gated signal coming from the recording attenu-
ator and the constant signal from the opposite
channel were both fed to both mixers. For the
monotic intensity condition, the mixed constant
and varying signal was delivered to one earphone,
but no signal went to the other.

Stimulus

Range of Intensity. Although previous studies
specified the intensities at which discriminations
were made in terms of sensation levels*1*?%, the
fact that more parameters were included in the
present study dictated a fixed physical reference
—hearing level (HLL) was chosen. Since the sub-
ject population and the conditions of testing were
similar to those used for the ISO standard, it
was selected as the reference.

Three levels, 20, 35, and 50 dB HL, comprised
the baseline levels of the experiment. Apparent
azimuth and AI the intensity increment) were
calculated from these baseline levels.

Range of Frequency. Three frequencies were
selected to represent the low, middle, and high
portions of the human auditory range: 250, 2000,
and 6000 Hz.

Apparent Azimuth. The DL(?)sn, was ex-
amined for situations in which the reference por-
tion of the duty cycle appeared to be off center
and for situations in which the varying signal
was either more or less intense than the constant
signal, The constant signal was presented at 20,
25, 30, 35, 40, 45, and 50 dB HL. With the vary-
ing signal to one ear at a baseline of 20 dB HL,
the range of the constant signal presented to the
contralateral ear provided differences between the
two signals from 0 to +30 dB. With the vary-
ing signal at a baseline of 35 dB HL, the differ-
encse ranged from —15 to +15 dB, and with
the varying signal at a baseline of 50 dB HL, the
range of differences was from —30 to 0 dB.

Phase. All signals in the dichotic and diotic
intensity conditions were presented at’ both 0° and
180° interaural phase.

Subjects

The subjects were five men between the ages of
18 and 27. None had a history of ear pathology
and all had an air-conduction threshold of 15 dB
HL (ISO) or better between 250 and 8000 Hz.

Procedure

Subjects were instructed to listen for rhythmic
changes in a steady tone. They were told that

the changes might sound like fluctuations in loud-
ness or changes in the position of the sound.
They indicated when they heard the fluctuations
by pressing a switch; they released the switch
when the signal grew steady again. Each subject
received an initial practice period of at least 2
hours and was given a 15-min practice period
before each session.

Air-conduction thresholds were obtained be-
fore each session. If the 15-dB HL criterion was
exceeded, the subject. was rescheduled.

Ninety-six random presentations at one fre-
quency constituted one set of trials. The monotic
trials were presented randomly to the left and
to the right ear. The dichotic trials were pre-
sented randomly with the varying signal in the
left ear and the constant signal in the right, and
with the varying signal in the right ear and the
constant signal in the left. Each set was pre-
sented on two occasions of each frequency, mak-
ing a total of 576 trials per subject.

Response

The subject’s response was observed, via the
tracing of the recording attenuator, until the
tracing was stable for 4 maximum and 4 mini-
mum excursions. This procedure provided ap-
proximately 4600 data points per subject. Stimu-
lation was not continued for more than 3 min
per task.

The arithmetic mean of the 4 maximum and
4 minimum scores provided the average decibel
difference between the gated and steady tones that
made up the varying signal. This number was
then converted to AI in decibels with the help
of the table provided by Tobias.?°

II, Result

Data

A ¢ test was performed on the paired data (left
ear—right ear) for the monotic condition, and
another ¢ test was performed on the paired data
for the dichotic condition. No significant dif-
ferences were found. A ¢ test was used to com-
pare the two complete samples of data for all sub-
jects. No significant differences were found be-
tween the replications. Therefore, the scores pre-
sented in the tables (and the data points in the
graphs) are averaged for ears and for replica-
tions as well as for subjects. Four analyses of
variance were run.



O++++0 Churcher, King, and Davies, 800 Hz,
monaural DL (i) a8 a function of
3.0 sensation evel
Q—-—0 Churcher, King, and Davies, 800 Hz,

2.5

binaural DL (i) os a function of
sensation level

Present study, 250 Hz, monaural OL (i)

o as a function of hearing level
=
2.0 Present study, 250 Hz, binaural OL (i)
z a8 o function of hearing level
~ 1S
a
a

10

20 30 40
LEVEL

50
IN dB

60

Froure 4. Comparison of monaural and binaural DL (i) of present study with monaural
and binaural DL(i) from Churcher, King, and Davies’.

B——8 Gage, 300 Hz,
variable signal at 40 dB HL
e——® Present study, 250 Hz,
voriable signal at 20 4B HL
4.57
°
4.0F 7
235+
z
a 3.0F
2
4
ec
2
z
« _
ir]
fd .
z
ish
1 1 1. 1 A 1 A j
-5 0 +5 *10 +15 20. +25 +30 +35
CONSTANT SIGNAL MINUS VARIABLE SIGNAL
Figure 5. Comparison of low-frequency interaural DL(i) of present study with low-

frequency interaural DL(i) from Gage’

8


Comparison of the Intensity Difference
Limens

Effect of Condition. Table 1 and Fig. 2 show
that the values obtained under dichotic conditions
are larger than those of the monotic condition
except for 6000 Hz at 35 dB HL. The diotic
DL (i) is consistently the smallest. Differences
between conditions are significant (p<.01).

Taste 1, Mean DL(i) values (in dB) for 3 stimulus
conditions at 3 levels and at 3 frequencies

Frequency Condition

(Hz) Level (dB)

Hearing

Dichotic Monotic Diotic

20 2.70 1.80 151

250 35. 1.44 1.28 0,93
50 115 0.88 0.60

20 1.33 1.08 0.95,

2000 35 117 1.06 0.75
50 0.72 0.65 0.53

20 1.95 1.88 114

6000 35, 1.25, 1,82 0.90
50 0.92 0.93 0.60

Effect of Level. The largest scores are ob-
tained at the lowest level (20 dB HI) ; the
smallest are at the highest level (50 dB HL)
(Fig. 2). This tendency of the DL() to de-
crease with an increase in level is significant
(p<.01).

Effect of Frequency. The values in Table 1
and the curves in Fig. 2 show the DL(#) to be
smaller at. 2000 Hz than at 250 and 6000 Hz.
The level of significance of the main effect for
frequency did not support this trend (p<.10).
However, the interaction of condition and fre-
quency suggested an effect related to condition
(p<.05). A separate analysis of the simple main
effects for frequency*t produced the following
levels of significance for each condition: dichotic
(p<.01), monotic (p<.01), and diotie (p<.05).

Phase

Significant phase effects (p<.05) were obtained
for only one group of tasks—the 20 dB baseline
data for apparent azimuth. Predictably, the
analysis of simple effects of phase for each fre-
quency (see the first graph of Fig. 2) shows the
effect to be limited to 250 Hz (p<.01). The
DL(é) iat obtained under a condition of 180° in-
teraural phase relation is smaller than the
DL(Z)int obtained under a 0° interaural phase
condition when the constant signal and varying
signal are presented at the same level. However,

as the constant signal is increased relative to the
varying signal, the DL(¢)int for 180° interaural
phase increases in size until it is equal to the
DL (4) ine for 0° interaural phase, when the con-
stant and varying signals are 30 dB apart.

Apparent Azimuth

Baseline of 20 dB HL. The results for 20-
dB baseline portion of the study are presented
in Table 2; graphic representation of the results
for all 3 baseline levels is found in Fig. 3. As the
intensity of the constant signal is increased rela-
tive to the variable signal, the size of the DL() int
increases. The function becomes asymptotic for
250 Hz when the difference reaches 15 dB. The
method of moving averages produces a rather
smooth curve for the line representing 250 Hz.
However, an inspection of the data points shows
a peak when the difference is 15 dB, followed by
a slight decline. This “ripple” in the data, al-
though less exaggerated, is like the ones reported
by Gage.* The effect is least at 6000 Hz. The
differences for the effect of difference level (ap-
parent azimuth) are significant (p<.01).

Taste 2, Mean DL(i),,; values (in db) for decibel dif-
ferences between the constant and variable (20-db
baseline) intensities for 3 frequencies (250, 2000, and
6000 Hz) and for 2 interaural phase conditions (0°
and 180°)

Difference Phase Frequency
Decibel 250Hz 2000Hz 6000 Hz

6 0° 270 «1881.95
180° 210 «154 2.28
45 0° B23 «1641.98
180° 292 «M9 AL
0° B73 «1.85.07

+
#0 180° 274 «1661.94
0° 423 1862.25

+
% 180° 369 211 2.10
499 0° 415 2082.88
180° 359 «218 = 2.04
o 395 «1.06.81

+
°5 180° 415 1092.56
‘90 0° 407 2.238 2.78
180° 406 208.

Baseline of 35 db HL. The results for the 35-
dB baseline portion of the study are presented in



3.0)

250 Hz

2.5F

2.0F O--O interaural OL (i)

4—-4 Monaural OL (i)
| %—X Binaural OL (i)

rT rr a 50
BASELINE HEARING LEVEL (dB)

OL (i) in dB

20 35 50
BASELINE HEARING LEVEL (dB)

20 35 50
BASELINE HEARING LEVEL (dB)

ROWLAND & TOBIAS
96s

Frevre 2, Comparison of the monaural, binaural, and interaural DL(i) as a function of hearing level and frequency.

4


Table 3. The second graph of Fig. 3 shows that,
as the constant signal is increased from —15 dB
to +15 dB relative to the varying signal, the size
of the DL(é);a: increases most at 250 Hz and
least..at 6000 Hz. Of all the variables, only the
overall effect of difference level is significant
(p<.0l).

Tame 8. Mean DL(#),,, values (in dB) for decibel dif-
ferences between the constant and variable (35-dB
baseline) intensities for 3 frequencies (250, 2000,
and 6000 Hz) and for 2 interaural phase conditions
(0° and 180°)

Difference Phase Frequency

Decibel 250Hz 2000Hz 6000Hz

_ o 1.25 1.02 i

180° 1.80 0.98 1.29

wo 0° 145 0.99 1.25

180° 1.38 1.01 1,22

5 0° 151 1.10 1.26

180° 1.45 108 118

° 0° 144 147 1.25

180° 1.56 1.10 117

‘os 0 1.66 124 128

180° 1.69 120 1.28

0 0 1.86 1.19 182

180° 1.738 1.80 1.27

2 ; 1 .
+15 ° m a i
180° 78 1.41 126

Base line of 50 dB HL. The results for the 50-
dB baseline portion of the study are presented
in Table 4. The data in Table 4 and the third
graph of Fig. 8 (for the 50-dB baseline) show
a very slight change in size of the DL(7) int as
the constant signal increases relative to the vary-
ing signal. This change is not significant. No
significant effects were demonstrated for the other
variables.

IV. Discussion

The major conclusion to be drawn from these
experiments is that intensity discrimination un-
der dichotic conditions differs sufficiently from
discrimination under monotic and diotic condi-
tions to warrant its differentiation as the in-
teraural intensity difference limen. Had the
DL(a)mon and the DL(i)si, Gage’s® and Cho-
cholle’s' earlier descriptions of it as simply a
DL (é) mon in the presence of a contralateral sound
of equal frequency would have seemed more

‘Taste 4, Mean DL(i),,, values (in dB) for decibel dif-
ferences between the constant and variable (50-dB
baseline) intensities for three frequencies (250, 2000,
and 6000 Hz) and for 2 interaural phase conditions
(0° and 180°)

Decibel Frequency

Difference Phase 250He 2000H2 6000 He

90 0 090 (019

180° 095 «0.78004

95 0 080 © 0.76.4

180° 098 = 0.79.94

_99 0 1060.78 (0.89

180° 090 0.78 1.00

- o ost = 082082

180° LoT(7T 86

10 0° 10107881

180° 094 0.78096

5 0° 098 = 0851

180° 104084 1.00

° 0 115072092

180° 106 0830.90

logical. However, the difference between the
DL (4) ine and the DL(#)mon at 250 Hz, and their
eventual equivalence at 6000 Hz, indicate an in-
dependent nature.

The differential response to frequency is what
might be expected of a system that need not de-
pend upon intensity differences alone for locali-
zation cues at lower frequencies,

Although the previous reports of Churcher,
King, and Davies? and of Upton and Holway** #*
show that intensity discrimination under diotic
conditions is superior to discrimination under
monotic conditions, our finding that this differ-
ence is significant is worth noting. Figure 4 pro-
vides a comparison of the present study’s 250-Hz
data with the data for 800 Hz from Churcher,
King, and Davies.

The prediction that the DL(?)in: would de-
crease as overall level was increased is clearly
demonstrated by the data. The predicted “level-
ing off” effect of the increasing intensity is less
definite, but the tendency is there,

The predicted increases in the DL(¢) int with
relative increases in the constant signal were veri-
fied. Our data are similar to Gage’s* and Cho-



AM 67-10

INTERAURAL INTENSITY DIFFERENCE LIMEN

Roy C. Rowland, Jr., Ph.D.
and
Jerry V. Tobias, Ph.D.

Approved by Released by
~ oe
Bilt Pte (USiage€ m0
J. R. Diniz, M.D. P. V. Srecen, M.D.
Cuter, Crvi, AEROMEDICAL Feprrat Arr Surcron
Instrrute
May 1967

FEDERAL AVIATION ADMINISTRATION
Office of Aviation Medicine



=

10.

uu.

12.

1B.

REFERENCES

. CHocnotte, R, La sensibilite auditive differentielle

@intensite en presence d’un son contralateral de
meme frequence. Acustica, 7, 75-83 (1957).

. CHURCHER, B. G., Kine, A. J., and Davies, H, The

minimum perceptible change of intensity of a pure
tone. Lond. Edinb. Dubl. Phil. Mag., 18, ser. 7, 927-
989 (1934).

Dn«atick, F. L, and Orson, Rura M. The intensive
difference limen in audition, J. acoust. Soc. Am., 12,
517-525 (1941).

. Foro, A. Dynamic auditory localization. J. aooust.

Soe, Am., 13, 367-872 (1942),

. Gace, F, H. The variation of the uniaural differ-

ential threshold with simultaneous stimulation of the
other ear by tones of the same frequency. Br. J.
Psych., 25, 458-464 (1935).

Harris, J. D, Loudness discrimination. J. Speech
Hear. Dis, Monogr. Suppl. No. 11 (1963).

. Kiumep, R. G. and Hapy, H. R. Some measure-

ments of interaural time difference thresholds. J.
acoust, Soc. Am., 28, 859-860 (1956).

. Knupsen, V. O. The sensibility of the ear to small

differences in intensity and frequency,
21, ser. 2, 84-102 (1928),

Phys. Rev,

. Licker, J. ©. R., Wesster, J. C., and Hevtun, J. M.

On the frequency limits of binaural beats. J. acoust.
Soc, Am., 22, 468-47 (1950).

Luscuer, E. and Zwistockr, J. Comparison of var-
ious methods employed in determination of reeruit-
ment phenomenon, J. Lar. Otol., 65, 187-195 (1951).
Mis, A. W. On the minimum audible angle. J.
acoust. Soc. Am., 30, 237-246 (1958).

Mitts, A. W. Lateralization of high-frequency tones.
J. acoust, Soc. Am., 32, 132-134 (1960).
Monroomery, H. C. Influence of experimental tech-
nique on the measurement of differential intensity
sensitivity of the ear. J. acoust. Soc. Am., 7, 30-43
(1985).

10

14, Riesz, R. R. Differential intensity sensitivity of the
ear for pure tones, Phys. Rev., 31, ser. 2, 867-875
(1928).

Srencer, E. Simulation und dissimulation von ohr
krankheiten und deren festellung. Dt, med. Wschr.,
83, 970-973 (1907).

Stevens, S. S. and Newman, E. B. The localization
of actual sources of sound. Am. J. Psychol., 48, 297-
306 (1936).

Srewart, G. W. The function of intensity and phase
in the binaural location of pure tones. I. Part I.
Intensity, Phys. Rev., 15, ser. 2, 425-482 (1920a).
Srewart, G. W. The function of intensity and phase
in the binaural location of pure tones. I. Part II.
Phase, Phys, Rev., 15, ser. 2, 482-445 (1920b).
Taytor, G, J, An experimental study of tests for
the detection of auditory malingering. J. Speech
Hear, Disorders, 14, 119-130 (1949).

‘Tostas, J. V, A table of intensity increments. Of.
Aviat, Med, Rep., No. AM 66-4 (1966).

15.

16.

VW.

18.

19.

20.

21, Upton, M. Differential sensitivity in sound localiza-
tion, Proc. natn, Acad. Sci. U.S.A. 22, 409-412
(1936).

22. Upron, M. and Hotway, A. H. On the psychophysics
of hearing. I, Monaural differential sensitivity and
exposure-time. Proc. natn. Acad. Sci. U.S.A., 28, 29-
32 (1937a).

Upron, M. and Horway, A. H, On the psychophysics
of hearing. II, Binaural differential sensitivity and
exposure-time, Proc, natn, Acad. Sci. U.S.A., 23, 32-
34 (1937b).

Winer, B. J. Statistical Principles in Experimental
Design. New York: McGraw-Hill (1962).
Zwistocki, J. and FexpMan, R. S. Just noticeable
differences in dichotic phase. J. acoust. Soc. Am., 28,
860-864 (1956).

23.

25,


cholle’s' (Figs. 5 and 6), even including Gage’s zation task such as the DL(é)int than on a non-
“ripple” at the low frequency. localization binaural task such as the DL(é)vin

Phase effects were not found at the two higher _is reflected in the data. The decrease in the
frequencies, just as was predicted. The assump- DL(é)in for the 180° interaural phase relation
tion that phase might have more effect on a locali- _is the reverse of the prediction.

@—-—B8 Chocholle, 1000 Hz,
variable signal at 10 dB
sensation level

o——@ Present study, 2000 Hz,
variable signal ot 20 dB

hearing level

2.57
a
a]
<
a Lec —--8
a
a
4
5 0p -- 7
20°
«
Wy)
tod
205+

1 1 1 n 1 1 j

1
{e) +5 +10 +15 +20 +25 +30 +35 +40
CONSTANT SIGNAL MINUS VARIABLE SIGNAL

Ficure 6. Comparison of mid-frequency interaural DL(i) of present study with interaural DL(i) from Chocholle*.



CONSTANT MINUS VARIABLE HL

oO. +5 +10 +! +20 +25 +30

40)

3.5)

3.0

25

a

WTERAURAL OL in dB
iJ
o

o
c=}

i]

BASELINE HL-2008 par} oe

20 25 30 35 40 45 30
HEARING LEVEL of CONSTANT TONE (dB)

CONSTANT MINUS VARIABLE HL
-10 =5 oO +5 +10.

=15 +15
BASELINE HL=3508 oe
0"
pa On — 4
e oe s
an

‘ 1 ste 1 n

20. 2 30 35 40 #45 50
HEARING LEVEL of CONSTANT TONE (dB)

CONSTANT MINUS VARIABLE HL
=20, 15. bal’) 5

BASELINE HL*50d8

eG —— Banas fecen=

— ‘-

20 25 30 35 40 45 50
HEARING LEVEL of CONSTANT TONE (da)

ROWLAND & TOBIAS
1965

Ficvre 3. The interaural (DL (i) as a function of phase, frequency, and apparent azimuth.

6



Phase

> Attenuator} Reversing

Switch
Oscillator
Phase Electronic Recording
> shitter Switch [attenuates > }>J Attenuator +> Attenuator
Interval
Timer

Fioure 1, Simplified block diagram of apparatus.

In 1935, Gage® reported the effect on the
DL(¢) mon of simultaneously introducing a tone of
the same frequency at a different level to the op-
posite ear. Such a condition satisfies our defini-
tion of an apparent-azimuth study of the
DL(z)int. Using a tone with varying intensity
in one ear and the same tone but with constant
intensity in the other, Gage found that the
DL(‘)int is larger when the constant-intensity
signal is greater than the varying-intensity signal
and smaller when the constant-intensity signal is
smaller. In a similar study, Chochollet reported
an increase in the size of the DL(4)int with a
concurrent increase in the constant signal. Data
from the present study were expected to show
similar trends.

II. Method

Instrumentation

Figure 1 presents a simplified block diagram
of the apparatus used. Three intensity condi-
tions of stimulation—dichotic, diotic, and monotic
—were provided.

Each of three test frequencies—250, 2000, and
6000 Hz—was produced by a Hewlett-Packard
201-CR audio oscillator. The tone from the oscil-
lator was split into two channels. One channel
was split and fed through two mixers: one part
of this split signal was combined with the signal
in the opposite channel to become the varying
signal and one part remained as the constant
signal. This constant signal went next to a Hew-
lett-Packard 350-D attenuator set, then to a
phase-reversing switch, then to one of a matched

pair of Telephonics TDH-39 earphones with
NAF 48490-1 cushions. In the other channel, the
tone was first fed to a Grason-Stadler E3520B
phase shifter, which compensated for unwanted
shifts in relative phase between the constant and
varying signals. The tone proceeded to a Grason-
Stadler electronic switch, model E3382B, which
was activated by a Grason-Stadler 471-1 interval
timer. The electronic switch was set for a rise-
decay time of approximately 50 msec, and the
interval times was set to gate the signal in a 0.5
sec on, 0.5 sec off cycle. The gated signal was
fed to a Grason-Stadler recording attenuator,
E4554A, with a rate-of-change of 1 dB/sec, then
to a mixer where it was combined with the tone
split from the opposite channel. Alternate half-
seconds, the varying signal from the mixer was
equal to the constant signal in the opposite chan-
nel in frequency, phase, and intensity; then for
the other 0.5 sec, the two signals differed in in-
tensity. The varying signal proceeded to a Hew-
lett-Packard 350-D attenuator. This attenuator
and the 350-D attenuator in the constant-signal
channel controlled the overall signal levels. A
Daven T-326-F attenuator with a 2.0 dB range
in 0.1 dB steps was placed between the 350-D
attenuator and the remaining TDH-89 earphone
to correct for differences in acoustic output be-
tween the channels. The earphones were matched
for phase with a special coupler: the actual
acoustic phase difference was less than 2° for
any of the test frequencies.

For the diotic intensity condition, with the
varying signal delivered to both earphones, the


INTERAURAL INTENSITY DIFFERENCE LIMEN*

I. The Problem

This study was designed to examine intensity
discrimination under dichotic intensity condi-
tions. The measure may be termed the inter-
aural intensity difference limen (DL (i) int). The
DL(¢)int can be interpreted in two ways—as
differential sensitivity and as sound-localization
discrimination. As a phenomenon of differential
sensitivity, it was investigated as a function of
frequency and level. To determine the relation
of the dichotic DL(2) int to other measures of in-
tensity discrimination, it was compared with
monotic (signal to one ear) and diotic (identical
signal to both ears) conditions, which yield the
monaural intensity difference limen (DL (i) mon)
and the binaural intensity difference limen
(DL(‘)in) respectively. As a phenomenon of
sound localization, the DIL(¢) int was studied as a
function of two additional variables—interaural
phase and apparent azimuth.

The DL(2)int has been studied variously but
incompletely. In 1936, Upton! examined it as a
function of level at one frequency. Later, Mills*
looked at it as a function of frequency at one
level. It has been studied as a function of ap-
parent azimuth at low- and mid-range frequen-
cies.»* But no investigation has systematized
the DL(i)int as a function of level, frequency,
and apparent azimuth. Also, the diversity of re-
sults and methods found in previous DL/(i)
studies®**1014 suggests the need for an empirical
comparison of the DL(i)moa, DL(é)in, and DL
(iin using an equivalent method. Finally, for
completeness, an interaural phase relation of 180°
was included to determine whether phase affects
intensity discrimination as it does the perceived
image.

Predicted Results

Effect of Level. The DL(2)mon is small and
stable at levels well above threshold, but at levels

*Dr. Roy C. Rowland is the Supervisor of the Speech and
Hearing Unit of the Oklahoma Department of Public Welfare;
Dr, Jerry V. Tobias is the Chief of Communication Processes
Research, Psychology Laboratory, Civil Aeromedical Institute.

slightly above threshold it is much larger®**, It
was expected that the DL(i)in. would behave
similarly.

Effect of Frequency. In sound-field localiza-
tion studies, (1) phase (or time) cues dominate
the low-frequency range, (2) localization ability
is poor in the middle range, and (3) actual in-
tensity differences are large in the high-freqency
rangeTM*, None of these findings suggests a
strong basis for assuming any large variation in
intensity discrimination throughout the range.
However, Ford’s‘ report of better localization at
2000 Hz than at 200 Hz suggests that the effect of
frequency on the DL(é)in: might resemble the
effects reported by Riesz't for the DL(¢) mon.
Thus it was suspected that the DL(¢) ia. might
be smaller in the mid-range than at lower or
higher frequencies.

Effect of Phase. Since usable temporal infor-
mation from pure-tone signals is restricted to fre-
quencies below 1500 Hz%?TM1825, it was predicted
that no phase effect would be demonstrated above
that frequency. Also, since the dichotic intensity
condition in the present study can be assumed to
measure a minimum shift in localization from a
centered sound image, the dichotic phase condi-
tion could increase the size of the DL(Z)int at
low frequencies.

Apparent Azmuth. Stenger", suggesting a test
to detect simulated unilateral hearing loss, re-
ported that when tones from two matched tun-
ing forks are presented to a subject with one tone
louder (nearer its ear) than the other, the sub-
ject does not perceive the softer (farther) fork
at all. Therefore, a tone simultaneously pre-
sented to both ears suggests a perceived or ap-
parent azimuth related to the intensity difference
between ears. Investigating the Stenger effect
as a test of malingering, Taylor’? verified. the
laterization effect; but the data suggest that, if
the intensity of the tone is gradually changed at
one ear, sophisticated subjects are able to use in-
tensity localization cues to simulate deafness suc-
cessfully despite the test.



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