Intensity Difference Limens Research Articles

Intensity discrimination: Relation of auditory‐nerve activity to psychophysical performance

In order to test the notion that saturation of auditory‐nerve fibers limits psychophysical performance in intensity discrimination at high stimulus levels, intensity difference limens (DL) of single auditory‐nerve fibers in anesthetized cats were measured for tones at the CF, using stimulus paradigms and detectability measures similar to those of psychophysics. The physiological DL reaches a minimum in the range of stimulus levels where discharge rate increases rapidly. In a narrow range of levels around the minimum, single‐fiber DLs approach psychophysical DLs. An optimum‐processor model that combines intensity information from an array of 30 000 auditory‐nerve fibers with a realistic threshold distribution predicts intensity DLs that are well below psychophysical DLs over a 90‐dB range of levels. This result implies that psychophysical performance is not limited by saturation in auditory‐nerve fibers, but by central factors. Suboptimal processors that give more weight to intensity information from high‐threshold fibers than to information from low‐threshold fibers can closely predict psychophysical DLs for both tones and broadband noise over a wide range of stimulus levels. Moreover, such processing schemes, which are based on average discharge rates, provide a stable representation of the spectra of speech sounds with respect to variations in intensity. [Work supported by NIH.]

Absolute and intensity-difference taste thresholds in the rat: Evaluation of an automated multi-channel gustometer

An operant conditioning procedure is described for assessing taste detection and discrimination in rats. Rats were trained to lick at a 10-barrel stimulus delivery tube for a 0.005-ml sample of a positive (S+) or negative (S−) stimulus and to switch to a water delivery tube after presentation of S+. The system is computer controlled and selection of stimuli can be made interactive with performance. After preliminary training, absolute and intensity-difference thresholds can be determined in 1–2 sessions. Mean absolute thresholds in 10 rats for NaCl, sodium-saccharin, and sucrose were 0.0049%, 0.0012%, and 0.035%, respectively. Intensity-difference thresholds (not reported previously in the rat) for NaCl and saccharin were 16.1% and 12.3%, respectively. With these procedures rats also rapidly acquired 2-tastant discriminations, including those involving taste mixtures.

Sound intensity processing by the goldfish.

Capacities of the goldfish for intensity discrimination were studied using classical respiratory conditioning and a staircase psychophysical procedure. Physiological studies on single saccular (auditory) nerve fibers under similar stimulus conditions helped characterize the dimensions of neural activity used in intensity discrimination. Incremental intensity difference limens (IDLs in dB) for 160-ms increments in continuous noise, 500-ms noise bursts, and 500-ms, 800-Hz tone bursts are 2 to 3 dB, are independent of overall level, and vary with signal duration according to a power function with a slope averaging - 0.33. Noise decrements are relatively poorly detected and the silent gap detection threshold is about 35 ms. The IDLs for increments and decrements in an 800-Hz continuous tone are about 0.13 dB, are independent of duration, and are level dependent. Unlike mammalian auditory nerve fibers, some goldfish saccular fibers show variation in recovery time to tonal increments and decrements, and adaptation to a zero rate. Unit responses to tone increments and decrements show rate effects generally in accord with previous observations on intracellular epsp's in goldfish saccular fibers. Neurophysiological correlates of psychophysical intensity discrimination data suggest the following: (1) noise gap detection may be based on spike rate increments which follow gap offset; (2) detection of increments and decrements in continuous tones may be determined by steep low-pass filtering in peripheral neural channels which enhance the effects of spectral "splatter" toward the lower frequencies; (3) IDLs for pulsed signals of different duration can be predicted from the slopes of rate-intensity functions and spike rate variability in individual auditory nerve fibers; and (4) at different sound pressure levels, different populations of peripheral fibers provide the information used in intensity discrimination.

Intensity difference limens at high frequencies.

Thresholds for amplitude modulation detection were obtained from four subjects at frequencies of 2, 4, 6, 8, and 10 kHz for sensation levels of 15, 30, 45, and 60 dB and modulation rates of 2, 4, and 8 Hz. High-frequency difference limens calculated from amplitude modulation thresholds were found to change nonmonotonically as a function of sensation level, independent of modulation rate. This nonmonotonic relation stemmed mainly from a gradual reduction of the difference limen at the lowest sensation level with increasing frequency. Difference limens for pulsed tone discrimination were also measured in two of the subjects at 2, 6, and 10 kHz and sensation levels of 15, 30, 45, and 60 dB. The relation between intensity discrimination and sensation level was similar to that found for amplitude modulation detection. These findings are interpreted as indicating that the nonmonotonic relation between sensation level and intensity resolution is a general characteristic of stimulus processing at higher frequencies.

Investigations of the residual hearing capacity of severely hearing-impaired and profoundly deaf subjects.

Speech reception functions (maximum discrimination score for phonemes, speech reception threshold) and auditory functions (pure-tone audiogram, difference limens for frequency and intensity, temporal modulation transfer function, critical ratio and temporal integration) have been investigated in a group of severely hearing-impaired and deaf subjects (median Fletcher index: 80 dB), for different frequency regions (250, 1 000 and 2 000/4 000 Hz). The correlations between the different functions were calculated on the basis of characteristic numbers, derived from the originally measured functions, in the indicated frequency regions. The residual hearing capacities could be described best with two factors, one reflecting the distortion term in the speech reception threshold and the other representing the frequency discrimination for high-frequency tones. It was found that residual hearing capacities are present, at least up to a Fletcher index of 105 dB.

Olfactory discrimination in rats with anterior amygdala lesions.

In Experiment 1, rats with posterior lateral olfactory tract/anterior amygdala lesions or with control neocortical lesions were tested for retention of a preoperatively learned odor detection task and for learning on new odor discrimination problems. All rats had perfect or near-perfect retention of the detection task, and there were no discernible differences between groups in learning on the new odor discrimination problems. In Experiment 2, an intensity-difference threshold for olfaction was determined in 4 rats before and after lesions of the posterior lateral olfactory tract and anterior amygdala. There were no apparent differences between pre- and postoperative performances on this psychophysical test. These results indicate that lateral olfactory tract projections to the amygdala and posterior olfactory cortex are not essential for normal performance on simple olfactory discrimination tasks.

The relations among critical ratios, critical bands, and intensity difference limens in man.

Band-narrowing estimates of the critical bandwidth (CB) are consistently larger than critical-ratio (CR) estimates for the same signal frequency. Bilger [in Hearing and Davis: Essays Honoring Hallowell Davis, edited by S.K. Hirsh et al. (Washington U.P., St. Louis, 1976), p. 191] proposed that this difference could be accounted for by reference to intensity-discrimination performance [CR(Hz)/CB(Hz) = delta I/I]. To test this hypothesis, band-narrowing, critical-ratio, and intensity-discrimination data were collected for four normally hearing, well-trained listeners. Signal frequency was 2000 Hz and two noise levels were used: 20 and 50 dB N0. The relations proposed by Bilger among critical-bandwidth estimates from band-narrowing experiments, critical-ratio estimates from pure-tone detection in wideband noise, and intensity discrimination for a critical-band-wide noise in wideband noise are not supported by the results of individual listeners, or results averaged across listeners.

Interaural intensity discrimination: insensitivity at 1000 Hz.

Recent data from three laboratories have replicated Mills' [J. Acoust. Soc. Am. 32, 132-134 (1960)] finding that interaural intensity discrimination is relatively poorer for tones of 1000 Hz than for tones of either higher or lower frequencies. To get a finer look at this frequency effect, interaural intensity difference thresholds were obtained from four subjects for tones of several frequencies around 1000 Hz. An adaptive two-interval forced-choice procedure was employed, in which the overall intensity of the signals was varied randomly in order to prevent subjects from listening to monaural loudness changes. Despite large intersubject differences in overall sensitivity to interaural intensity differences, all four subjects showed a local peak in their threshold functions at or near 1000 Hz. This curious "1000-Hz effect" might be explained by imagining that an interaural intensity comparator operates more efficiently as frequency increases, but that a peripheral interaural intensity difference to interaural-time difference conversion contributes to laterality judgments for low-frequency tones, thus acting to lower thresholds again for frequencies below 1000 Hz.

Operating ranges and intensity psychophysics for cochlear implants. Implications for speech processing strategies.

This report reviews human and monkey psychophysical data related to the detection and perception of intensity information by subjects with cochlear implants. The threshold contour for sinusoidal stimuli is characterized by a slope near zero at frequencies below about 100 Hz, a slope of 5 to 15 dB per octave at frequencies from about 100 Hz to between 0.2 and 2.0 kHz, and a slope of 4 dB per octave at higher frequencies. Equal loudness (or latency) contours follow the threshold contour at low intensities but change gradually as intensity is increased, assuming a shape that can be characterized by zero slope below 100 to 250 Hz and a 3-dB-per-octave slope at higher frequencies. Loudness growth functions and intensity difference limens are also dependent on stimulus frequency and intensity. These psychophysical data suggest that membrane characteristics and other factors impose marked alterations on incoming electrical stimuli, which must be considered carefully when developing speech encoding strategies.

Frequency and intensity difference limens for harmonics within complex tones.

A two-interval, two-alternative forced choice task was used to estimate frequency difference limens (DLs) for individual harmonics within complex tones, and DLs for the periodicity (i.e., number of periods per s) of the whole complexes. For complex tones with equal-amplitude harmonics, the DLs for the lowest harmonics were small (less than one percent). The DLs increased rather abruptly around the fifth to seventh harmonic. The highest harmonic in each complex was also well discriminated, and the discriminability of a single high harmonic was markedly improved by increasing its level relative to the other components. The DL for a complex tone was generally smaller than the frequency DL of its most discriminable component. The DL for a complex was found to be predictable from the DLs of the harmonics comprising the complex, using a formula derived by Goldstein [J. Acoust. Soc. Am. 54, 1496-1516 (1973)] from his optimum processor theory for the formation of the pitch of complex tones. The DL for a complex is sometimes primarily determined by high harmonics, such as the highest harmonic, or a harmonic whose level exceeds that of adjacent harmonics. We also measured intensity DLs for individual harmonics within complex tones. The intensity DLs were smallest for low harmonic numbers, and for the highest harmonic in a complex. An excitation-pattern model was used to determine whether the frequency DLs of harmonics within complex tones could be explained in terms of place mechanisms, i.e., in terms of changes in the amount of excitation at appropriate frequency places. We conclude that place mechanisms are not adequate, and that information about the frequencies of individual harmonics is probably carried in the time patterning of neural impulses.

Reliability of auditory function tests in severely hearing-impaired and deaf subjects.

The test-retest variability of a series of auditory functions has been investigated in a group of severely hearing-impaired and deaf subjects (64 ears, median Fletcher index: 80 dB) and in a group of 10 controls with normal hearing. An adaptive forced-choice procedure was used for both groups. The functions were: tone audiogram, difference limen for intensity, difference limen for frequency, modulation transfer function and critical ratio. In spite of sometimes strongly deviating function values within the hearing-impaired group, the test-retest variability of the two groups was found to be of the same order of magnitude, except for the tone audiogram where the variability in the hearing-impaired group was twice that for the control group.

Olfactory thresholds in unilaterally bulbectomized rats

Five normal and five unilaterally bulbectomized rats were tested for their intensity difference threshold, absolute threshold and ability to acquire a two-odor discrimination. There were no significant differences between groups for mean threshold values or the rate at which the detection and discrimination tasks were acquired. These results do not support the notion proposed in earlier studiesthat central summation of birhinically inhaled odors increases sensitivity to threshold levels of odorants.

The relation between loudness growth and intensity discrimination

The notion that the intensity-difference limen can be used as an indirect test of loudness recruitment is an old idea. However, to date, a firm quantitative relation between the two measures has not been established. We consider the problem as one in which loudness is related to the overall stimulus magnitude and intensity discrimination is a reflection of the accuracy with which a loudness judgment can be made. Since for pure tones loudness and intensity discrimination covary as functions of intensity, for a specific intensity one quantity can be inferred from the other. To test this relation, adaptive ABLB loudness-growth functions were obtained for a 2000-Hz, 500-ms pure tone from a normally hearing listener (partially masked monaurally) and a listener with a moderate, unilateral sensorineural hearing loss. Intensity-discrimination performance was measured for the same 2000-Hz tone, at several levels, in both ears of each subject. Intensity-recruitment functions derived from these data are very similar to the obtained loudness-recruitment functions. [Work supported by DRF and NINCDS.]

Intensity difference thresholds assessed with eighth nerve and auditory cortex potentials: compared values from cochlear and vascular responses.

In normal guinea pigs the eighth nerve compound action potential shows a latency of about 1 ms and the evoked early potential at the auditory cortex occurs after about 10 ms. Determination of auditory sensitivity with both responses using tone-bursts of variable rise/fall time and plateau duration showed they are responses to the onset of the stimulus since the existence of a plateau has no effect. Increases in stimulus rise time diminish the synchrony of both responses, the limits were found to be 2 ms for eighth nerve responses and 10 ms for cortical potentials. Amplitude modulation of continuous tones using these time values evokes clear responses at levels which apparently correspond to intensity difference thresholds measured behaviorally by other authors. Similar measures were performed on saccular acoustic responses, using the model of selective cochlear destruction. Evoked responses could also be observed at levels of amplitude modulation similar to those of normal animals. This fine sensitivity reinforces the idea of a functionality of the saccular acoustic reception.

Auditory intensity discrimination in human infants

Auditory intensity difference limens were measured in 40 human infants (ages 7–9 months) and several adults using a go, no—go operant head‐turning technique and an adaptive staircase (tracking) discrimination procedure. Subjects were visually reinforced for responding to intensity increments in a repeating background signal either a 1.0‐kHz tone or a vowel /a/, presented free‐field at a level of 60 dB SPL. Stimulus control in infants was monitored through the use of randomly interleaved “probe” and “catch” trials. Infant thresholds ranged from 3.0 to 12.0 dB for tones and from 2.5 to 12.0 dB for vowels, with the most sensitive infants falling within the range of adults. It is noteworthy that we were unsuccessful in training any infants to respond to intensity decrements, although adults experienced no difficulty with this task. [Supported by NIH grant HD 11915.]

Intensity discrimination with cochlear implants.

Intensity difference limens were measured for various frequencies and intensities of sinusoidal and pulsatile electrical stimulation in monkeys with electrodes implanted in the scala tympani, scala vestibuli, modiolus, or middle ear. Difference limens decreased, as a function of initial stimulus intensity, from values of 1.5-3 dB near threshold to as low as 0.5 dB near the upper limit of the dynamic range. If sensation level was held constant, difference limens decreased as a function of frequency up to about 500 Hz, and then remained constant. They were similar across a variety of electrode placements and separations if differences in threshold and dynamic range were taken into account. However, difference limens measured in severely damaged ears were slightly smaller than those in moderately damaged ears. The near miss to Weber's law, characteristic of acoustic difference limens, was not seen with electrical stimuli. Differences limens for electrical stimuli were roughly one-half those for acoustic stimuli; thus, part of the deficit in dynamic range for electrical stimulation compared with acoustic stimulation is countered by the smaller intensity differences limens for electrical stimuli.

The effects of lesions of nucleus rotundus on visual intensity difference thresholds in turtles (Chrysemys picta)

Intensity difference thresholds were assessed behaviorally in 7 painted turtles ( Chrysemys picta) before and after lesions of nucleus rotundus thalami or control lesions. Three subjects with control lesions and two subjects with slight bilateral damage to nucleus rotundus showed no permanent elevation of threshold postoperatively. In contrast, two subjects with severe damage to nucleus rotundus showed threshold elevations postoperatively and did not recover with further training. The impairment shown by these subjects with damage to nucleus rotundus appeared to be only on the more difficult problems; they performed as they had preoperatively on easy discriminations.

Monaural and binaural intensity discrimination in normal and cochlear-impaired listeners.

Monaural and binaural intensity difference limens for 75-dB SPL pure tones were determined for 6 normal subjects and for 6 subjects with cochlear hearing loss. The magnitude of binaural masking level difference (BMLD) was also determined. Both normal and hearing-impaired subjects showed a 0.45-dB binaural advantage for intensity discrimination. In contrast, the BMLD was appreciably reduced with hearing impairment. Results are discussed in terms of specific effects of hearing loss on binaural hearing.

Auditory signal processing in hearing-impaired persons with residual high-frequency hearing

A number of persons with severe sensorineural hearing losses in the frequency range below 6000 Hz have been shown to have near-normal thresholds for frequencies above 6000 Hz. This paper reports on measures of high-frequency signal processing in seven persons with this type of heating impairment. Measures taken include estimates of difference limens for intensity and frequency, gap detection, rate discrimination, and narrow-band masking functions. Although little normative data exist for comparative purposes, measures of frequency, intensity, and time resolution are within the range of what one would anticipate by extrapolation. Intensity and frequency differences limens, and wideband and narrow-band masking functions also were obtained in the region of impaired hearing {i.e., 3–6 kHz}; narrow-band masking measures and difference limens indicated impaired frequency processing and varied between subjects. Intensity discrimination fell within previously published ranges. [Work supported by the National Institutes of Health.]

The relations among critical bands, critical ratios, and intensity difference limens in man

Band‐narrowing estimates of the critical bandwidth (CB) are consistently larger than critical‐ratio (CR) estimates for the same signal frequency. Bilger [in Hearing and Davis: Essays Honoring Hallowell Davis, edited by S. K. Hirsh et al. (Washington U. P., St. Louis, 1976), p. 191] hypothesized that this difference can be accounted for by reference to intensity‐discrimination performance (CF[Hz]/CB[Hz] = ΔI/I). To test this hypothesis, band‐narrowing, critical‐ratio, and intensity‐discrimination data were collected from four subjects. Signal frequency was 2000 Hz and two noise spectrum levels were used: 20 and 50 dB. The psychophysical procedure was an adaptive, two‐interval, forced‐choice, converging on 70.7% correct. Based on critical‐bandwidth estimates obtained from the band‐narrowing experiments using common two‐line regression techniques, critical‐ratio estimates for pure‐tone detection in broadband noise, and intensity‐discrimination for critical bands of noise, the results across experimental conditions for individual subjects are not well described by Bilger's equation. [Work supported by NIH and the Deafness Research Foundation.]