6
Relations among temporal resolution, forward masking, and simultaneous masking Richard A. Smiarowski* and Raymond Carhart Auditory Research Laboratory, Northwestern University, Evanston, Illinois 60201 (Received 29 August 1973; revised 10 February 1975) The data from this investigation suggest that both temporal resolution and forward masking are the products of a single underlying mechanism of auditory persistence. Persistence of auditory activity from a burst of noise not only limits the listener'sability to perceivea trailing noise burst as temporally separate (temporal resolution), but also interferes with his detection of a click that follows the burst of noise (forward masking). After a relatively stable initial period of about 2.5 msec, the magnitude of this persisting residual activity decays linearly over log time, as evidenced by the progressive reduction in the level of both the trailing noise burst in temporal resolution and the click threshold in forward masking. The data also indicate that forward masking of the click bears the same relationshipto masker level as does simultaneous masking of the click. The mechanisms of forward masking may be essentially the same as those for simultaneous masking, except that for the former the masking process is a residue that persistsfrom prior stimulation. Subject Classification: 65.58, 65.75. INTRODUCTION A comparison of the data from a number of independent investigations suggests that temporal resolution and for- ward masking are related manifestations of auditory per- sistence. Up until the present study, no one has reported measuring both temporal resolution and forward masking under comparable conditions with the same set of listeners. Information about the time course of temporal resolu- tion comes from an experiment by Plomp (1964) in which subjects listened for the presence of a separation between two successive bursts of wide-band noise. Plomp main- tains that a listener's ability to perceive the bursts as temporally separate is determined by the persistence of auditory excitation which takes a short time to die away following the cessation of the leading noise burst. If the trailing noise burst follows close enough in time, its onset will come while the residual excitation from the leading burst is still present in the auditory system. The additional stimulation resulting from the introduc- tion of the trailing burst will then add to this residual activity. If the trailing burst is intense enough, there will be a noticeable increase in auditory excitation and the listener will perceive the bursts as being temporally separate. In contrast, if the trailing burst is not intense enough, the amount of additional excitation will be insuf- ficient and the listener will experience a fused continuum. Somewhere between these two extremes, a certain criti- cal intensity of trailing burst will make the temporal separation just perceptible to the listener. Therefore, according to Plomp's model of temporal resolution, this critical trailing burst intensity is taken as an indicant of the level of residual activity persisting from stimulation by the leading noise burst. Plomp presents a set of decay functions which show the rela- tionship between the trailing burst level and the just noticeable interval separating the two successive bursts of noise. In general, with the just noticeable temporal separation plotted along a logarithmic scale, the trailing burst level in decibels is seen to decline in a linear fashion when the temporal separation exceeds 3 msec. Thus, Plomp's findings suggest a two-stage pattern of residual activity that holds the auditory system in a high state of excitation for about 3 msec following the termination of the leading noise burst, and then decays linearly over log time. Furthermore, an extrapolation of his auditory decay functions shows that regardless of the intensity of the leading noise burst, its poststimula- tory effect terminates in about 225 msec. A parallel with Plomp's two-stage time course of residual auditory excitation can be found in the data on the forward masking of a short-duration probe by a pre- ceding burst of wide-band noise (Elliott, 1962; Stein, 1960; Wilson and Carhart, 1971). In general, these forward-masking studies reveal that the auditory system remains in a relatively high state of masking for a few milliseconds following termination of the burst of mask- ing noise; thereafter, the degree of masking becomes progressively less as the interval between noise burst and probe is increased. This progressive decline iri decibels of forward masking approaches a linear trend when plotted over log time. Furthermore, this post- stimulatory masking effect generally dies out within about 250 msec regardless of the intensity of the noise burst that prooucea Such a close a•reement withthegeneral time course of temporal resolution reinforces the view that forward masking of the probe results from the residual excita- tion produced by the noise burst. In other words, the noise burst sets up auditory processes that continue to interfere with detection of the probe for a short time after the noise burst has beenterminated. Plomp'stemporal resolution paradigm offers a method for estimating the magnitude of this decaying residual excitation, whereas forward masking can be taken as the correlated effect that this residual activity has on detectability of a probe. Although, in general, these data on temporal resolu- tion and forward masking do indeed reveal a similar poststimulatory time course, they are derived from independent investigations and are therefore not entirely comparable. This fact prompted us to confirm the re- lationship with a single group of subjects who were tested 1169 J. Acoust. Soc. Am., Vol. 57, No. 5, May 1975 Copyright ¸ 1975 by the AcousticalSociety of America 1169 Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 132.206.7.165 On: Mon, 24 Nov 2014 15:53:15

Relations among temporal resolution, forward masking, and simultaneous masking

Embed Size (px)

Citation preview

Page 1: Relations among temporal resolution, forward masking, and simultaneous masking

Relations among temporal resolution, forward masking, and simultaneous masking

Richard A. Smiarowski* and Raymond Carhart

Auditory Research Laboratory, Northwestern University, Evanston, Illinois 60201 (Received 29 August 1973; revised 10 February 1975)

The data from this investigation suggest that both temporal resolution and forward masking are the products of a single underlying mechanism of auditory persistence. Persistence of auditory activity from a burst of noise not only limits the listener's ability to perceive a trailing noise burst as temporally separate (temporal resolution), but also interferes with his detection of a click that follows the burst of noise (forward masking). After a relatively stable initial period of about 2.5 msec, the magnitude of this persisting residual activity decays linearly over log time, as evidenced by the progressive reduction in the level of both the trailing noise burst in temporal resolution and the click threshold in forward masking. The data also indicate that forward masking of the click bears the same relationship to masker level as does simultaneous masking of the click. The mechanisms of forward masking may be essentially the same as those for simultaneous masking, except that for the former the masking process is a residue that persists from prior stimulation.

Subject Classification: 65.58, 65.75.

INTRODUCTION

A comparison of the data from a number of independent investigations suggests that temporal resolution and for- ward masking are related manifestations of auditory per- sistence. Up until the present study, no one has reported measuring both temporal resolution and forward masking under comparable conditions with the same set of listeners.

Information about the time course of temporal resolu- tion comes from an experiment by Plomp (1964) in which subjects listened for the presence of a separation between two successive bursts of wide-band noise. Plomp main- tains that a listener's ability to perceive the bursts as temporally separate is determined by the persistence of auditory excitation which takes a short time to die away following the cessation of the leading noise burst. If the trailing noise burst follows close enough in time, its onset will come while the residual excitation from the

leading burst is still present in the auditory system. The additional stimulation resulting from the introduc- tion of the trailing burst will then add to this residual activity. If the trailing burst is intense enough, there will be a noticeable increase in auditory excitation and the listener will perceive the bursts as being temporally separate. In contrast, if the trailing burst is not intense enough, the amount of additional excitation will be insuf- ficient and the listener will experience a fused continuum. Somewhere between these two extremes, a certain criti- cal intensity of trailing burst will make the temporal separation just perceptible to the listener.

Therefore, according to Plomp's model of temporal resolution, this critical trailing burst intensity is taken as an indicant of the level of residual activity persisting from stimulation by the leading noise burst. Plomp presents a set of decay functions which show the rela- tionship between the trailing burst level and the just noticeable interval separating the two successive bursts of noise. In general, with the just noticeable temporal separation plotted along a logarithmic scale, the trailing burst level in decibels is seen to decline in a linear

fashion when the temporal separation exceeds 3 msec. Thus, Plomp's findings suggest a two-stage pattern of

residual activity that holds the auditory system in a high state of excitation for about 3 msec following the termination of the leading noise burst, and then decays linearly over log time. Furthermore, an extrapolation of his auditory decay functions shows that regardless of the intensity of the leading noise burst, its poststimula- tory effect terminates in about 225 msec.

A parallel with Plomp's two-stage time course of residual auditory excitation can be found in the data on the forward masking of a short-duration probe by a pre- ceding burst of wide-band noise (Elliott, 1962; Stein, 1960; Wilson and Carhart, 1971). In general, these forward-masking studies reveal that the auditory system remains in a relatively high state of masking for a few milliseconds following termination of the burst of mask- ing noise; thereafter, the degree of masking becomes progressively less as the interval between noise burst and probe is increased. This progressive decline iri decibels of forward masking approaches a linear trend when plotted over log time. Furthermore, this post- stimulatory masking effect generally dies out within about 250 msec regardless of the intensity of the noise burst that prooucea

Such a close a•reement with the general time course of temporal resolution reinforces the view that forward masking of the probe results from the residual excita- tion produced by the noise burst. In other words, the noise burst sets up auditory processes that continue to

interfere with detection of the probe for a short time after the noise burst has beenterminated. Plomp'stemporal resolution paradigm offers a method for estimating the magnitude of this decaying residual excitation, whereas forward masking can be taken as the correlated effect that this residual activity has on detectability of a probe.

Although, in general, these data on temporal resolu- tion and forward masking do indeed reveal a similar poststimulatory time course, they are derived from independent investigations and are therefore not entirely comparable. This fact prompted us to confirm the re- lationship with a single group of subjects who were tested

1169 J. Acoust. Soc. Am., Vol. 57, No. 5, May 1975 Copyright ̧ 1975 by the Acoustical Society of America 1169

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 132.206.7.165 On: Mon, 24 Nov 2014 15:53:15

Page 2: Relations among temporal resolution, forward masking, and simultaneous masking

1170 Smiarowski and Carhart: Temporal resolution and temporal masking 1170

under conditions of both temporal resolution and for- ward masking. We also addressed ourselves to the notion that forward masking is a manifestation of the short-lived persistence of those same processes that operate to produce the traditional simultaneous masking that occurs when the probe is concurrent with the mask- ing stimulus. Our experimental strategy and the result- ing data are discussed below.

I. PROCEDURE

Six listeners were tested in this study. None had his- tory of ear pathology or surgery, and each was given an otological examination to affirm freedom from ear abnormality. All had normal hearing as determined by a screening test at octave frequencies from 125 through 8000 Hz at 15 dB HL re ISO 1964. Each subject was tested with ihree sets of monaural conditions that served to measure temporal resolution, forward masking, and simultaneous masking, respectively.

Temporal resolution was assessed with the sequence of wide-band noise bursts depicted in Fig. 1. This figure shows that, on each trial, the listener was pre- sented with two sequential pairs of noise bursts. Both pairs were the same in terms of the intensity of their leading and trailing members. The only difference between the two pairs was that there was no elapsed time between the leading and trailing bursts in the first pair, whereas the bursts in the second pair were separated by a brief time delay (At). Each noise burst in the se- quence was 500 msec in duration and had fast (about 10 •sec) rise and fall times. 1

At the outset of a test run, the intensity of the leading noise bursts were set at either 60 or 80 dB SPL, and the At was set for 5, 10, 20, 40, or 80 msec. On each trail the listener was asked to judge whether the sec- ond pair of bursts did not seem to him to be a fused con-

tinuum like the first pair. From trial to trial, the ex- perimenter systematically varied the intensity of the trailing burst in accordance with the double-random staircase method (Cornsweet, 1962). This procedure was used to determine the critical trailing burst intensi- ty at which the temporal separation (At) between the sec- ond pair of bursts was just perceptible to the listener. This was done for each of the following combinations of leading burst intensity and At: 80 dB SPL at 5, 10, 20, 40, and 80 msec, and 60 dBSPL at 5, 10, 20, and 40 msec.

An additional set of temporal resolution conditions was employed to determine the briefest At that was percepti- ble as a temporal separation when the leading and trail- ing bursts were of the same intensity. The aforemen- tioned sequence of noise bursts was used, except that in this case all four bursts were at the same fixed intensity (60 or 80 dB SPL), and it was the duration of the At be- tween the second pair that was varied from trail to trail in accordance with the double-random staircase strategy.

The listeners were also tested with a forward-masking paradigm that was comparable to the temporal resolu- tion conditions described above. In this case, the mask- ing stimulus was a burst of wide-band noise which had

the same characteristics as the leading bursts used in the temporal resolution paradigm--60 or 80 dB SPL, 500-msec duration, and fast (about 10-•sec) rise and fall times. The termination of the masking noise burst was followed, at a preselected time delay (At), by a clickwhich served as the probe. The threshold of the click was established at each At by varying the intensity of the click from trial to trial in accordance with the

aforementioned double-random staircase technique. Forward masking of the click was measured at noise levels of 60 and 80 dB SPL for each of the following At'S: 0.1, 1, 2.5, 5, 10, 20, 40, 80, and 160 msec.

Simultaneous masking was also ascertained with the staircase method by presenting the click concurrently with the masking-noise burst. More specifically, the onset of the click occurred I msec before the termination

of the 500-msec burst of noise. With this paradigm, simultaneous masking of the click was determined at each of seven noise levels: 20, 30, 40, 50, 60, 70, and 80 dB S PL.

Details on the equipment that was used to control the test conditions in this experiment can be found in Smiarowski (1970). Basically, the instrumentation consisted of the following. The wide-band noise was generated by a white-noise source (Grason-Stadler, model E5539A), and each 500-msec burst of the noise was produced by electronic switching (Grason-Stadler, model 829D). A brief electrical pulse was used to pro- duce the click probe in the masking paradigms. Acous- tically, the click that emanated from the TDH-39 ear- phone was characterized by an initial rarefaction-con- densation cycle having a period of about 0.3 msec. This initial cycle was followed by a decremental afterringing that died out in about 0.2 msec. The logarithmic dec- rement of the waveform was 0.9.

The experimenter controlled the intensity of the noise and click by means of a dual-channel amplification-at- tenuation system (Grason-Stadler, model 162). All of the testing was monaural via a TDH-39 earphone en- cased in a MX-41/AR cushion. The same type of cushion mounted on a dummy earphone was used to cover the non, test ear. The listener was seated in a sound-treated

test suite.

II. RESULTS AND DISCUSSION

Data depicting the time course of both forward masking and temporal resolution appear in Fig. 2. The leftmost ordinate scale in Fig. 2 indicates the amount of forward masking of the click, whereas the rightmost ordinate scale gives the level of the trailing noise burst in the

FIRST PAIR

500 • 5oo---• 1000-

SECOND PAl R

•-I,e--- 500--")'1 At 14--500 •

TIME IN MSEC

FIG. 1. The sequence of noise bi•rsts presented on each trial during measurement of temporal resolution.

J. Acoust. Soc. Am., Vol. 57, No. 5, May 1975

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 132.206.7.165 On: Mon, 24 Nov 2014 15:53:15

Page 3: Relations among temporal resolution, forward masking, and simultaneous masking

1171 Smiarowski and Carhart: Temporal resolution and temporal masking 1171

•5

•0

• 35

• 30

• 25

20

15

lo

5

0 0.1

ß !

80 dB SPL A

60 dB SPL n

I i I ! I I

1 2.5 5 lO 2o #o

AT IN NSEC

8o

75 •

'o•

65 z o

60 m

z

•o •

35

80 160

FIG. 2. Time course of forward masking (circles) compared with that of temporal resolution (triangles). l•arameter is the intensity of the masking- and leading-noise bursts used in the forward masking and temporal resolution paradigms, respec- tively. Each triangle and circle represents the mean score for the group of subjects. The range of scores around the means are indicated with brackets for forward masking data (•) and temporal resolution data (v ̂ ).

temporal resolution condition. The At values are ar- ranged logarithmically along the abscissa of the figure.

Let us consider the forward-masking data first. The circular data points in Fig. 2 indicate the shift in the threshold of the click at each At given on the abscissa. The closed circles show the forward masking produced when the masking noise was at 60 dB SPL, whereas the open circles indicate the forward masking resulting from 80 dB SPL noise. In each case, the general time course of forward masking has been traced with straight lines. Overall, it can be seen that forward-masking of the click was somewhat stable initially, and then began to decline markedly after a few milliseconds into the poststimula- tory period. This two-stage time course confirms the findings of earlier investigators (Elliott, 1962; Stein, 1960; Wilson and Carhart, 1971).

With regard to the initial stage of forward masking, Fig. 2 shows a slight reduction in the threshold of the click during-the first_ 2.5 m_•.ee• nf fhe poststimulatory_ _ period. Furthermore, these data reveal that masking in this early stage was nearly as great as that obtained in the simultaneous masking condition. In the latter case, there was a 31.7-dB threshold shift in the presence of 60 dB SPL noise, and a 50.2-dB shift when the noise was at 80 dB S PL.

In contrast to this relatively stable initial stage, the late component of forward masking is characterized by a marked reduction in masking as a function of increas- ing the At. Note in Fig. 2 that this progressive decline in decibels of masking approaches a linear trend when plotted over log At. Moreover, these more steeply sloping segments of the 60 and 80 dB SPL functions show substantial convergence, and if extrapolated they would reach 0-dB masking, or termination of the forward-

masking effect, at around 250 msec.

Turning now to the results of the temporal resolution conditions, we find these data represented by the tri- angles in Fig. 2. The set of triangles show the level of the trailing noise burst in relation to the just notice- able interburst interval, or At, which is given on the abscissa of the figure. The open triangles show the re- sults that were obtained when the leading noise burst was at 60 dB SPL, whereas the closed triangles indicate the data for the 80 dB SPL leading burst. •It should be noted that the level of the trailing noise burst can be found on the rightmost ordinate scale in Fig. 2. The position of this scale is such that the maximum trailing burst level (80 dB SPL) coincides with the maximum forward masking (50 dB) that was obtained. 3 Matching the two scales in terms of the maximum poststimulatory effect provides us with a format for comparing the time course of forward masking with that of temporal resolu- tion.

It will be recalled that the listeners were tested with

two sets of temporal resolution conditions. In one case, the level of the trailing noise burst was equal to that of the leading one, and it was the At that was systematical- ly varied in order to determine the minimum perceptible separation between the two bursts. At 60 dB SPL, the uppermost open triangle in Fig. 2 shows that the mini- mum perceptible At was 2.8 msec; whereas at 80 dB SPL, the uppermost closed triangle, it was 2.7 msec. These minimum At values are in good agreement with Plomp's (1964) findings, and further confirm the mini- mum time constant for temporal auditory acuity which has been demonstrated to be on the order of 2 msec in

a variety of related experiments dealing with the resolu- tion of transients and interrupted noise (Green, 1971).

To complete the time course of temporal resolution, recall that another set of temporal resolution conditions was employed. In this case, we selected At's that were used in the forward-masking paradigm--these included 5, 10, 20, 40, and 80 msec. With the At fixed at one of these values, and the leading burst at either 60 or 80 dB SPL, the level of the trailing burst was systematical- ly varied in order to determine the critical level of trail- ing burst at which the separation, or At, was just per- ceptible to the listener. The remainder of the triangles in Fig. 2 give these critical trailing burst levels as a function of •h• •-e•l•cFe•[ •/'•. T-a•n as a w--hoTe; • the set of triangles in Fig. 2 depict a time course for temporal resolution that closely parallels that of forward masking. Note in this regard how the triangles cluster along the line depicting the second stage of forward masking.

If one assumes, as Plomp (1964) did, that the time course of auditory decay is measured by the temporal resolution paradigm, then the data in Fig. 2 support the view that forward masking is also a manifestation of auditory persistence. More specifically, we find that a burst of noise sets up auditory activity that continues to hold the auditory system i•. a relatively high state of excitation for about 2.5 msec after the burst ceases.

Thereafter, this residual activity decays linearly over log time as evidenced by the decline in both click thresh-

J. Acoust. Soc. Am., Vol. 57, No. 5, May 1975

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 132.206.7.165 On: Mon, 24 Nov 2014 15:53:15

Page 4: Relations among temporal resolution, forward masking, and simultaneous masking

1172 Smiarowski and Carhart: Temporal resolution and temporal masking 1172

old and trailing burst level.

A further question arises with regard to whether the forward masking of a probe results from the persistence of those same auditory processes that produce the tradi- tional simultaneous masking of the probe when it is con- current with the masking stimulus. This question brings us to the simultaneous masking phase of this study, which is discussed next.

Figure 3 gives the masking that resulted when the click was presented simultaneously with the noise burst. In this figure, the broken line traces the simultaneous masking of the click over seven different noise levels ranging from 20 to 80 dB SPL. Note that, as expected, the general configuration that resulted conforms to the classical Hawkins and Stevens (1950) function for simul- taneous masking of a signal by wide-band noise.

Now we return to the question of whether forward masking bears the same relationship to masker level as does simultaneous masking. To gain some insight into this matter, we turn to Fig. 4 which includes the simul- taneous masking function from Fig. 3. This function appears as it did in Fig. 3, a set of closed circles fitted with a broken line. In addition to this simultaneous

masking function, Fig. 4 includes a set of squares and triangles fitted with a solid line. This latter set of data points is taken to represent forward masking as a func- tion of the magnitude of the residual excitation persisting from the masking noise burst. Based on Plomp's (1964) model of temporal resolution, the level of the trailing burst is our estimate of the magnitude of this residual masker. Trailing burst level is given on the uppermost horizontal •cale in Fig. 4. The trailing burst levels, and the corresponding forward-masking values against

55

5o

•o

-- 30

10

50 60

I•ISE BURST IN DB SPL

7o 8o

FIG. 3. Simultaneous masking as a function of noise level. Solid circles fitted with broken line show the resultsofthe pres- ent study. Each of these circles represents the mean score for the group of subjects; brackets (un) indicate the range of scores around the means. The solid line gives the classical Hawkins- Stevens function.

20 55 ,

50

•1o

• 35

_z 30

z 25

( 20

15

lo

TRAILING NOISE BURST IN DB SPL

30 qo 50 60 70 80 , ß i , I i ' ß i i , I ß

0 20 30 •0 5•0 0 70 0 SIMULTANEOUS MASKING NOISE BURST IN DB SPL

FIG. 4. Relationship between simultaneous masking and simul- taneous masking noise burst level (closed circles fitted with broken line) compared with the relationship between forward- masking and trailing noise-burst level (squares and t. riangles fitted with a solid line).

which they are plotted in Fig. 4, were taken from the time course data which appear in Fig. 2. To explain, the squares in Fig. 4 represent the data from the set of Fig. 2 conditions in which the masking- and leading- noise bursts were presented at 60 dB SPL; whereas, the triangles in Fig. 4 are based on the 80 dB SPL data from Fig. 2. The number next to each square and tri- angle in Fig. 4 indicates the At at which the trailing burst level and corresponding forward masking value were obtained from Fig. 2. For example, take the square which is marked with a 5 in Fig. 4. This data point is based on the forward masking of the click that was obtained at a at of 5 msec when the masking noise was at 60 dB SPL (see Fig. 2). The trailing noise burst level for this data point comes from the temporal res- olution condition in which the leading burst was 60 dB SPL and the At separating the two bursts was at 5 msec (see Fig. 2). Likewise, the plot of the remaining squares and triangles in Fig. 4 is based on data that ap- pear in Fig. 2.4

The two sets of data that emerged in Fig. 4 are in .good agreement. The relationship between forward- masking and trailing-burst level is represented by the solid line. The slope of this best-fit line reveals 0.96- dB shift in forward masking per decibel change in trail- ing-burst level, the latter being used as the index of residual masker level. This function closely parallels the linear portion of the simultaneous masking function which appears as a broken line in Fig. 4. The slope of this best-fit line shows 0.94-dB shift in simultaneous

masking per decibel change in masker level.

Such a close agreement with respect to the relation- ship between amount of masking and level of masker, lends support to the notion that the phenomena of simul- taneous and forward masking behave in a similar man-

J. Acoust. Soc. Am., Vol. 57, No. 5, May 1975

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 132.206.7.165 On: Mon, 24 Nov 2014 15:53:15

Page 5: Relations among temporal resolution, forward masking, and simultaneous masking

1173 Smiarowski and Carhart: Temporal resolution and temporal masking 1173

ner. This notion is further reinforced by previous studies (Gardner, 1947; Harris, 1959; Li{scher and Zwislocki, 1949; Munson and Gardner, 1950; Samoilova, 1961) which demonstrate a spread-of-masking pattern in forward masking that is similar to the one found in simultaneous masking. These findings coincide with Gardner's (1947) suggestion that monaural forward masking may be thought of as the persistence of those same processes that produce simultaneous masking.

As a final comment, the shape of the forward-masking function depicted in Fig. 2 suggests that two processes were operating following physical cessation of the mask- ing noise burst. For approximately the first 2.5 msec thereafter, forward masking remained close to the same level as simultaneous masking. The mechanism that holds the auditory system in such a high state of masking for such a brief period of time must be presumed to be very peripheral. A reasonable hypothesis seems to be that it is a manifestation of continuing mechanical agita- tion of the cochlear partition, albeit the process of transduction to neural excitation might also be involved. By contrast, when the At appreciably exceeded 2.5 msec, a more rapid linear decrease in masking with log At emerged. Even though this second process continued for about 200 msec, Liischer and Zwislocki (1949) have considered this latter stage of forward masking to be the result of a peripheral process since the phenomenon is demonstrably monaural. However, the time course of forward masking is sufficiently long so that one might question their interpretation. More evidence is needed before the matter is resolved. Meanwhile, one probably should entertain the possibility that within the central nervous system, possibly at a level as low as the coch- lear nuclei, persistence of activity is set up by a masker which limits sensitivity for new stimuli during the die- away phase of this residual activity. In this regard it is imperative to remind ourselves that monaural simul- taneous masking is not wholly determined by the inter- action of the two stimulus trains within the peripheral auditory system of the side receiving stimulation. Ef- fects occurring within the central nervous system also must be considered. This fact is demonstrated by the MLD phenomenon in audition. Recall that a monaural signal which is just inaudible in the presence of a mon- aural masker becomes audible when the masker is added

to the contralateral ear. Obviously, the monaural threshold obtained when masking is present only in the same ear does not represent the reduction in sensitivity attributable to peripheral factors alone. The peripheral resolving power Per se is better than this monaural "•-•'•^'• but the addition of stim,,la+im• nf the eontrnl

nervous system by a masker via the second ear is needed to demonstrate that such is the case.

ACKNOWLEDGMENT

This study was supported by the National Institute of Neurological Diseases and Stroke through Training Grant NS 05329 and Career Award K6 NS 16224.

*Present address: Auditory Research Laboratory, Veterans Administration Outpatient Clinic, 425 South Hill St., Los

Angeles, CA 90013. 1The procedure for measuring temporal resolution in this study

had the effect of making the total time slightly greater during the second pair of noise bursts because of the addition of az•t. Thus, the question arises as to whether durational cues might have influenced subject judgment. It seems safe for two rea- sons to discount this possibility. First, the level of the trail- ing burst that yielded temporal resolution at each • de- creased with increased At in systematic fashion which sug- gests that the magnitude of residual response was the primary determinant of listener judgment. Second, since 80 msec was the longest At used in this study, the largest At/t was 0.08. This ratio is slightly less than the 0.1 value which various studies indicate as approximately the Weber fraction for per- ception of durational differences (Creelman, 1964; Small and Campbell, 1.962). The conditions in this study with shorter A t's are correspondingly less suspect.

2A preliminary study revealed that the 80 dB SPL leading burst afforded reliable measurement of temporal resolution up to a At of 80 msec, whereas the maximum usable At was around 40 msec for the 60 dB SPL leading burst. At longer At's, the sensation level of the trailing burst became so low as to make it difficult for the listener to make a reliable judgment as to whether he perceived the separation between bursts.

3To simplify the alignment of the two ordinate scales in Fig. 2, the maximum forward masking of 49.8 dB was rounded to 50 dB. Note in Fig. 2 that 49.8 dB of forward masking occurred at a At of 0.1 msec when the masking noise was at 80 dB SPL.

4Data points for At's of 5, 10, 20, 40, and 80 msec appear in Fig. 4 since both forward masking and temporal resolution were measured at those time intervals (see Fig. 2). Two additional data points appear in Fig. 4, a triangle marked 2.7 and a square marked 2.8. Recall from the earlier discussion of the temporal resolution data in Fig. 2, that these two At's represent the minimum perceptible separation between leading and trailing noise bursts of the same intensity. Since forward masking was not measured directly at 2.7 and 2.8 msec, we derived the forward-masking values for these two data points in Fig. 4 from the best-fit lines that represent the time course of forward masking in Fig. 2.

Cornsweet, T. N. (1962). "The Staircase Method in Psycho- physics," Am. J. Psychol. 75, 485-491.

Creeman, C. D. (1964). "Human Discrimination of Auditory Duration," in Signal Detection and Recognition by Human Ob- servers, J. A. Swets, Ed. (Wiley, New York), Chap. 12, pp. 265-290.

Elliott, L. L. (1962). "Backward and Forward Masking of lh-obe Tones of Different Frequencies," J. Acoust. Soc. Am. 34, 1116-1117.

Gardner, M. B. (1947). "Short Duration Auditory Fatigue as a Method of Classifying Hearing Impairment," J. Acoust. Soc. Am. 19, 178-190.

Green, D. M. (1971). '•remporal Auditory Acuity," Psychol. Rev. 78, 540-551.

Harris, C. M. (1959). "Residual Masking at Low Frequencies," J. Acoust. Soc. Am. 31, 1110-1115.

Hawkins, J. E., Jr., and Stevens, S.S. (1950). '•rhe Masking of Pure Tones and of Speech by White Noise," J. Acoust. Soc. Am. 22, 6-13.

Lllscher, E., and Zwislocki, J. (1949). "Adaptation of the Ear to Sound Stimuli," J. Acoust. Soc. Am. 21, 135-1.39.

Munson, W. A., and Gardner, M. B. (1950). "Loudness Pat- terns--a New Approach," J. Acoust. Soc. Am. 22, 177-190.

Plomp, R. (1964). "Rate of Decay of Auditory Sensation," J. Acoust. Soc. Am. 36, 277-282.

Samoilova, I. K. (1961). Preceding Masking of Short Tonal Signals," in l•roc. Third International Congress on Acoustics, L. Cremer, Ed. (Elsevier, Amsterdam), Vol. I, pp. 139- 141.

Smiarowski, R. A. (1970). "Relations Among Temporal Res- olution, Forward Masking, and Simultaneous Masking,"

J. Acoust. Soc. Am., Vol. 57, No. 5, May 1975

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 132.206.7.165 On: Mon, 24 Nov 2014 15:53:15

Page 6: Relations among temporal resolution, forward masking, and simultaneous masking

1174 Smiarowski and Carhart: Temporal resolution and temporal masking 1174

Ph.D. dissertation, Northwestern U. Small, A.M., and Campbell, R. A. (1962). "Temporal Dif-

ferential Sensitivity for Auditory Stimuli," Am. J. Psyehol. 75, 401-410.

Stein, H. J. yon. (1960). "Das Absinken der Mith•rschwelle

nach dem Abschalten yon Weissem Rauschen," Acustica 10, 116-119.

Wilson, R. H., and Carhart, R. (1971). "Forward and Back- ward Masking: Interactions and Additivity," J. Acoust. Soe. Am. 49, 1254-1263.

J. Acoust. Soc. Am., Vol. 57, No. 5, May 1975

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 132.206.7.165 On: Mon, 24 Nov 2014 15:53:15