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PubMed1992

Binaural suppression of nonechoes.

A brief diotic conditioner has been shown to effectively disrupt lateralization of a brief dichotic probe presented after a short interval (4-10 ms, onset to onset) with an interaural time delay that is clearly discriminable in the absence of the conditioner [e.g., P. M. Zurek, J. Acoust. Soc. Am. 66, 1750-1757 (1980)], even when the conditioner and the probe are different sounds [P. L. Divenyi and J. Blauert, in Auditory Processing of Complex Sounds, edited by W. A. Yost and C. S. Watson (Erlbaum, Hillsdale, NJ, 1987), pp. 146-155]. The present experiments investigated lateralization suppression of a narrow-band dichotic probe centered at 2 kHz by a narrow-band diotic conditioner, in situations in which the probe could not be regarded as an echo of the conditioner. In one case, the conditioner was identical to the probe but was presented in opposite phase; in other conditions, the center frequency of the conditioner was different, with the center frequency to bandwidth ratio remaining constant. The effects of frequency and temporal separation between the conditioner and the probe on the suppression of the lateralization of the probe were explored. Suppression of lateralization was measured as the increase in the just-noticeable interaural time difference (jnd), with respect to the interaural time jnd obtained for the probe alone. Low-frequency (0.5 to 1.5 kHz) conditioners were more effective in suppressing the lateralization of the probe than those at or above the probe frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed2004

Sample discrimination of frequency differences with distracters.

This study examined the effect of the type and frequency range of remote frequency distracters on sample discrimination of frequency differences (SD-F). For baseline SD-F conditions, normal-hearing listeners judged frequency differences between pairs of target tones drawn from Gaussian frequency distributions near 2000 Hz. In experiment 1, the distracters were pairs of random-frequency tones, fixed-frequency tones, or noise bands, with one distracter above and one below the target region. Three frequency separations of targets and distracters were tested, none overlapping the target region. Effects of fixed-frequency or noise-band distracters were small compared to that of random-frequency distracters, which drove performance to near chance. In experiment 2, dominance of the low-frequency distracter was supported by the effects of changing distracter level, by presenting only the higher- or lower-frequency distracter, and by the pattern of weights derived from trial-by-trial responses. Performance recovered only when the lower-frequency distracter was attenuated 40-50 dB relative to the targets. In experiment 3, all stimulus distributions were shifted 2 octaves higher in frequency; the stronger influence of the distracter frequency below the target remained. The results demonstrate the importance of both stimulus variability and frequency relationships in the interaction of targets and distracters for SD-F.

Acoustic Stimulation↗
PubMed1990

Response of auditory-nerve fibers to intensity increments in a multitone complex: neural correlates of profile analysis.

Recent psychophysical studies have shown that the detection of an intensity increment superimposed on the center component (1 kHz) of a multitone complex (1, 3, 7, or 11 components) improves as more components are added outside of the critical band. It has been suggested that this form of intensity discrimination is based on a change in the neural profile. To test this hypothesis, neural profiles were constructed by plotting the degree of phase locking to the 1-kHz tone as a function of each unit's characteristic frequency (CF). Neural phase-locking profile to the 1-component signal at 1 kHz had a broad peak; however, the neural profile became narrower as the number of components in the signal increased. The just detectable increment for the 1-component condition was -5 dB re: 1000-Hz component level (3.86-dB increment plus component level re: component level), whereas, for the 3-, 7-, and 11-component conditions, it was -15 dB re: component level (1.42 dB). The neural and psychophysical IDL for the chinchilla were similar for the 1-component condition. However, the overall trends in the psychophysical and neural data are different. In the psychophysical studies IDL is typically poorest in the 3-component condition and improves when more components are added. By contrast, the neural IDL was poorest in the 1-component condition and improved when more components were added. In the multicomponent conditions, units with CFs in 492-1380 Hz were found to be more sensitive in detecting the intensity increment to the 1000-Hz component.

Animals↗
PubMed1989

Event-related perturbations in an electrophysiological measure of auditory function: a measure of sensitivity during orienting?

The effects of salient foreground stimuli in evoked potentials to weak background probe stimuli were examined in situations requiring passive observation or discriminative judgments of foreground tone stimuli. The background probe stimuli consisted of a continual train of weak acoustic stimuli presented at a rate of about 40 stimuli per second. Under such conditions, a 40-Hz steady-state rhythm (SSR) is established, which has been proposed to consist of the algebraic summation of individual middle-latency components evoked by stimuli in the train. The 40-Hz SSR was averaged over trials and extracted from the composite event-related potential signal using narrow-band digital filtering, for continuous examination of latency and amplitude during the course of the period immediately preceding and following the foreground stimulus. The foreground stimulus was followed by a brief period (peaking at about 200 ms) during which the latency of response to the background probe stimuli was reduced. The extent of this latency reduction was in proportion to the magnitude of the simultaneous slow-wave ERP responses and, to a lesser extent, heart rate responses. It is proposed that the results may reflect a transient period of sensitization during orienting, at a presumably early level in the auditory system, and that the method thus offers a means for determining the extent and temporal course of such effects.

Adult↗
PubMed1994

Frequency and duration discrimination of short first-formant speechlike transitions.

Frequency and duration discrimination thresholds of short rising and falling one-formant speechlike transitions without a steady state were determined by means of same/different paired comparison tasks in two experiments. When frequency extent is varied (experiment 1), just noticeable differences decrease with increasing transition duration. Expressed in Hz, thresholds are, on average, 70, 63, and 58 Hz for 20, 30, and 50 ms, respectively. However, when transition duration is varied at a constant frequency extent (experiment 2), difference limens increase with increasing duration and are, on average, 2.7, 4.5, and 4.9 ms for standard transitions of 20, 30, and 50 ms, respectively. The thresholds determined in the two experiments indicate that different psychoacoustical cues are used depending on whether final frequency (experiment 1) or transition duration (experiment 2) are varied. Both experiments were performed at two different frequency regions (between 200 and 700 Hz and between 500 and 1000 Hz), but the results did not differ per region. In addition, no significant differences were found between rising and falling transitions. Particular attention was paid to a methodological issue, viz., the extent to which sensitivity changes as a result of different proportions of catch trials. It was found that the listeners maintained the same response strategies throughout the tests, as their performance is similar, irrespective of the number of catch trials included in the testing sessions.

Adult↗
PubMed2003

Spectral shape discrimination by hearing-impaired and normal-hearing listeners.

The ability to discriminate between sounds with different spectral shapes was evaluated for normal-hearing and hearing-impaired listeners. Listeners discriminated between a standard stimulus and a signal stimulus in which half of the standard components were decreased in level and half were increased in level. In one condition, the standard stimulus was the sum of six equal-amplitude tones (equal-SPL), and in another the standard stimulus was the sum of six tones at equal sensation levels re: audiometric thresholds for individual subjects (equal-SL). Spectral weights were estimated in conditions where the amplitudes of the individual tones were perturbed slightly on every presentation. Sensitivity was similar in all conditions for normal-hearing and hearing-impaired listeners. The presence of perturbation and equal-SL components increased thresholds for both groups, but only small differences in weighting strategy were measured between the groups depending on whether the equal-SPL or equal-SL condition was tested. The average data suggest that normal-hearing listeners may rely more on the central components of the spectrum whereas hearing-impaired listeners may have been more likely to use the edges. However, individual weighting functions were quite variable, especially for the HI listeners, perhaps reflecting difficulty in processing changes in spectral shape due to hearing loss. Differences in weighting strategy without changes in sensitivity suggest that factors other than spectral weights, such as internal noise or difficulty encoding a reference stimulus, also may dominate performance.

Adult↗
PubMed1991

Discriminating between coherent and incoherent frequency modulation of complex tones.

A series of experiments measured the discrimination by human listeners of frequency-modulated complex tones which differed only in the coherence of frequency modulation (FM). For the coherently modulated tones all components were modulated by the same 5-Hz sinusoid, and by the same percentage of their starting frequencies, whereas for the incoherently modulated tones the modulation of one (target) component differed from that of the rest. When the 400-ms complex was composed of consecutive harmonics of a common fundamental, performance improved monotonically with increases in modulator delay, and was nearly perfect at the longest delays. When the complex was inharmonic, performance was near chance at all modular delays, both for component frequencies between 1500 and 2500 Hz, and for component frequencies between 400 and 800 Hz. It is argued that listeners detected incoherence in harmonic complexes by detecting the resulting mistuning of the target component. This conclusion was supported by the finding that listeners were usually at least as good at detecting a fixed mistuning of the center component of a harmonic complex as they were at detecting a modulator phase delay imposed on it. A final experiment, with a stimulus duration of 1 s and slower modulation rates, showed that listeners could detect incoherence for some inharmonic complexes. However, detection was worse than for harmonic complexes and was, it is argued, based on weak harmonicity cues. The results of all experiments point to the absence of an across-frequency mechanism specific to the detection of FM incoherence.

Attention↗
PubMed2004

The role of phase changes in sound signals in localization of sound sources.

The auditory system in humans and animals makes virtually no discrimination of phase changes in the structure of monaurally presented sound signals. However, electrophysiological studies have demonstrated marked changes in the responses of the central parts of the auditory system when the phase structure of the signal changes during presentation of the same type of stimulation. We have suggested that this inconsistency is due to the preparative role of phase effects during monaural stimulation for subsequent operations in the auditory system involved in determining the location of a sound source in space. This report presents experimental data on defined changes (increases in amplitude) in the electrical responses of the midbrain center of the auditory system (inferior colliculus) in antiphase binaural presentation of series of sound impulses (comparison with synphase presentation). These changes may be part of the mechanism underlying the interference resistance of the auditory system during determination of the location of a sound source (binaural release from masking). Neuronal cortical activity is sensitive and selective to dynamic interaural changes in the phase spectrum of the signal, which may provide the basis of the mechanism for locating a moving sound source. Auditory evoked potentials in humans demonstrate memorizing of the direction of movement of a sound image, as shown by the changes in parameters on presentation of stimuli of different locations (deviant stimuli) differing from the standard parameters of mismatch negativity.

Acoustic Stimulation↗
PubMed2004

Electrophysiologic abnormalities of auditory and visual information processing in patients with traumatic brain injury.

OBJECTIVE: Through both auditory and visual modalities, we sought to explore the efficacy of event-related potentials in detecting residual cognitive impairments in patients with traumatic brain injury (TBI). DESIGN: Control subjects and TBI patients with favorable recovery were recruited. Pure tone and primary color discrimination tasks were utilized to elicit auditory and visual event-related potentials, respectively. All subjects were instructed to push a response button when they detected the target stimuli. Both behavioral and electrophysiologic responses were obtained simultaneously. We analyzed the event-related potential waveforms and examined the differences in amplitude, latency, behavioral reaction time, and response accuracy. RESULTS: A total of 11 TBI patients and 11 control subjects were tested. Results showed that (1) TBI patients had significantly lower P300 amplitude in both auditory (11.2 vs. 22. 7 microV) and visual (11.6 vs. 20.9 microV) domains, (2) TBI patients had significantly longer P300 latency in both auditory (355 vs. 294 msecs) and visual (376 vs. 341 msecs) modalities, and (3) although there was no significant difference in response accuracy (97.7%vs. 100%), reaction time for both auditory and visual tasks were significantly longer in TBI patients (auditory, 404 vs. 277 msecs; visual, 397 vs. 346 msecs). CONCLUSION: Although TBI patients with good recovery showed similar response accuracy when compared with control subjects, they demonstrated significantly poorer performance in both electrophysiologic and behavioral responses. Diminished amplitudes and prolonged latencies in P300 responses indicate impaired organization and categorization of incoming sensory information; prolonged behavioral reaction times suggest slowing in the response execution process. Clinical and theoretical implications and goals for continued research are discussed.

Adult↗
PubMed2001

Probing the human hippocampus using rCBF: contrasts in schizophrenia.

Regional cerebral blood flow (rCBF) data from two PET-15O water schizophrenia studies were analyzed using individually placed, magnetic resonance (MR)-guided hippocampal volumes of interest (VOI). In one study, normal (N = 10) and schizophrenic (N = 18) volunteers performed an overlearned auditory discrimination task in rest, control, and decision conditions. In the other study, schizophrenic and normal volunteers received the noncompetitive NMDA receptor antagonist ketamine and placebo and had sequential rCBF evaluations. Moreover, the schizophrenic volunteers were off drug in one study and on antipsychotic drug in the second study, allowing an additional comparison of medication status. VOIs were placed on anterior, middle, and posterior hippocampal areas in each PET image from both studies, redirected from an MR scan, and individually adjusted. While no hippocampal activation was apparent in either the normal or schizophrenic group in the task vs. condition comparison, rCBF was higher in the schizophrenic than in the normal hippocampus in both task and control conditions, independently. In addition, at rest rCBF was significantly higher in the unmedicated group of schizophrenics than in the group of medicated patient volunteers and higher than in the normal comparison group. This suggests that schizophrenia is associated with elevated rCBF in the hippocampus, which "normalizes" with antipsychotic drug treatment. Ketamine, the noncompetitive NMDA receptor antagonist, was more potent in reducing rCBF in the schizophrenic group compared to the normal volunteer group. These data are consistent with a previous report from our laboratory of reduced NMDA receptor NR1 subunit expression and possible abnormal NMDA receptor composition in schizophrenia. These data show an abnormality of hippocampal function in schizophrenia and suggest that this abnormality may be associated with the pathophysiology of the illness.

Acoustic Stimulation↗
PubMed1994

Discrimination of random, time-varying spectra with statistical constraints.

In the present study, a sample-discrimination procedure is used to examine how various statistical constraints on random, time-varying spectra might aid listeners in discriminating among these signals. Time-varying spectra were constructed by adding five sequences of tones, five tones in each sequence, centered at the octave frequencies from 250 to 4000 Hz. The levels, frequencies, and durations of the tones varied randomly from one presentation to the next. The signal to be detected was an increment in the level of tones comprising the sequence centered at 1000 Hz (the target sequence). Three main conditions were examined in which the parameter values of tones varied independently of one another (equal-variance condition), covaried across tone sequences (cross-channel covariance condition), or covaried among themselves for each tone (cross-parameter covariance condition). For the latter two conditions, two special cases were examined in which the target sequence varied independently of nontarget sequences or varied according to the same statistical rules. For five of six listeners, comparisons of performance to that of an ideal, within-channel observer gave clear evidence of an ability to take advantage of cross-channel statistical constraints, but only when the target sequence varied independently of its context. Results offer no evidence for listeners' ability to take advantage of cross-parameter constraints. Trial-by-trial analyses in the equal-variance condition identify factors other than the appropriate weighting of information as primarily responsible for suboptimal performance, though individual differences complicate this interpretation. A failure of simple detection-theoretic models to account for these results is discussed in terms of possible perceptual rules governing sound source identification of real-world objects.

Adult↗
PubMed2002

Informational masking with small set sizes.

Informational masking refers to interference in the detectability of a sound, or discrimination of some property of a sound, beyond that which can be attributed to interactions at the auditory periphery. In the current experiments the signal to be detected was a tone added to a 6-tone masker, and informational masking was introduced by randomly choosing the frequencies of the tones that comprise the masker. The primary question was whether small numbers of maskers could replace randomly drawn maskers without sacrificing the underlying detection schemes adopted by observers. Similar to the method used by Wright and Saberi [J. Acoust. Soc. Am. 105, 1765-1775 (1999)], detection thresholds were measured for different masker set sizes, where set size refers to the number of 6-tone maskers from which any one masker was drawn. Set sizes of 3, 6, 12, and 24 were tested as well as conditions in which the maskers were chosen at random. In addition, observers' memory for maskers was coarsely evaluated. Large differences in thresholds were found across observers and across different masker sets. Even for set sizes of 24, the memory test suggests some recognition of maskers for some observers. Post hoc analysis of the data included an evaluation of the relative contribution of different frequencies using a single linear model. As a base for comparison, a linear model fitted to each condition was also evaluated. Although the data were fitted better using many rather than one linear model, the reduction in quality of fit was modest. This result suggests substantial consistency in decision strategies regardless of masker set size.

Adult↗
PubMed2004

Frequency discrimination training: is there ear symmetry?

While it is well documented that significant improvements in a frequency discrimination task occur following training of normal-hearing adult subjects, less is known about the symmetry between the ears. The objectives of the present study were (1) to compare the first obtained DLF thresholds between left and right ears, (2) to determine whether single-session training would result in similar improvements in those subjects trained in the right ear versus those trained in the left ear, and (3) to evaluate the generalization of learning to the untrained ear and compare its extent between the left and right ears. Two groups of 10 normal-hearing subjects participated in a single-session training. One group of 10 subjects was trained in the left ear and the second group of 10 additional subjects was trained in the right ear. Single-session training consisted of 10 difference limen frequency (DLF) thresholds for 1 kHz using a two-interval, two-alternative forced choice paradigm. Generalization to the untrained ear was tested 24 hours post-training. Our results show that: (1) No significant differences were found in the first obtained DLF thresholds between left and right ears; (2) Similar improvement in DLF thresholds occurred in both ears for single-session training; and (3) Twenty-four hours post-training, learning generalized to the untrained ear with similar generalization to both ears. Future studies are required in order to establish whether this symmetry is maintained with verbal stimuli and/or after multi-session training.

Acoustic Stimulation↗
PubMed2001

Interaural correlation sensitivity.

Sensitivity to differences in interaural correlation was measured for 1.3-ERB-wide bands of noise using a 2IFC task at six frequencies: 250, 500, 750, 1000, 1250, and 1500 Hz. The sensitivity index, d', was measured for discriminations between a number of fixed pairs of correlation values. Cumulative d' functions were derived for each frequency and condition. The d' for discriminating any two values of correlation may be recovered from the cumulative d' function by the difference between cumulative d''s for these values. Two conditions were employed: the noisebands were either presented in isolation (narrow-band condition) or in the context of broad, contiguous flanking bands of correlated noise (fringed condition). The cumulative d' functions showed greater sensitivity to differences in correlation close to 1 than close to 0 at low frequencies, but this difference was less pronounced in the fringed condition. Also, a more linear relationship was observed when cumulative d' was plotted as a function of the equivalent signal-to-noise ratio (SNR) in dB for each correlation value, rather than directly against correlation. The equivalent SNR was the SNR at which the interaural correlation in an NoS(pi) stimulus would equal the interaural correlation of the noise used in the experiment. The maximum cumulative d' declined above 750 Hz. This decline was steeper for the fringed than for the narrow-band condition. For the narrow-band condition, the total cumulative d' was variable across listeners. All cumulative d' functions were closely fitted using a simple two-parameter function. The complete data sets, averaged across listeners, from the fringed and narrow-band conditions were fitted using functions to describe the changes in these parameters over frequency, in order to produce an interpolated family of curves that describe sensitivity at frequencies between those tested. These curves predict the spectra recovered by the binaural system when complex sounds, such as speech, are masked by noise.

Attention↗
PubMed1993

Antimasking effects of the olivocochlear reflex. II. Enhancement of auditory-nerve response to masked tones.

1. The antimasking effects of olivocochlear (OC) efferent feedback were studied in anesthetized or decerebrate cats by comparing responses of single auditory-nerve fibers (ANFs) to tone bursts in continuous masking noise seen with and without addition of a moderate-level contralateral noise known to activate the OC reflex. Responses were measured as a function of tone-burst intensity, tone-burst frequency, and masker intensity and were analyzed so as to allow quantitative estimates of the detectability of the tone bursts against the noise background. 2. Addition of the contralateral OC elicitor both increased the maximum discharge rates to the masked tone bursts and decreased the rates to the ipsilateral masker. The rate increases to the tone bursts could be explained on the basis of a decrease in adaptation caused by decreasing the steady response to the masker. The result is a steepening of the rate-versus-level function for masked tone bursts and a concomitant increase in the estimated discriminability of small increments of tone-burst intensity. 3. For tone bursts at the fiber's characteristic frequency (CF), the OC effects on detection threshold for the masked tone bursts depended on masker level, with small increases in threshold for low masker levels and somewhat larger decreases in threshold for higher masker levels. For tone bursts below CF, OC effects, when present, always decreased the detection threshold. 4. The largest antimasking effects were seen for fibers with CFs between 6 and 12 kHz and for masker levels within 20 dB of the fiber's threshold to the masker. These trends appeared to hold for fibers of all spontaneous rates (SRs). 5. Enhancement of the response to unmasked tone bursts and concomitant decrease in the "spontaneous rate" was elicited by OC activation in fibers if threshold sensitivity approached -10 dB SPL. This "enhancement-in-quiet" appears to arise when an animal-generated noise produces a continuous response (in the absence of purposely applied sound) that is suppressed by OC activity. This finding raises questions as to the range of "true" spontaneous rates in the cat. 6. The results highlight two important distinctions between the effects of OC feedback in quiet versus those in noise. In quiet, the effects are predominately suppressive and are restricted to stimuli at frequencies near a fiber's CF and at intensities within its dynamic range. In continuous background noise, the OC reflex can enhance the responses to transient stimuli. Such effects are seen throughout the fiber's response area.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2001

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.

Audiometry, Pure-Tone↗
PubMed1999

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

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

Animals↗
PubMed2003

Discrimination of the fundamental frequency of complex tones with fixed and shifting spectral envelopes by normally hearing and hearing-impaired subjects.

Difference limens for the fundamental frequency (F0) of complex tones (DLCs) were measured for four normally hearing subjects and three subjects with cochlear hearing loss. The nominal F0 was 100, 200 or 400 Hz. The two tones to be compared contained either low resolved harmonics (RES), harmonics with intermediate resolvability (INT) or high unresolved harmonics (UNRES). For one set of stimuli (fixed harmonics, FH), the tones to be compared contained three harmonics of fixed number, so the changes in F0 were associated with spectral cues. For a second set of stimuli (Shaped), spectral cues were minimized by filtering stimuli through a fixed passband. For the INT and UNRES conditions, the excitation patterns evoked by the Shaped stimuli hardly changed when F0 was altered. To prevent subjects from comparing the frequencies of individual harmonics in the RES condition, subjects were required to detect F0 differences between two tones with non-overlapping harmonics. It was not possible to obtain repeatable results for the hearing-impaired subjects in this condition. The normally hearing subjects had smaller DLCs for the FH than for the Shaped stimuli for the RES condition, and the UNRES condition at the two higher F0s. However, DLCs were similar for the FH and Shaped stimuli for the INT condition and the UNRES condition at the 100-Hz F0, suggesting that spectral cues were not used in these conditions. Except for one subject with F0=400 Hz, the hearing-impaired subjects had smaller DLCs for the FH than for the Shaped stimuli, for both INT and UNRES conditions (although the difference was small for F0=200 Hz in the UNRES condition), suggesting that they used spectral cues for the FH stimuli. For the Shaped stimuli, DLCs were similar in the INT and UNRES conditions for the hearing-impaired subjects, but were smaller in the INT than the RES condition for the normally hearing subjects. We suggest that, in the INT condition with Shaped stimuli, normally hearing subjects used temporal fine structure cues to perform the task. The hearing-impaired subjects appeared to use only temporal envelope cues.

Acoustic Stimulation↗