Auditory temporal summation in infants and adults: Effects of stimulus bandwidth and masking noise

A visually reinforced operant procedure was employed to determine the behavioral thresholds of 6- to 7-month-old infants and adults for stimuli of various bandwidths and durations. Experiment 1 compared absolute thresholds for broadband and 1/3-octavefiltered clicks and 300-msec noise bursts. For adult subjects, the difference in threshold for clicks and noise bursts was -quite comparable in the two bandwidth conditions, but infants’ click-noise threshold differences were significantly larger for broadband than for 1/3-octave stimuli. In Experiment 2, 2-point threshold-duration functions were compared for 4-kHz tones and octave-band noise bursts presented in backgrounds of quiet and continuous noise. Infants’ threshold-duration function for octave-band noise bursts was significantly steeper than the comparable adult function in quiet, but not in masking noise. These results suggest that young infants may have particular difficulty detecting low intensity broadband sounds when durations are very short.     

Auditory sensitivity in school-age children

Thresholds for octave-band noises with center frequencies of 0.4, 1, 2, 4, and 10 kHz and 1/3-octave-band noises centered at 10 and 20 kHz were obtained from children 6 to 16 years of age. Such thresholds, combined with those obtained previously for infants, preschool children, and adults, provide a detailed picture of developing auditory sensitivity between infancy and maturity. Continuing improvements in sensitivity are evident from infancy through the preschool period, well into the school years. For stimuli with center frequencies of 0.4 and 1 kHz, maximal sensitivity is achieved at about 10 years of age, compared to 8 years for stimuli of 2 and 4 kHz. For 10-kHz stimuli, there is little change beyond 4 or 5 years of age. Finally, 20-kHz stimuli yield maximal sensitivity at about 6 or 8 years of age, followed by a progressive decline to adult levels. These findings are considered in relation to auditory sensitivity in nonhuman species, to structural and functional development of the ear, and to possible changes in the efficiency of neural processing.     

Developmental changes in infants' sensitivity to octave-band noises

Localization responses to octave-band noises with center frequencies at 200, 400, 1000, 2000, 4000, and 10,000 Hz were obtained from infants 6, 12, and 18 months of age. During an experimental trial, an octave-band noise was presented on one of two speakers located 45° to each side of the infant. A head turn to the noise (correct response) was rewarded by activating an animated toy on top of the speaker. The intensity of the noise was varied over trials (method of constant stimuli) to determine thresholds at each center frequency. Thresholds for the lower frequencies were approximately 5–8 db higher in the 6-month-old infants compared to the older infants. However, there were no consistent differences among groups at the higher frequencies. Infant thresholds were found to be 20–30 db higher than adult thresholds at the lower frequencies. At the higher frequencies thresholds for infants were approaching those of adults.     

Effect of masker level on infants’ detection of tones in noise

In adult listeners, the signal-to-noise ratio at masked threshold remains constant with increases in masker level over a wide range of stimulus conditions. This relationship was examined in 7-month-old infants by obtaining masked thresholds for .5- and 4-kHz tones presented in four levels of continuous masking noise. Adults were also tested for comparison. Masker spectrum levels ranged from 5 to 35 dB/Hz for .5-kHz tones, and from ?5 to 25 dB/Hz for 4-kHz stimuli. Thresholds were determined for stimuli of both 10 and 100 msec in duration. The results indicated that infants’ performance was more adultlike for 4-kHz stimuli. Although mean thresholds for both 10- and 100-msec, 4-kHz tones were approximately 7 dB higher in infants than in adults, E/N₀ at threshold remained essentially constant over the 30-dB range of maskers employed. By contrast, infants’ thresholds for .5-kHz tones were exceptionally high at lower levels of the masker. Threshold E/N0 decreased significantly as masker level increased from 5 to 35 dB/Hz, and this decrease was significantly greater for 10- than for 100-msec stimuli. Temporal summation of .5-kHz tones, measured as the difference between thresholds obtained at the two signal durations, was greater for infants than for adults at low levels of the masker. However, because infants’ thresholds improved more rapidly with level for 10- than for 100-msec tones, age differences in temporal summation were no longer significant when masker spectrum level was 35 dB/Hz. These results suggest that the relationship between signal-to-noise ratio at masked threshold and level of the masker is dependent on both signal frequency and duration during infancy.     

Size of critical band in infants, children, and adults

Masked thresholds at two signal frequencies (0.8 and 4 kHz) were obtained from listeners aged 6.5 months, 2 years, 5 years, and 20.5 years in the presence of constant spectrum level, narrowband maskers of differing bandwidths. Consistent with the classical results of Fletcher (1940), masked threshold for all age groups increased with bandwidth up to a critical width, beyond which further increases in bandwidth were ineffective in increasing threshold. These critical widths (estimates of critical band size) did not change substantially with age (critical widths for infants were no more than 50% larger than those of adults) despite substantial changes in masked thresholds with age. Thus, contrary to previous claims, changes in auditory filter width cannot account for developmental changes in masked or absolute thresholds.     

Temporal summation of 500-Hz tones and octave-band noise bursts in infants and adults

A visually reinforced operant procedure was employed to obtain 2-point threshold-duration functions in 7-month-old infants and adults in two experimental paradigms. In Experiment 1, thresholds were determined for 10- and 100-msec, 500-Hz tones and octave-band noise bursts presented in quiet and noise backgrounds. Threshold-duration functions were significantly steeper for infants than for adults under all experimental conditions, and did not differ in slope as a result of differences in either stimulus bandwidth or masking noise. In Experiment 2, thresholds for trains of brief 500-Hz tone pulses were examined in infant and adult subjects. Infant functions were adult-like for integration of multiple-pulse stimuli, suggesting that the traditional, long-term process of temporal summation is mature by 7 months of age. Differences between the present results and those previously obtained for 4-kHz stimuli appear to be consistent with the view that different mechanisms are involved in the detection of low- and high-frequency signals.     

Detection by the guinea pig of acoustic stimulation and of electrical stimulation of the brain

A threshold procedure using operant behavioral techniques with positive reinforcement was developed after initial efforts with avoidance behavioral procedures proved unsatisfactory. In the first of three experiments the operant threshold procedure was tested by determining masked auditory thresholds for trains of clicks. In a second experiment, similar techniques were used to measure thresholds for electrical stimulation of the brain. The last experiment, again an auditory problem, involved a determination of the absolute thresholds for trains of short noise bursts as a function of the time between bursts. Middle ear malfunction proved to be a more severe problem than had been anticipated on the basis of reports in the literature. The threshold procedure, however, seems to be adequate for determination of absolute or masked thresholds with auditory or electrical stimuli.     

Effect of masker level on infants' detection of tones in noise

In adult listeners, the signal-to-noise ratio at masked threshold remains constant with increases in masker level over a wide range of stimulus conditions. This relationship was examined in 7-month-old infants by obtaining masked thresholds for .5- and 4-kHz tones presented in four levels of continuous masking noise. Adults were also tested for comparison. Masker spectrum levels ranged from 5 to 35 dB/Hz for .5-kHz tones, and from -5 to 25 dB/Hz for 4-kHz stimuli. Thresholds were determined for stimuli of both 10 and 100 msec in duration. The results indicated that infants' performance was more adult-like for 4-kHz stimuli. Although mean thresholds for both 10- and 100-msec, 4-kHz tones were approximately 7 dB higher in infants than in adults, E/N0 at threshold remained essentially constant over the 30-dB range of maskers employed. By contrast, infants' thresholds for .5-kHz tones were exceptionally high at lower levels of the masker. Threshold E/N0 decreased significantly as masker level increased from 5 to 35 dB/Hz, and this decrease was significantly greater for 10- than for 100-msec stimuli. Temporal summation of .5-kHz tones, measured as the difference between thresholds obtained at the two signal durations, was greater for infants than for adults at low levels of the masker. However, because infants' thresholds improved more rapidly with level for 10- than for 100-msec tones, age differences in temporal summation were no longer significant when masker spectrum level was 35 dB/Hz. These results suggest that the relationship between signal-to-noise ratio at masked threshold and level of the masker is dependent on both signal frequency and duration during infancy.     

Infants’ detection of increments in low- and high-frequency noise

A visually reinforced operant paradigm was employed to examine the relationship between the difference limen (DL) for intensity and level of the standard during infancy. In Experiment 1,7-month-old infants and adults detected increments in continuous noise presented via headphones at each of four levels ranging from 28 to 58 dB SPL. Noise stimuli were 2-octave bands centered at either 400 or 4000 Hz, and increments were 10 and 100 msec in duration. Infants’ DLs were significantly larger than those of adult subjects and significantly larger for low- than for high-frequency stimuli. For the high-frequency noise band, infants’ DLs were generally consistent with Weber’s law,remaining essentially constant for standards higher than 28 dB SPL (3 dB SL) for 100-msec increments and 38 dB SPL (13 dB SL) for 10-msec increments. For low-frequency noise, infants’ absolute thresholds were exceptionally high, and sensation levels of the standards were too low to adequately describe the relationship. In Ex-periment 2, 7-month-old infants detected 10- and 100-msec increments in 400-Hz noise stimuli presented in sound field. Infants’ low-frequency DLs were large at low intensities and decreased with increases in level of the standard up to at least 30 dB SL. For both low- and high-frequency noise, the difference between DLs for 10- and 100-msec increments tended to be large at low levels of the standard and to decrease at higher levels. These results suggest that the relationship between the DL and level of the standard varies with both stimulus frequency and duration during infancy. However, stimulus-dependent immaturities in increment detection may be most evident at levels within approximately 30 dB of absolute threshold.     

Spectral cues provided by the pinna for monaural localization in the horizontal plane

Six subjects located, monaurally, 1.0-kHz-wide noise bursts whose source originated on the side of the functioning ear and whose center frequency ranged from 4.0 through 9.0 kHz (Part 1). Irrespective of their actual locations, the stimuli appeared to migrate from the frontal sector of the arc toward the side as the center frequency was increased above 4.0 kHz. For some subjects, the sounds appeared again in front at the higher center frequencies. Comparable data were obtained with noise bursts 2.0 kHz in width. We referred to these constellations of location judgments, influenced by the frequency composition of the stimuli, as spatial referent maps. In Part 2, we measured, by means of a miniature microphone placed at the entrance of the external ear canal, the pinna amplification function for these same stimuli emanating from the same locations. The results showed a positive relation between the apparent location of noise bursts centered at 6.0 kHz and above and the relative amplification provided by the pinna. Localization performances by two subjects, chosen on the basis of their noncorresponding spatial referent maps, were examined for stimuli of wider bandwidths IPart 3). Their proficiency differed markedly from one another, which we accounted for in terms of different spatial referent maps that were associated with differences in the pinna amplification function.     

Sources of auditory masking in infants: Distraction effects

Previous work has demonstrated that infants’ thresholds for a pure tone are elevated by a masker more than would be predicted from their critical bandwidths. The present studies explored the nature of this additional masking. In Experiment 1, detection thresholds of 6-month-old infants and of adults for a 1-kHz tone were estimated under three conditions: in quiet, in the presence of a 4- to 10-kHz bandpa] noise at 40 dB SPL, and in the presence of the same noise at 50 dB SPL. The noise was gated on at the beginning of each trial. Adult thresholds were the same in all three conditions, indicating that little or no sensory masking took place in the presence of the noise. Infant thresholds were about 10 dB higher in the presence of the noise. We term this effectdistraction masking. In Experiment 2, the effect of gating the noise on at trial onset was examined. Thresholds for the same tone were estimated in quiet and in the presence of the bandpass noise at 40 dB SPL, but the noise was presented continuously during the session. Under these conditions, distraction masking was still observed for infants. These findings suggest that a masker can have nonsensory effects on infants’ performance in a psychoacoustic task.     

Correlations among within-channel and between-channel auditory gap-detection thresholds in normal listeners

We obtained data on within-channel and between-channel auditory temporal gap-detection acuity in the normal population. Ninety-five normal listeners were tested for gap-detection thresholds, for conditions in which the gap was bounded by spectrally identical, and by spectrally different, acoustic markers. Separate thresholds were obtained with the use of an adaptive tracking method, for gaps delimited by narrowband noise bursts centred on 1.0 kHz, noise bursts centred on 4.0 kHz, and for gaps bounded by a leading marker of 4.0 kHz noise and a trailing marker of 1.0 kHz noise. Gap thresholds were lowest for silent periods bounded by identical markers--'within-channel' stimuli. Gap thresholds were significantly longer for the between-channel stimulus--silent periods bounded by unidentical markers (p < 0.0001). Thresholds for the two within-channel tasks were highly correlated (R = 0.76). Thresholds for the between-channel stimulus were weakly correlated with thresholds for the within-channel stimuli (1.0 kHz, R = 0.39; and 4.0 kHz, R = 0.46). The relatively poor predictability of between-channel thresholds from the within-channel thresholds is new evidence on the separability of the mechanisms that mediate performance of the two tasks. The data confirm that the acuity difference for the tasks, which has previously been demonstrated in only small numbers of highly trained listeners, extends to a population of untrained listeners. The acuity of the between-channel mechanism may be relevant to the formation of voice-onset time-category boundaries in speech perception.     

Gains, losses and lateral differences in the hearing of schizophrenic patients.

Schizophrenic patients, whether newly admitted to hospital or institutionalized, exhibited ear differences in absolute threshold. Right ear thresholds were superior to left ear thresholds especially at frequencies above 2 kHz, but deteriorated in the course of the day or as a result of repeated testing. These effects were attributed to the dynamics of left hemisphere processes which in schizophrenia appear susceptible to inhibition and fatigue, effects that may be endocrine related. Relative to control subjects institutionalized schizophrenic patients showed superior hearing below 1 kHz and inferior hearing above 2 kHz.     

Hearing in passerine and psittacine birds: a comparative study of absolute and masked auditory thresholds

Operant conditioning and a psychophysical tracking procedure were used to measure auditory thresholds for pure tones in quiet and in noise for seven species of small birds--the budgerigar, canary, cockatiel, European starling, song sparrow, swamp sparrow, and the zebra finch. Audibility curves are roughly similar among the seven birds, with the maximum sensitivity between 2 and 5 kHz and poorer sensitivity outside this narrow region. Critical ratios (signal-to-noise ratio at masked threshold) were calculated from pure-tone thresholds in noise. Except for the budgerigar, the critical ratio functions of all birds increase at the rate of 3 dB/octave. This pattern is typical of that observed in most vertebrates. Critical ratios in the budgerigar, on the other hand, decrease gradually from 0.5 kHz to 2.8 kHz and increase dramatically above 2.8 kHz. The present research demonstrates that the critical ratio function for the budgerigar is not only different from other vertebrates but also different from other birds.     

Tactile sensitivity of children: effects of frequency, masking, and the non-Pacinian I psychophysical channel

Tactile perception depends on the contributions of four psychophysical tactile channels mediated by four corresponding receptor systems. The sensitivity of the tactile channels is determined by detection thresholds that vary as a function of the stimulus frequency. It has been widely reported that tactile thresholds increase (i.e., sensitivity decreases) as a function of age. However, there is controversial evidence with regard to the progressive loss of sensitivity starting from childhood. In this study, the tactile thresholds of children (n=9, ages 7-11 years) were measured and compared with the thresholds of young adults (n=11, ages 21-27 years). The stimuli consisted of sinusoidal bursts of mechanical displacements, which were applied to the left index fingertips of the participants by using a cylindrical probe (base area=0.126 cm2) without a contactor surround. Absolute thresholds were measured at frequencies of 2, 10, 40, 100, 250, and 500 Hz without masking. The absolute thresholds decreased at high frequencies and were similar to data from the literature except for some discrepancy because of methodological differences. In addition, the threshold of the non-Pacinian I channel was measured at 40 Hz by elevating the thresholds of the Pacinian channel by forward masking. The effects of forward masking in children were similar to results in young adults. In conclusion, there were no significant differences between the tactile thresholds of children and those of young adults at key frequencies: 40 Hz for the Pacinian and non-Pacinian I channels and 250 Hz for the Pacinian channel. These findings contradict the hypothesis that there is gradual loss of tactile sensitivity starting from childhood to early adulthood. The loss of sensitivity due to aging probably is more abrupt and occurs at a later age.     

Behavioral limits of auditory temporal resolution in the rat: amplitude modulation and duration discrimination

Thresholds for detecting the presence of amplitude modulation in a noise carrier were determined for rats using conditioned avoidance procedures. There was a progressive increase in threshold with modulation rates between 5 Hz and 2 kHz. Further tests were conducted to determine difference thresholds for detecting an increase in modulation rate for standard rates of 10, 50, and 100 Hz. The size of the difference threshold increased progressively as the standard rate increased. In addition, thresholds for detecting an increase in the duration of a noise burst were determined for various standard durations. The difference thresholds were constant for values between 10 and 50 ms but increased progressively, with standard durations between 0.1 and 1.0 s.     

The masking of octave-band noise by broad-spectrum noise: A comparison of infant and adult thresholds

Localization responses to a 4,000-Hz octave-band noise in a background of broad-spectrum noise were obtained from infants, 6, 12, 18, and 24 months of age, and adults. A two-alternative, forced-choice procedure was used to determine thresholds at each of two levels of masking noise, 42 and 60 dBC. Adults were also tested for their localization of pure tones in noise and their detection of octave-band noises with the more traditional two-interval, forced-choice task. Increasing the masking noise from 42 to 60 dBC resulted in comparable threshold shifts for all age groups. However, infant thresholds were 16–25 dB higher than those obtained for adults. The theoretical implications of these findings are discussed.     

The spatial attributes of stimulus frequency and their role in monaural localization of sound in the horizontal plane

Listeners, whose right ears were blocked, located low-intensity sounds originating from loudspeakers placed 15 deg apart along the horizontal plane on the side of the open, or functioning, ear. In Experiment 1, the stimuli consisted of noise bursts, 1.0 kHz wide and centered at 4.0 through 14.0 kHz in steps of .5 kHz. We found that the apparent location of the noise bursts was governed by their frequency composition. Specifically, as the center frequency was increased from 4.0 to about 8.0 kHz, the sound appeared to move away from the frontal sector and toward the side. This migration pattern of the apparent sound source was observed again when the center frequency was increased from 8.0 to about 12.0 kHz. Then, with center frequencies of 13.0 and 14.0 kHz, the sound appeared once more in front. We referred to this relation between frequency composition and apparent location in terms of spatial referent maps. In Experiment 2, we showed that localization was more proficient if the frequency content of the stimulus served to connect adjacent spatial referent maps rather than falling within a single map. By these means, we have further elucidated the spectral cues utilized in monaural localization of sound in the horizontal plane.     

Monaural and binaural temporal integration of noise bursts

Summary A comparison was made between monaural and binaural temporal integration of noise bursts at threshold. The data indicate partial integration, with approximately a 6 dB decrease in threshold per decade increase in noise burst duration for both conditions of stimulation (i.e., parallel functions) for durations ranging from 4 to 256 msec. When thresholds in dB are plotted as a function of log duration, the linear component accounts for 99% of the data indicating no essential change in the partial integration functions up to at least 256 msec. The intercept difference between the monaural and binaural integration functions is 2.5 dB.     

Infant auditory perception: Tonal masking

The results of an earlier study (Olsho, 1984) indicated that 5- to 8-month-old infants were relatively better at discriminating among high-frequency pitches than low. In the present study, sensory and nonsensory explanations for that effect were evaluated by examining infants' performance in a task requiring similar sensory processing but differing in the demands placed on processes such as memory. Infants' ability to resolve frequency was tested using a tonal masking paradigm, the psychophysical tuning curve. Twenty-four infants were tested at probe frequencies ranging from 500 to 4000 Hz; a group of young adults served as a comparison. Masked and unmasked thresholds were estimated using the visually reinforced head turn procedure in conjunction with an adaptive psychophysical method. Although infants' tuning curves fell below those of adults (indicating poorer performance), the widths and slopes of the infants' curves were not different from the adults'. Moreover, the difference between age groups remained constant across probe frequencies. These findings imply that by 5 months of age, the infant's ability to resolve sound frequency is similar to the adult's.