Neuropsychologia 41 (2003) 666–675
Significant relation between MR measures of planum temporale area and
dichotic processing of syllables in dyslexic children
Kenneth Hugdahl
a,∗
, Einar Heiervang
b
, Lars Ersland
e
, Arvid Lundervold
f
,
Helmuth Steinmetz
c
, Alf Inge Smievoll
d
a
Department of Biological and Medical Psychology, University of Bergen, Årstadveien 21, N-5009 Bergen, Norway
b
Department of Psychiatry, University of Bergen, Bergen, Norway
c
Department of Neurology, Johann Wolfgang Goethe-University, Frankfurt, Germany
d
Department of Radiology, Haukeland University Hospital, Bergen, Norway
e
Department of Clinical Engineering, Haukeland University Hospital, Bergen, Norway
f
Department of Physiology, University of Bergen, Bergen, Norway
Received 17 April 2002; received in revised form 7 October 2002; accepted 7 October 2002
Abstract
In the present study, we investigated differences between dyslexic and normal reading children in asymmetry of the planum temporale
area in the upper posterior part of the temporal lobe and dichotic listening performance to consonant–vowel syllables. The current study
was an extension of previous studies in our laboratory on the same participants, now including also girls and left-handers. There were 20
boys and 3 girls in the dyslexic group and 19 boys and 4 girls in the normal reading group. The age of the participants was 10–12 years
for both groups. The participants were screened from a population of 950 students in the fourth school grade in the greater Bergen district.
The planum temporale area was measured in sagittal magnetic resonance (MR) images. Mean left and right area and asymmetry index
were compared between the groups. Dichotic presentations of consonant–vowel syllables made it possible to separately probe left and right
hemisphere phonological function, and to correlate this with planum temporale area. The results showed a significantly larger left than right
planum temporale area for both groups. However, while the right planum temporale area was similar for the dyslexic and control groups,
the left planum temporale was significantly (one-tailed t-test) smaller in the dyslexic group. Both groups also showed a significant right ear
advantage to the consonant–vowel syllables in the dichotic listening test. The relation between planum temporale and dichotic listening
asymmetry showed a significant correlation for the dyslexic group only, indicating a positive relation between brain structure and function
in dyslexic children. The results are discussed in terms of important subject characteristics with regard to brain markers of dyslexia.
© 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Phonological function; Magnetic resonance; Dichotic
1. Introduction
Dyslexic children are impaired in perceptual discrimina-
tion of phonemes [34,39]. They also show impaired speed
and accuracy for lexical access to verbal working memory
[37], and they lack phonological awareness for tasks manip-
ulating phoneme cues [15]. Thus, dyslexics have a deficit
in speech perception, or more specifically, in phonological
decoding of written language (cf. [40]), which translates
into an impairment of reading and writing [49]. Current
hypotheses about the mechanisms behind deficits in pro-
cessing of phonological stimuli have focused on deviant
or absent brain laterality in the classic speech areas in the
∗
Corresponding author.
E-mail address: hugdahl@psych.uib.no (K. Hugdahl).
upper posterior part of the left temporal lobe. In particular,
structural abnormality of the planum temporale (PT) area
has been implicated (see e.g. [2,13]).
The PT is a region situated in the superior temporal gyrus
between Heschl’s sulcus anteriorly and the end of the Syl-
vian fissure (SF) posteriorly. It is typically defined as the
transition of the posterior wall of the descending onto the
ascending ramus of the Sylvian fissure, and it is larger in
the left than in the right hemisphere in about 70% of nor-
mal individuals [11,51]. The PT is an auditory association
area involved in the processing of verbal and non-verbal
stimuli, and includes Wernicke’s area in the left hemisphere
[10,51]. There are many studies that have addressed the
question whether dyslexic children show reduced or reversed
leftward PT asymmetry, unfortunately, with varying results
[8,9,13,22,29,30,32,44–46]. However, what may be more
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