Auditory masking experiments in schizophrenia

Abstract

Twelve schizophrenic subjects with acoustic hallucinations in their case histories were compared with 12 healthy reference subjects and eight subjects with panic disorder in a test of three auditory masking tasks, simultaneous masking (SM), forward masking (FM) and backward masking (BM). The schizophrenic subjects showed no differences from reference subjects on SM but had higher thresholds for the two other conditions (FM and BM). Schizophrenics with very increased thresholds (n=6) had a significantly higher need for residential treatment. Thresholds for SM and BM were not, as for reference subjects, related to age for schizophrenics. No statistically significant differences regarding any masking experiments were found between the panic disorder subjects and the reference subjects. Simultaneous masking, reflecting functions of the basilar membrane and those of elementary brainstem processing, showed no signs of dysfunction in schizophrenic subjects. Schizophrenics showed aberrations in FM and BM, possibly influenced by more central (cortical) processes.

Full text

Psychiatry Research 113 (2002)115–125
0165-1781/02/$ - see front matter 䊚2002 Elsevier Science Ireland Ltd. All rights reserved.
PII: S0165-1781Ž02.00248-2
Auditory masking experiments in schizophrenia
Johan Kallstrand, Peter Montnemery, Soren Nielzen, Olle Olsson*
¨´¨´
Clinical Psychoacoustic Laboratory, Division of Psychiatry, Department of Clinical Neuroscience, University Hospital,
SE-221 85 Lund, Sweden
Received 5 June 2001; received in revised form 11 November 2001; accepted 2 June 2002
Abstract
Twelve schizophrenic subjects with acoustic hallucinations in their case histories were compared with 12 healthy
reference subjects and eight subjects with panic disorder in a test of three auditory masking tasks, simultaneous
masking (SM), forward masking (FM)and backward masking (BM). The schizophrenic subjects showed no
differences from reference subjects on SM but had higher thresholds for the two other conditions (FM and BM).
Schizophrenics with very increased thresholds (ns6)had a significantly higher need for residential treatment.
Thresholds for SM and BM were not, as for reference subjects, related to age for schizophrenics. No statistically
significant differences regarding any masking experiments were found between the panic disorder subjects and the
reference subjects. Simultaneous masking, reflecting functions of the basilar membrane and those of elementary
brainstem processing, showed no signs of dysfunction in schizophrenic subjects. Schizophrenics showed aberrations
in FM and BM, possibly influenced by more central (cortical)processes.
䊚2002 Elsevier Science Ireland Ltd. All rights reserved.
Keywords: Schizophrenia; Simultaneous masking; Forward masking; Backward masking; Psychoacoustics
1. Introduction
1.1. The schizophrenic context
A great variety of symptoms characterize schizo-
phrenia illness. They emanate from different net-
works of the nervous system, namely those that
handle motor, sensory, hearing, visual, thinking
and talking functions, i.e. higher mental activities.
Acoustic hallucinations and sound events experi-
enced as inadequate in relation to the auditory
scene are common, a fact that underlines the need
*Corresponding author. Tel.: q46-46-1738-88; fax: q46-
46-1738-84.
E-mail address: Olle.Olsson@psykiatr.lu.se (O. Olsson).
for further investigations of psychoacoustic pro-
cesses in this disorder.
Indirect evidence for the existence of the schiz-
ophrenic disorder may be drawn from cognitive
testing (Hentschel and Smith, 1980; Heinrichs et
al., 1997). A few neurophysiological measures
have been found to be typical for the diagnosis.
The best known is perhaps the dishabituation of
the sensory-motor system, a general deficit of
function that may be related to some other find-
ings: retarded orienting response, unchanged startle
reflex and lessened accuracy of smooth pursuit eye
movements. Abnormalities in EEG measures such
as P300, P50 and pre-pulse inhibition have been
assessed as typical for the diagnosis (Freedman et

116 J. Kallstrand et al. / Psychiatry Research 113 (2002) 115–125
¨
al., 1996; Romani et al., 1987). An important
finding regarding hearing and schizophrenia was
reported by Lindstrom et al. (1987), who demon-
¨
strated an aberration of the auditory pathway in
schizophrenics by means of ABR (auditory brain-
stem response).
Several perceptual deficits have been discovered
it this laboratory regarding psychoacoustical func-
tioning in schizophrenics. These regard streaming
(sorting into frequency layers depending on ampli-
tude relations), restoration of missing sounds and
abnormal final percepts after complex sound stim-
ulation (Olsson, 2000). The brainstem and subcor-
tical structures together with cortical networks are
at work in auditory processing. McKay et al.
(2000)found differences between reference sub-
jects and schizophrenics on tests of filtered speech
perception and dichotic speech tests. They argued
that the deficits might be associated with central
auditory processing in the right hemisphere. Rai-
nowicz et al. (2000)found an impaired delayed
tone-matching performance in schizophrenia. They
further showed that schizophrenic subjects had no
increased susceptibility to auditory distraction and
concluded that the dysfunction originates in tem-
poral rather than prefrontal cortex. Green et al.
(1999)found visual backward masking deficits in
schizophrenia. These dysfunctions inspired us to
look into masking processes in the auditory field.
1.2. Auditory masking
Auditory masking refers to the increase in detec-
tion threshold of a certain stimulus, when presented
in the presence of another distracting sound (Kohl-
rausch and Houtsma, 1989). Qualitative similari-
ties and temporal relations between the stimulus
and the masker determine the potency of masking
effect. The masker is presented at the same time
as the target tone (simultaneous masking, SM),
preceding the target tone (forward masking, FM)
or presented after the target tone (backward mask-
ing, BM). Usually the stimulus consists of a sine-
wave tone and the masker either of a sine-wave
tone or a filtered noise.
For SM it has been concluded that suppression
in the cochlea accounts for most of the effect
(Moore, 1997; Delgutte, 1990), although regions
more centrally located in the auditory system might
be involved (Nieder and Klump, 1999). Further-
more, dendritic filtering mechanisms intrinsic to
the dorsal cochlear nucleus (DCN)might contrib-
ute to the facilitation of stimulus detection in noise
(Frisina et al., 1994).
FM has been explained by ringing of the basilar
membrane (Carlyon, 1988), but the large discrep-
ancy between spike counts in single auditory nerve
neurons and behavioral thresholds (Relkin and
Turner, 1988)suggests a role for more centrally
located functions. One such mechanism could be
echo-suppression in the DCN, wherein spectral
and temporal properties of auditory information
are modulated (Kaltenbach et al., 1993).FMis
present for ISIs (interstimulus intervals)up to 200
ms (Scharf, 1970).
The distracting effect of BM is less potent than
for FM and is generally thought to reflect cortical
functions (Soderquist et al., 1981). Elliot (1962)
suggested that the effect of BM is achieved when
the masker catches the stimulus tone up, because
the higher energetic masker is more rapidly and
dominantly coded than the target tone. This is
supposed to happen at some place central to the
cochlea (Elliot 1962). The BM effect is present
for ISIs up to 25 ms (Scharf, 1970). Backward
and simultaneous masking abilities deteriorate with
age (Gehr and Sommers, 1999; Moore, 1997).
This study was undertaken as a consequence of
earlier studies involving processes basically resting
on masking as a fundamental component. Espe-
cially, the role of aberrations in perception of
restoration of missing sounds, where schizophren-
ics have altered thresholds, pointed to the need for
further investigations on masking and schizophre-
nia (Olsson and Nielzen, 1999; Nielzen and Ols-
´´
son, 1997; Warren et al., 1972).
1.3. Aims of study
This study aims at investigating several aspects
of auditory masking in schizophrenia. It is assumed
that schizophrenic subjects may show impairments
of masking functions, as they have earlier shown
aberrations in experiments where masking plays
an integrated role. Comparisons with reference

117J. Kallstrand et al. / Psychiatry Research 113 (2002) 115–125
¨
subjects are carried out, and the influence of
anxiety is controlled.
2. Methods
2.1. Stimuli and masker
In all experiments a 1000-Hz sine-wave tone
with 0.010 s impulse length and 0.002 s rise and
fall time was used as the target stimulus. A 1500-
Hz low pass filtered noise (Butterworth filter)was
used as the masker. The duration of the masking
noise was 0.750 s with 0.005 s rise and fall time
(cf. Fig. 1). The duration of the masking noise
was chosen to avoid effects of temporal integra-
tion, which are present at durations shorter than
0.53 s (Brahe-Pedersen and Elberling, 1972). The
stimuli and noise were constructed using MAT-
LAB (5.1), Signal Processing Toolbox and a Sili-
con Graphics (O2)workstation. In the
simultaneous masking (SM)experiments the stim-
uli and masker were presented simultaneously with
the stimuli tone placed in the center of the masking
noise. In the forward masking (FM)and backward
masking (BM)experiments, the ISIs were 0.020
s. The test material was presented with the use of
the software SoundEdit 16 (44.1 kHz sampling
rate)on a Macintosh Powerbook G3. The output
of the Powerbook was connected to headphones
(TDH 39)with cushions (MX41yAR). Presenta-
tions were made binaurally with the stimuli in
phase over headphones. In all tests the level of the
masking noise was kept constant at 83 dB SPL.
The amplitude level of the sine-wave tone was
varied (cf. Section 2.2). Calibration of the sound
levels was performed using a Bruel & Kjaer 2203
¨
sound-level meter with a Bruel & Kjaer 4152
¨
artificial ear.
2.2. Procedure
All tests were performed in a satisfactorily
soundproofed room. Before the presentations the
subjects were verbally informed of the nature of
the experiments, and the sine-wave tone they
should respond to was presented to them. The
subjects were tested one at a time. A positive
response to each stimulus was verbally indicated
by the subjects and transformed to the test protocol
by the test leader. Thus, a staircase method was
used rather than forced-choice paradigm common-
ly used in masking experiments (Penner, 1995).
One reason was the inclusion of schizophrenic
subjects who may not tolerate long-duration and
monotonous procedures. However, the testing was
done by means of two staircases going up and
down, in order that the subjects could learn the
test. Only the second staircase was used for sub-
sequent analysis in the study.
At the start of each experiment the level of the
stimuli was kept well below the level of detection
w73 dB SPL (hearing level)for SM; 46 dB SPL
for FM; and 46 dB SPL for BMxand increased in
steps of 3 dB to a level well above detection (91
dB SPL for SM; 91 dB SPL for FM; and 91 dB
SPL for BM). This sequence was then repeated in
a descending order. Each test was repeated twice,
because the first part of the test was suspected to
become biased by unwanted differences in learning
and cognition. As indicated above, the second part
of the presentations (SM presentations no. 15–28,
FM and BM no. 33–64)is referred to as ‘stabilized
thresholds’ and in the analyses these are defined
as the mean of the ascending and the descending
ones.
2.3. Subjects
Twelve hallucinating schizophrenic outpatients
participated in the experiments, six females and
six males. The diagnosis of schizophrenia was
established by a senior psychiatric physician
according to DSM-IV (American Psychiatric Asso-
ciation, 1994, pp. 285–286). The hallucinations
were anamnestically documented by chief physi-
cians, and during the test the subjects neither
subjectively reported nor objectively displayed any
signs of ongoing hallucinations. Patients with a
history of organic brain disease, alcoholic or drug
abuse, or the presence of additional psychiatric
diagnosis were excluded. All subjects were tested
to exclude hearing impairment (Peter Montne-
mery). The medication given is the only one and
´
not confounded with other pharmaceutical sub-
stances. A formal consent was ascertained in accor-

118 J. Kallstrand et al. / Psychiatry Research 113 (2002) 115–125
¨
Fig. 1. Experimental design. Grey areas represent the masking noise. White squares indicate the sine-wave tone and broken lines
indicate the variations of the amplitude of the target tone.
dance with the requirements of the ethical
committee at the University of Lund (171–94).
An equal number of healthy individuals matched
for age (cf. Table 1)and gender were also inves-
tigated and used as reference subjects. Further-
more, eight subjects with the diagnosis panic
disorder, four males and four females (mean ages
44.24 and S.D.s14.43), were recruited to control
for a possible influence of anxiety.
3. Results
As can be seen in Figs. 2–4, no difference
between schizophrenics and reference subjects

119J. Kallstrand et al. / Psychiatry Research 113 (2002) 115–125
¨
Table 1
Some identifying and background variables of the subjects with schizophrenic symptoms
Ind. no. Age Sex Medication No. of Duration of DSM-IV No. of
mgyweek admissions illness diagnosis months
(years)as inpatient
1 44 M Roxiam 2450 mg 13 20 295.30 26
2 42 F Perphenazine 900 mg 1 14 295.30 1
3 38 F Haloperidol 28 mg 5 3 295.30 5
4 61 M Flupenthixol 3.5 mg 15 25 295.30 36
Clozapine 1400 mg
5 45 M Risperidone 35 mg 15 295.30 8
6 53 M Perphenazine 28 mg 7 25 295.10 25
7 32 M Flupenthixol 28 mg 1 3 295.30 1
8 37 M Haloperidol 28 mg 5 11 295.20 31
9 57 F Risperidone 35 mg 5 29 295.30 15
10 43 F Zuclopenthixol 250 mg 3 24 295.30 11
11 33 F Clozapine 4200 mg 1 17 295.10 54
12 43 F — 3 11 295.10 1.5
MsMale, FsFemale.
emerged for SM, while there were differences
between schizophrenics and reference subjects
regarding FM and BM.
As mentioned earlier, the results are based on
the values of the second staircase. No statistically
significant differences were observed regarding
SM. Schizophrenics showed significantly raised
thresholds compared to reference subjects regard-
ing FM (Mann–Whitney U-test, Zsy3.38, P-
0.001). Likewise, in the BM experiment, the
schizophrenics were less sensitive to the target
tone; they needed a higher sound level to detect it
(Mann–Whitney U-test, Zsy2.40, P-0.05)
Table 2.
In Table 3 a division of the reports of the
schizophrenics in the FM experiment was made.
Those who did not hear the target tone when all
reference subjects did (exposition 40)were cate-
gorized into one group (very increased threshold,
VIT; ns6)and the rest of the schizophrenics into
another (increased threshold, IT; ns6)(cf. Fig.
3). The comparison IT schizophrenics (ns6)and
the references (Ns12)were significantly different
regarding FM (Mann–Whitney U-test, Zsy2.10,
P-0.05). Still more significant was the difference
between the VIT schizophrenics and the reference
subjects (Mann–Whitney U-test, Zsy3,41, P-
0.001). The comparison between the VIT and IT
schizophrenic subgroups was significant regarding
FM (Mann–Whitney U-test, Zsy2.90, P-
0.005). A comparison between the IT and VIT
subgroups concerning months as inpatients showed
a significant difference (Mann–Whitney U-test,
Zsy2.25, P-0.05). For BM the only significant
difference was revealed when references were
compared with schizophrenics with VIT (Mann–
Whitney U-test, Zsy2.30, P-0.05).
In the reference group, simultaneous and back-
ward masking thresholds were positively correlated
with age (Spearman, simultaneous masking; rs
0.67, P-0.05: backward masking; rs0.81, P-
0.01). No significant correlations with age were
present for the schizophrenic group. Neither were
there any significant differences between patients
medicated with high dose neuroleptics vs. those
medicated with low dose neuroleptics.
Comparisons between reference subjects, schi-
zophrenics and panic disorder subjects showed the
following results: panic disorderyreferences, SM
Mann–Whitney U-test, Zsy0.12, Ps0.91; FM,
Mann–Whitney U-test, Zsy0.50, Ps0.61; BM,
Mann–Whitney U-test, Zsy0.08, Ps0.94.
Between panic disorder and schizophrenics the
following values resulted: SM, Mann–Whitney U-
test, Zsy0.27, Ps0.79; FM, Mann–Whitney U-
test, Zsy2.12, Ps0.03; BM, Mann–Whitney
U-test, Zsy1.50, Ps0.13. These results indicate
that no specific differences may be related to the
influence of anxiety as it is manifested in panic
disorder and in schizophrenia as well.

120 J. Kallstrand et al. / Psychiatry Research 113 (2002) 115–125
¨
Fig. 2. Simultaneous masking condition: Frequency distribution of the number of individuals who heard the sine-wave tone for each
presentation (abscissa). Unfilled squaressreference subjects (Ns12)and filled squaressschizophrenics (Ns12).
4. Discussion
Before mentioning the main findings, a brief
comment will be made on matters of design and
rating. Data from tests on schizophrenics often
show great variability (Nielzen, 1982). However,
´
when experiments are constructed in series of
events in regular patterns, aberrations in schizo-
phrenics may emerge due to their specific percep-
tual and behavioral characteristics. That is why
such a design was used in this study. It may also
explain why recent studies have failed to demon-

121J. Kallstrand et al. / Psychiatry Research 113 (2002) 115–125
¨
Fig. 3. Forward masking condition: Frequency distribution of the number of individuals who have heard sine-wave tone for each
presentation (abscissa). Unfilled squaressreference subjects (Ns12)and filled squaressschizophrenics (Ns12).
strate auditory masking aberrations in schizophren-
ics (March et al., 1999). Another problem is that
schizophrenics often are said to be unable to
understand or perform experimental testing. There-
fore, their ratings are claimed to be invalid or not
reliable in many contexts (Steinberg, 1986). How-
ever, in a circumscribed experimental situation,
these arguments have been shown to be misleading
because it even happens that schizophrenics may
be more reliable in their reports than reference
subjects (Hemsley, 1993). Psychotic processes
influence the former group, but these effects may

122 J. Kallstrand et al. / Psychiatry Research 113 (2002) 115–125
¨
Fig. 4. Backward masking condition: Frequency distribution of the number of individuals who have heard the sine-wave tone for
each presentation (abscissa). Unfilled squaressreference subjects (Ns12)and filled squaressschizophrenics (Ns12).
be counterbalanced by dishabituation, which is
combined with rigidity and resistance against
swiftly changing circumstances. The latter group
is influenced by environmental and perhaps per-
sonally upsetting factors, which lead to uncertainty
of ratings. With the design of this study the ratings
on simultaneous masking by the schizophrenics
were very close to those of the reference group
(cf. Table 2), which strengthens validity and
reliability.
The first of the main findings is that schizo-
phrenic and reference subjects responded in the
same manner regarding simultaneous masking.
Most researchers (Moore, 1997)are convinced

123J. Kallstrand et al. / Psychiatry Research 113 (2002) 115–125
¨
Table 2
Group differences for thresholds between reference subjects
and schizophrenic subjects
References Schizophrenics P-value
Means S.D. Means S.D.
SM 5.0 1.1 4.7 1.0 0.58 (n.s.)
FM 4.5 1.5 8.0 2.8 0.001***
BM 3.1 2.1 7.9 5.1 0.02*
SMsSimultaneous masking (8 stimulus steps);FMsfor-
ward masking (16 stimulus steps);BMsbackward masking
(16 stimulus steps).*sSignificance below 0.05; ***sSignif-
icance below 0.005. Reference subjects, Ns12; schizophrenic
subjects, Ns12.
Table 3
Group comparisons between reference subjects and schizophrenic subjects with increased and very increased thresholds, regarding
forward masking
References Schizophrenics Group comparisons
IT VIT Ref.yRef.yschiz. Schiz. ITy
schiz. IT VIT schiz. VIT
SM 5.0 (1.1)5.0 (0.9)4.4 (1.2)n.s. n.s. n.s.
FM 4.5 (1.5)5.8 (1.1)10.4 (1.3)0.036* 0.0006**** 0.0038***
BM 3.1 (2.1)6.6 (4.9)9.3 (5.3)n.s. 0.021* n.s.
No. of admissions — 2.8 (1.8)7.3 (5.6)— — n.s.
Duration of illness — 14.7 (10.6)18.2 (6.3)— — n.s.
(years)
Inpatient (months)— 6.8 (5.6)28.9 (17.1)— — 0.025*
Age 42.0 (14.2)42.8 (8.3)45.2 (10.3)n.s. n.s. n.s.
SMsSimultaneous masking; FMsforward masking; BMsbackward masking. ITsIncreased threshold; VITsvery increased
threshold. References, Ns12; schizophrenics IT, ns6; schizophrenics VIT, ns6. *sSignificance below 0.05; ***sSignificance
below 0.005, ****sSignificance below 0.001. The three left-handed columns refer to means and S.D.
that simultaneous masking mainly depends on
mechanical functions of the basilar membrane and
low brainstem functions, but this process may also
be influenced by central modulation. Our finding
is in concordance with those by McKay et al.
(2000), who observed several aberrations at cor-
tical levels in schizophrenics but not at low brain-
stem levels.
The second main finding consists of a significant
difference between schizophrenic and reference
subjects regarding forward masking. Schizophren-
ics detected the target tone at a higher signal to
noise ratio in comparison with the reference sub-
jects. A mechanistic interpretation may be intro-
duced as an explanation for the forward masking
phenomenon (Carlyon, 1988). According to this,
ringing of the basilar membrane would hamper the
later detection of the target tone. However, an
argument that schizophrenics should have a defi-
cient function of the basilar membrane is not
upheld in any context. It has been shown that the
signal to noise ratio (now in terms of spike counts)
in the auditory nerve is well-preserved (Relkin
and Turner, 1988). It is equally unlikely that
schizophrenics should display any aberrations of
the auditory nerve. Kaltenbach et al. (1993)argues
that the main mechanisms for forward masking
consists of shaping of the acoustical input by
mainly DCN in line with evolutionary demands
for suppression of redundant information, espe-
cially of successive more or less insignificant
elements (echoes). The difference of FM between
the studied groups could be explained by the
assumption that the disintegration of the schizo-
phrenic nervous system extends even to low
regions of brainstem nuclei. However, as it is
known that schizophrenics often have cortical dys-
functions in the temporal and frontal lobes, it is
more reasonable to postulate that FM is regulated
by cortical activity to a substantial extent.
The third important finding was discovered in
connection with backward masking. The schizo-
phrenics detected the sine-wave tone later than the
reference subjects. Although the view has been
advanced that backward masking does not exist
(Moore, 1997; Miyazaki and Sasaki, 1984), but

124 J. Kallstrand et al. / Psychiatry Research 113 (2002) 115–125
¨
much effort has been made by others to explain
the phenomenon, whose existence they convinc-
ingly demonstrate (Scharf, 1970; Crawley et al.,
1994). According to the mainstream of theories,
backward masking necessarily involves handling
of the auditory input by high central nervous
network systems, notably those represented by
temporal and frontal cortical areas. Schizophrenics
surely have deficits in these systems (Kuperberg
and Heckers, 2000), a fact which is further sup-
ported by the present findings.
The group with very increased thresholds
showed the highest impact of illness, as measured
by months of inpatient care and, therefore, the
deficient functioning may be related to the degree
of illness.
There were no significant differences related to
age, sex and the other background factors includ-
ing medication between subgroups of schizophren-
ics. Reference subjects showed a significant
correlation between SM and BM thresholds and
age. The age effect is attributed to successively
deteriorating brain processes (Gehr and Sommers,
1999; Moore, 1997). A possible explanation for
the lack of this in schizophrenics could be that
schizophrenic adaptation aims at compensating
attentive deficits of the illness while reference
subjects might become more suppressive with
regard to unimportant stimuli with age. Another
possibility is that impairment with age causes the
correlation while in schizophrenics the impact of
the already existing illness plays the main role—
and there may be other explanations.
The normal auditory masking pattern was not
influenced by panic disorder or the anxiety related
to this illness. This, together with the lack of major
influences from studied background factors among
the schizophrenics and the increasing deterioration
of forward masking threshold with greater need of
inpatient care speaks for the likelihood that the
results here dealt with are related to the schizo-
phrenic disease.
While SM may be considered to reflect mechan-
ical cochlear mechanisms and neural processes at
low levels, FM may be seen as a more elaborate
functional mechanism involving mainly the co-
chlear nucleus and probably cortical areas. Con-
trary to this and according to experimental
assessment, BM involves higher structures, the
temporal lobe and frontal cortical structures. The
importance of the findings in this study is that
they reveal varying degrees of dysfunction in
essential processes of auditory function in schizo-
phrenics. However, a more basic function such as
simultaneous masking is not affected here. In the
present sample of schizophrenics, the impaired
performance connected to the forward masking
experiment hints at the possibility that dysfunc-
tions exist not only in cortical networks but in
brainstem networks as well.
Acknowledgments
This study was supported by grants from the
Sjobring Foundation at the Division of Psychiatry
¨
of the Lund University, The Royal Swedish Acad-
emy of Sciences, The Royal Physiographic Society
of Lund and Pfannenstill Foundation of Lund.
References
American Psychiatric Association, 1994. Diagnostic and Sta-
tistical Manual of Mental Disorders, 4th ed. Washington,
DC.
Brahe-Pedersen, C., Elberling, C., 1972. Temporal integration
of acoustic energy in normal hearing persons. Acta Otolar-
yngologica (Stockholm)74, 398–402.
Carlyon, R.P., 1988. The development and decline of forward
masking. Hearing Research 32, 65–79.
Crawley, E.J., Kallman, H.J., Neely, J.H., 1994. A test of an
interruptionytemporal-uncertainty theory of auditory back-
ward recognition masking of target duration. Acta Psychol-
ogica (Amsterdam)87, 1–18.
Delgutte, B., 1990. Physiological mechanisms of psychophys-
ical masking: observations from auditory-nerve fibers. Jour-
nal of the Acoustical Society of America 87, 791–809.
Elliot, L.L., 1962. Backward and forward masking of probe
tones of different frequencies. Journal of the Acoustical
Society of America 34, 1116–1118.
Freedman, R., Adler, L.E., Worsley-Myles, M., Nagamoto,
H.T., Miller, C., Kilsy, M., McRae, K., Cawthra, E., Waldo,
M., 1996. Inhibitory gating of an evoked response to
repeated auditory stimuli in schizophrenic and normal sub-
jects. Archives of General Psychiatry 53, 1114–1121.
Frisina, R.D., Walton, J.P., Karcich, K.J., 1994. Dorsal cochlear
nucleus single neurons can enhance temporal processing
capabilities in background noise. Experimental Brain
Research 102, 160–164.
Gehr, S.E., Sommers, M.S., 1999. Age differences in backward
masking. Journal of the Acoustical Society of America 106,
2793–2799.

125J. Kallstrand et al. / Psychiatry Research 113 (2002) 115–125
¨
Green, M.F., Nuechterlein, K.H., Breitmeyer, B., Mintz, J.,
1999. Backward masking in unmedicated schizophrenic
patients in psychotic remission: possible reflection of aber-
rant cortical oscillation. American Journal of Psychiatry
156, 1367–1373.
Heinrichs, R.F., Ruttan, L., Zakzanis, K.K., Case, D., 1997.
Parsing schizophrenia with neurocoginitive tests: evidence
of stability and validity. Brain and Cognition 35, 207–224.
Hemsley, D.H., 1993. A simple (or simplistic?)cognitive
model for schizophrenia. Behaviour Research and Therapy
31, 633–645.
Hentschel, U., Smith, G., 1980. Experimentelle Personlichkeit-
¨
spsychologie. Akademische Verlagsgesellschaft, Wiesbaden.
Kaltenbach, J.A., Meleca, R.J., Falzarano, P.R., Myers, S.F.,
Simpson, T.H., 1993. Forward masking propertiesof neurons
in the dorsal cochlear nucleus: possible role in the process
of echo suppression. Hearing Research 67, 35–44.
Kohlrausch, A., Houtsma, A., 1989. Description of complex
sounds, section III: masking. In: Nielzen, S., Olsson, O.
´
(Eds.), Structure and Perception of Electroacoustic Sound
and Music. Elsevier Science Publishers, Amsterdam, pp.
149–151.
Kuperberg, G., Heckers, S., 2000. Schizophrenia and cognitive
function. Current Opinions in Neurobiology 10, 205–210.
Lindstrom, L., Klockhoff, I., Svedberg, A., Bergstrom, K.,
¨¨
1987. Abnormal auditory brain-stem responses in halluci-
nating schizophrenic patients. British Journal of Psychiatry
151, 9–14.
McKay, C.M., Headlam, D.M., Copolov, D.L., 2000. Central
auditory processing in patients with auditory hallucinations.
American Journal of Psychiatry 157, 759–766.
March, L., Cienfuegos, A., Goldbloom, L., Ritter, W., Cowan,
N., Javitt, D.C., 1999. Normal time course of auditory
recognition in schizophrenia, despite impaired precision of
the auditory sensory (‘echoic’)memory code. Journal of
Abnormal Psychology 108, 69–75.
Miyazaki, K., Sasaki, T., 1984. Pure-tone masking patterns in
non-simultaneous masking conditions. Japanese Psychology
Research 26, 110–119.
Moore, D.R., 1997. The Psychology of Hearing. Oxford
University Press, Oxford.
Nieder, A., Klump, G.M., 1999. Time course of simultaneous
masking in the starling’s auditory forebrain. Experimental
Brain Research 124, 311–320.
Nielzen S., 1982. Music, mind and mental illness. Doctoral
´
thesis, Lund, Sweden.
Nielzen, S., Olsson, O., 1997. Perceptual grouping due to pitch
´
and amplitude in hallucinating schizophrenics. Psychopa-
thology 30, 140–148.
Olsson, O., Nielzen, S., 1999. Perceptual restoration of missing
´
sounds in a group of hallucinating schizophrenics. Percep-
tual and Motor Skills 89, 315–326.
Olsson O., 2000. Psychoacoustics and hallucinating schizo-
phrenics: a psychobiological approach to schizophrenia.
Doctoral thesis, Lund, Sweden.
Penner, M.J., 1995. Psychophysical methods. In: Klump, G.M.,
Dooling, R.J., Fay, R.R., Stebbins, W.S. (Eds.), Methods in
Comparative Psychoacoustics. Birkhauser Verlag, Basel, pp.
¨
47–57.
Rainowicz, E.F., Silipo, G., Goldman, R., Javitt, D.C., 2000.
Auditory sensory dysfunction in schizophrenia: imprecision
or distractibility? Archives of General Psychiatry 57,
1149–1155.
Relkin, E.M., Turner, C.W., 1988. A reexamination of forward
masking in the auditory nerve. Journal of the Acoustical
Society of America 84, 584–591.
Romani, A., Merello, S., Gozzoli, L., Zerbi, F., Grassi, M.,
Cosi, V., 1987. P300 and CT scan and patients with chronic
schizophrenia. British Journal of Psychiatry 151, 506–513.
Scharf, B., 1970. Critical bands. In: Tobias, J.V. (Ed.), Foun-
dations of Modern Auditory Theory. Academic Press, New
York and London, pp. 157–202.
Soderquist, D.R., Carstens, A.A., Frank, G.J., 1981. Backward,
simultaneous, and forward masking as a function of signal
delay and frequency. Journal of Auditory Research 21,
227–245.
Steinberg R., 1986. Musikpsychopathologie. Musikalischer
Ausdruck und Psychische Krankheit. Doctoral thesis,
Munchen, Germany.
¨
Warren, R.M., Obusek, C.J., Ackroff, J.M., 1972. Auditory
induction: perceptual synthesis of absent sounds. Science
176, 1149–1151.