound discrimination. Water-deprived rats were exposed to one of two sounds (S+ or S-) in a trial, and licking a spout only during the presentation of S+ was rewarded with water. The percentage of trials in which licking occurred was calculated separately for S+ and S-, and sound discrimination was estimated from the difference in the percentage. S+ and S- were significantly discriminated during an 8 h period. In the second test after 1-2 weeks, sound discrimination for the same S+ and S- was significantly better than that for the S+ of the previous S- and S- of the previous S+. These findings indicate that the memory of the sounds combined with reward in the first test was maintained for 1-2 weeks.

[Rare audiometry findings in progressive, monosymptomatic low frequency deafness]

BACKGROUND

Objective audiometric methods such as the measurement of otoacoustic emissions and auditory evoked potentials (click evoked and notched-noise auditory evoked brainstem potentials, auditory evoked cortical potentials) can provide helpful information. However, information derived from the individual test is limited and each method contains specific restrictions. To illustrate the possible audiometric pitfalls this work presents a case history.

CASE REPORT

The puretone audiometry showed a severe hearing loss at low frequencies up to 2 kHz, a slight hearing loss for higher frequencies up to 6 kHz and almost normal hearing thresholds above 6 kHz. Transitory evoked otoacoustic emissions could not be detected. While the auditory evoked brainstem responses (ABR) using click-stimuli showed a normal pattern no reproducible responses could be derived using the notched-noise technique. Auditory evoked cortical potentials exhibited a normal N1/P2-complex and were detectable down to stimulus levels 0-20 dB above the individual hearing threshold.

DISCUSSION

Click-evoked auditory evoked brainstem potentials are widely regarded as the "gold-standard" of objective audiometry. The example presented in this work shows that in special cases it may provide false negative results. In these special cases auditory tone-evoked cortical potentials may provide better objective information. When objectively assessing the determination of the hearing threshold it may be necessary not only to use all electrophysiological measurements but also to know their methodical restrictions and pitfalls.

Psychophysical suppression measured with bandlimited noise extended below and/or above the signal: effects of age and hearing loss

The objectives of this study were to measure suppression with bandlimited noise extended below and above the signal, at lower and higher signal frequencies, between younger and older subjects, and between subjects with normal hearing and cochlear hearing loss. Psychophysical suppression was assessed by measuring forward-masked thresholds at 0.8 and 2.0 kHz in bandlimited maskers as a function of masker bandwidth. Bandpass-masker bandwidth was increased by introducing noise components below and above the signal frequency while keeping the noise centered on the signal frequency, and also by adding noise below the signal only, and above the signal only. Subjects were younger and older adults with normal hearing and older adults with cochlear hearing loss. For all subjects, suppression was larger when noise was added below the signal than when noise was added above the signal, consistent with some physiological evidence of stronger suppression below a fiber's characteristic frequency than above. For subjects with normal hearing, suppression was greater at higher than at lower frequencies. For older subjects with hearing loss, suppression was reduced to a greater extent above the signal than below and where thresholds were elevated. Suppression for older subjects with normal hearing was poorer than would be predicted from their absolute thresholds, suggesting that age may have contributed to reduced suppression or that suppression was sensitive to changes in cochlear function that did not result in significant threshold elevation.

Temporal modulation transfer functions obtained using sinusoidal carriers with normally hearing and hearing-impaired listeners

Temporal modulation transfer functions were obtained using sinusoidal carriers for four normally hearing subjects and three subjects with mild to moderate cochlear hearing loss. Carrier frequencies were 1000, 2000 and 5000 Hz, and modulation frequencies ranged from 10 to 640 Hz in one-octave steps. The normally hearing subjects were tested using levels of 30 and 80 dB SPL. For the higher level, modulation detection thresholds varied only slightly with modulation frequency for frequencies up to 80 Hz, but decreased for high modulation frequencies. The decrease can be attributed to the detection of spectral sidebands. For the lower level, thresholds varied little with modulation frequency for all three carrier frequencies. The absence of a decrease in the threshold for large modulation frequencies can be explained by the low sensation level of the spectral sidebands. The hearing-impaired subjects were tested at 80 dB SPL, except for two cases where the absolute threshold at the carrier frequency was greater than 70 dB SPL; in these cases a level of 90 dB was used. The results were consistent with the idea that spectral sidebands were less detectable for the hearing-impaired than for the normally hearing subjects. For the two lower carrier frequencies, there were no large decreases in threshold with increasing modulation frequency, and where decreases did occur, this happened only between 320 and 640 Hz. For the 5000-Hz carrier, thresholds were roughly constant for modulation frequencies from 10 to 80 or 160 Hz, and then increased monotonically, becoming unmeasurable at 640 Hz. The results for this carrier may reflect "pure" effects of temporal resolution, without any influence from the detection of spectral sidebands. The results suggest that temporal resolution for deterministic stimuli is similar for normally hearing and hearing-impaired listeners.

Binaural processing model based on contralateral inhibition. III. Dependence on temporal parameters

This paper and two accompanying papers [Breebaart et al., J. Acoust. Soc. Am. 110, 1074-1088 (2001); 110, 1089-1104 (2001)] describe a computational model for the signal processing of the binaural auditory system. The model consists of several stages of monaural and binaural preprocessing combined with an optimal detector. Simulations of binaural masking experiments were performed as a function of temporal stimulus parameters and compared to psychophysical data adapted from literature. For this purpose, the model was used as an artificial observer in a three-interval, forced-choice procedure. All model parameters were kept constant for all simulations. Model predictions were obtained as a function of the interaural correlation of a masking noise and as a function of both masker and signal duration. Furthermore, maskers with a time-varying interaural correlation were used. Predictions were also obtained for stimuli with time-varying interaural time or intensity differences. Finally, binaural forward-masking conditions were simulated. The results show that the combination of a temporal integrator followed by an optimal detector in the time domain can account for all conditions that were tested, except for those using periodically varying interaural time differences (ITDs) and those measuring interaural correlation just-noticeable differences (jnd's) as a function of bandwidth.

Binaural processing model based on contralateral inhibition. II. Dependence on spectral parameters

This and two accompanying articles [Breebaart et al., J. Acoust. Soc. Am. 110, 1074-1088 (2001); 110, 1105-1117 (2001)] describe a computational model for the signal processing in the binaural auditory system. The model consists of several stages of monaural and binaural preprocessing combined with an optimal detector. In the present article the model is tested and validated by comparing its predictions with experimental data for binaural discrimination and masking conditions as a function of the spectral parameters of both masker and signal. For this purpose, the model is used as an artificial observer in a three-interval, forced-choice adaptive procedure. All model parameters were kept constant for all simulations described in this and the subsequent article. The effects of the following experimental parameters were investigated: center frequency of both masker and target, bandwidth of masker and target, the interaural phase relations of masker and target, and the level of the masker. Several phenomena that occur in binaural listening conditions can be accounted for. These include the wider effective binaural critical bandwidth observed in band-widening NoS(pi) conditions, the different masker-level dependence of binaural detection thresholds for narrow- and for wide-band maskers, the unification of IID and ITD sensitivity with binaural detection data, and the dependence of binaural thresholds on frequency.

Forward- and simultaneous-masked thresholds in bandlimited maskers in subjects with normal hearing and cochlear hearing loss

Forward- and simultaneous-masked thresholds were measured at 0.5 and 2.0 kHz in bandpass maskers as a function of masker bandwidth and in a broadband masker with the goal of estimating psychophysical suppression. Suppression was operationally defined in two ways: (1) as a change in forward-masked threshold as a function of masker bandwidth, and (2) as a change in effective masker level with increased masker bandwidth, taking into account the nonlinear growth of forward masking. Subjects were younger adults with normal hearing and older adults with cochlear hearing loss. Thresholds decreased as a function of masker bandwidth in forward masking, which was attributed to effects of suppression; thresholds remained constant or increased slightly with increasing masker bandwidth in simultaneous masking. For subjects with normal hearing, slightly larger estimates of suppression were obtained at 2.0 kHz rather than at 0.5 kHz. For hearing-impaired subjects, suppression was reduced in regions of hearing loss. The magnitude of suppression was strongly correlated with the absolute threshold at the signal frequency, but did not vary with thresholds at frequencies remote from the signal. The results suggest that measuring forward-masked thresholds in bandlimited and broadband maskers may be an efficient psychophysical method for estimating suppression.

Effects of low-pass filtering on the intelligibility of speech in quiet for people with and without dead regions at high frequencies

A dead region is a region of the cochlea where there are no functioning inner hair cells (IHCs) and/or neurons; it can be characterized in terms of the characteristic frequencies of the IHCs bordering that region. We examined the effect of high-frequency amplification on speech perception for subjects with high-frequency hearing loss with and without dead regions. The limits of any dead regions were defined by measuring psychophysical tuning curves and were confirmed using the TEN test described in Moore et al. [Br. J. Audiol. 34, 205-224 (2000)]. The speech stimuli were vowel-consonant-vowel (VCV) nonsense syllables, using one of three vowels (/i/, /a/, and /u/) and 21 different consonants. In a baseline condition, subjects were tested using broadband stimuli with a nominal input level of 65 dB SPL. Prior to presentation via Sennheiser HD580 earphones, the stimuli were subjected to the frequency-gain characteristic prescribed by the "Cambridge" formula, which is intended to give speech at 65 dB SPL the same overall loudness as for a normal listener, and to make the average loudness of the speech the same for each critical band over the frequency range important for speech intelligibility (in a listener without a dead region). The stimuli for all other conditions were initially subjected to this same frequency-gain characteristic. Then, the speech was low-pass filtered with various cutoff frequencies. For subjects without dead regions, performance generally improved progressively with increasing cutoff frequency. This indicates that they benefited from high-frequency information. For subjects with dead regions, two patterns of performance were observed. For most subjects, performance improved with increasing cutoff frequency until the cutoff frequency was somewhat above the estimated edge frequency of the dead region, but hardly changed with further increases. For a few subjects, performance initially improved with increasing cutoff frequency and then worsened with further increases, although the worsening was significant only for one subject. The results have important implications for the fitting of hearing aids.

Is there pre-attentive memory-based comparison of pitch?

The brain's responsiveness to changes in sound frequency has been demonstrated by an overwhelming number of studies. Change detection occurs unintentionally and automatically. It is generally assumed that this brain response, the so-called mismatch negativity (MMN) of the event-related brain potential or evoked magnetic field, is based on the outcome of a memory-comparison mechanism rather than being due to a differential state of refractoriness of tonotopically organized cortical neurons. To the authors' knowledge, however, there is no entirely compelling evidence for this belief. An experimental protocol controlling for refractoriness effects was developed and a true memory-comparison-based brain response to pitch change was demonstrated.

Role of mammalian auditory cortex in the perception of elementary sound properties

Studies in several mammalian species have demonstrated that bilateral ablations of the auditory cortex have little effect on simple sound intensity and frequency-based behaviors. In the rat, for example, early experiments have shown that auditory ablations result in virtually no effect on the rat's ability to either detect tones or discriminate frequencies. Such lesion experiments, however, typically examine an animal's performance some time after recovery from ablation surgery. As such, they demonstrate that the cortex is not essential for simple auditory behaviors in the long run. Our study further explores the role of cortex in basic auditory perception by examining whether the cortex is normally involved in these behaviors. In these experiments we reversibly inactivated the rat primary auditory cortex (AI) using the GABA agonist muscimol, while the animals performed a simple auditory task. At the same time we monitored the rat's auditory activity by recording auditory evoked potentials (AEP) from the cortical surface. In contrast to lesion studies, the rapid time course of these experimental conditions preclude reorganization of the auditory system that might otherwise compensate for the loss of cortical processing. Soon after bilateral muscimol application to their AI region, our rats exhibited an acute and profound inability to detect tones. After a few hours this state was followed by a gradual recovery of normal hearing, first of tone detection and, much later, of the ability to discriminate frequencies. Surface muscimol application, at the same time, drastically altered the normal rat AEP. Some of the normal AEP components vanished nearly instantaneously to unveil an underlying waveform, whose size was related to the severity of accompanying behavioral deficits. These results strongly suggest that the cortex is directly involved in basic acoustic processing. Along with observations from accompanying multiunit experiments that related the AEP to AI neuronal activity, our results suggest that a critical amount of activity in the auditory cortex is necessary for normal hearing. It is likely that the involvement of the cortex in simple auditory perceptions has hitherto not been clearly understood because of underlying recovery processes that, in the long-term, safeguard fundamental auditory abilities after cortical injury.

Determination of auditory thresholds in the brushtail possum (Trichosurus vulpecula)

The auditory abilities of the brushtail possum (Trichosurus vulpecula) have been measured by cochlear potential readings, but no behavioural determinations of their auditory abilities have been published. Six experimentally naive possums were trained to perform a two-response conditional discrimination between the presence and absence of an 880-Hz tone (at 80 dB). All six possums readily learned this task. The behavioural threshold for this tone was determined using a modified tracking procedure and found to be similar to that reported using cochlear microphonic potentials. One concern with the current method was the communal nature of the experimental environment so a further threshold determination in a sound-attenuating chamber was conducted. No substantial difference was noted between the results obtained in the two threshold determinations. The success of the current method means that a full, behavioural audiogram for the brushtail possum, which would complement the existing cochlear potential data, is now possible.

P300 response under active and passive attentional states and uni- and bimodality stimulus presentation conditions

P300 evoked potential patterns were evaluated for two active and two passive attentional states in which stimuli were presented in either one or two sensory modalities. Latency to an infrequent auditory tone was shortest when counting the frequent stimulus, longer when counting the infrequent stimulus, longer still under a passive condition, and longest in a non-P300 condition. P300 amplitudes identifiable in the passive attentional state were enhanced when stimuli were from two, rather than from one, sensory modality. Implications for understanding the mechanism of attention are discussed. Preliminary findings in traumatic brain injury patients suggest that passive P300 responses may be useful in assessing the extent and severity of brain dysfunction.

A model of frequency coding in the central auditory nervous system

A phenomenological model for neural coding in the central auditory system is presented. This model is based on average rate-place codes and the hypothesis is that the rate-place code present in the population of low spontaneous rate nerve fibres is adequate to account for frequency discrimination thresholds across the entire audible frequency range. The activity of a population of nerve fibres in response to an input pure tone is calculated and a neural spike train pattern is generated. An optimal central observer estimates the input frequency from the spike train pattern. The model output is the frequency difference limen at the specific input frequency, determined from the estimated input frequency. It is shown that a rate-place code can account for psychoacoustically observed frequency difference limens. The model also supports the hypothesis that a human listener does not make full use of all the information relevant to frequency that is available in auditory nerve spike trains.

Multiple bottlenecks in information processing? An electrophysiological examination

When two stimuli are to be processed in rapid succession, reaction time (RT) to the second stimulus is delayed. The slowing of RT has been attributed to a single processing bottleneck at response selection (RS) or to a central bottleneck following the initiation of the first response. The hypothesis of a response initiation bottleneck is mainly based on reports of underadditive interactions between stimulus onset asynchrony (SOA) and the number of stimulus-response alternatives (simple vs. two-choice response). The present study tested the hypothesis of a response initiation bottleneck by recording the lateralized readiness potential (LRP), a brain wave, emerging during or immediately following RS. The LRP findings were consistent with a central bottleneck but did not support the late bottleneck hypothesis. Instead, the LRP provided direct evidence that the underadditive interaction of number of alternatives and SOA is due to an increase of response anticipations in the simple response condition.

The amplitude-modulation following response in young and aged human subjects

The amplitude-modulation following response (AMFR) is a steady-state auditory response which may be an objective measure of intensity discrimination. Aged subjects with normal hearing have poorer intensity discrimination for low-frequency tones measured behaviorally, which would predict poorer AMFRs for low-frequency carriers. Experiment 1 was designed to assess age-related differences in AMFR characteristics. Response amplitudes were not significantly different among the young and aged groups for either carrier frequency (520 or 4000 Hz) or modulation depth (0--100%). Response phase did not vary systematically among groups. These results suggest that the AMFR may not be directly comparable to behavioral measures of intensity discrimination in aged subjects with normal hearing. To assess the contribution of high-frequency hearing loss on the AMFR in aged subjects, Experiment 2 compared AMFR amplitudes in aged subjects and in young subjects under the condition of high-pass masking. The amplitude of the AMFR was reduced at 520 Hz for both aged subjects and masked young subjects compared to unmasked young subjects, suggesting that reduced amplitudes in aged subjects with high-frequency hearing loss were associated with threshold elevations. Furthermore, the results suggest that the base of the cochlea contributes to the AMFR for low carrier frequencies.