Psychological and physiological study of auditory maskin

Coats, A. C.

Kellaway, P.

English

NASA Technical Reports Server

AUDITORY MASKING - A STUDY O F  ITS PHYSIOLOGICAL 
MECHANISM AND OF CORRELATIONS BETWEEN PHYSIOLOGICAL 
AND PSYCHOLOGICAL OBSERVATIONS 
Re search Grant #NSG -3 90 
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Progress Report, First Quarter, Y e a r  Ended April 1, 1966 
Principal Investigator: Peter Kellaway, A. M. , Ph. D. 
Professor of Physiology 
Baylor University College of Medicine 
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Associate Investigator: A. C. Coats, M. S. , M.D. 
Assistant Professor of Physiology 
Baylor University College of Medicine 
Address of University:  Baylor University College af Medicine 
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INAS-R O R  TMX O R  AD N U M B E R )  1 '  
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1200 Moursund Avenue 
Houston, Texas 77025 
I T H R U I  
Hard copy (HC) *P 
Microfiche (MF) /&3 
ff 653 July 65 
https://ntrs.nasa.gov/search.jsp?R=19660006290 2020-03-16T21:42:51+00:00Z
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19 
Introduction 
/s579 
Work during the last quarter has consisted primarily of a continuation of 
the psychological study of auditory masking which was begun during the latter 
part of the previous year. 
been accumulated: 1) The time course of recovery of masked click threshold 
following cutoff of the masking stimulus, 
and 3) The effect of masking intensity, 
Psychological data in the following three areas  has 
2) The effect of masking duration, 
Results in these three areas  and the ob- 
served similarities and differences with comparable physiological observations 
a re  discussed in this report. Also, control experiments to determine the effect 
of a "warning flash" on the recovering click threshold a re  discussed. 
In addition, some further physiological studies have been done. These are: 
1) Further study of masked and unmasked click input-output curves, and 2 )  A 
study of the effect of masking intensity utilizing white noise instead of pure 
tones, as was usually done in previous studies. The results of these physiolo- 
gical experiments a re  also discussed in this report. 
Psychological Studies of Auditory Masking 
I. The Effect of a "Warnine Flash" Delivered Immediately Prior to  the Recover- 
ing Test Click 
Very soon after the psychological studies were begun, it became apparent 
that something other than the masking stimulus affected the masking-recovery 
curve. For lack of a better term, we have termed this the "expectancy factortt,  
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In essence, the expectancy factor is based on the following: If it is possible to 
predict exactly when the test click will arrive, the threshold of the test click 
will be lower than if the test click arrives at an "Unexpected" time. A s  outlined 
in previous reports, "psychological" masking-recovery curves a re  obtained by 
deliverin'g a single test click after masking cutoff and tracking its threshold with 
a recording attenuator. 
a r e  obtained. 
stimulus as his ''cue" to time the arrival of the test click. This is done by mak- 
ing a judgment of the time interval separating the masking stimulus (actually the 
cutoff of the masking stimulus) and the test click. 
Test click thresholds at several post-rr~asking ixitervals 
We have found that in this situation the subject uses the masking 
This interval judgment is 
lrsetlt with the first few pairs of stimuli, and on subsequent pairs is used to pre- 
dict the time of arrival of the test click, and thereby to lower test click threshold. 
Since, however, long intervals a r e  more difficult to judge accurately than short 
intervals, the "expectancy factor" operates to increase test -click threshold at 
longer post-masking intervals. Thus, because of the "expectancy factor", 
masking-recovery curves which were obtained early in the course of this study 
showed a distinct tendency to  It sag1( at longer post-masking intervals. 
In order to hold the "expectancy factor" constant, a "warning flash" has 
been placed into the stimulus sequence. 
200 msec prior to the test click. 
This consists of a light flash delivered 
The 200-msec interval remains constant 
throughout the experimental run, and the interval between masking cutoff and 
the flash-click pair is varied. The subject is instructed to ignore the masking 
stimulus (as best he can) and to use the flash to determine the exact time of 
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arrival of the test click. 
W e  have completed several control experiments in which masking-recovery 
curves have been obtained with and without the warning flash. 
results of these experiments: 
Following a re  the 
1) No qualitative alteration in  the results so far obtained (i. e. the masking- 
duration and masking-intensity studies de scribed below) is preckced ?q the -warn- 
ing flash. 
2) However, the following quantitative alterations in the results obtained 
were consistently observed: 
a) The variability of recovering click threshold (i. e. the excursion 
of the attenuator pen when click threshold is being traced) is significantly re- 
duced - particularly at long cutoff -test click intervals. 
b) The tendency for click threshold to rise at long cutoff-test click 
intervals was completely eliminated. 
These control experiments have led us to the conclusion that the addition 
of the "warning flash" to  the masking stimulus-test click sequence significantly 
increases the quality of the data obtained. 
adopted as a standard part of our experimental procedure. 
The warning flash has therefore been 
11. Time Course of Recovery from Masking of Click Threshold 
Typical human masking-recovery curves a re  shown in Figs. 1, 3, and 
4. 
rithmic increment" form that the animal recovery curves show. 
It is apparent that the human masking-recovery curves show the same "loga- 
An occasional 
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curve shows some irregularity, usually occurring very shortly after cutoff of 
the masking stimulus (see Fig. 4). However, such irregularities did not appear 
consistently, and the recovery time course is therefore interpreted as essen- 
curves obtained from animals, we have not as yet excluded the possibility that 
a significant quantitative difference exists between the two. For example, it 
appears that psychological masking recovery may be significantly faster than 
pny siological recovery. 
111. The Effect of Duration of the Masking Stimulus 
The effect of masking duration in the psychological experimental situation 
appears to be very similar to its effect in the physiological situation. We have 
previously reported that increasing masking duration has no effect on masked 
click action-potential amplitude, but slows recovery. Thus, A. P. masking- 
recovery curves following short- and long-duration masking start from the 
same point (i. e. the masked click A. P. amplitude), diverge in the early phases 
of recovery, then reconverge as recovery nears  completion. A s  illustrated by 
Fig. 1, recovery of human click threshold following long- and short-duration 
masking shows an identical pattern. 
Also, a psychological masking-duration effect identical with the previously 
de scribed physiological masking-duration effect was consistently observed. An 
example is shown in Fig. 2. 
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SUBJECT: A.C. 
66 MASKING OURATiO N 
0.2 SEC 
#.- - ---. 4.5 SEC #e- 
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34- 0 IO0 260 300 4bO 500 660 760 860 900 IO00 
MSEC AFTER COMPLETE MASKING CUTOFF 
Fig. 1. Effect of masking duration on the time course of recovery from 
masking. 
ring at different post-masking intervals. 
tained with a recording attenuator. 
driving a PDR-600 earspeaker with a square-wave pulse 60 V in amplitude and 
0.01 msec in duration. 
uator. 
fall time was 5 msec. 
These curves represent plots of the thresholds of single clicks occur- 
The masked (click) stimuli were produced by 
The single click thresholds were ob- 
This driving pulse was led through the recording atten- 
The masked stimulus was a 2,500 cps pure tone of 75 db intensity. Rise- 
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SUBJECT A. C. 
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MASKING TONE DURATION (SEC) 
Fig. 2. Psychological masking-duration effect. To obtain this plot, the 
masking cutoff -test click interval was held constant (100 msec) and the duration 
of the masking stimulus was varied. 
click is plotted against the duration of the masking stimulus. 
produced by driving a PDR-600 earspeaker with an 0.01 msec, 60 V square- 
wave pulse. Masking stimulus: 70 db, 2,000 cps tone with rise-fall time of 
5 msec. 
The threshold of the test ("recovering") 
The test click was 
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IV. The Effect of Intensity of the Masking Stimulus 
As previously reported, physiological studies have shown that increasing 
masking intensity lowers the point from which the masking-recovery curve 
starts (i. e. reduces the amplitude of the masked action potential). However, 
increasing masking intensity does not alter the time course of the early phase 
of masking recovery. Thus, previously obtained physiological dala show that 
the early part of the recovery curve following the more intense of two masking 
s t inul i  will lie beneath the recovery curve following less  intense masking by a 
constant amount. As illustrated by Figs. 3 and 4, the same holds true for psy- 
chological data. 
Physiological Studies of Auditory Masking 
I. Further Studies of Masked and Recovering Click Input-Output Curves 
In previous progress reports, the results of masked (or "recovering") and 
unmasked (or "control") click input -output experiments have been outlined. These 
experiments showed that if masking intensity is less  than about 60 db, the dif- 
ference between the masked and unmasked input-output curves is constant above 
a certain click intensity. 
and unmasked input-output curves is plotted, this "difference curve1' will show a 
"plateau" above a certain click intensity. 
70 db, however, the "difference curve" shows no such "plateau". 
In other words, if the difference between the masked 
At masking intensities above about 
All of the above observations were made with pure-tone masking. In the 
last quarter, these observations have been repeated with white -noise masking. 
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64: 
20 60 loo 200 300 400 
MSEC AFTER COMPLETE MASKING CUTOFF 
Fig. 3. The effect of masking intensity on the recovery of masked click 
Test clicks were produced by driving a PDR-600 earspeaker A. P. threshold. 
with square-wave pulses 0.1 msec in duration and 75 volts in amplitude. 
square waves were led through the recording attenuator. 
was a 1.7 sec burst of white noise with rise-fall time of 5 msec. 
The 
The masking stimulus 
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SUBJECT: B.N. 
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6 4,4 
0 100 200 300 400 
MSEC AFTER COMPLETE MASKING CUTOFF 
Fip. 4. 
masked click A. P. threshold. 
as for Fig. 3. 
ent subject. 
The effect of masking intensity on the recovery of 
However, these curves were obtained from a differ- 
All stimulus parameters are the same 
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Tne results of one such experiment a re  shown in Fig. 5. In this figure a re  
plotted the unmasked click input-output curve and difference curves for a series 
of masking intensities. 
steps. 
As can be seen, masking intensity was varied by 5 - d ~  
The following a re  apparent from an examination of Fig. 5:  
1) " P l a t e ~ u s "  CZ"Y be seer; in the d i f f e r ~ i ~ e  C U T V ~ S  obtained with masking 
intensities of 40, 3 5 ,  and 30 db attenuation. Thus, a "plateau" in the masked- 
unmasked difference curve is obtained with white-noise as well a s  with pure-tone 
masking. 
2 )  The transition between "plateau" and "no plateau" is not sharp, and, in 
fact, one might interpret the curves at 5 and 0 db as showing a slight plateau 
(though beginning at a much higher click intensity). 
3 )  The masking intensity at which a distinct "difference plateaut1 ceases 
t o  be readily apparent is in the range of 26-36  db (30-40 db attenuation from r e -  
ference intensity of 66 db). 
tensity" observed with pure -tone masking. 
This contrasts sharply with the 70 db "transition in- 
These observations a re  in agreement with the generalization, made from 
previous observations, that the only difference between pure -tone and white - 
noise masking (where clicks a re  the masked stimuli) is that white noise is a much 
more effective masking stimulus than is a pure tone. 
ence between white -noise and pure -tone masking is quantitative and not qwlita- 
tive. 
In other words, the differ- 
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UNMASKED 
INPUT-OUTPUT 
CURVE 
DIFFERENCE CURVES ,p 20 *..-..-*-- 
....... ............... 
............. ..--- ..... ....... ......".... ..... 4q/---< 
_*_-._.-. ---.-.-- 
/---------.-- 
WHITE NOISE INTENSITY 
(db ATTENUATION) 
60 55 5 0  45 40 35 30 25 20 
CLICK INTENSITY (db ATTENUATION) 
Click input-output "difference curves'' at different masking inten- - Fig. 5. 
sities. Each difference curve was constructed by subtracting the appropriate re - 
covering input-output curve (not shown) from the unmasked input-output curve. 
The test click stimuli were produced by driving a PDR-600 earspeaker with an 
0.01 msec, 40 V square wave. 
uator. 
off with a 5-msec rise-fall time. 
uation was 66 db. 
was complete. 
The driving pulse was led through a decade atten- 
The masking stimulus was a 1.25 sec burst of white noise turned on and 
The intensity of the white noise at 0 db atten- 
The test clicks were delivered 5 msec after masking cutoff 
1 7 
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11. The Effect of Masking Intensity on Recovering A. P. Amplitude 
In Fig. 6 is shown a plot of recovering action-potential amplitude versus 
masking intensity. The same data plotted in Fig. 5 were used to obtain these 
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'1 plots. 
In Fig. 7 is shown a comparable plot where a pure tone rather than white 
noise was the masking stimulus. We have suggested that the %nee1' in the re- 
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a transition zone in this region. The white-noise intensity curves (Fig. 6) also 
show a possible llkneel' in the region of 50-60 db. However, as is readily appar- 
ent, the form of the curves above and below the "knee" is exactly opposite the 
form above and below the "knee1' when pure tones a re  used as the masking sti- 
muli. With pure tones, the rate of decrease of masked A. P. amplitude above 
the knee is clearly greater than the rate of decrease below the knee. With white 
noise, exactly the opposite is true. 
Thus, when the effect of masking intensity on recovering A. P. amplitude 
is studied, we find a qualitative difference between white -noise and pure-tone 
masking, and the previously suggested generalization that only quantitative dif- 
ferences between these two stimulus types exist appears not to  hold in this in- 
stance. 
Future exploration of this apparent difference is planned. Of great impor- 
tance will be experiments in which masking intensity-masked A. P. amplitude 
curves a re  obtained with pure tone and white noise from the same preparation. 
Also of considerable importance will be an extension of the white noise 
intensity range studied above that shown in Fig. 6.  
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MASKING (WHITE NOISE) 
30 25 20 
INTENSITY 
15 , I O  5 0 
(db ATTENUATION) 
- 
Fig. 6. Recovering action-potential amplitude versus masking 
intensity. A replot of the same data presented in Fig. 5. 
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./*-. POST-STI MU LATORY :2=:-- - : - -.- - - -- -- :,,,e-.- 
'-'-'%. A.P. AMPLITUDE 
\*e- - - -.-. -.-..---.--,. 
--*---.- - _ . _ _ _ *  \:\ .- --._ '-. ---.-- -.--_._ '('-%. 5th click ---. \ -. -- --.-.-. 
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PER -STI MU LATO R< %.,, 
A. P. AMPLITUDE ' 
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Fig, 7. Masked ("per -stimulatory") and recovering ("post - stimulatory") 
action-potential amplitude versus masking intensity. 
a pure tone. 
where white noise was the masking stimulus. The test clicks were produced by 
driving a PDR-600 earspeaker with a train of 0.15 V square-wave pulses, 0.01 
msec in duration. 
tone, 0.3 sec in duration. 
The masking stimulus was 
Compare.the knee at 20 db attenuation with the %nee'' in Fig. 6 
Click rate was 20/sec. Masking stimulus was a 2,500 cps 
Intensity at 0 db attenuation was 82. 5 db. 
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It is possible that these planned experiments will lead to a further elucida- 
tion of the nature of the "transition zone" found in the 50-60 db intensity range. 
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Auditory masking - A study of its physiological mechanism and of correlations between physiological and psychological observations  First quarterly progress report, period ending 30 Jun. 1965

Auditory masking - A study of its physiological mechanism and of correlations between physiological and psychological observations First quarterly progress report, period ending 30 Jun. 1965

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