Intensity representation

Transcription

1 Intensity representation 1 Representation of the intensity of sound (or is it something else about efficiency?) Remember that when we left frequency and temporal resolution, we concluded that frequency resolution and temporal resolution are mature at a fairly early age. But at the same time, masked thresholds and modulation detection thresholds are immature. Intensity resolution is one of the things that could contribute to those immature thresholds. 1

2 Resolution and efficiency Broad filter Threshold standard listener Inefficient listener Threshold- 20 log m Poor temporal resolution standard listener Inefficient listener Notch Width ( f/f) Modulation Frequency (Hz) So here is the situation. Let s say we measure frequency resolution using the auditory filter width (notched noise) method. Compared to a standard (mature) listener, an individual with poor frequency resolution, a broad auditory filter has higher thresholds, but the slope of the function describing threshold v. notch width will be shallower. An individual who has normal frequency resolution may require a higher signal;-to-noise ratio to detect the probe than the standard listener does. That listener's curve will indicate higher thresholds, but it will be parallel to the standard curve. We say that the listener is inefficient. Several studies show that infants and children have mature frequency resolution, but are inefficient listeners. The same situation holds for temporal resolution, where the measure is the temporal modulation transfer function. A listener with poor temporal resolution will have poorer thresholds than the standard listener, and his TMTF will have a lower high-frequency cutoff-- I.e, his performance will begin to deteriorate at lower modulation frequencies than the standard listener s does. A listener might have normal temporal resolution, but need a bigger modulation depth to tell that there is modulation. That person s TMTF will be shifted to poorer thresholds, but his TMTF will parallel that of the standard listener. That is the inefficient listener. Again, this is what we think is going on with young children. So thresholds are determined by both resolution and efficiency, and we have at least two examples that suggest that older infants and younger children have mature resolution but poor efficiency. 2

3 Contributors to inefficient listening Intensity resolution Inattentiveness Listening strategy Physiological noise The top item on the list of potential causes of inefficient listening is intensity resolution. So here we consider how intensity resolution develops. 3

4 Problem with measuring intensity resolution How do you separate bad performance from bad resolution? 4

5 Resolution and efficiency Broad filter Poor resolution Threshold Inefficient listener Threshold Don t know standard listener standard listener Notch Width ( f/f) Stimulus We don t have a measure of intensity resolution that lets us pull intensity resolution away from other sources on inefficiency. We will depend on the existence of Age X Stimulus interactions: cases where children are adult like or more adult like in some stimulus conditions than others. Then we use the argument: If they are attentive enough to perform the task for stimulus 1, then they should be attentive enough to perform the task for stimulus 2. Their poor performance on stimulus 2 must be due to immature resolution. Of course, if it turns out that the infant or child produces a curve that is just a shifted parallel version of the adult s, then we don t have a basis for arguing that it is resolution is immature. We just don t know. In fact, if we consistently see that every stimulus manipulation we make changes the child s performance just as It changes the adult s, then we may make the argument that resolution is mature. That is, the child s resolution behaves : just like the adult s, even though their overall level of performance is poorer. 5

6 Topics in intensity processing Absolute sensitivity Intensity discrimination Loudness 6

7 Prenatal absolute sensitivity: measurement problems What is the stimulus that reaches the fetal ear? What is the stimulus that reached the fetal inner ear? What is the message in the fetal auditory nerve? Is the response to the sound or to the maternal response? We should address the question of absolute sensitivity in the prenatal period. After all, the ear is thought to begin to function around weeks of gestation, and we know we can record an ABR at 28 weeks. So it is natural to wonder what the fetus thresholds are like. If you give this a moments thought, you realize that this is not an easy question to answer. Sound is filtered through maternal tissue and fluid whether it comes from the mother or from an external source. These structures act as a low-pass filter. But then the sound has to actually get into the inner ear, probably by bone conduction, and not much is known about fetal bone conduction. Over the third trimester of gestation, the inner ear is continuing to develop, so the message in the auditory nerve is likely to be immature. And even if we get a response of some kind from the fetus, it is next to impossible to keep the mother from hearing the stimuli. The mother may react to the sound, which in turn leads to a reaction in the fetus even if the fetus didn t hear the sound. There are in fact many papers in the literature, many in which the stimulus was presented with an artificial larynx. Because such a device produces a fairly intense very low frequency vibration, it is not even clear that any response to it is a response to sound rather than a skin response to the vibration. (Skin senses tend to be more mature than audition during development.) 7

8 References: Barden, T., Peltzman, P., and Graham, J. (1968). "Human fetal electroencephalographic response to intrauterine acoustic signals," Am J Obst & Gynec, 100, Bernard, J., and Sontag, L. W. (1947). "Fetal reactivity to tonal stimulation: A preliminary report," J Genet Psychol, 70, Birnholz, J. C., and Benacerraf, B. R. (1983). "The development of human fetal hearing," Science, 222, Bradley, R. M., and Mistretta, C. M. (1975). "Fetal sensory receptors," Physiol. Rev., 55, Cook, C. J., Williams, C., and Gluckman, P. D. (1987). "Brainstem auditory evoked potentials in the fetal sheep, in utero," J. Dev. Physiol., 9, Fox, H. E., and Badalian, S. S. (1993). "Fetal movement in response to viroacoustic stimulation: A review," Obstet Gynecol Surv, 48, Gerhardt, K. J., Abrams, R. M., Kovaz, B. M., Gomez, K. J., and Conlon, M. (1988). "Intrauterine noise levels produced in pregnant ewes applied to the abdomen," Am J Obstet Gynecol, 159, Gerhardt, K. J., Abrams, R. M., and Oliver, C. C. (1990). "Sound environment of the fetal sheep," Am. J. Obstet. Gynecol., 162,

9 Gerhardt, K. J., Huang, X., Arrington, K. E., Meixner, K., Abrams, R. M., and Antonelli, P. J. (1996). "Fetal sheep in utero hear through bone conduction," Am J Otolaryngol, 17, Gerhardt, K. J., Otto, R., Abrams, R. M., Colle, J. J., Burchfield, D. J., and Peters, A. J. (1992). "Cochlear microphonics recorded from fetal and newborn sheep," Am. J. Otolaryngol., 13, Grimwade, J. C., Walker, D. W., Bartlett, M., Gordon, S., and Wood, C. (1971). "Human fetal heart rate change and movement in response to sound and vibration," Am J Obstet Gynecol, 109, Hepper, P. G., and Shahidullah, B. S. (1994). "Development of fetal hearing," Archives of disease in childhood, 71. Jensen, O. H. (1984). "Fetal heart rate response to a controlled sound stimulus after propranolol administration to the mother," Acta Obstet. Gynecol. Scand., 63, Johansson, B., Wedenberg, E., and Westin, B. (1992). "Fetal heart rate response to acoustic stimulation in relation to fetal development and hearing impairment," Acta Obstet. Gynecol. Scand., 71, Lecanuet, J.-P., and Granier-Deferre, C. (1993). "Speech stimuli in the fetal environment," in Developmental neurocognition: Speech and face processing in the first year of life, edited by D. d. boysson-bardies, S. Schonen, P. Jusczyk, P. McNeilage and J. Morton (Kluwer Academic Press, London). 9

10 Lecanuet, J.-P., Granier-Deferre, C., and Busnel, M.-C. (1988). "Fetal cardiac and motor responses to octave-band noises as a function of central frequency, intensity and heart rate variability," Early Hum Devel, 18, Lecanuet, J.-P., Granier-Deferre, C., Cohen, H., Le Houezec, R., and Busnel, M.-C. (1986). "Fetal responses to acoustic stimulation depend on heart rate variability pattern, stimulus intensity and repetition," Early Hum Devel, 13, Lecanuet, J. C., Granier-Deferre, C., and Busnel, M. C. (1989). "Differential fetal auditory reactiveness as a function of stimulus characteristics and state," Sem. Perinatol., 13, Lecanuet, J. P. (1996). "Fetal sensory competencies," Eur. J. Obstet. Gynecol., 68, Querleu, D., Renard, X., Versyp, F., Paris-Delrue, L., and Crepin, G. (1988). "Fetal hearing," Eur. J. Ob. Gyn. Repro. Biol., 29, Smith, S. L., Gerhardt, K. J., Griffiths, S. K., and Huang, X. (2003). "Intelligibility of sentences recorded from the uterus of a pregnant ewe and from the fetal inner ear," Audiol Neurootol, 8, Staley, K., Iragui, V., and Spitz, M. (1990). "The human fetal auditory evoked potential," Electroencephalogr Clin Neurophysiol, 77, 1-5. Tanaka, Y., and Arayama, T. (1969). "Fetal responses to acoustic stimuli," Pract. Oto-Rhino-Laryngol., 31, Walker, D., Grimwade, J., and Wood, C. (1971). "Intrauterine noise: A component of the fetal environment," Am. J. Obstet. Gynecol., 109, Zappasodi, F., Tecchio, F., Pizzella, V., Cassetta, E., Romano, G. V., Filligoi, G., et al. (2001). "Detection of fetal auditory evoked responses by means of magnetoencephalography," Brain Research, 917,

11 Lecanuet et al. (1988) wk gestational age fetuses 500, 2000, 5000 Hz octave bands of noise Speaker 20 cm above maternal abdomen (mother listens to music) 100, 110, 115 db SPL 1 cm above maternal abdomen 5 second duration 5-15 min interstimulus interval High ( awake ) and low ( asleep ) HR variability Cardiac and motor responses (ultrasound) Given all of these caveats, then, let s consider one example of a study of prenatal sensitivity by Lecanuet et al. They presented octave bands of noise to fetuses who were 37 to 40 weeks gestational age. They used three different frequencies and presented each at three intensities, 100, 110 and 115 db SPL. They measured the level of the stimuli 1 cm above the maternal abdomen. They get points for calibrating at all. The speaker playing the sounds was 20 cm above the mother s abdomen, and they tried to keep the mother from hearing the sound by having her listen to music. (I m guessing that the music would really have to be at 120 db SPL to mask the noise bands, so I don t think it worked very well.) The stimuli were 5 s long, and after each presentation, they waited at least 5 min so that the fetus returned to baseline before the next presentation. They also considered whether the fetus was awake or asleep, by measuring HR variability. HR variability tends to be low when we re asleep and high when we re awake. They recorded two types of response to sound, HR change and movement of the lower limb. Both responses were measured via ultrasound. 11

12 Prenatal cardiac responses to sound Lecanuet et al s results on HR are shown here, where the vertical stripes are HR accelerations, and the slanted stripes are HR decelerations. HR decelerations are part of the orienting response to stimulation; they indicated interest rather than a startle. Thus, the researchers are trying to say that they are looking at responses that are not just startle responses to very intense sound. The widely spaced stripes are for periods of high HR variability, while the closely spaced ones are for periods of low HR variability. Each bar represents the percent of presentations on which that response was recorded. The intensity increases from left to right and frequency increases from bottom to top. Let s just consider the responses in high HR variability, when the fetus is awake. For 500 Hz at 100 db, 36% of responses were HR decelerations, while 28% were HR accelerations-- so it was more likely that the fetus was interested in the sound than startled by it. But for both 2000 and 5000 Hz at 100 db more of the responses were accelerations than decelerations. The implication is that these sounds are louder to the fetus than the 500 Hz noise band--that they are further above threshold. As intensity increases for all frequencies, the proportion of acceleration responses increase. That suggests that the sounds are getting louder and that they are all pretty loud. So the general conclusion might be that these sounds are well above threshold, and that the fetus is more sensitive to the higher frequency sounds. 12

13 Prenatal motor responses to sound These are the proportion of times the fetus produced a motor response. This response is produced less often than a HR change. At any intensity, response probability increases as the frequency increases. Response probability also increases as the intensity increases. So again, it looks like the fetus is more sensitive to the higher frequency. So you might think based on this that maybe the threshold is db SPL, but keep in mind that these sounds are actually presented in a background of noise, and that the spectrum of that noise is weighted toward to low frequencies. Thus, the fetus may look more sensitive to high frequency sounds because there is no noise at high frequencies. What we know then is that the fetus responds to intense external sounds, but we really do not have a good idea of what the absolute threshold is like. 13

14 Weir (1979): Pure-tone thresholds of newborns Pure tones Hz, various intensities Delivered with insert phones Recorded respiration, heart rate and motor responses; scored as response or not offline Sensitivity expressed as d 14

15 Pure tone thresholds of newborns Weir compared the newborns threshold with those reported by Watson et al. (1972), who tested adults with similar procedure, in a yes-no paradigm. Weir found that the newborn s thresholds were most like those of adults at 125 Hz, and that the immaturity in threshold increased with increasing frequency. This pattern of results is more like we might expect from Gottlieb s analysis of the frequencies that young organisms first respond to-those in the low-mid range of the species' range of hearing. References: Weir, C. (1976). "Auditory frequency sensitivity in the neonate: A signal detection analysis," J. Exp. Child Psychol., 21, Weir, C. (1979). "Auditory frequency sensitivity of human newborns: Some data with improved acoustic and behavioral controls," Percept. Psychophys., 26, Weir, C. G. (1985). "Use of behavioural tests in early diagnosis of hearing loss," Acta Otolaryngol. (Stockh.), Suppl. 421,

16 Werner & Gillenwater (1990): Pure-tone thresholds at 2-5weeks Observer-based method, but without reinforcement of infant response Tones presented with insert earphones Frequencies 500, 1000, and 4000 Hz, various intensities 16

17 Pure-tone thresholds of 2-5-weekolds The results showed that infant thresholds (individual infant) were about the same compared to adults at all frequencies, although the difference was a little bigger at 4000 Hz. There was more of difference at 4000 Hz if the group thresholds were considered.the differences between infants and adults are about db, compared to the db differences reported by Weir (1979). So either infants sensitivity improves by 20 db in the first couple of weeks of life, or the observer-based method is more sensitive than the response method used by Weir. Werner, L. A., and Gillenwater, J. M. (1990). "Pure-tone sensitivity of 2- to 5-week-old infants," Infant Behav. Dev., 13,

18 Trehub et al. (1991): Thresholds for octave-band noises, mo Observer-based method, with no reinforcement 4-kHz noise band alternated from left to right speakers Observer responded signal or no signal 18

19 Third-octave band thresholds, months Sensitivity was estimated as d at three intensities, as shown here. The threshold for the 1.5 month olds was estimated (d =1) at around 40 db SPL, about 7 db lower than reported by Werner and Gillenwater for a 4 khz tone. Whether that difference has to do with the bandwidth of the stimulus, differences in method or the passage of 3 more weeks of postnatal experience, is not clear. Notice that between 1.5 and 3.5 months, the threshold improves by about 8 db. Trehub, S. E., Schneider, B. A., Thorpe, L. A., and Judge, P. (1991). "Observational measures of auditory sensitivity in early infancy," Dev. Psychol., 27,

20 Olsho et al. (1988) Pure-tone thresholds 3-12 months Observer-based method (with reinforcement) Adaptive thresholds, Hz ear bud earphones 20

21 Pure-tone thresholds 3-12 months The results showed that at 3 months infants thresholds were 20 db higher than adults at 250 Hz, but about 30 db higher at 8000 Hz. In general, the infant adult difference gets bigger as the frequency increases at 3 months. At 6 and 12 months, the 250 Hz threshold is about the same as a 3-month--olds, but thresholds at higher frequencies have improved by db. By 6 months, the absolute threshold at 1000 Hz is about 15 db higher than adult threshold; at higher frequencies, the difference is 10 db or a little less. So the big improvement between 3 and moths is at higher frequencies. Note that the 4 khz threshold here is only a few db better than that reported by Trehub et al for the octave band noise. Olsho, L. W., Koch, E. G., Carter, E. A., Halpin, C. F., and Spetner, N. B. (1988). "Pure-tone sensitivity of human infants," J. Acoust. Soc. Am., 84,

22 Thresholds for speech-filtered noise Tharpe and Ashmead also used an observer-based method to trace the development of absolute sensitivity from birth to over 9 months. Their stimulus was a noise shaped to match the spectrum of speech; it was presented in sound field. What they found is that there was a threshold improvement of about 25 db over this time period, and that nearly all of it occurred by 6 months of age. This is in general agreement with the results of previous cross-sectional studies. Tharpe, A. M., and Ashmead, D. H. (2001). "A longitudinal investigation of infant auditory sensitivity," Am. J. Audiol., 10,

23 Third-octave bands, 6 mo-20 yr Finally, Trehub et al (1988) published the results of a 2-spatial alternative forced choice conditioned head turn/button press procedure for infants and children as old as 16 years. Each of these graphs shows the threshold as a function of age for a different frequency, 400 to Hz. Notice that at 10 khz, there is only a little improvement in infancy and early childhood before the threshold is adult like, The 4-kHz threshold falls to adult level by 5 years of age, and then as the frequency gets lower, it takes longer and longer for the threshold to reach adult levels. So again, high frequencies improve quickly early; low frequencies improve more slowly over a longer time period. Trehub, S. E., Schneider, B. A., Morrengiello, B. A., and Thorpe, L. A. (1988). "Auditory sensitivity in school-age children," J. Exp. Child Psychol., 46,

24 Development of the audibility curve: Summary So this is a figure I made to summarize the result of these studies. At birth, behavioral thresholds are high pretty much across the frequency range, although they are closer to adults at lower frequencies. There is a big drop in threshold between the newborn (study) and the 1-monthold (studies); the drop is bigger at the higher frequencies, and it is the high frequency threshold that improves the most from 1 month to 3 months to 6 months. By 4 years, if not before, the high frequency thresholds are adult like, but then it takes another 6 years before the low frequency thresholds approach adult levels. So rapid early drop in high frequency sensitivity, followed by slower, longer development of low frequencies. 24

25 Intensity discrimination: Adults and children Amplitude 1 2 Which one is more intense, 1 or 2? Amplitude Time 1 2 When did the increment occur, 1 or 2? Time The second topic under intensity representation is intensity discrimination. There are two ways that we generally test intensity discrimination in adults, and the same methods are general used to test children. We can present to discrete tones of different intensities to the listener and ask him to indicate the one that was more intense. This is referred to as gated intensity discrimination because the tones are gated on and off. The other method is increment detection. The sound is on continuously. Two intervals are indicated to the listener; the listener chooses the interval that had an increment in intensity. This is also referred to as continuous intensity discrimination. Adults generally do better in continuous than gated conditions, although all the reasons for that have not been identified. 25

26 Intensity discrimination: Infants Respond when the intensity changes Amplitude gated Time Respond when the intensity changes Amplitude continuous increment detection Time When we test infants, we do things a little differently. We may present a sound at one intensity repeatedly in the background, and on a trial the intensity of the sound goes up or down. This is analogous to the gated intensity discrimination shown in the last slide. We can also do increment detection or continuous intensity discrimination with infants. The sound is presented continuously, and the infant learns to respond when the intensity increases. 26

27 Intensity discrimination: infants and children This is a summary of intensity discrimination jnd obtained in various labs. The unfilled symbols at.5 years are for pure tones; the filled symbols are for broadband noise or speech. The triangles with the huge error bars are from our friends Jensen and Neff (who kept the data of sleeping children) and the circles are from Maxon and Hochberg. Maxon presented two tones from the audiometer and asked the kids whether the sounds were the same or different. It looks like even 4-year-olds are only a db or so worse than adults and certainly by 6 years everyone looks like adults. All of the development in intensity discrimination seems to happening in the first few years. Bench, J. (1969). "Audio-frequency and audio-intensity discrimination in the human neonate," Int Audiol, 8, Bull, D., Eilers, R. E., and Oller, D. K. (1984). "Infants' discrimination of intensity variation in mulitsyllabic stimuli," J. Acoust. Soc. Am., 76, Jensen, J. K., and Neff, D. L. (1993). "Development of basic auditory discrimination in preschool children," Psychol. Sci., 4, Maxon, A. B., and Hochberg, I. (1982). "Development of psychoacoustic behavior: Sensitivity and discrimination," Ear Hear., 3, Moffitt, A. R. (1973). "Intensity discrimination and cardiac reaction in young infants," Dev Psychol, 8, Sinnott, J. M., and Aslin, R. N. (1985). "Frequency and intensity discrimination in human infants and adults," J. Acoust. Soc. Am., 78,

28 Detection in noise parallels intensity discrimination Detection of tone or a noise band in noise is basically increment detection: the listener responds when he hears an increase in the intensity of the sound in some frequency region. So we might expect that detection in noise would develop along a time course that is similar to that seen for intensity discrimination. In Schneider et al s data (which we looked at when I talked about frequency representation), 4-year-olds aren t much better than 18 month olds and they still have a good 5-10 db to reach adult levels. Schneider, B. A., Trehub, S. E., Morrongiello, B. A., and Thorpe, L. A. (1989). "Developmental changes in masked thresholds," J. Acoust. Soc. Am., 86,

29 Detection in noise in standard psychophysical procedure But these are data obtained in a 3-alternative forced choice procedure for a tone in two different bandwidth maskers, The masked threshold is plotted as a function of age over the label simultan. The number at the bottom of the graph shows the slope of the best fitting line to the data. Masked threshold is improving by about.7 db/year between 5 and 11 years-- which really isn t much. In other studies, Hall, Buss and Grose have found that 4-year-olds are only a little worse than adults in detecting tones in noise. So in general the results are consistent with the idea that detection in noise develops along a time course that is similar to that for intensity discrimination. Buss, E., Hall, J. W., Grose, J. H., and Dev, M. B. (1999). "Development of adult-like performance in backward, simultaneous, and forward masking," J. Speech Lang. Hear. Res., 42,

30 Evidence for immature resolution v. other sorts of efficiency? Poor resolution Threshold Don t know standard listener Stimulus So far, just about all we know about intensity discrimination in infants and young children is that it is not that great. Is there any evidence that this is anything besides a lack of attentiveness or something else like that? 30

31 Increment detection: Infants Berg and Boswell examined increment detection in 7-9 month old infants and adults for octave bands of noise centered at 400 Hz or 400 Hz, with increment durations of 10 or 100 ms. In previous work, Berg and her colleagues had found that infants tended to be quite poor when dealing with short-duration sounds. These graphs show the intensity jnd as a function of the intensity of the sound. The adults are plotted in triangles, the infants in squares; the short increment is in the unfilled symbols and the long increment is in the filled symbols.. Look at the 4000 Hz data on the right. As the intensity goes up, the jnd is constant for the adults and for the infants with the long increment as Weber s Law would predict. The adults do a little better than the infants for the long duration. But basically, the infant results just parallel the adult results, just a little worse. For the short duration increment, the infant jnd is still worse than the adults and it is particularly bad at low intensities-- this may be because the standard tone is close to the infants quiet threshold for this sound at 30 db SPL. The results are different for the 400 Hz noise band, on the left.the infants jnd gets better dramatically as the level of the sound increases. The infants are more like the adults at high intensities than they are at low intensities. No jnd is plotted at the lower intensities at 400 Hz, because those sound levels were again too close to the infants quiet threshold for those sounds. Berg and Boswell suggest that this could be because for the low-frequency sound, anyway, excitation grows more slowly as the intensity of the sound is increased for the infants, or that the amount of excitation created by a given intensity is more variable for infants than it is for adults, so they need a bigger change to be sure that it was really a change and not just random variation. Berg, K. M., and Boswell, A. E. (1998). "Infants' detection of increments in lowand high-frequency noise," Percept. Psychophys., 60,

32 Increment detection: Children Berg and Boswell subsequently tested 1 to 3 year old children using the conditioned-head turn sort of procedure with 200-ms increments at the same two frequencies, again with octave bands of noise. At 4000 Hz, the 2-and 3-year-olds, pretty much have constant DLs across the intensity range, although they look a little worse at the lowest intensity, and they are a little poorer than adults in general. There is a steeper improvement with level among 1 year olds, which is interesting, because this wasn t evident in the results of the infants in the last study. However, at 400 Hz, all of the kids are showing a bigger improvement with level than the adults, and this is exaggerated in the 1 year olds. So again (in general) it seems like the effect of intensity on increment detection is different for the kids, at least for the lowfrequency sound. Berg, K. M., and Boswell, A. E. (2000). "Noise increment detection in children 1 to 3 years of age," Percept. Psychophys., 62,

33 Intensity discrimination: Summary Intensity jnd improves from about 5-7 db at 6 months to 1-2 db in adulthood. Intensity jnd is mature by about 5-6 years of age. Detection in noise follows the same time course as intensity discrimination. There is some evidence that intensity resolution is poorer during development for low-frequency sounds. 33

34 Loudness The last topic under intensity representation is loudness-- the perceived magnitude of sound. You may recall that Stevens introduced the method of magnitude estimation to quantify loudness, and that he showed that a 10 db increase in level led to a doubling of loudness. The question is whether loudness grows in the same way for infant and children as it does for adults. 34

35 Reaction time as a measure of loudness Reaction time (ms) Intensity It is hard to get infants to perform magnitude estimation or even crossmodality matching. One possible way to get around this is to look at how an infant's reaction time to a sound changes with the intensity of sound. Adults reaction time decreases as the sound intensity increases; past studies have shown that reaction time is related to loudness for adults. Leibold and Werner decided to see if infants reaction time changed with sound intensity in a similar way. 35

36 Loudness: Infants Reaction time to tone bursts of either 400 or 1000 Hz were measured to different sound intensities. The infants were trained to make a conditioned head turn when they heard the sound. Adults reaction times are plotted at the bottom of each panel and you can see that their reaction time decreased as the intensity increased. Infants had a much longer reaction time than adults, but their reaction time also decrease as the intensity increases. But notice that their reaction time decreases more as the intensity increases than adult's reaction time does. This suggests that loudness may grow faster with intensity for infants than it does for adults, but this is the only study that has examined this issue. Leibold, L., and Werner, W. A. (2002). "Relationship between intensity and reaction time in normal hearing infants and adults," Ear Hear., 23,

37 Loudness: Children Line length Intensity By the time a child is 4 or 5 years old, they can reliably perform magnitude estimation-style tasks. In particular they can do what is called cross modality matching. The listener is asked to draw a line that is as long as the sound is loud. 37

38 Loudness: Children Collins and Gescheider performed this experiment with children aged 4-7 years and with adults. From these line-length-intensity functions, you can figure out what the exponent of the loudness growth function is. The results are shown here, with the adults in unfilled symbols and the children with filled symbols. You can see that the exponents are pretty much the same for the children and adults, suggesting that at least by age 4, loudness grows in the same way for children and adults. Collins, A. A., and Gescheider, G. A. (1989). "The measurement of loudness in individual children and adults by absolute magnitude estimation and cross-modality matching," J. Acoust. Soc. Am., 85, Serpanos, Yula Cherpelis; Gravel, Judith S. Revisiting Loudness Measures in Children Using a Computer Method of Cross- Modality Matching (CMM). [References]. Journal of the American Academy of Audiology. Vol 15(7) Jul-Aug 2004,

39 Loudness discomfort levels: Children Another loudness-related measure is the loudness discomfort level-- how loud is too loud. This is an important measure in the clinic. By using a system like that illustrated here, various investigators have examined loudness discomfort levels for children. These are from a paper by Kawell et al. (1988), who tested hearing impaired adults and children. Kawell, M. E., Kopun, J. G., and Stelmachowicz, P. G. (1988). "Loudness discomfort levels in children," Ear Hear, 9,

40 Loudness discomfort levels Their results are shown here. The absolute thresholds are plotted in db SPL for children (circles) and adults (squares) in the unfilled symbols. Their subjects were matched on the basis of their audiogram. The loudness discomfort levels are plotted in the filled symbols. No question that the kids and the adults are the same. 40

41 Intensity representation: Summary Absolute sensitivity at high frequencies is adult like in the preschool period, but at low frequencies continues to mature into the school years. Intensity discrimination is mature by 5-6 years, and there is a little evidence indicating immature intensity resolution in infants and toddlers. Loudness may grow faster with increasing intensity in infants than adults, but loudness growth is mature by 4 years. 41