The functional analysis of auditory discrimination

Mammals have evolved the ability to acquire auditory discriminations. The characteristics of this discriminative ability presumably fit the natural conditions under which discriminations are normally acquired. The purpose of this paper is to review experiments which were directed at showing that auditory discriminations are most rapidly acquired when natural features are incorporated into the experiments. The experiments were also directed at discovering the underlying characteristics of the discriminative ability. When animals were trained to discriminate the position of a sound source in which natural features were incorporated into the experiment, the discrimination was acquired in one trial. Manipulation of the natural features suggested that one trial acquisition depends upon the following. (1) Stimulus novelty; the effect of reinforcement is stronger in the presence of novel than familiar stimuli. (2) Specific behavioral effect of reinforcement; the effect of reinforcing a response in the presence of a novel auditory stimulus is to increase the strength of approaching and manipulating the sound source.     

Detection and discrimination of spectral peaks and notches at 1 and 8 kHz

The ability of subjects to detect and discriminate spectral peaks and notches in noise stimuli was determined for center frequencies fc of 1 and 8 kHz. The signals were delivered using an insert earphone designed to produce a flat frequency response at the eardrum for frequencies up to 14 kHz. In experiment I, subjects were required to distinguish a broadband reference noise with a flat spectrum from a noise with either a peak or a notch at fc. The threshold peak height or notch depth was determined as a function of bandwidth of the peak or notch (0.125, 0.25, or 0.5 times fc). Thresholds increased with decreasing bandwidth, particularly for the notches. In experiment II, subjects were required to detect an increase in the height of a spectral peak or a decrease in the depth of a notch as a function of bandwidth. Performance was worse for notches than for peaks, particularly at narrow bandwidths. For both experiments I and II, randomizing (roving) the overall level of the stimuli had little effect at 1 kHz, but tended to impair performance at 8 kHz, particularly for notches. Experiments III-VI measured thresholds for detecting changes in center frequency of sinusoids, bands of noise, and spectral peaks or notches in a broadband background. Thresholds were lowest for the sinusoids and highest for the peaks and notches. The width of the bands, peaks, or notches had only a small effect on thresholds. For the notches at 8 kHz, thresholds for detecting glides in center frequency were lower than thresholds for detecting a difference in center frequency between two steady sounds. Randomizing the overall level of the stimuli made frequency discrimination of the sinusoids worse, but had little or no effect for the noise stimuli. In all six experiments, performance was generally worse at 8 kHz than at 1 kHz. The results are discussed in terms of their implications for the detectability of spectral cues introduced by the pinnae.     

Minimum audible movement angle: marking the end points of the path traveled by a moving sound source

Four experienced subjects were tested on their ability to discriminate the direction of motion or the order of events in a single-interval, two-alternative, forced-choice adaptive paradigm. Two conditions, employing a broadband "pink" noise (500-8000 Hz), were examined: (1) A continuous noise was available from the moving sound source during the entire period of travel; and (2) 10-ms noise pulses were presented from the moving source at the beginning and end of the arc traveled (during the interpulse interval the source was inactive). Minimum audible movement angle (MAMA) thresholds were significantly lower when the moving source was active throughout the period of travel (0.914 degrees) than when only the end points of the arc of travel were "marked" (1.604 degrees). These results do not support the notion that the discrimination of motion can be reduced to a simple comparison of the location of the source at signal onset and the position of the source at signal offset. The MAMA thresholds obtained with broadband noise in the current experiment are considerably lower than the thresholds previously observed with tonal targets.     

Factors affecting vowel formant discrimination by hearing-impaired listeners

The goal of this study was to measure the ability of adult hearing-impaired listeners to discriminate formant frequency for vowels in isolation, syllables, and sentences. Vowel formant discrimination for F1 and F2 for the vowels /I epsilon ae / was measured. Four experimental factors were manipulated including linguistic context (isolated vowels, syllables, and sentences), signal level (70 and 95 dB SPL), formant frequency, and cognitive load. A complex identification task was added to the formant discrimination task only for sentences to assess effects of cognitive load. Results showed significant elevation in formant thresholds as formant frequency and linguistic context increased. Higher signal level also elevated formant thresholds primarily for F2. However, no effect of the additional identification task on the formant discrimination was observed. In comparable conditions, these hearing-impaired listeners had elevated thresholds for formant discrimination compared to young normal-hearing listeners primarily for F2. Altogether, poorer performance for formant discrimination for these adult hearing-impaired listeners was mainly caused by hearing loss rather than cognitive difficulty for tasks implemented in this study.     

The role of the envelope in processing iterated rippled noise.

Iterated rippled noise (IRN) is generated by a cascade of delay and add (the gain after the delay is 1.0) or delay and subtract (the gain is -1.0) operations. The delay and add/subtract operations impart a spectral ripple and a temporal regularity to the noise. The waveform fine structure is different in these two conditions, but the envelope can be extremely similar. Four experiments were used to determine conditions in which the processing of IRN stimuli might be mediated by the waveform fine structure or by the envelope. In experiments 1 and 3 listeners discriminated among three stimuli in a single-interval task: IRN stimuli generated with the delay and add operations (g = 1.0), IRN stimuli generated using the delay and subtract operations (g = -1.0), and a flat-spectrum noise stimulus. In experiment 2 the listeners were presented two IRN stimuli that differed in delay (4 vs 6 ms) and a flat-spectrum noise stimulus that was not an IRN stimulus. In experiments 1 and 2 both the envelope and waveform fine structure contained the spectral ripple and temporal regularity. In experiment 3 only the envelope had this spectral and temporal structure. In all experiments discrimination was determined as a function of high-pass filtering the stimuli, and listeners could discriminate between the two IRN stimuli up to frequency regions as high as 4000-6000 Hz. Listeners could discriminate the IRN stimuli from the flat-spectrum noise stimulus at even higher frequencies (as high as 8000 Hz), but these discriminations did not appear to depend on the pitch of the IRN stimuli. A control experiment (fourth experiment) suggests that IRN discriminations in high-frequency regions are probably not due entirely to low-frequency nonlinear distortion products. The results of the paper imply that pitch processing of IRN stimuli is based on the waveform fine structure.     

Sensitivity of school-aged children to pitch-related cues

Two experiments investigated the ability of 17 school-aged children to process purely temporal and spectro-temporal cues that signal changes in pitch. Percentage correct was measured for the discrimination of sinusoidal amplitude modulation rate (AMR) of broadband noise in experiment 1 and for the discrimination of fundamental frequency (F0) of broadband sine-phase harmonic complexes in experiment 2. The reference AMR was 100 Hz as was the reference F0. A child-friendly interface helped listeners to remain attentive to the task. Data were fitted using a maximum-likelihood technique that extracted threshold, slope, and lapse rate. All thresholds were subsequently standardized to a common d' value equal to 0.77. There were relatively large individual differences across listeners: eight had relatively adult-like thresholds in both tasks and nine had higher thresholds. However, these individual differences did not vary systematically with age, over the span of 6-16 yr. Thresholds were correlated across the two tasks and were about nine times finer for F0 discrimination than for AMR discrimination as has been previously observed in adults.     

Gap detection and masking in hearing-impaired and normal-hearing subjects

Subjects with cochlear impairments often show reduced temporal resolution as measured in gap-detection tasks. The primary goals of these experiments were: to assess the extent to which the enlarged gap thresholds can be explained by elevations in absolute threshold; and to determine whether the large gap thresholds can be explained by the same processes that lead to a slower-than-normal recovery from forward masking. In experiment I gap thresholds were measured for nine unilaterally and eight bilaterally impaired subjects, using bandlimited noise stimuli centered at 0.5, 1.0, and 2.0 kHz. Gap thresholds were usually larger for the impaired ears, even when the comparisons were made at equal sensation levels (SLs). Gap thresholds tended to increase with increasing absolute threshold, but the scatter of gap thresholds was large for a given degree of hearing loss. In experiment II threshold was measured as a function of the delay between the onset of a 210-ms masker and the onset of a 10-ms signal in both simultaneous- and forward-masking conditions. The signal frequency was equal to the center frequency of the bandlimited noise masker, which was 0.5, 1.0, or 2.0 kHz. Five subjects with unilateral cochlear impairments, two subjects with bilateral impairments, and two normal subjects were tested. The rate of recovery from forward masking, particularly the initial rate, was usually slower for the impaired ears, even when the maskers were presented at equal SLs. Large gap thresholds tended to be associated with slow rates of recovery from forward masking.     

Range resolution and the possible use of spectral information in the echolocating bat, Eptesicus fuscus

Individuals of the echolocating bat Eptesicus fuscus were trained to discriminate simulated two-wave-front targets with internal time delays of 0 to 100 microns between the wave fronts from a one-wave-front target. The ability of bats to discriminate between such targets can be referred to as range resolution. In Eptesicus fuscus, this ability is limited to distinct internal time delays (12, 32-40, and 52-100 microns) between the two wave fronts of a double-wave-front target. Analysis of the simulated two-wave-front echoes reveals periodic frequency minima in the spectrum. Position and separation of these spectral minima depend on the time delay between the two wave fronts. The occurrence of spectral minima within the frequency range of the first harmonic in the echo of the bats' echolocation call correlates to the bats' ability to discriminate a one-wave-front echo from two-wave-front echoes, suggesting that Eptesicus fuscus uses spectral differences within the first harmonic in echoes for range resolution.     

Vocal control of acoustic information for sonar discriminations by the echolocating bat, Eptesicus fuscus

This study aimed to determine whether bats using frequency modulated (FM) echolocation signals adapt the features of their vocalizations to the perceptual demands of a particular sonar task. Quantitative measures were obtained from the vocal signals produced by echolocating bats (Eptesicus fuscus) that were trained to perform in two distinct perceptual tasks, echo delay and Doppler-shift discriminations. In both perceptual tasks, the bats learned to discriminate electronically manipulated playback signals of their own echolocation sounds, which simulated echoes from sonar targets. Both tasks utilized a single-channel electronic target simulator and tested the bat's in a two-alternative forced choice procedure. The results of this study demonstrate changes in the features of the FM bats' sonar sounds with echolocation task demands, lending support to the notion that this animal actively controls the echo information that guides its behavior.     

Dolphin (Tursiops truncatus) echoic angular discrimination: effects of object separation and complexity

A bottlenose dolphin was tested on its ability to echoically discriminate horizontal angular differences between arrays of vertically oriented air-filled PVC rods. The blindfolded dolphin was required to station in a submerged hoop 2 radial m from the stimuli and indicate if an array with two rods (S+) was to the right or the left of a single rod (S-). The angular separation between the two rods (thetaw) was held constant within each experiment while the angle between the S+ and the S-stimuli (thetab) varied to produce angular differences (deltatheta= thetab-thetaw) ranging from 0.25 to 4 degrees. In experiment I, thetaw was maintained at 2 degrees and in experiment II, thetaw was maintained at 4 degrees. Resulting 75% correct thresholds (method of constant stimuli) were 1.5 and 0.7 degrees, respectively. The two main findings of this study are: (1) decreasing the number of targets does not aid in localization, and (2) increasing the space between the rods enhances localization. Taken as a whole, the experiments suggest dolphins have a well-developed ability to resolve spatial information through sonar.     

Phonemic and phonetic factors in adult cross-language speech perception

Previous research has indicated that young infants can discriminate speech sounds across phonetic boundaries regardless of specific relevant experience, and that there is a modification in this ability during ontogeny such that adults often have difficulty discriminating phonetic contrasts which are not used contrastively in their native language. This pattern of findings has often been interpreted as suggesting that humans are endowed with innate auditory sensitivities which enable them to discriminate speech sounds according to universal phonetic boundaries and that there is a decline or loss in this ability after being exposed to a language which contrasts only a subset of those distinctions. The present experiments were designed to determine whether this modification represents a loss of sensorineural response capabilities or whether it shows a shift in attentional focus and/or processing strategies. In experiment 1, adult English-speaking subjects were tested on their ability to discriminate two non-English speech contrasts in a category-change discrimination task after first being predisposed to adopt one of four perceptual sets. In experiments 2, 3, and 4 subjects were tested in an AX (same/different) procedure, and the effects of both limited training and duration of the interstimulus interval were assessed. Results suggest that the previously observed ontogenetic modification in the perception of non-native phonetic contrasts involves a change in processing strategies rather than a sensorineural loss. Adult listeners can discriminate sounds across non-native phonetic categories in some testing conditions, but are not able to use that ability in testing conditions which have demands similar to those required in natural language processing.     

Detection and rate discrimination of amplitude modulation in electrical hearing

Three experiments were designed to examine temporal envelope processing by cochlear implant (CI) listeners. In experiment 1, the hypothesis that listeners' modulation sensitivity would in part determine their ability to discriminate between temporal modulation rates was examined. Temporal modulation transfer functions (TMTFs) obtained in an amplitude modulation detection (AMD) task were compared to threshold functions obtained in an amplitude modulation rate discrimination (AMRD) task. Statistically significant nonlinear correlations were observed between the two measures. In experiment 2, results of loudness-balancing showed small increases in the loudness of modulated over unmodulated stimuli beyond a modulation depth of 16%. Results of experiment 3 indicated small but statistically significant effects of level-roving on the overall gain of the TMTF, but no impact of level-roving on the average shape of the TMTF across subjects. This suggested that level-roving simply increased the task difficulty for most listeners, but did not indicate increased use of intensity cues under more challenging conditions. Data obtained with one subject, however, suggested that the most sensitive listeners may derive some benefit from intensity cues in these tasks. Overall, results indicated that intensity cues did not play an important role in temporal envelope processing by the average CI listener.     

Distortion product otoacoustic emission (DPOAE) in tinnitus patients

The aim of this study was to determine the effect of tinnitus (experiment I) and the combined effect of tinnitus and sensorineural hearing loss (experiment II) on the distortion product otoacoustic emission (DPOAE) for two age groups of tinnitus patients. Tinnitus patients with normal earing, along with normal-hearing control subjects, participated in experiment I. They were divided into two age groups, below 50 and above 50 years. Experiment I showed that the DPOAE levels in the tinnitus patients were lower than those in the normal-hearing (nontinnitus) subjects. The differences depended on the frequency and the age of the patients, suggesting the confounding influence of presbyacusis. The second group of tinnitus patients with increasing and notch-like hearing loss participated in experiment II. They were also divided into two age groups, below 50 and above 50 years. The data from experiment II showed that DPOAE activity well reflects the increasing and notch-like hearing loss functions up to about 40 dB HL. The effect of age on the DPOAE level was clearly noted only for the tinnitus patients with clinically normal-hearing thresholds and was ambiguous for the tinnitus patients with hearing loss.     

Discrimination of sound source velocity in human listeners

The ability of six human subjects to discriminate the velocity of moving sound sources was examined using broadband stimuli presented in virtual auditory space. Subjects were presented with two successive stimuli moving in the frontal horizontal plane level with the ears, and were required to judge which moved the fastest. Discrimination thresholds were calculated for reference velocities of 15, 30, and 60 degrees/s under three stimulus conditions. In one condition, stimuli were centered on 0 degrees azimuth and their duration varied randomly to prevent subjects from using displacement as an indicator of velocity. Performance varied between subjects giving median thresholds of 5.5, 9.1, and 14.8 degrees/s for the three reference velocities, respectively. In a second condition, pairs of stimuli were presented for a constant duration and subjects would have been able to use displacement to assist their judgment as faster stimuli traveled further. It was found that thresholds decreased significantly for all velocities (3.8, 7.1, and 9.8 degrees/s), suggesting that the subjects were using the additional displacement cue. The third condition differed from the second in that the stimuli were "anchored" on the same starting location rather than centered on the midline, thus doubling the spatial offset between stimulus endpoints. Subjects showed the lowest thresholds in this condition (2.9, 4.0, and 7.0 degrees/s). The results suggested that the auditory system is sensitive to velocity per se, but velocity comparisons are greatly aided if displacement cues are present.     

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.     

The influence of carrier level and frequency on modulation and beat-detection thresholds for sinusoidal carriers

This paper is concerned with modulation and beat detection for sinusoidal carriers. In the first experiment, temporal modulation transfer functions (TMTFs) were measured for carrier frequencies between 1 and 10 kHz. Modulation rates covered the range from 10 Hz to about the rate equaling the critical bandwidth at the carrier frequency. In experiment 2, TMTFs for three carrier frequencies were obtained as a function of the carrier level. In the final experiment, thresholds for the detection of either the lower or the upper modulation sideband (beat detection) were measured for "carrier" frequencies of 5 and 10 kHz, using the same range of modulation rates as in experiment 1. The TMTFs for carrier frequencies of 2 kHz and higher remained flat up to a modulation rate of about 100-130 Hz and had similar values across carrier frequencies. For higher rates, modulation thresholds initially increased and then decreased rapidly, reflecting the subjects' ability to resolve the sidebands spectrally. Detection thresholds generally improved with increasing carrier level, but large variations in the exact level dependence were observed, across subjects as well as across carrier frequencies. For beat rates up to about 70 Hz (at 5 kHz) and 100 Hz (at 10 kHz), beat detection thresholds were the same for the upper and the lower sidebands and were about 6 dB higher than the level per sideband at the modulation-detection threshold. At higher rates the threshold for both sidebands increased, but the increase was larger for the lower sideband. This reflects an asymmetry in masking with more masking towards lower frequencies. Only at rates well beyond the maximum of the TMTF did detection for the lower sideband start to be better than that for the upper sideband. The asymmetry at intermediate frequency separations can be explained by assuming that detection always takes place in filters centered above the stimulus spectrum. The shape of the TMTF and the beat-detection data reflects a limitation in resolving fast amplitude variations, which must occur central to the inner-ear filtering. Its characteristic resembles that of a first-order low-pass filter with a cutoff frequency of about 150 Hz.     

Observer weighting strategies in interaural time-difference discrimination and monaural level discrimination for a multi-tone complex

Two experiments measured listeners' abilities to weight information from different components in a complex of 553, 753, and 953 Hz. The goal was to determine whether or not the ability to adjust perceptual weights generalized across tasks. Weights were measured by binary logistic regression between stimulus values that were sampled from Gaussian distributions and listeners' responses. The first task was interaural time discrimination in which listeners judged the laterality of the target component. The second task was monaural level discrimination in which listeners indicated whether the level of the target component decreased or increased across two intervals. For both experiments, each of the three components served as the target. Ten listeners participated in both experiments. The results showed that those individuals who adjusted perceptual weights in the interaural time experiment could also do so in the monaural level discrimination task. The fact that the same individuals appeared to be analytic in both tasks is an indication that the weights measure the ability to attend to a particular region of the spectrum while ignoring other spectral regions.     

Differences in auditory performance between monaural and dichotic conditions. I: masking thresholds in frozen noise.

Thresholds of a 5-ms, 1-kHz signal were determined in the presence of a frozen-noise masker. The noise had a flat power spectrum between 20 Hz and 5 kHz and was presented with a duration of 300 ms. The following interaural conditions were tested with four listeners: Noise and signal monaural at the same ear (monaural condition, NmSm), noise and signal identical at both ears (diotic condition, NoSo), noise identical at both ears and signal monaural (dichotic condition, NoSm) and uncorrelated noise at the two ears and signal monaural (NuSm). The signal was presented at a fixed temporal position with respect to the frozen noise in all measurements and thresholds were determined for different starting phases of the carrier frequency of the signal. Variation of the carrier phase strongly influenced the detection in the diotic condition and the masked thresholds varied by more than 10 dB. The pattern of thresholds for the monaural condition was less variable and the thresholds were generally higher than for the diotic condition. The monaural-diotic difference for specific starting phases amounted to as much as 8 dB. Comparison measurements using running noise maskers revealed no such difference. This relation between monaural and diotic thresholds was further investigated with eight additional subjects. Again, monaural and diotic thresholds in running noise were identical, while in frozen noise, diotic thresholds were consistently lower than monaural thresholds, even when the ear with the lower NmSm threshold was compared. For the starting phase showing the largest monaural-diotic difference, the thresholds for NoSm lay between the monaural and the diotic values. At other starting phases, the NoSm threshold was clearly lower than both the NmSm and the NoSo threshold. One possible explanation of the observed monaural-diotic differences relates to contralateral efferent interaction between the right and the left hearing pathway. A prediction based on this explanation was verified in a final experiment, where frozen-noise performance for NmSm was improved by simultaneously presenting an uncorrelated running noise to the opposite ear.     

Auditory lateralization in monkeys: an examination of two cues serving directional hearing.

In assigning binaural ongoing time differences (phase) as the cue for localization of low frequencies, and binaural intensity differences as the cue for localization of high frequencies, the duplex theory has successfully accounted for human directional hearing of tones. Sensitivity of monkeys to these cues was examined in two experiments. The dependencies on frequency of interaural intensity difference thresholds (lateralization experiment I) and time difference thresholds (lateralization experiment II) were determined behaviorally on three monkeys (M. nemestrina). The range of frequencies was from 125 Hz to 8 kHz in experiment I and from 250 Hz to 2 kHz in experiment II. The results indicate that the duplex theory is applicable to monkeys. However, monkeys are less sensitive than man to both binaural cues. The shortest time disparity monkeys discriminate is 42 microseconds at 1.5 kHz and the smallest intensity difference is 3.5 dB at 500 Hz. Good agreement between the present findings and localization measurements [C. H. Brown et al., J. Acoust. Soc. Am. 63, 1484-1492 (1978)] suggests: (a) that monkeys utilize time disparity cues through higher frequencies than man; and (b) that inaccurate localization by monkeys at high frequencies reflects decreasing sensitivity to interaural intensity cues.     

Particle Size Determination by Impact Measurement in Pneumatically Conveyed Solids

A technique is described for measuring particle size in pneumatically conveyed powders. Particle size is inferred from measurement of the peak compression of a specially designed ultrasonic transducer subject to impact of the particles. The technique is well adapted to measure the coarser fractions of the particle size distribution, which have the largest compressions. Monitoring coarse fractions of powder distributions is essential for control of dry grinding processes such as those used in the cement and coal industries. In the laboratory the impact size monitor (ISM) has demonstrated the ability to discriminate between powders with a difference in peak particle size of 2% at an approximate peak size of 150 microns. The ISM operated successfully in the laboratory at a loading of 0.5 kg/m³ of powder and measured particles down to 50 microns in size. As well as size information the ISM can provide powder distribution across a pneumatic transport pipe.