masking-level differences with continuous and with burst masking noise

6
4.3, 4.7, 4.8, 4.15 Received 15 July 1966 Msking-Level Differences with Gontinuous with Burst Msking Noise* DENNIS MCFADDEN Hearing andCommunication Laboratory, Indiana University, Bloomington, Indiana 47401 Psychometric functions were obtained for several interaural phase combinations with bothcontinuous and burst masking noise. In the burst conditions, the signal(400 cps, 125 msec) and the wide-band masker (45 dB/cycle) were gated simultaneously; in the continuous conditions, only the signal was gated.Per- formance on burst NO-SO was only about 0.5 dB worse than that on continuous NO-SO, but the masking- level differences (MLD's) for N0-S•r, N•r-S0, NO-Sm, and N•r-Sm were 4-6 dB smaller with burst than with continuous noise. In an additional experiment, the noise burst (NO) wasgated0, 75, 150, 250, 400, 600, and 1000 msec before the onset of the signal (S•r). These MLD's increased gradually between 0 and 600 msec and then leveledoff at approximately the value obtained with a continuous masker. A single- interval Y•.s-Noprocedure was used in theseexperiments. When two-alternative forced choice was used, the difference between continuous and burst noise was considerably diminished. INTRODUCTION ESEARCH on masking-level differences has pro- vided a basis for classifying the conditionsof binaurallistening into two groups. In one groupare those dichotic conditionsin which a listener's per- formance in a signal-detection task is better than his performance under monaural conditions at the same signal-to-noise ratio. This improvement inperformance, when expressed in decibels, isknown as a masking-level difference or MLD, andthe conditions resulting in such an improvement are called MLD conditions. In the second group are those binaural conditions in which the detectability of a signal is essentially the same as that under monaural conditions, and these are called non-MLD conditions. Using a non-MLD condition, Watson • compared the detectability of a signal when the masker wasa con- tinuously present noise to its detectability when the masker wasa burstof noise gated on every trial for the durationof the signal. He found performance with burst noise slightly inferior to performance with continuous noise. Probably for reasons of convenience, * Presented at the Seventy-First Meeting of the Acoustical Society of America, 1-4 June 1966, Boston, Mass. [J. Acoust. Soc. Am. 3•}, 1231 (A) (1966)•. • C. S. Watson, "Signal Detection andCertain Physical Char- acteristics of the Stimulus during the Observation Interval," Ph.D. thesis, Hearing and Communication Laboratory, Ind. Univ. (1962). 1414 Volume 40 Number 6 1966 moststudies of MLD's have used a continuous masking noise. In the experimentsto be reported here, both continuous noise and burst noise were used with both non-MLD and MLD conditions. With burst noise, detection performance in the MLD conditions was degraded considerably more than performance in the non-MLD conditions. That large MLD's are routinely obtained indicates that in the MLD conditions the auditory systemcan operate upon someparametersof the acoustic stimulus which are either not available or not of value in the non-MLD conditions. Apparently, in the MLD conditions, either the parameters of interestor the listener's ability to usethese parameters are altered by burst noise, whereasin the non-MLD conditions little changein these factors results from using burst noise instead of continuous noise. I. GENERAL PROCEDURE AND APPARATUS Each subject was seated in a separate, sound- deadened booth and was provided with a set of di- chotically wired Permoflux PDR-10 earphones. The noisesource was a Grason-Stadler 455-B noise gener- ator.The spectrum levelwas 45 dB re 0.0002t•bar, and the bandwidth was determined by the characteristics of the earphones. The signal wasa 400-cps tonewith a duration of 125 msec. All gating was done without respect to phase and without a controlled rise time. Figure 1 shows the ON-O• response of a PDR-10 Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 150.214.146.47 On: Mon, 24 Nov 2014 11:36:37

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Page 1: Masking-Level Differences with Continuous and with Burst Masking Noise

4.3, 4.7, 4.8, 4.15 Received 15 July 1966

Msking-Level Differences with Gontinuous with Burst Msking Noise*

DENNIS MCFADDEN

Hearing and Communication Laboratory, Indiana University, Bloomington, Indiana 47401

Psychometric functions were obtained for several interaural phase combinations with both continuous and burst masking noise. In the burst conditions, the signal (400 cps, 125 msec) and the wide-band masker (45 dB/cycle) were gated simultaneously; in the continuous conditions, only the signal was gated. Per- formance on burst NO-SO was only about 0.5 dB worse than that on continuous NO-SO, but the masking- level differences (MLD's) for N0-S•r, N•r-S0, NO-Sm, and N•r-Sm were 4-6 dB smaller with burst than with continuous noise. In an additional experiment, the noise burst (NO) was gated 0, 75, 150, 250, 400, 600, and 1000 msec before the onset of the signal (S•r). These MLD's increased gradually between 0 and 600 msec and then leveled off at approximately the value obtained with a continuous masker. A single- interval Y•.s-No procedure was used in these experiments. When two-alternative forced choice was used, the difference between continuous and burst noise was considerably diminished.

INTRODUCTION

ESEARCH on masking-level differences has pro- vided a basis for classifying the conditions of

binaural listening into two groups. In one group are those dichotic conditions in which a listener's per- formance in a signal-detection task is better than his performance under monaural conditions at the same signal-to-noise ratio. This improvement in performance, when expressed in decibels, is known as a masking-level difference or MLD, and the conditions resulting in such an improvement are called MLD conditions. In the second group are those binaural conditions in which the detectability of a signal is essentially the same as that under monaural conditions, and these are called non-MLD conditions.

Using a non-MLD condition, Watson • compared the detectability of a signal when the masker was a con- tinuously present noise to its detectability when the masker was a burst of noise gated on every trial for the duration of the signal. He found performance with burst noise slightly inferior to performance with continuous noise. Probably for reasons of convenience,

* Presented at the Seventy-First Meeting of the Acoustical Society of America, 1-4 June 1966, Boston, Mass. [J. Acoust. Soc. Am. 3•}, 1231 (A) (1966)•.

• C. S. Watson, "Signal Detection and Certain Physical Char- acteristics of the Stimulus during the Observation Interval," Ph.D. thesis, Hearing and Communication Laboratory, Ind. Univ. (1962).

1414 Volume 40 Number 6 1966

most studies of MLD's have used a continuous masking noise. In the experiments to be reported here, both continuous noise and burst noise were used with both

non-MLD and MLD conditions. With burst noise, detection performance in the MLD conditions was degraded considerably more than performance in the non-MLD conditions. That large MLD's are routinely obtained indicates that in the MLD conditions the

auditory system can operate upon some parameters of the acoustic stimulus which are either not available or

not of value in the non-MLD conditions. Apparently, in the MLD conditions, either the parameters of interest or the listener's ability to use these parameters are altered by burst noise, whereas in the non-MLD conditions little change in these factors results from using burst noise instead of continuous noise.

I. GENERAL PROCEDURE AND APPARATUS

Each subject was seated in a separate, sound- deadened booth and was provided with a set of di- chotically wired Permoflux PDR-10 earphones. The noise source was a Grason-Stadler 455-B noise gener- ator. The spectrum level was 45 dB re 0.0002 t•bar, and the bandwidth was determined by the characteristics of the earphones. The signal was a 400-cps tone with a duration of 125 msec. All gating was done without respect to phase and without a controlled rise time. Figure 1 shows the ON-O• response of a PDR-10

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Page 2: Masking-Level Differences with Continuous and with Burst Masking Noise

MLD'S- CONTINUOUS AND WITH BURST NOISE

Fro. 1. The os-o•? response of a PDR-10 earphone through a 6-cc coupler. The signal used for this photograph was a 4000-cps tone, gated without respect to phase and without a controlled rise time, through a mercury-wetted relay. The amplitude of this tone was about 10 times that actually used in any of the experiments.

earphone through a 6-cc coupler (B&K model 4109) to a signal gated through the mercury-wetted relay used in the burst-noise conditions. In order to better illus-

trate the envelope, 4000 cps at a level approximately 20 dB greater than actually run in any of the experi- ments was used for Fig. 1. Apparently all irregularities in the envelope at onset were over in about 2 msec; decay time was also approximately 2 msec.

Figure 2 shows the timing sequence for a signal-plus- noise (SN) trial and a noise-alone (N) trial for both the continuous-noise and the burst-noise conditions. A

trial consisted of a warning light (500 msec), a pause (500 msec), an observation interval (125 msec), a response interval (2000 msec), and one of two feedback lights (500 msec). During the response interval, the subject pushed one of two keys to indicate whether he felt that the trial was an SN or an N trial. These

responses were recorded on counters. The order of the trials was randomly determined, with p (SN) =p (N) =0.5. Counterbalancing of the different conditions of noise and of interaural phase was employed both within sessions and over sessions for all experiments.

Trials were run in blocks of 80, the number of blocks per session varying between eight and 12. There were 2-min breaks between blocks and a 5-7-min rest after

every third or fourth block. The subjects were in- structed to remove their headsets during both the breaks and the rests. Numerous short practice blocks were run at signal levels 2-3 dB greater than the highest used for that condition and, at the beginning of each block, the tone was briefly presented 10 dB up from the level to be used on that block.

For each subject on every block of trials the pro- portions p(Y•.sl SN) and p(Y•.s [N) were computed and used to determine a d' (Ref. 2). This d' was transformed into _P(C)m, which is the maximum proportion of

2 p. B. Elliott, "Tables of d'," Electronics Defense Group Tech. Rept. No. 97, Univ. Mich. (1959).

CONTINUOUS

__•"' /•-I NOISE

BURST

I I I I

FRINGED

SN TRIAL N TRIAL.

Fro. 2. A signal-plus-noise (SN) and a noise-alone (N) trial are shown for each of the methods of presenting the masker. In the continuous-noise conditions, the masker was present throughout the block of trials. In the burst-noise conditions, the masker was gated during the observation interval. In the "noise-fringed" conditions, the masker was gated prior to the observation interval by a period of time that was varied from one block of trials to the next.

correct decisions possible if the subject were operating on underlying distributions which were normal and had equal variances. The value of P (C)m is obtained from a table of the normal curve by reading off the area cor- responding to a z score of «d' For each subject, mean _P (C)m was plotted against 10 log(E/N0), and for each of the resulting psychometric functions an ogive of the ap- propriate k (Ref. 3) was adjusted to the left or right un- til the best fit was achieved. The value of 10 log(E/No) necessary for the subject to respond correctly on 80% of the trials was then determined to the nearest 0.25 dB, and these values were used to compute MLD's. The burst-noise MLD conditions were referred to the burst-

noise non-MLD condition, and the continuous-noise MLD conditions were referred to the continuous-noise non-MLD condition. Since the same k was used for all

of a particular subject's functions, the MLD's would have been the same no matter what level of performance was selected. Eighty percent correct was chosen arbitrarily.

Some of the symbols used to characterize the various interaural conditions are: N for the masking noise, S for the signal, 0 for an interaural phase shift of 0 ø, •r for an interaural phase shift of 180 ø, and m for a manaural stimulus. Thus, Nm-Sm designates the manaural listening condition, NO-SO means that the interaural phase shift of both the noise and the signal is 0 ø, and

a For values of d' between 0 and about 4, a listener's performance can be approximated by d'=m (E/No) k, where E is the energy of the signal, No is the noise power per unit bandwidth, and m and k are constants •see J.P. Egan, "Masking-Level Differences as a Function of Interaural Disparities in Intensity of Signal and of Noise," J. Acoust. Sac. Am. 38, 1043-1049 (1965); and J.P. Egan, W. A. Lindner, and D. McFadden, "Masking-Level Differences and the Form of the Psychometric Function," J. Acoust. Sac. Am. 37, 1181 (A) (1965)•. The value of k varies over subjects but is usually between 1.0 and 2.0, the larger k's being associated with steeper psychometric functions.

The Journal of the Acoustical Society of America 1415

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Page 3: Masking-Level Differences with Continuous and with Burst Masking Noise

D. McFADDEN

N0-S,r means that the noise has a 0 ø interaural phase shift and the signal has a 180 ø interaural phase shift. In all the experiments reported in this paper, NO-SO was used as the non-MLD condition.

H. EXPERIMENT 1. BURST VS CONTINUOUS NOISE

In order to establish the effect of burst noise upon those conditions that do and those that do not lead to

MLD's, psychometric functions were obtained for NO-SO, N,r-S0, N0-S,r, N,r-Sm, and N0-Sm, with both burst and continuous masking noise.

A. Procedure

Three young women between 16 and 19 years old were employed as subjects. All had clinically normal hearing. With the exceptions noted below, six 80-trial blocks were run for each of four signal levels for every combination of noise and interaural phase.

B. Results

The results of this experiment are shown in Fig. 3 and Table I. Figure 3 shows that burst noise had practically no effect on the non-MLD condition. Specifically, performance on burst NO-SO was about 0.4 dB worse than performance on continuous NO-SO. This result is consistent with the data of Watson, 1 who used a 1000-cps tone and a wide-band masking noise and found a difference of about 1 dB with burst noise of 250 msec.

Unlike performance in the non-MLD condition, per- formance in the two MLD conditions shown in Fig. 3, N0-S,r and N,r-S0, was degraded considerably by burst noise. Table I shows the magnitude of this burst-noise effect. The entries in this table are the averages of the MLD's of the three subjects. The MLD's obtained with continuous noise are very similar to those reported by previous investigators.

ioo

90

z 70

n 60

-8 -6 -4 -2 0 4 6 8 I0 I I

I0 log (E/No)

Fro. 3. Psychometric functions relating percent correct detec- tions to signal energy. Burst-noise (closed data points) and continuous-noise (open points) data are shown for NO-SO (e,o), N•r-S0 (A, /x) and N0-S•r(I,F1). Each point is a mean based on data from three subjects. All ogives are k- 1.0. A Y•,s-so psycho- physical method was used. Signal: 400 cps, 125 msec. Masker: 45 dB sound-pressure level (SPL)/cycle.

N,r-Sm and N0-Sm are not plotted in Fig. 3, but MLD's for these conditions are shown in Table I. These data are less reliable than those for the other conditions since all the data for a condition were collected in a

single session and only 400 trials at each of two signal levels were run. Nevertheless, the same effect is evident. The MLD shown for burst N,r-Sm is the mean for two

subjects. The third subject was omitted because she had a negative MLD (--3.0 dB) while the other two subjects both had MLD's of 1.0 dB.

A point that should be noted is that the data for burst noise in Fig. 3 have a slightly lower slope than those for continuous noise. This was true for most of the

subjects in all the experiments reported in this paper. All ogives in Fig. 3 are k- 1.0. There is some indication in Table I that the N,r conditions are affected more by burst noise than the NO conditions.

III. EXPERIMENT 2. NOISE FRINGING

The results of Expt. 1 showed that, for the MLD conditions, gating the noise and signal simultaneously led to a smaller MLD than gating the signal against a background of noise. MLD's intermediate to those obtained with burst and with continuous noise should

be obtained if the noise is gated prior to the observation interval. When this "fringe" of noise is very short, the MLD should not be very different from that obtained with simultaneously gated noise and signal, and when the fringe becomes quite long the MLD should be equal to that obtained with continuous noise. Intermediate

values of fringe should lead to intermediate values for the MLD. Experiment 2 was designed to determine the functional relation between the MLD and the duration

of the noise fringe.

A. Procedure

Two female college students, ages 19 and 20, were employed as subjects. Neither was used in the previous experiment; both had clinically normal heating. In addition to burst and to continuous NO-SO, N0-S,r was run both with continuous noise and with the noise gated either 0, 75, 150, 250,400, 600, or 1000 msec prior to the observation interval. Of course, a noise fringe of 0 msec is the same as the burst condition used in Expt. 1.

TA•,•, I. Mean MLD's for the three subjects of Expt. 1. The reference condition for the continuous-noise MLD's was con-

tinuous NO-SO, and the reference condition for the burst-noise conditions was burst NO-SO. Performance on burst NO-SO was about 0.4 dB worse than performance on continuous NO-SO. Only two subjects (entry in parentheses) are represented in the MLD for burst N•r-Sm because the third subject's performance was judged to be atypical (see text).

Condition Continuous Burst Difference (dB) (dB) (dB)

N0-Sr 15.1 10.2 4.9 Nr-S0 11.5 5.7 5.8 N0-Sm 9.3 4.2 5.1 Nr-Sm 6.5 (1.0) 5.5

1416 Volume 40 Number 6 1966

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Page 4: Masking-Level Differences with Continuous and with Burst Masking Noise

MLD'S' CONTINUOUS AND WITH BURST NOISE

........ , , //--r-- I00 ß ß

i I i i i i i i I I i // i 0 200 400 600 800 I000 CONT.

NOISE FRINGE (MSEC)

Fro. 4. The magnitude of the MLD for N0-S•r as a function of the duration of the noise fringe. Each point is a mean based on data from two subjects. All the noise-fringed MLD's were referred to burst NO-SO, and the one continuous-noise condition was referred to continuous NO-SO. For this crew, performance on continuous NO-SO was about 0.25 dB worse than performance on burst NO-SO. A xms-so psychophysical method was used. Signal: 400 cps, 125 msec. Masker: 45 dB SPL/cycle.

90

• 70

a. 60

i i i I i i i i i i

_

_

o /

/' i i i I I I i I i i i i

-8 -6 -4 -2 0 2 4 6 8 I0 12 14

IOIog (E/N o )

Fro. 5. Psychometric functions relating percent correct detec- tions to signal energy. Data obtained with burst noise (closed data points) and with continuous noise (open points) are shown for NO-SO and for N0-S•r. Two psychophysical methods, Y•.s-so and 2AFC, were used for each condition. Each point is a mean based upon data from four subjects. All ogives are k-- 1.0. Signal: 400 cps, 125 msec. Masker: 45 dB SPL/cycle. Left-hand curves, N0-S•r: •:•.s-so (I,[:3); 2AFC (V,V). Right-hand curve, NO-SO: xms-•o (e,¸); 2AFC (A,/x).

Figure 2 shows the timing sequence for both an SN and an N trial in the noise-fringed conditions. The same duration of noise fringe was used for all of the trials in a block. In an attempt to eliminate uncertainty about the time of signal onset as an important factor in this experiment, the observation-interval light always coincided with the time of signal onset, not the onset of the noise. Four 80-trial blocks at each of two signal levels were run for each condition and the data were

treated as described above. As in Expt. 1, a YES-NO method was used.

B. Results

The mean MLD for each of the noise-fringed condi- tions is plotted in Fig. 4. The MLD's obtained in the noise-fringed conditions are referred to burst NO-SO, and the one point obtained with continuous noise is referred to continuous NO-SO. Performance on the

latter was about 0.25 dB worse than performance on burst NO-SO. For this crew of listeners, the difference between the MLD's for continuous and for burst noise was about 1 dB smaller than that for the crew in

Expt. 1. Figure 4 shows that the MLD's increased gradually as the length of the noise fringe was increased. Apparently, about 600 msec of noise prior to signal onset was sufficient to restore the MLD to its value with continuous noise.

IV. EXPERIMENT 3. YES-NO vs 2AFC

When a two-alternative forced-choice (2AFC) method was used, the burst-noise effect was consider- ably diminished. Since no other instance of such a discrepancy between YES-NO and 2AFC data was known, the results are reported here.

A. Procedure

Four female college students between the ages of 20 and 23 were employed as subjects. None was a subject

in the prior experiments, and all had clinically normal hearing. Six 80-trial blocks were run at each of two signal levels for burst and for continuous NO-SO and for burst and for continuous N0-S•r. The data for these

conditions were first collected with the 2AFC method

and then with the YES-NO method. The timing sequence of a trial was the same as that described above, except that, in 2AFC, there were two 125-msec observation intervals separated by a 500-msec pause. A signal was presented on every trial, and it was equally likely to be in the first or the second observation interval. The

subjects' task was to indicate which observation interval contained the signal.

B. Results

The da.ta are shown in Fig. 5 and Table II (Ref. 4). The entries in Table II are means of the MLD's of the

four subjects. As before, the burst-noise MLD condition is referred to the burst non-MLD condition and the continuous-noise MLD condition is referred to the

continuous non-MLD condition. The ogives in Fig. 5

4 In order to make the xms-•o and the 2AFC data comparable in Fig. 5, the 2AFC data were treated in the following way. The proportions p (one[I) and p (one llI) were computed for each block and these were used to obtain a d' from the table (Ref. 2). This d' was transformed into a/' (C)m, and for each subject the/' (C)m'S were averaged for each signal level. This mean/' (C)m was trans- formed back into a d', which was divided by the V2, since d'2xF½ --V2d'•:s. [-See J.P. Egan and F. R. Clarke, "Psychophysics and Signal Detection," in Experimental Methods and Instrumentation in Psychology, J. B. Sidowski, Ed. (McGraw-Hill Book Co., New York, 1966), Chap. 5, pp. 211-246.-], This corrected d' was trans- formed into a /' (C)m, which was used as the measure of per- formance for that subject for that signal level. When d' could not be computed for a particular block, /' (C)m was taken to be the number of correct decisions divided by the number of trials on that block. The only reason for transforming the 2AFC data in this way was to make these data and the x•.s-•o data comparable in Fig. 5. The entries in Table II would be the same without this transformation.

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Page 5: Masking-Level Differences with Continuous and with Burst Masking Noise

D. McFADDEN

TABLE II. Mean MLD's on N0-S•- for the four subjects of Expt. 3. The reference condition ior the continuous-noise MLD's was continuous NO-SO, and the reference condition for the burst-noise MLD's was burst NO-SO. For the YEs-so method, performance on burst NO-SO was about 0.8 dB worse than performance on continuous NO-SO. For the 2AFC method, per- formance on burst NO-SO was about 0.2 dB worse than per- formance on continuous NO-SO.

M e thod Con tinuous Burst Difference (dB) (dB) (dB)

xms-so 15.5 12.1 3.4 2AFC 15.4 13.9 1.5

are all k-1.0. All the NO-SO conditions are within 0.5 dB of the single curve through them. The 2AFC and the Y•.s-so data for continuous N0-S•r are also quite similar. The two burst N0-S•r conditions, however, are rather discrepant, there being about a 2-dB difference in the MLD's. A comparison of Tables I and II and Fig. 4 shows that the Y•.s-so MLD for this crew (12.1 dB) was more like that obtained with the crew in Expt. 2 (12.25 dB) than that obtained with the crew in Expt. 1 (10.2 dB).

V. DISCUSSION

The results of the three experiments are briefly summarized. Detection performance in the MLD conditions was degraded considerably when the noise and the signal were gated simultaneously. The decrease in the MLD was between 4 and 6 dB. There was some

indication that the N• conditions were affected slightly more than the NO conditions. Detection in the non-

MLD condition was essentially the same with burst as with continuous noise. The performance o/the sub- jects in Expts. 1 and 3 averaged about 0.4 dB worse with burst NO-SO than with continuous NO-SO. The

performance of the subjects in Expt. 2 was slightly worse (0.25 dB) with continuous NO-SO. When the noise was gated 600 msec prior to signal onset, the MLD was approximately equal to the MLD obtained with continuous noise. The difference between the MLD's with burst and with continuous noise was about 2 dB smaller with a 2AFC method than with a x•.s-so

method.

In the delay-line, vector model of Jeffress, 5 the basis for detection in the MLD conditions is assumed to be

the phase angle 0 between the signal-plus-noise in one ear and the signal-plus-noise in the other ear. This interaural phase shift is constantly changing; and its rate of change is a function of the effective bandwidth

• L. A. Jeffress, "Binaural Signal Detection: Vector Theory," Defense Res. Lab. Acoust. Rept. No. 245, Univ. Tex. (1965).

of the noise, the center frequency of the effective band, and the length of the signal vector. Neurally, a particular 0 corresponds to a point in the dday line where impulses coming from the two ears coincide. There are assumed to be some small fluctuations in the inter-

aural phase shift even when there is no signal added to the noise. The addition of a signal tends to move the point of coincidence away from the locus of neural ac- tivity associated with noise-alone. Large movements are caused by large O's, and these are presumed to be easily detectable, small O's being discriminated from the noise- alone activity only with difficulty.

The burst-noise effect may be explained in terms of the Jeffress model. There is no reason to assume that the distribution of O's obtained during the observation intervals would differ in any way in the continuous and the burst conditions. But at the onset of an observation

interval there is a difference in the two conditions.

Unlike the continuous-noise conditions, there are no O's prior to onset in the burst-noise conditions because there is no noise present. On an SN trial in the burst- noise conditions, the signal and masker are added prior to onset so that, at onset, a value of 0 simply appears somewhere in the delay line, and the subject's task is to decide which of the two events gave rise to that value of 0. In the continuous-noise conditions, however, O's associated with noise-alone occur both prior to and subsequent to the signal. When a signal is added, there is a tendency for the point of coincidence to move away from the center of activity associated with noise-alone, and it is this movement at onset in the continuous-noise

conditions which is absent in the burst conditions. The

advantage of continuous noise, then, may simply be that, due to the presence of noise-alone prior to and subsequent to the observation interval, the subject's "memory" for the characteristics of the noise-alone activity is better than it is in the burst conditions. This better "memory" would aid performance by making values of 0 occurring on SN trials more easily dis- criminable from similar O's arising from noise-alone. The results of Expt. 2 indicate that about 600 msec of noise prior to the observation interval makes the "memory" about as good as it is with continuous noise.

The discrepancy between the results with the 2AFC and the x•.s-so methods may also be attributed to a better "memory." In 2AFC the subject receives a noise- alone burst on every trial--twice as many per block as he receives in the x•.s-so method--and perhaps this helps maintain his "memory" for the noise-alone activity enough to lead to the larger MLD.

An implication of this explanation of the burst-noise effect is that continuous noise should be of more benefit in the NO noise conditions than in the N,r conditions. This is because the noise-alone activity associated with NO fluctuates around the point in the delay line cor-

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Page 6: Masking-Level Differences with Continuous and with Burst Masking Noise

MLD'S: CONTINUOUS AND WITH BURST NOISE

responding to the median plane, and the Jeffress model assumes that the density of neural elements is greatest near the median plane. Therefore, with continuous NO, the changes in the point of coincidence resulting from the addition of the signal occur where the mechanism is most sensitive. The results of Expt. 1 do show the NO conditions to be slightly less affected by burst noise than the N•r conditions. The difference is not large, but it is in the right direction.

ACKNOWLEDGMENTS

Without the patient guidance of Professor James P. Egan this research would not have been possible. D. E. Robinson and D. S. Emmerich read and criticized

the manuscript. The author is a Public Health Service predoctoral fellow. This research was supported in part by the Air Force Office of Scientific Research, U.S. Air Force. The apparatus was constructed with funds granted by the National Science Foundation.

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