The role of frequency selectivity in measures of auditory and vibrotactile temporal resolution.

The purpose of this study was to compare the role of frequency selectivity in measures of auditory and vibrotactile temporal resolution. In the first experiment, temporal modulation transfer functions for a sinusoidally amplitude modulated (SAM) 250-Hz carrier revealed auditory modulation thresholds significantly lower than corresponding vibrotactile modulation thresholds at SAM frequencies greater than or equal to 100 Hz. In the second experiment, auditory and vibrotactile gap detection thresholds were measured by presenting silent gaps bounded by markers of the same or different frequency. The marker frequency F1 = 250 Hz preceded the silent gap and marker frequencies after the silent gap included F2 = 250, 255, 263, 310, and 325 Hz. Auditory gap detection thresholds were lower than corresponding vibrotactile thresholds for F2 markers less than or equal to 263 Hz, but were greater than the corresponding vibrotactile gap detection thresholds for F2 markers greater than or equal to 310 Hz. When the auditory gap detection thresholds were transformed into filter attenuation values, the results were modeled well by a constant-percentage (10%) bandwidth filter centered on F1. The vibrotactile gap detection thresholds, however, were independent of marker frequency separation. In a third experiment, auditory and vibrotactile rate difference limens (RDLs) were measured for a 250-Hz carrier at SAM rates less than or equal to 100 Hz. Auditory RDLs were lower than corresponding vibrotactile RDLs for standard rates greater than 10 Hz. Combination tones may have confounded auditory performance for standard rates of 80 and 100 Hz. The results from these experiments revealed that frequency selectivity influences auditory measures of temporal resolution, but there was no evidence of frequency selectivity affecting vibrotactile temporal resolution.     

Vibrotactile forward masking: evidence for channel independence

Threshold shifts for the detection of vibrotactile test stimuli were determined as a function of the intensity of a masker. A 50-ms sinusoidal test stimulus was applied to the thenar eminence of the hand 25 ms after the termination of a 700-ms sinusoidal masker applied to the same site. The intensity of the masker was varied over a range of 0-44 dB SL. The frequency of the masker was either 15 or 250 Hz and the frequency of the test stimulus was either 15, 25, 100, or 250 Hz. The results support the hypothesis that the detection of vibrotactile stimuli is mediated by at least two receptor systems which do not mask each other.     

Testing the concept of a modulation filter bank: the audibility of component modulation and detection of phase change in three-component modulators

Two experiments were performed to test the concept that the auditory system contains a "modulation filter bank" (MFB). Experiment 1 examined the ability to "hear out" the modulation frequency of the central component of a three-component modulator applied to a 4-kHz sinusoidal carrier. On each trial, three modulated stimuli were presented. The modulator of the first stimulus contained three components. Within a run the frequencies of the outer two components were fixed and the frequency of the central ("target") component was drawn randomly from one of five values. The modulators of second and third stimuli contained one component. One had a frequency equal to that of the target and the other had a frequency randomly selected from one of the other possible values. Subjects indicated whether the target corresponded to the second or third stimulus. Scores were around 80% correct when the components in the three-component modulator were widely spaced and when the frequencies of the target and comparison differed sufficiently. Experiment 2 examined the ability to hear a change in the relative phase of the components in a three-component modulator with harmonically spaced components, using a 31FC task. The frequency of the central component, f(c), was either 50 or 100 Hz. Scores were 80%-90% correct when the component spacing was < or = 0.5 f(c), but decreased markedly for greater spacings. Performance was only slightly impaired by randomizing the overall modulation depth from one stimulus to the next. The results of both experiments are broadly consistent with what would be expected from a MFB with a Q value of 1 or slightly less.     

Child and adult vibrotactile thresholds for sinusoidal and pulsatile stimuli

Three experiments were performed to obtain vibrotactile sensitivity thresholds from hearing children and adults, and from deaf children. An adaptive two-interval forced-choice procedure was used to obtain estimates of the 70.7% point on the psychometric sensitivity curve. When hearing children of 5-6 and 9-10 years of age and adults were tested with sinusoids and haversine pulse stimuli, at 10, 100, 160, and 250 Hz or pps, respectively, only the 10-Hz stimulus resulted in an age effect. For this stimulus, young children were significantly less sensitive than adults. When sinusoids were again tested at 20, 40, 80, and 160 Hz, a small overall effect of age was observed with a significant effect only at 20 Hz. Two prelingually profoundly deaf children were tested with haversine pulse trains at 10, 50, 100, 160, and 250 pps. Both children were at least as sensitive to the tactile stimulation as were the hearing children and adults. Pulsatile stimulation, compared to sinusoidal stimulation, exhibited relatively flat threshold versus frequency functions. The present results, demonstrating no age effect for pulsatile stimulation and similar performance for deaf and hearing children, suggest that pulsatile stimulation would be appropriate in vibrotactile speech communication aids for the deaf.     

Vibrotactile forward masking as a function of age.

Thresholds for the detection of a 50-ms test stimulus delivered to the thenar eminence were measured as a function of the time interval between the offset of a 500-ms masking stimulus and the onset of the test stimulus (delta t). The frequency of the masker and the test stimulus was the same during a particular testing session and was either 25 or 250 Hz. At all values of delta t, older subjects exhibited significantly more masking than did young subjects. The effects of age were greater for stimuli that primarily affect the Pacinian system (250 Hz) than those that primarily affect non-Pacinian systems (25 Hz). Psychophysical measurements of the apparent duration of tactile sensations suggest that both sensory persistence and adaptation are affected by aging. Since adaptation seemed to be the more dominant factor for stimuli with durations as long as 500 ms, it was concluded that the effects of aging on forward masking seen in our study were due mainly to increased amounts of adaptation produced by the masker.     

Gap detection in multiple narrow bands of noise as a function of spectral configuration.

This study sought to differentiate between the effect of stimulus bandwidth and the effect of number of activated auditory channels on gap detection in narrow bands of noise. The aim was to clarify the role of across-frequency analysis in temporal processing. Experiment 1 established that when total noise bandwidth is held constant at 100 Hz, gap detection improves as stimulus energy is distributed to both lower and higher frequencies. Experiment 2a showed that the effect was smaller, or was absent, when the cumulative stimulus bandwidth was increased from 100 to 200 Hz. Experiment 2b confirmed that the benefit of spectral dispersion for the narrower cumulative bandwidth also held for a higher frequency region. The results suggest that in conditions where the cumulative stimulus bandwidth is relatively narrow and, concomitantly, gap detection is relatively poor, there is an advantage in dispersing the stimulus across a number of auditory channels. The advantage for the distribution of energy across a range of auditory channels may be offset when the spectral spacing of bands exceeds a critical value.     

Masking of short stimuli by noises with spiked spectra: II. Time summation and frequency selectivity of hearing in a narrow frequency range

The thresholds of masking of short high-frequency pulses with either different durations (1.25–25 ms) and similar central frequency or different central frequencies (3.6–4.4 kHz) but similar durations were measured to reveal manifestations of the properties of peripheral encoding in auditory perception. Noises with a spiked amplitude spectrum structure were used as maskers. The central frequency and the frequency band of a masker were 4 and 1 kHz, respectively. The central frequencies of a stimulus and a masker being equal, the noise the central frequency of which coincided with the frequency corresponding to a dip of an indented spectrum was called an off_(rip)-frequency masker. Owing to the off_(rip)-masker, stimuli-induced masking thresholds were formed taking into account excitation in a narrow region of a basila membrane and auditory nerve fibers with characteristic frequencies from a narrow range. High-frequency pulses with an envelope in the form of the Gaussian function and sinusoidal filling were used as stimuli. At masker levels of 30 dB above the auditory threshold, frequencies of off_(rip)-masker spectra spikes of 500–2000 Hz, and a central stimulus frequency of 4 kHz, the thresholds of tonal stimuli (25 ms in duration) masking in two out of three probationers were higher than the thresholds of masking of compact stimuli (1.25 ms in duration). In the third probationer, on the contrary, the thresholds of tonal stimuli masking were lower than the thresholds of compact stimuli masking. At masker levels of 50 dB, individual threshold differences disappeared. The obtained results were interpreted in the context of implementation of different methods of auditory encoding of the intensity. The methods were based on either the average frequency of auditory nerve pulsations or the number of fibers participating in the response. The interpretation was also carried out in the context of revealing manifestations of nonlinear properties of basila membrane displacements in auditory thresholds. The fact that the dependence of detection thresholds of compact stimuli on their central frequency in one of the two probationers did not reveal the minimum in case of coincidence of off_(rip)-masker and stimulus frequencies pointed to the presence of an auditory “problem zone” that was likely to be localized at the periphery of the auditory system.     

The effect of broadband noise on the human brainstem auditory evoked response. II. Frequency specificity

A series of experiments evaluated the effects of broadband noise (ipsilateral) on wave V of the brainstem auditory evoked response (BAER) elicited by tone bursts or clicks in the presence of high-pass masking noise. Experiment 1 used 1000- and 4000-Hz, 60-dB nHL tone bursts in the presence of broadband noise. With increasing noise level, wave V latency shift was greater for the 1000-Hz tone bursts, while amplitude decrements were similar for both tone-burst frequencies. Experiment 2 varied high-pass masker cutoff frequency and the level of subtotal masking in the presence of 50-dB nHL clicks. The effects of subtotal masking on wave V (increase in latency and decrease in amplitude) increased with increasing derived-band frequency. Experiment 3 covaried high-pass masker cutoff frequency and subtotal masking level for 1000- and 4000-Hz tone-burst stimuli. The effect of subtotal masking on wave V latency was reduced for both tone-burst frequencies when the response-generating region of the cochlear partition was limited by high-pass maskers. The results of these three experiments suggest that most of the wave V latency shift associated with increasing levels of broadband noise is mediated by a place mechanism when the stimulus is a moderate intensity (60 dB nHL), low-frequency (1000 Hz) tone burst. However, the interpretation of the latency shifts produced by broadband noise for 4000-Hz tone-burst stimuli is made more complex by multiple technical factors discussed herein.     

Vibrotactile forward masking: psychophysical evidence for a triplex theory of cutaneous mechanoreception

Threshold shifts for the detection of vibrotactile test stimuli were determined as a function of the intensity of a masker. A 50-ms sinusoidal test stimulus was applied to the thenar eminence of the hand 25 ms after the termination of a 700-ms sinusoidal masker applied to the same site. The frequency of the test stimulus and the frequency of the masker were varied. To eliminate the influence of the Pacinian receptor system, stimuli were delivered through a 0.01-cm2 contactor. The results support the hypothesis that the detection of vibration delivered through a small contactor is determined by two separate populations of non-Pacinian receptors. The study constitutes a psychophysical demonstration of the existence of three receptor systems responsible for the detection of vibration.     

A comparison of steady-state evoked potentials to modulated tones in awake and sleeping humans.

Steady-state evoked potential responses were measured to binaural amplitude-modulated (AM) and combined amplitude- and frequency-modulated (AM/FM) tones. For awake subjects, AM/FM tones produced larger amplitude responses than did AM tones. Awake and sleeping responses to 30-dB HL AM/FM tones were compared. Response amplitudes were lower during sleep and the extent to which they differed from awake amplitudes was dependent on both carrier and modulation frequencies. Background EEG noise at the stimulus modulation frequency was also reduced during sleep and varied with modulation frequency. A detection efficiency function was used to indicate the modulation frequencies likely to be most suitable for electrical estimation of behavioral threshold. In awake subjects, for all carrier frequencies tested, detection efficiency was highest at a modulation frequency of 45 Hz. In sleeping subjects, the modulation frequency regions of highest efficiency varied with carrier frequency. For carrier frequencies of 250 Hz, 500 Hz, and 1 kHz, the highest efficiencies were found in two modulation frequency regions centered on 45 and 90 Hz. For 2 and 4 kHz, the highest efficiencies were at modulation frequencies above 70 Hz. Sleep stage affected both response amplitude and background EEG noise in a manner that depended on modulation frequency. The results of this study suggest that, for sleeping subjects, modulation frequencies above 70 Hz may be best when using steady-state potentials for hearing threshold estimation.     

Sensitivity to amplitude-modulated vibrotactile signals

Temporal modulation transfer functions were measured for sinusoidally amplitude-modulated vibratory stimuli delivered to the thenar eminence of the hand. Results for sinusoidal carriers at 25, 50, 100, and 250 Hz reflected greater sensitivity to modulation than those for either broadband or narrow-band noise carriers. The correspondence of these results to other measures of temporal sensitivity in the tactile system was examined. In addition, findings are discussed in view of their relevance to comparisons across sensory modalities, and to the design of vibrotactile aids for hearing-impaired persons.     

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.     

Object-related brain potentials associated with the perceptual segregation of a dichotically embedded pitch

The cortical mechanisms of perceptual segregation of concurrent sound sources were examined, based on binaural detection of interaural timing differences. Auditory event-related potentials were measured from 11 healthy subjects. Binaural stimuli were created by introducing a dichotic delay of 500-ms duration to a narrow frequency region within a broadband noise, and resulted in a perception of a centrally located noise and a right-lateralized pitch (dichotic pitch). In separate listening conditions, subjects actively discriminated and responded to randomly interleaved binaural and control stimuli, or ignored random stimuli while watching silent cartoons. In a third listening condition subjects ignored stimuli presented in homogenous blocks. For all listening conditions, the dichotic pitch stimulus elicited an object-related negativity (ORN) at a latency of about 150-250 ms after stimulus onset. When subjects were required to actively respond to stimuli, the ORN was followed by a P400 wave with a latency of about 320-420 ms. These results support and extend a two-stage model of auditory scene analysis in which acoustic streams are automatically parsed into component sound sources based on source-relevant cues, followed by a controlled process involving identification and generation of a behavioral response.     

Functional MRI of working memory and selective attention in vibrotactile frequency discrimination

Background  

Focal lesions of the frontal, parietal and temporal lobe may interfere with tactile working memory and attention. To characterise the neural correlates of intact vibrotactile working memory and attention, functional MRI was conducted in 12 healthy young adults. Participants performed a forced-choice vibrotactile frequency discrimination task, comparing a cue stimulus of fixed frequency to their right thumb with a probe stimulus of identical or higher frequency. To investigate working memory, the time interval between the 2 stimuli was pseudo-randomized (either 2 or 8 s). To investigate selective attention, a distractor stimulus was occasionally presented contralaterally, simultaneous to the probe.     

Vibrotactile masking: effects of stimulus onset asynchrony and stimulus frequency

Vibrotactile thresholds for the detection of a 50-ms vibratory stimulus on the thenar eminence of the hand were measured in the presence of and in the absence of a 700-ms suprathreshold vibratory masking stimulus. When thresholds were measured in the presence of the masking stimulus, stimulus onset asynchrony (SOA) was varied so that backward, simultaneous, and forward masking could be measured. The amount of masking, expressed as threshold shift, was greatest when the test stimulus was presented near the onset or offset of the masking stimulus. For both backward and forward masking, the amount of masking decreased as a function of increasing stimulus onset asynchrony. Comparisons were made of the amounts of masking measured when the test and masking stimuli were both sinusoids, and when the test stimulus was a sinusoid and the masking stimulus was noise. In all conditions, the masked threshold decreased approximately 4.0 dB when SOA was increased from 100 to 650 ms with reference to the onset of the 700-ms masking stimulus. More simultaneous masking was observed when sinusoidal test stimuli were detected in the presence of noise than when they were detected in the presence of sinusoidal maskers of the same frequency. The functions were essentially identical for detection of a low-frequency (20 Hz) test stimulus mediated by a non-Pacinian channel and detection of a high-frequency (250 Hz) test stimulus mediated by the Pacinian channel.     

Thresholds for the detection of inharmonicity in complex tones

Thresholds were measured for the detection of inharmonicity in complex tones. Subjects were required to distinguish a complex tone whose partials were all at exact harmonic frequencies from a similar complex tone with one of the partials slightly mistuned. The mistuning which allowed 71% correct identification in a two-alternative forced-choice task was estimated for each partial in turn. In experiment I the fundamental frequency was either 100, 200, or 400 Hz, and the complex tones contained the first 12 harmonics at equal levels of 60 dB SPL per component. The stimulus duration was 410 ms. For each fundamental the thresholds were roughly constant when expressed in Hz, having a mean value of about 4 Hz (range 2.4-7.3 Hz). In experiment II the fundamental frequency was fixed at 200 Hz, and thresholds for inharmonicity were measured for stimulus durations of 50, 110, 410, and 1610 ms. For harmonics above the fifth the thresholds increased from less than 1 Hz to about 40 Hz as duration was decreased from 1610-50 ms. For the lower harmonics (up to the fourth) threshold changed much less with duration, and for the three shorter durations thresholds for each duration were roughly a constant proportion of the harmonic frequency. The results suggest that inharmonicity is detected in different ways for high and low harmonics. For low harmonics the inharmonic partial appears to "stand out" from the complex tone as a whole. For high harmonics the mistuning is detected as a kind of "beat" or "roughness," presumably reflecting a sensitivity to the changing relative phase of the mistuned harmonic relative to the other harmonics.(ABSTRACT TRUNCATED AT 250 WORDS)     

Multicomponent stimulus interactions observed in basilar-membrane vibration in the basal region of the chinchilla cochlea.

Multicomponent stimuli consisting of two to seven tones were used to study suppression of basilar-membrane vibration at the 3-4-mm region of the chinchilla cochlea with a characteristic frequency between 6.5 and 8.5 kHz. Three-component stimuli were amplitude-modulated sinusoids (AM) with modulation depth varied between 0.25 and 2 and modulation frequency varied between 100 and 2000 Hz. For five-component stimuli of equal amplitude, frequency separation between adjacent components was the same as that used for AM stimuli. An additional manipulation was to position either the first, third, or fifth component at the characteristic frequency (CF). This allowed the study of the basilar-membrane response to off-CF stimuli. CF suppression was as high as 35 dB for two-tone combinations, while for equal-amplitude stimulus components CF suppression never exceeded 20 dB. This latter case occurred for both two-tone stimuli where the suppressor was below CF and for multitone stimuli with the third component=CF. Suppression was least for the AM stimuli, including when the three AM components were equal. Maximum suppression was both level- and frequency dependent, and occurred for component frequency separations of 500 to 600 Hz. Suppression decreased for multicomponent stimuli with component frequency spacing greater than 600 Hz. Mutual suppression occurred whenever stimulus components were within the compressive region of the basilar membrane.     

Development of absolute thresholds in chickens

Absolute auditory thresholds were estimated in chickens at 0 and 4 days after hatching. Momentary suppressions of the chicks' regular peeping, following the onset of a tone, were used as indications of stimulus detection. In the first experiment a staircase procedure was used to estimate thresholds. The absolute thresholds of both ages were the same at low frequencies (250-500 Hz), but at higher frequencies (1-2 kHz) 4-day-old chicks had lower thresholds than the 0-day-old chicks. The estimates of thresholds at 1 kHz were corroborated in the second experiment with a method of constant stimuli. A more efficient modified method of limits was used to replicate the age by frequency interaction in the third experiment. These changing thresholds are likely to reflect a developmental process somewhere in the auditory system and not some nonsensory artifact for two reasons: similar thresholds at low frequencies show that developmental differences are not due to differences in the sensitivity of the testing procedure at the two ages and thresholds obtained from the 4-day-old birds are similar to estimates from mature birds. In conclusion, responsiveness to low frequencies develops before responsiveness to higher frequencies, showing that the development of absolute thresholds is correlated with other measures of functional maturation in the auditory system.     

Perceptual interactions in the loudness of combined auditory and vibrotactile stimuli

The loudness of auditory (A), tactile (T), and auditory-tactile (A+T) stimuli was measured at supra-threshold levels. Auditory stimuli were pure tones presented binaurally through headphones; tactile stimuli were sinusoids delivered through a single-channel vibrator to the left middle fingertip. All stimuli were presented together with a broadband auditory noise. The A and T stimuli were presented at levels that were matched in loudness to that of the 200-Hz auditory tone at 25 dB sensation level. The 200-Hz auditory tone was then matched in loudness to various combinations of auditory and tactile stimuli (A+T), and purely auditory stimuli (A+A). The results indicate that the matched intensity of the 200-Hz auditory tone is less when the A+T and A+A stimuli are close together in frequency than when they are separated by an octave or more. This suggests that A+T integration may operate in a manner similar to that found in auditory critical band studies, further supporting a strong frequency relationship between the auditory and somatosensory systems.     

An effect of temporal asymmetry on loudness

A set of experiments was conducted to examine the loudness of sounds with temporally asymmetric amplitude envelopes. Envelopes were generated with fast-attack/slow-decay characteristics to produce F-S (or "fast-slow") stimuli, while temporally reversed versions of these same envelopes produced corresponding S-F ("slow-fast") stimuli. For sinusoidal (330-6000 Hz) and broadband noise carriers, S-F stimuli were louder than F-S stimuli of equal energy. The magnitude of this effect was sensitive to stimulus order, with the largest differences between F-S and S-F loudness occurring after exposure to a preceding F-S stimulus. These results are not compatible with automatic gain control, power-spectrum models of loudness, or predictions obtained using the auditory image model [Patterson et al., J. Acoust. Soc. Am. 98, 1890-1894 (1995)]. Rather, they are comparable to phenomena of perceptual constancy, and may be related to the parsing of auditory input into direct and reverberant sound.