Suppression effects in backward and forward masking.

Suppression effectsin backward andforwardmasking Daniel L. Webera) and David M. Green Laboratoryof Psychophysics, Harvard University,Cambridge,Massachusetts 02138 (Received12 July 1978;revised1 December1978)

The differences in the suppression effectobserved in forwardandbackwardmaskingare consistent with an interpretationthat suppression in forwardmaskingresultsfrom a reductionof the effectivelevel of the maskerin the aditoryperiphery,and that the suppression in backwardmaskingis influenced by these peripheral processes, but is dominated by additional, centralprocesses. This conclusion is supported by experiments that showdifferences in the effectof ipsilateralversuscontralateral presentation of the suppressor, and differences in the amountof the suppression observedas a functionof the level,duration, and frequencyof the suppressor. PACS numbers: 43.66.Mk, 43.66.Dc

for which suppression effects can be observed differ in

INTRODUCTION

Houtgast (1972) provided the first psychophysical demonstration

that

the effectiveness

of a nonsimultane-

forward and backward masking.

(4) In backward masking, contralateral presentation

ous masker, in his work a sinusoid, can be reduced by adding a second sinusoidal signal during the masking interval. He further demonstrated that this phenomenon occurs only if the second sinusoid, the suppressor, is slightly above or below the frequency of the masker and has a greater intensity than the masker. His assumption that the suppressor acts as if it simply reduces the effective level of the masker has been supported by later experiments (Shannon, 1976; Weber

of the suppressor is nearly as effective as ipsilateral presentation, whereas contralateral presentation is hardly effective in forward masking.

and Green, 1978) for forward masking. We present

We argue that these differences between forward and backward masking reflect differences in the processes underlying the suppression effects in the two types of masking. In particular, the suppression effects in backward masking appear to be determined primarily

e,vidence•consistent with this simpleassumption in backward masking using both ipsilateral

and contralateral

suppressors.

Psychophysical demonstrations of suppression commonly use the pulsation threshold technique (Houtgast,

1972, 1973, 1974; Vogten, 1974) and forward masking (Shannon,1976; Terry and Moore, 1977; O'Malley and Feth, 1978). Although suppression effects have been demonstrated in backward masking (Shannon, 1975;

Tyler and Small, 1977), there are few data providing a direct comparison between forward and backward

masking (Leshowitz and Zurek, 1977; Weber, 1978). Moreover, the studies making a direct comparison used band-limited

noise which

makes

the exact

identification

of the suppression region difficult. A recent paper compared differences in the temporal properties of the suppression effect observed in forward

and backward

masking (Weber and Green, 1978). forward and backward masking and also study more extensively certain aspects of the suppression of backward masking. In particular we demonstrate that:

(1) Suppressioneffects are much larger in backward than in forward masking. is different

for noise and sinusoidal

maskers and the delay between signal and masker produces different effects in forward and backward masking.

(3) The suppressorintensity (relative to the masker)

a)Nowat MRCAppliedPsychology Unit, Cambridge,England. 1258

they dominate the peripheral effects of suppression common to both forward and backward masking. I. METHODS

All of the experiments employ an adaptive two-alter-

native forced-choice (2AFC)procedure to estimate the threshold of a brief 2-kHz sinusoidal signal. The duration of the signal was sometimes 2 ms, sometimes

9 ms, depending on the experiment. • It was filtered in all experiments to reduce transients associated with offset and onset. Two other stimuli, essential in the studies of suppression, are a masker and a suppressor. standard

noise

masker

was

a narrow

band

of

noise 200 Hz wide at 3-dB down points with an equivalent rectangular width of 250 Hz centered at 2 kHz. The spectrum level was 40 dB (total noise power 64 dB). Its duration was always 500 ms. The reason for using this

noise

al masker

masker

will

rather

than

be discussed

the more

usual

sinusoid-

in Sec. II A 3.

The suppressor signal was often a band-pass noise

(2) The increase in maskingproducedby a changein of the masker

by central processes,whoseeffects are so large that

The

In this paper, we expioreotherdifferencesbetween

level

(5) In backward masking, a sinusoid or noise suppressor produces slightly different amounts of suppression as a function of suppressor level and these differences depend on whether the suppressor is presented in the ipsilateral or contralateral ear.

J. Acoust. Soc. Am. 65(5), May 1979

extendingfrom about 2.3 to 3.7 kHzf' •We will use the phrase noise suppressor to describe this stimulus. other experiments the suppressor was a sinusoid.

In

A. Stimulus generation Stimulus generation was essentially tlie same in all the experiments. First we describe the typical system,

0001-4966/79/051258-10500.80

¸ 1979 Acoustical Society of America

1258

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 138.26.31.3 On: Tue, 23 Dec 2014 09:32:22

and then note the exceptions.

The 2-kHz sinusoidal signal (Wavetek 116) was pulsed for 8 ms and bandpass filtered (GR 1952 Universal fil-

ter, 400 Hz wide centeredat 2 kHz), thenpassedto separate manual and programmable attenuators for each observer.

The manual

attenuators

set the maximum

signal intensity. The programmable attenuators were computer-controlled and adaptively adjusted the intensity for each observer before the signal was added to the noise (s). A PDP-15 computer controlled all timing, stimulus presentation, data collection, and data storage.

Two separate noise sources (Elgenco602A)were used to generate the masker and the suppressor. Exact details of masker generation are described elsewhere

(WeberandGreen, 1978). The noisesuppressorwas gated through an electronic switch and bandpass filter and added to the signal.

For each observer, the output of the adder (inputs

of signal, masker, and suppressor)was amplified for transmission

to individual

sound-treated

observation

chambers (IAC-1200A). The stimuli were presented to each observer's left ear through TDH-39 earphones. In some cases--e.g., the contralateral suppression con-

dition-the suppressor was not added to the signa! and masker, but was independently amplified and transmitted to the observer's

right ear.

second observation interval, observers responded by pushing one of two buttons corresponding to the observation intervals. After all the observers (maximum of

three) responded, feedback was provided (150 mS) by illuminating the light corresponding to the correct interval and the next trial began. C. Observers Different

numbers

of observers

were

used in these

experiments but all were college students with normal hearing. All were given two weeks practice before data collection began. The observers were initially selected on the basis of their performance in a simple intensity-discrimination task (discriminating a 3-dB increment to a 60-riB,

2-kHz sinusoidal signal in a 2AFC paradigm). After 5 rain familiarization with the task, each of nine potential observers achieved at least 95% correct resportses. During the training period, one of these observers failed to demonstrate a suppression effect for the standard masker and suppressor condition, although even this observer did show a suppression effect if one compared the masking produced by broadband and narrow-band noise. To facilitate the comparison of data across experiments, this observer was replaced. Suppression results for another observer changed over a

10-rib range, although the large standard errors of the

In one set of experiments, the signal duration was 2 ms. Two values of the signal-masker

delay were ex-

measurements (6-12 dB) made the results ambiguous. This observer was also replaced?

amined in a forward-masking (3.8 and 30 ms) and in a backward-masking (3.8 and 10 ms) condition. In this set of experiments a sinusoidal masker was compared

A. Ipsilateral results

with a noise masker.

I. Suppress/on in backwardand forwardrnask/ng

The sinusoidal

masker

was gen-

erated by substituting a secondWaVetek set at 2 kHz for the standard noise masker typically employed.

Several experiments employed sinusoidal suppressors. These were generated by substituting a second Wavetek for the noise source used for the suppressor.

The envelope of the sinusoidal suppressors was shaped by passing the gated signal through two bandpass filters

(Krohn-Hite 3343) approximately«-octavewide cen-

II.

RESULTS

AND DISCUSSION

,

The masking produced by a narrow band of noise, the standard noise masker, is reduced by the presentation of another band of noise, the noise suppressor,

in both backward and forward masking (Table I). The signal was a 9-ms sinusoid that immediately preceded (in backwardmasking) or immediately followed (in forward masking) the masker, and the masker and suppressor were presented simultaneously. The threshold

tered at the suppressor frequency. B. Procedure

TABLE I.

In the adaptive procedure, the signal intensity increased by 2 dB after each incorrect response and decreased by 2 dB after two consecutive correct responses at a given intensity. The transition from consecutive correct responses to an incorrect response

(andvice versa) defined a "turn-around." The first four turn-arounds

were discarded

and'the average in-

[on.•it'v nf [ho cigna 1 •t tha remaining +,,,•n_o,•,,,,,ao in o 100-trial

block was used to estimate the signal thres-

Ipsilateral

suppression.

Backward masking Observer

Forward masking

M

M +S

Diff.

M

M +S

Diff.

PA DB DC EF MH PM JN DW

64.6 66.1 67.5 69.0 66.1 66.6 65.4 67.6

35.4 33.5 25.2 31.9 36.0 41.2 27.2 36.5

29.2 32.6 42.3 37.1 30.1 25.4 38.2 31.1

67.1 66.7 69.0 69.7 62.6 68.4 68.6 70.1

53.4 49.9 60.9 56.7 58.3 60.0 50.6 60.3

13.7 16.8 8.1 13.0 4.3 8.4 18.0 9.8

Average

66.6

33.4

33.2

67.8

56.3

11.5

hold. Typically, 25-30 turn-arounds occurred in 100 trials.

The data are the mean of at least

estimates per observer. was tested

for

three

Typically,

consecutive

three

such

the same condition

blocks

of trials.

The sequence of events within each trial was the same for all experiments. The first of two 510-ms observation intervals, separated by ?50 ms, occurred 200 ms, after the offset of a 300-ms warning light. After the 1259

J. Acoust.Soc.Am., Vol. 65, No. 5, May 1979

Intra

•

1.4

5.2

2.4

4.4

Inter

•

1.5

3.8

0.9

1.4

Signal: Sinusold, 9 ms, 2kHz, zero delay. Masker:

Standard masker

(200 Hz wide).

Suppressor: Noise suppressor band (2.3-3.7 kHz), 40-dB spectrum level, simultaneous with rnasker.

D.L. WeberandD. M. Green' Suppression in masking

1259


,

values in the masker-alone and masker-plus-suppressor conditions, and the difference, are presented for eight observers. The standard deviation for the three threshold determinations for each observer, averaged across observers, is referred to as the intraobserver standard deviation. The standard deviation computed across the threshold estimates for the eight individuals is labeled

the interobserver

standard

deviation.

TABLE II. Comparison of the effect of an ipsilateral suppressor when gated with masker, when continuous, or when presented alone.

Gated

S•

Continuous

M

(M + S)

gated M

DB DC EF MH PM DW

66.6 67.5 69.0 63.0 66.6 67.6

30.5 3Q.5 34ø8 40.2 41.2 43.7

68.9 69.6 67.6 69.6 67.8 67.3

27.2 21.4 39.6 28.8 38.6 36.3

Average

66.7

36.8

68.4

32.0

1.4 2.0

2.8 5.7

1.9 1.0

The

thresholds in the masker-plus-suppressor condition show more variability both within and between observers than the masker-alone condition. The average difference between the masker-alone and masker-plussuppressor condition, or suppression effect, for backward masking is 33.2 dB and for forward masking, 11.5 dB. The suppression effect is considerably greater in backward masking than in forward masking.

Continuous

Observer

Intra Inter

• •

S

alone

0.95 7.3

Signal: Sinusold, 9 ms, 2 kHz, zero delay.

2. Suppress/onas a reduction in masking level in backward masking For some time it has been argued (e.g., I-Ioutgast, 1972) that the ipsilateral suppressor, in effect, reduces the level of activity at the periphery in the region of frequency near the signal (and masker). In simultaneous masking both signal and masker are reduced where•

Masker: Standard noise masker (200 Hz wide). Suppressor: Sinusold, 2.5 kHz at 80 dB, simultaneous with masker in gated (M + S) condition.

as in' nonsimultaneous masking only the masker is reduced; hence one can measure a reduction in the masker's effectiveness only in nonsimultaneous masking.

usoid as the masker and observed considerable suppression in backward masking. Informal experiments indicated that little or no suppression was measured using a sinusoidal masker in the backward-masking condition. One explanation for this difference might be the change in signal threshold with changes in the level of the

One way to pursue this simple idea is to contrast two conditions. In the first suppression condition the masker and suppressor are presented simultaneously for a brief period immediately after the signal is presented as in our previous experiment; in a second condition the suppressor is presented continuously. Both masker and signal are presumably reduced in level with the continuous suppressor and thus we examine a condition analogous to simultaneous masking. This analogy suggests that the signal threshold should be approximately equal to the masker-alone condition. We also measure

3. Growth of masking In both of the previous experiments we used a narrow

noise

band

rather

than

the more

conventional

sin-

masker; what we call "growth of masking" functions. Figure I compares growth of masking functions for I 70

i

i

i

,•l

i

]

i

/i

FORWARD /•i/•[ bFORWARD

- NOISE //•/" SINUSOlD /

/

a. 6o

)-

5o

the signalthresholdwhenonly the suppressoris pre-

II

-

-

-

4o

sented continuously to determine if the continuous suppressor is not, in itself, elevating the signal threshold, perhaps by virtue of its longer duration.

30

_

_

o T=3.8

o

/

Table II presents the results of this experiment. It is similar to the previous experiment except that only six

cBACKWARD // dBACKWARD ' r•

observers participated and the suppressorwas a 2500-

0

Hz sinusoid at 80 dB SPL, rather than the noise suppressor used in the previous experiment.

I-

The suppression effect is only evident when the suppressor overlaps temporally with the masker and not the signal. When the suppressor is present during both signal and masker intervals the signal threshold is nearly identical to that produced by the masker alone. The continuous suppressor alone hardly masks the 2kHz signal; the threshold is lower than the gated masker plus suppressor for five of the six observers. Thus this experiment is consistent with the assumption that suppression operates by changes in the effective level of the masker. This result leads naturally to the next question: how does masking depend upon the level of the masker? We also investigate how the type of masker and temporal separation between the signal and masker influence the detectability of the signal. 1260

J. Acoust.Soc. Am., Vol. 65, No. 5, May 1979

•

6o

5o

-NOISE ///// SINUSOlD _

_

F- 40

30

0 ß

•/1

i

ABeS0

15

SPECTRUM

T•IO

I

i

I

30

45

40

LEVEL

MASKER

OF

i

i

i

55

70

85

MASKER

LEVEL

( SPL )

FIG. 1. Threshold as a function of masker level. A delay between signal and masker of 3.8 ms occurs for all conditfons (open circles). (a) Forward masking, noise masker. Delay equals 30 ms (closed circles). (b) Forward masking, sinusoidal masker. Delay equals 30 ms (closed circles). (c) Backward masking, noise masker. Delay equals 10 ms (closed circles). (d) Backward masking, sinosoidal masker. Triangle in panel C indicates absolute threshold. The data are means over three observers. The dashed line shows a slope of 1. Signal: Sinusoid, 2 ms, 2 kHz, various delays. Masker: Standard noise masker or sinusold at 2 kHz. Suppressor: None.

D.L. Weberand D. M. Green: Suppressionin masking

1260


backward and forward masking, using both sinusoidal and noise maskers for each. The gated signal duration was shortened from 9 to 2 ms so that short delays be-

tween the entire signal and masker could be explored.

Two temporal separations (•) be[ween the masker and signal were tested. The independent variable is the intensity of the masker. The dashed line has a slope of 1--a

10-dB

increase

in the masker

raises

the threshold

In forward masking, the 3.8-ms delay condition for both the sinusoidal and noise maskers produces nearly one-to-one increases in signal level as a function of

maskinglevel [Figs. l(a) and l(b)]. However, whenthe delay between masker and signal is increased to 30 ms, a 10-dB increase in the masker may produce only a 5-dB change in the detectability of the signal. Thus

introducing a suppressor might produce twice as much change in signal threshold if the signal is delayed 3.8 ms from the masker than if the signal is delayed 30 ms, even though the suppressor produced the same change in Terry

and Moore

(1977) show growth of masking functions in forward masking for both a noise and sinusoidal masker. Their signal duration was much longer than any of ours (a

linear ramp 20 ms on, 20 ms off with no steady portion)

andthe signallevel changed 5 dBfor-a 10-dBchangein masker level. Thus the suppression effects, measured as a change in signal threshold, are consistently smaller in their study than in ours.

Although not directly related to our main point, we may compare the effectiveness of the sinusoid and noise masker in the two studies. One must first realize that the overall level of our noise masker is about 24 dB

higher than its spectrum level. Thus the overall level of the highest noise masker is 45 + 24 = 69 dB and produces a signal threshold of about 75 dB in the 3.8-ms delay condition and about 50-55 dB in the 30-ms delay condition. A sinusoidal masker of nearly the same lev-

el (70 dB) produces a signal threshold of only 60 dB in the 3.8-ms delay condition and a 45-50-dB threshold in the 30-ms delay condition. Therefore,

our noise

band of approximately 200 Hz at 2 kHz produces about 10 dB more masking than a sinusoid of the same over-

all power at all levels andbothdelays.4 Terry and Moore (1977) found a sinusoid to be a more effective masker than a «-octave band noise. Their stimulus had a much longer duration than our 2-ms signal. Such long durations contribute to the importance of a pitch cue in their experiment which is probably not as important in our study. In any case the two studies are contradictory--the total discrepancy in their results and ours is nearly 25 dB.

Both studies agree that it makes little difference if one uses sinusoidal or noise maskers to study suppression of forward masking since changes in the masker

intensity produce the same changes in masked threshold for both types of maskers. Our data for forward masking show that for a given change in the masker intensity,

a larger changein signalthresholdoccursfor Signals temporally closer to the masker. This relation has

been knownfor some time (Plomp, 1964) and was ap1261

For backward masking the results are very different

[Fig. l(c) and l(d)]. Althoughthe 3.8-ms delay condition has nearly a slope of one for the noise masker [Fig. l(c)] the correspondingslope for the sinusoidmasker [Fig. l(d)] is very shallow. Indeed, for a sinusoid masker

there

is at most

a 3-dB

increase

in threshold

as the masker level changes from 40 to 85 dB.

of the signal 10 dB.

the effective level of the masker.

preciated in Houtgast's(1972) original study.


If it is correct that suppression reduces the effective level of the masker, then using a noise masker and short delays between signal and masker in a backward masking condition should be the best condition to observe any potential suppression effects. This follows because the growth of masking is nearly one-for-one with an increase

in masker

level and a 30-dB

reduction

in the effective level of the masker would be expected to reduce the signal threshold by a similar amount.

However, if the masker is a sinusoid, the result of effectively reducing the masker level, from say 70 to 40 riB, would produce little or no change in the signal threshold, since the signal threshold changeslittle with changes in masker level.

The experimental results depicted in Fig. 1, at least for forward masking, also have implication for the decay of sensationproducedby a masking stimulus. This method of analysis has a long history in nonsimultaneous masking (Plomp, 1964; Miller, 1948; Munson, 1947). When considered from this point of view, one important implication is that the decay of sensation produced by either a sinusoidal or noise masker appears to be similar [the similar thresholdsfor both 3.8-ms

and 30-ms delays for each type of masker, Figs. l(a) and l(b)]. For the two experiments whose results are presented in Table I and II, we used a relatively long signal, 9 ms, as compared with the 2-ms signal in the data just discussed. For four observers we measured the growth of masking using this 9-ms signal and either a noise or sinusoid as the masker in backward masking. The resuits are presented in Fig. 2. All observers are similar for the noise masker; the growth of masking is nearly one-for-one. Thus we would expect the suppression

results presented in Table I or II to represent the effective reduction in masker caused by applying the suppressor. For the sinusoidal masker, however, the resuits should be quite different. Even if the suppressor were equally effective, observers DB and DC should show little effect of suppression, whereas observers MH and DW should show roughly half the suppression observed

with

a noise

masker.

These

data and their

interpretation suggestwhy Tyler and Small (1977) observed a relatively small amount of suppression. In their paper one can compare the backward masking produced by a 40-dB and 70-dB sinusoidal masker (masker

alone versus masker plus 2-kHz suppressor).The increment in be[ween sonable servers

threshold caused by this 30-dB change ranges 6 and 22 dB over their five observers, in reaagreement with the range observed for our obin Fig. 2.

D. L. Weberand D. M. Green: Suppression in masking

1261


BACKWARD

SPECTRUM 0 !

i0 20 i

MASKING

LEVEL

30 40

!

0 I

i

7o

i0 2o 30 40 i

i

I

!

//?

DS

/!

//I//

50

•iiiiiii1 i

I

i

!

/

u• 60

• 50 40-

i

II

II

/

•60

MH

II

II

Il

I

l

/

///•

DC

DW w

// MEAN •- 40 •o

I

I

I

I

I

I

I

1

M

-20

-IO

O

IO

20

30

40

SPECTRUM

LEVEL

OF

SUPPRESSOR

20

OVERALL

MASKER

LEVEL

(SPL)

FIG[ 2. Threshold as a function of masker level in backward masking. Masking produced by noise maskers (opencircles) and sinusoidal maskers (closed circles) for four observers. The dashed line shows a slope of 1. Signal: Sinusold, 9 ms, 2 kHz.

Masker:

Suppressor:

Standard noise masker

or sinusold at 2 kHz.

FIG. 3. Effect of suppressor level in forward (squares) and backward (circles) masking. The points labeled "M" indicate the amount of masking by the masker alone. The averages are for four observers, DB, DC, MH, and DW. The individual data for the backward masking condition are shown in Fig. 6. Signal: Sinusold, 9 ms, 2 kHz, 3.8 ms delay. Masker: Standard noise masker (200 Hz wide). Suppressor: Noise suppressor band (2.3-3.7 kHz) simultaneous with masker.

None. our earlier

4. Effect of suppressor/eve/

results

but with

the standard

masker

and

9-ms signal in one ear and a 2.5-kHz sinusoidal sup-

These differences in the growth of masking functions

in forward andbackwardmaskingled us to explore the effects of suppressor level in backward and forward masking. The masker was the standard noise masker. The signal was 9 ms long, with a 3.8-ms separation between masker and signal. The suppressor, our band

pressor (80 dB) in the opposite ear. Shannon(1975) had already shown that there is little contralateral suppression in forward masking but the phenomenon had not been examined in backward masking. Table III presents the results;

effect on the amount of suppression. Furthermore, reducing the noise spectrum level to -20 dB (total noise

even the smallest effect of contralateral suppression in backward masking, 22.4 dB for observer PA, is larger than the largest ipsilateral suppression measured in

In previous studies of the effect of suppressor level,

either in pulsation threshold (Houtgast, 1974), or in forward masking (Shannon, 1976), both masker and suppressors were sinusoids. In those studies no suppression was apparent unless the suppressor was at

least 10 dB greater than the masker and full suppression required a 20-dB d•ference.

Our results for a

noise masker and noise suppressor show full suppression at a spectrum level of 30 dB for the suppressor. At this level the overall masker level and suppressor level are roughly equal, 64 and 63 dB, respectively. For backward masking, this relation between suppressor and masker level had not been previously s•died. B. Contralateral suppression

1. Suppressionin backward and forward masking

as that

shown

in Table

I.

The

con-

of noise 2.3-3.7 kHz, was varied ilxlevel. In forward masking (Fig. 3, squares), suppression is evident only at the higher spectrum level of [he suppressor, 30 and 40 dB. In backward masking (circles), the changein the suppressor's spectrum level from 40 to -10 dB has little

power only 13 dB) still produces as much suppression as the highest (40-dB) suppressor in forward masking.

is the same

the masker-alone

dition

aver-

age effect of the contralateral suppressor in backward masking is 28.3 dB as opposed to 33.2 dB for the ipsilateral suppressor. Every observer shows somewhat

less contralateral than ipsilateral suppression,5 but

TABLE

HI.

Contralateral

suppression.

Backward masking

Forward masking

Observer

M

M + S

Diff.

M

M + S

Diff.

PA DB DC

64.6 66.1 67.5

42.0 34.5 30.8

22.4 31.6 36.7

67.1 66.7 69.0

64.4 55.1 68.5

2.7 11.6 0.5

EF MH PM JN DW

69.0 66.1 66.6 65.4 67.6

41.2 42.9 42.4 32.9 39.7

27.8 23.2 23.7 32.5 27.9

69.7 62.6 68.4 68.6 70.1

65.4 60.9 66.5 61.9 68.8

4.3 1.7 1.9 6.7 1.3

Average

66.6

38.3

28.3

67.8

63.9

3.8

1.4 1.5

4.1 2.8

2.4 0.9

4.5 2.2

Inter Intra

•r •r


ß

Masker:

The obvious differences between suppression in forward and backward masking led us to repeat some of 1262

J. Acoust.Soc.Am., ¾ol. 65, No. 5, May 1979

Standard noise masker (200 Hz wide). Suppressor: Sinusoid, 2.5 kHz at 80 dB SPL, simultaneous

with

masker.

D.L. WeberandD. M. Green: Suppression in masking

1262


forward masking, 18 dB for observer JN. In forward masking there is some evidence of contralateral suppression but it is relatively small, an average of 3.8 dB as opposed to an average of 11.5 for the ipsQateral condition. Again in forward as in backward masking each observer shows a larger suppression effect for the ipsilateral than for the contralateral

suppressor (compare Tables I and III). The large suppression effect produced by the contralateral suppressor clearly suggests the operation of some central processes underlying suppression in backward masking. The lack of much contralateral suppression in forward masking suggests that the processes underlying suppression in forward masking are largely peripheral. However, both peripheral and central processes are probably involved to some degree in producing suppression in both backward and forward masking.

As can be seen in Table IV, this difference is almost entirely due to a single observer, DC. His performance was extremely inconsistent in this condition, the standard

deviation

of the three

threshold

measurements

was 18 dB, and led also to the extremely high average

for the interobserver standard deviation (4.25 dB) shown

in Table

IV.

Discounting this one observer, the results are close replicas of the ipsilateral condition. Once again we can interpret the contralateral suppressor as simply reducing the effective level of the activity in the region of signal frequency. In backward masking, then, a suppressor reduces activity at the signal frequency in both the same and the opposite ear.

3. Maskfi•gpatterns A primary variable studied in previous research on suppression is the relative frequencies of the masker

and suppressor (Houtgast, 1974; Shannon, 1976). In a 2. Suppressionas a reduction in maskinglevel in backward masking For ipsilateral suppression we compared a continuous suppressor with one that was gated simultaneously with the masker. The continuous suppressor presumably suppressed both signal and masker. Those resuits, presented in Table II, were consistent with the argument that the suppressor reduced the effective level of whatever sound, masker or signal, that was simultaneously present.

A similar experiment was carried out for contralateral suppression and the results are presented in Table IV.

The

results

are similar

to those obtained

with

the

ipsilateral suppressor. For the same observers the average contralateral suppression is somewhat less. A contralateral suppressor gated with the masker reduces the signal threshold only 20.9 dB (66.7-45.8,

typical psychophysical experiment one varies the frequency of a sinusoidal suppressor with respect to a fixed-frequency, sinusoidal masker. The resulting masking patterns show an increase in threshold for suppressor frequencies in a region somewhat smaller than a conventional

critical

bandwidth

and a reduction

in

threshold for suppressor frequencies above and below this region. Suppressor frequencies outside these regions have no effect. We used a sinusoidal suppressor and our standard noise masker. The observers were DB, DC, and MH. The masking pattern in forward masking (Fig. 4, open

squares) is similar to that found in earlier studies. Two suppression regions are evident, one extending about one octave above the masker frequency, the other

I

'

'''''1

'

I

'

'''''1

Table IV) as opposedto 29.9 dB (66.7- 36.8, Table II) 7O

for the ipsilateral suppressor. The larger discrepancy, however, is that the continuous suppressor in the coniralateral

ear

lowers

the threshold

somewhat

more

F•0RWARD

than a continuous ipsilateral suppressor, 64 vs 68.4 dB.

TABLE IV. Comparison of the effects of a contralateral suppressor when gated with masker or when continuous. Gated

Observer

M

(M + S)

Continuous

S

Gated M

_

_J

o

BACKWARD

T

T

b.I

DB DC EF MH PM DW

66.6 67.5 69.0 63.0 66.6 67.6

46.9 38.7 42.3 50.3 43.2 53.5

65.1 53.7 67.0 61.3 69.6 67.4

T

66.7

Intr a (• Inter (•

1.4 2.0

45.8

RAL

40

t--

300 ß

Average

ß CONTRALATERAL

n,'

•ooo

SUPPRESSOR

3000

FREQUENCY

•oooo

Hz

64.0


FIG. 4. Masking patterns. Threshold as a function of suppressor frequency in forward masking (square s)-- and both tpsilateral (open circles) and contralateral (closed circles) in backward masking. Lines labeled For and Back indicate masking produced by the masker alone in forward and backward mask-

Masker: Standard masker (200 Hz wide). Suppressor: Sinusold, 2.5 kHz at 80 dB SPL, simultaneous'with masker in gated (M + S) condtion.

Masker: Standard noise band (200 Hz wide). Suppressor: Sinusold 80 dB SPL, simultaneous with masker.

1263

5.48 2.10

5.77 4.25

J. Acoust.Soc.Am., Vol. 65, No. 5, May 1979

ing, respectively. Signal: Sinusold, 9 ms, '2 kHz, zero delay.

D. L. Weberand D. M. Green: Suppression in masking

1263


BACKWARD

extending about two octaves below the masker frequency. The lower suppression region is somewhat deeper and wider than usually reported. The reasons for this discrepancy are not understood; perhaps it is because the masker

is a narrow-band

noise

rather

than

A

MASKING

DB

Z•

MH

a sinu-

sold. a. 4O

The masking patterns obtained in backward masking are totally different from the forward masking results

-

50

for both an ipsilateral (open circles) and a contralateral (closed circles) suppressor. The suppression effect is

',

:

:

•

',

',

I

70

larger for all suppressor frequencies and shows comparatively little change in the amount of suppression

0

with frequency (+5 dB in a full effect of about 30 dB).

60

o: 5o

Once more we would argue that this difference in backward and forward masking stronly suggests different mechanisms •n backward and forward masking.

-

4o

ß CONTRAlATERAl

Suppose the results in forward masking arise largely from peripheral processes that can be described by a region of excitation near the masker signal flanked by two suppression regions. Suppose further that two processes are at work in suppression of backward masking--one central, measured largely by the contralateral suppressor, the other peripheral, measured in addition to the central processes in the ipsilateral condition. If one compares the difference between the ipsilateral and contralateral conditions, one observes a small difference in the region near the signal frequency flanked by two regions of larger differences. These frequency characteristics are similar to the masking pattern in forward masking, although smaller in size. One may argue that the ipsilateral suppressor produces slightly greater suppression than the contralateral suppressor due to the additional influence of the peripheral processes. The large amount of contralateral suppression and the small difference between ipsilateral and contralateral conditions suggests that suppresion of backward masking is dominated by central

0

SPECTRUM

soidal

of the difference

and noise

maskers.

is not the same

For

their

for

sinusoidal

the conventional

critical

bandwidth.

SUPPRESSOR

Masker: Standard noise masker (200 Hz wide). Suppressor: Noise suppressor band (2.3-3.7 Hz) simultaneous with masker.

dard noise (Fig. 5) or a 2.5-kHz sinusold (Fig. 6). In either figure the point marked M represents the signal threshold for the masker alone. Some suppression

(lower threshold) is evident for practically all noise levels over a 60-dB range, although there are considerable differences in the form of the function among observers. Ipsilateral presentation of the suppressor is BACKWARD

MASKING i

i

i

I

i

i

i

i

!

I

I

I

iiiii

70

EF

60

J

40

ß

sinu-

CONTRALATERAL

0 I PSI LATERAL

masker :

the frequency region of the suppressor that produces an increase in signal threshold is roughly two to four times

OF

ing. The points labeled "M" indicate the amountof maskingby the masker alone. Signal: Sinusold, 9 ms, 2 kHz, zero delay.

Tyler and Small (1977) measured suppression using

the nature

LEVEL

FIG. 5. Effect of noise suppressor level. Threshold as a function of suppressor level for ipsilateral (open circles) and contralateral (closed circles) suppressors in backward mask-

processes.

a sinusoidal masker and sinusoidal suppressor in backward masking. They found, as do we, that the masking patterns in backward masking are qualitatively different from those obtained in forward masking. However,

IPSILATERAL

• 4o-,'o; ,; •'o•o

• 4o-,'o b ,b •'oio .o

In forward

70

:

:

:

:

:

I

I

I

I

I

I

I

I

I

_

o

ß.r. 60

masking this region is usually slightly less than one critical

bandwidth.

ß'r' 50 40

4. Effects of ipsilateral and contralateral suppressorlevel

_

I

We have already seen (Fig. 3) that the effects of the level of an ipsilateral noise suppressor are different in forward and backward masking. We conclude our experiments by presenting two studies of the effect of level for ipsilateral and contralateral suppressors in backward masking; The signal is a 2-kHz sinusoid 9 ms in duration; the masker is our 200-Hz band of noise centered at 2 kHz. The suppressor is either our stan1264


I

LEVEL

OF

SINUSOIDAL

SUPPRESSOR

(SPL)

FIG. 6. Effect of sinusoidal suppressor level. Threshold as a function of suppressor level for ipsilateral (open circles) and contralateral (closed circles) suppressors in backward mask-

ing. The points labeled "M" indicate the amount of masking by masker alone. Signal: Sinusoid, 9 ms, 2 kHz, zero delay. Masker: Standard noise masker (200 Hz wide). Suppressor: Sinusoid, 2.5 kHz, simultaneous with masker. D.L. Weberand D. M. Green: Suppressionin masking

1264


nearly always more effective than contralateral presentation at high suppressor levels, a result consistent with our previous findings. At lower suppressor levels the ipsilateral suppressor is more effective than a con-

iralateral suppressor for the noise suppressor (Fig. 5). These generalizations

are not true at low levels of

the suppressor when using a sinusoidal suppressor

(Fig. 6). The contralateral suppressorshowssuppression even at relatively low levels, e.g., 35 dB. At these low levels a contralateral stimulus produces suppression effects 20 to 25 dB larger than ipsilateral suppression,

a most surprising

result.

This result also

violates the general rule that suppression occurs only when the suppressor is much larger in intensity than the masker; here the suppressor is 35 dB, the masker has a noise spectrum of 40 dB and a total power of 64 dB.

The total power in the noise suppressor is about 33 dB greater than its noise spectrum level. Comparing noise and sinusoidal suppressors of equal power for the one observer common to both conditions, DW, indicates that the noise suppressor is always somewhat more effective than the sinusoid, although at low intensities (less than 35 dB) this difference is negligible for the contralateral presentation of the suppressor. At higher intensities, the noise suppressor reduces threshold by about 10 dB more than the sinusoid, in good agreement with our earlier comparison of noise and sinusoidal

III.

GENERAL

maskers.

DISCUSSION

The fact of differences

between

stimulus produces a drop in the rate below the spontaneous firing rate followed by an exponential rise back to the spontaneous rate. The decrease in rate at off-

set is directly proportional to the steady-state rate, thus the steady-state

rate can be called the effective

level of the masker. The time course of the recovery back to the spontaneous rate (Harris, 1977) is similar to that observed psychophysically (Elliott, 1967). The effect of masker duration is also similar; Harris estimates

one time

constant

to be about 200 ms.

Thus

there are good reasons to try to account for forward masking in terms of peripheral physiological events.

In another paper, Weber and Green (1978) showed that the effect of adding the suppressor for variable amounts of time in forward masking is closely simulated by simply reducing the masker level for the same time intervals. All these results then suggest that forward masking (and the suppression of forward

masking) is largely determined by peripheral processes. We would say "entirely" rather than "largely" but for the small effect of the contralateral suppressor in forward masking. Certainly the magnitude of this effect is small and suggests that such processes, if present, have very little influence. Unfortunately, our understanding of backward masking is much more primitive than our understanding of forward masking. Little physiological evidence is available; there is some suggestion that no backward masking is observed below the level of the cochlear nucleus, although some backward masking is reported

at higher levels (Watanabe and Simada, 1971). There forward

and back-

ences by altering values of the parameters of a model for peripheral frequency analysis, and he succeeded in predicting many common results in nonsimultaneous

are no specific models of backward masking. Some theories cite the very fact that the masker follows the signal as evidence that delays in the transmission of the signal relative to the masker events are implicated, else they would be forced to postulate noncausal processes. Other theories suggest the later masker interrupts or discontinues some ongoing processing of

masking (Duifhuis, 1973; 1976).

the signal. Whatever the cause of backward masking,

ward masking is not remarkable; most comparisons have shown the two to be different (e.g., Elliott, 1967).

Duifhuis (1973) attempted to accountfor these differ-

The idea that forward masking (and the suppression

of forward masking) is determined in the auditory periphery is supported by two general lines of evidence. First, there is similarity in the frequency effects. Combinations of suppressor frequency and intensity that produce a reduction in threshold define what is

our suppression results clearly implicate central events because their magnitude is nearly the same whether we apply the suppressor to the ipsilateral or contralateral

ear. Elliott (1962) studied ipsilateral and contralateral masking in forward and backward conditions. A contralateral masker produces essentially no effect in forward masking but raises thresholds 12-16 dB in back-

calledthesuppression region(Houtgast,1972;Shannon, ward masking. But terms such as central and peri1976). The suppressionregion in psychophysicalstudies closely resembles those obtained from recordings in peripheral auditory fibers in which the presentation of a second tone decreases the response elicited by a tone presented at the characteristic frequency of the fiber

(Sachsand.Kiang, 1968; Arthur, Pfeiffer, and Suga,

1971; Legouix, Remond,andGreenbaum,1973). Second, there is' similarity in the tempor.al effects observed in psychophysical studies and two recent physiological studies of eighth-nerve responses in a forward-mask-

ing stimulus condition (Smith, 1977; Harris,

1977).

pheral are hardly specific as to the nature of the phenomena. We could agree with Terry and Moore (1977) saying that the suppressor alters the relation between signal and nonsignal events and causes some new "qualitative differences between masker and probe" to emerge, but such descriptions are hardly quantitative.

Some have suggested that we should call backward suppression another name. Certainly we agree the processes of forward and backward suppression are different. But operationally the two are similar and until

The response of single units to the sinusoidal stimulus consists of an initially high response rate that decays

we can suggest some more specific mechanism

exponentially to a "steady-state" level determined by

informative. One would simply have to remember that this new word means the suppression effects observed

the level of the continuing signal. Termination 1265

J. Acoust. Soc. Am., Vol. 65, No. 5, May 1979

of the

the

choice of a different designation is not likely to be very

D. L. WeberandD. M. Green:Suppression in masking

1265


in backward masking--whereas the other results refer to the forward masking paradigm. Also, although there are many similarities betxveen the peripheral physio-

logical resultsandtheresuRs observed inpsychophy•ical studies of suppression in forward masking, data from both areas are comparatively recent. As more evidence

accumulates

ilarities

may become more evident.

IV.

the differences

as well

as sim-

CONCLUSION

Suppression effects in backward masking differ from those in forward masking. Unlike forward masking, the effect of the suppressor accumulates over a much longer time than can be simulated by a peripheral reduc-

tion in the masker level (Weber and Green, 1978). Unlike forward masking, a sinusoidal suppressor alters the signal threshold over a frequency region much larg-

er than a critical band (Tyler and Small, 1977) when a sinusoidal masker is used or produces a reduction in threshold at all frequencies when a noise masker is employed. Unlike forward masking, the contralateral suppressor is nearly as effective as the ipsilateral suppressor in many conditions, and more effective in some conditions. Unlike forward masking, the suppressor is effective even when its intensity is much less than the masker

intensityø All these experiments suggestthat the processes mediating suppression of backward masking are different from those in forward masking. In particular, these differences are consistent with the idea that suppression effects in forward masking are primarily determined by peripheral processes and that suppression', effects in backward masking are dominated by additional, central processes. ACKNOWLEDGMENTS

This research was supported in part by the National Institutes of Health, Public Health Service, United States Department of Health, Education, and Weftare. We thank M. Hartnett, who assisted in the data collection of several experiments. Extensive revision of this paper was prompted by the comments on an earlier draft of Drs. R. D. Luce, C. C. Wier, and W. A. Yost.

1Filteringthe 8-ms square-gated signalproducesa smooth envelopewith about 8 ms between the 3-dB downpoints, about 9 ms between the 6-dB down points, and about 10 ms betweenthe 40-dB downpoints. The last roughly correspondsto the limits of resolution on our oscilloscope (as used). Timing was judged on the oscilloscope (after calibration) and is specified in this paper in terms of the halfvoltage (6-dB down) points. Thus the nominal 8-ms signal is 9 ms when using the criterion applied to timing the onset-• offset delay. Similarly, the "2-ms" signal is 1.8 ms at the 3-dB down points and 2 ms at the 6-dB downpoints. In addition, the nominal 3.8-ms onset-offset delay (measured at the 6-dB points) between the signal and masker was 0 ms measured at the 40-dB down points. Using this last criterion, there was no quiet interval between the signal and masker.

2Thenoisesuppressor passed through Krohn-Hite filterswith cutoffs set at 2.3 and 3.7 kHz. The resulting noise was 1458

Hz wide at its 3-dB down points and had an equivalent rectangular bandwidth of roughly 2000 Hz. Thus the total noise power in the suppressor was about 33 dB above the noise spectrum level. Although the masker and the suppressor had overlapping spectra, the suppressor was over 10 dB down at the signal frequency.

SAppatently normal-hearing listenerswhofailedto showsuppression effects have been reported previously (Shannon, 1975).

4Itis importantto realizethatour standardnoisemasker, 200 Hz wide, is less than a critical band in width. In forward masking, increasing the width of the noise will deerease its effectiveness, presumably because the skirts fall in suppression areas (Houtgast, 197zi).

5Thisexperimentalconditionhasbeensuccessfully replicated by Dr. D. Me Fadden.

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Duifhuis, H. (1973). "Consequencesof peripheral frequency selectivity for nonsimultaneous masking," J. Aeoust. Soe. Am. 54, 1471-1488.

Duifhuis, H. (1976). "Coehlear nonlinearity and second filter: possible mechanisms and implications," J. Aeoust. Soc. Am. 59, 408-423. Elliott, L. L. (1967). '•Development of auditory narrow-band frequency contours," J. Aeoust. Soe. Am. 42, 143-153.

Elliott, L. L. (1962). "Backward masking: monotie and diehotic conditions," J. Aeoust. Soe. Am. 34, 1108-1115.

Harris, D. M. (1977). "Forward masking and recovery from short term adaptation in single auditory nerve fibers," Doctoral dissertation, Northwestern University, Evanston, IL.

Houtgast, T. (1972). "Psyehophysieal evidence for lateral inhibiton in hearing," J. Aeoust. Soe. Am. 51, 1885-1894. Houtgast, T. (1973). "Psyehophysieal experiments on 'tuning curves' and 'two-tone inhibiton,' "Acustica 29, 168-179. Houtgast, T. (1974). "Lateral suppression in hearing: A psychophysical study on the cat's capability to preserve and enhance spectral contrasts," Doctoral dissertation, Academische Pets B. V. Amsterdam, The Netherlands. Legouix, J.P., Remond, M. C., and Greenbaum, H. B. (1973). "Interference and two-tone inhibition," J. Acoust. Soc. Am. 53, 409-419.

Leshowitz, B., and Zurek, P.M.

(1977) (personal communica-

tion).

O'Malley, H., and Feth, L. (1978). "Relationship between auditory frequency selectivity and two-tone suppression," J. Acoust. Soc. Am. 63, S31(A). Miller, G. A. (1948). "The perception of short bursts of noise," J. Acoust. Soc. Am. 20, 160-170.

Munson, W. A. (1947). "The growth of auditory sensation," J. Acoust. Soc. Am. 19, 584-591. Plomp, R. (1964). "Rate of decay of auditory sensation," J. Acoust. Soc. Am. 36, 272-282. Sachs, M. B., and Kiang, N.Y. S. (1968). "Two-tone inhibition in auditory-nerve fibers," J. Acoust. Soc. Am. 43, 1120-1128.

Shannon,R. V. (1975). "Suppressionof forward masking," Doctoral dissertation, University of California at SanDiego, La Jolla, CA.

Shannon,R. V. (1976). "Two-tone unmasking and suppression in a forward masking situation," J. Acoust. Soc. Am. 59, 1460-1470.

Smith, R. L. (1977). "Short-term adaptationin single auditory nerve fibers: some poststimulatory effects," J. Neurophyslo1.40,

1098-1112 '

Terry, M., and Moore, B.C. J. (1977). '"Suppression' effects in forward masking," J. Acoust. Soc. Am. 62, 781-784 (L). Tyler, R. S., and Srtla11,A.M. (1977). "Two-tone suppression in backward masking," J. Acoust. Soc. Am. 62, 215-218 (L). Vogten, L. L. M. (1974). "Low-level pure-tone masking and two-tone suppression," IPO Annu. Prog. Rept. No. 9, 22-31. Watanabe, T., and Simada, Z. (1971). "Auditory temporal masking: an electrophysical study of single neurons in the ,

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cat's cochlear nucleus and inferior colliculus," Physiol. 21, 537-549.

Jpn. J.

Weber, D. L. (1978). "Suppressionand critical bands in band-limiting experiments,"

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Weber, D. L., and Green, D. M. (1978). "Temporal factors and suppressioneffects in backward and forward masking," J. Acoust. Soc. Am. 64, 1392-1399.

D.L. Weber and D. M. Green: Suppressionin masking

1267


"Suppression" effects in forward masking.

Frequency discrimination in forward and backward masking.

Dichotic forward and backward "masking" between CV syllables.

Backward recognition masking.

Laterality differences and practice effects under central backward masking conditions.

Intensity discrimination under backward masking.

Potential artifact in dichoptic backward masking.

Intensity discrimination in forward masking.

Relations among temporal resolution, forward masking, and simultaneous masking.

Forward and backward inference in spatial cognition.

Backward- and Forward-Looking Potential of Anaphors.

A dynamic look backward and forward.

Age-related differences in binocular backward masking with visual noise.

How color, regularity, and good Gestalt determine backward masking.

Evidence that adaptation of suppression cannot account for auditory enhancement or enhanced forward masking.

A modified multiplicative rule describes backward masking in a photoreceptor.

ALS--dying forward, backward or outward?

Phonological effects in forward and backward serial recall: qualitative and quantitative differences.

Forward and backward locomotion in individuals with dizziness.

Critical cardiac care in children: looking backward and looking forward.

Psychophysical tuning curves measured in simultaneous and forward masking.

The effects of forward and backward walking according to treadmill inclination in children with cerebral palsy.

U-shaped backward contour masking during stroboscopic motion.

Backward and forward recall by deaf and hearing children.