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Psychoacoustic tests; Central auditory processing; Patients with acquired brain lesions
Abstract:
The comprehension of spoken language is based on th
e central
auditory analysis of complex acoustic signals. Many
psychophysical studies have reported a direct rela
tion
between gradual, spectrotemporal modifications of s
peech sounds and the impairment of the comprehensio
n
of such altered sounds. Thus, it is reasonable to a
ssume that deficits in the understanding of speech
seen in
patients with acquired brain lesions may, to a cert
ain degree, be due to the impaired central processi
ng of
acoustic signals. In this study we report on the de
sign of psychoacoustic tests for evaluating signal
processing
at different levels of the central auditory system.
These tests allow a differentiation between signal
processing
in (i) the auditory periphery, (ii) the auditory br
ainstem and (iii) diencephalic or telencephalic aud
itory areas.
Specific care was taken to design reliable tests th
at can be administered with a minimal amount of ins
truction
and stress to the subjects. This is of special impo
rtance for the examination of patients with acquire
d brain
lesions who might suffer from cognitive deficits an
d/or have language comprehension problems that coul
d
make understanding complex psychophysical test inst
ructions difficult.
The main body of this report provides normative dat
a on age-depend-
ent alterations in auditory processing gathered fro
m one hundred normal-hearing subjects between 20 an
d 70
years of age. Monaural pure tone thresholds were us
ed to evaluate the performance of the peripheral he
aring
system and served as the basis for the subsequent t
ests. The integrity of auditory brainstem processin
g was
evaluated by quantifying interaural frequency, inte
nsity and duration discrimination. All the respecti
ve inter-
aural differences induce lateralized auditory perce
ptions that are critically dependent on binaural pr
ocessing.
Diencephalic/ telencephalic processing was tested b
y varying the same acoustic parameters (frequency,
inten-
sity and tone duration) as well as measuring discri
mination limens for successively presented signals.
Mon-
aural signal presentation permits the measurement o
f the respective processing in general, while prese
nting
these signals together with noise pulses to the oth
er ear (i.e. dichotic
signal/noise
stimulation) specifically measures
the contribution of the cortical hemisphere contral
ateral to the side where the signals were presented
. In nor-
mal-hearing subjects, the difference limens of the
monaural and dichotic
signal/noise
tests did not differ systemati-
cally, while the interaural thresholds were signifi
cantly lower at low frequencies. Both the pure tone
threshold
and all discrimination limens deteriorated with age
. Taking age dependency into consideration is indis
pens-
able to any neurological evaluation of central audi
tory performance. Data on patients with acquired le
sions in
the brainstem, lesions in the left or right superio
r temporal plane or subcortical lesions are present
ed in this
report as an indication for the specificity of the
tests.
