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Planum temporale asymmetry and ear advantage in dichotic listening in developmental dyslexia and Attention-Deficit/Hyperactivity Disorder (ADHD)
Published online by Cambridge University Press: 11 January 2002
Abstract
The planum temporale is clearly involved in language processing, for it serves as the auditory association cortex. Research has consistently demonstrated that 60 to 70% of the population has leftward asymmetry of the planum temporale. Research has also suggested that dyslexic individuals tend to have either rightward asymmetry or symmetrical plana. Moreover, many studies have found a relationship between the presence of dyslexia and/or language impairment and deficits in the normal right ear advantage found in dichotic listening paradigms. In this context, this study examined the relationship between planum temporale asymmetry and ear preference in dichotic listening performance in children with Developmental Dyslexia and Attention-Deficit/Hyperactivity Disorder (ADHD). Subjects included 19 children with dyslexia (10 of whom had a comorbid diagnosis of ADHD), 23 children with ADHD, and 12 diagnosed normal control children. Dichotic listening data were not collected for 8 of the 12 normal control children and for 3 of the 23 ADHD children. Results revealed no significant difference between ADHD and dyslexic subjects in regard to ear advantage on the free recall dichotic listening task. In addition, although the directed dichotic listening tasks were not related to degree of planum asymmetry, as predicted, results indicated that subjects who consistently displayed an atypical left ear advantage tended to have larger right bank lengths than those who consistently displayed a typical right ear advantage. These findings support the notion that some individuals with dyslexia or language deficits tend to have a larger right planum temporale and that performance on dichotic listening tasks may reflect this relatively unusual pattern. (JINS, 2002, 8, 22–36.)
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- Type
- Research Article
- Information
- Journal of the International Neuropsychological Society , Volume 8 , Issue 1 , January 2002 , pp. 22 - 36
- Copyright
- © 2002 The International Neuropsychological Society
INTRODUCTION
Numerous studies have demonstrated abnormal neuroanatomical patterns of asymmetry in individuals with dyslexia (e.g., Galaburda, 1989; Galaburda & Kemper, 1979; Galaburda et al., 1985; Humphreys et al., 1990; Hynd et al., 1990; Larsen et al., 1990; Morgan, 1996). Moreover, many studies have found a relationship between the presence of dyslexia and/or language impairment and deficits in the normal right ear advantage found in dichotic listening paradigms (e.g., Hugdahl et al., 1995; Hynd & Obrzut, 1981; Hynd et al., 1979; Mercure & Warren, 1978; Sparrow & Satz, 1970; Zurif & Carson, 1970). Because of these relatively consistent findings, it seemed that exploration of the relationship among planum temporale morphology, planum temporale asymmetry patterns, and auditory processing, as revealed through dichotic listening performance in dyslexia was warranted, as an improved understanding of the neurological and perceptual substrates of learning disorders may be revealed.
The planum temporale is considered to be a crucial structure in the language process, for it serves as the auditory association cortex and is a component of the temporal speech region of Wernicke's area (Geschwind & Levitsky, 1968; Geschwind, 1972; Geschwind & Galaburda, 1985). More specifically, as conceptualized in the Wernicke-Geschwind model for language, Wernicke's area, containing the planum, is thought to include an auditory comprehension center (Mayeux & Kandel, 1991) and is suspected to be the site in which rapid whole word comprehension takes place (Hynd, 1989; Hynd et al., 1998).
Past research has consistently demonstrated that 60 to 70% of the population has a longer left planum temporale than in the right hemisphere (i.e., leftward asymmetry; Galaburda et al., 1987; Geschwind & Levitsky, 1968; Rubens et al., 1976; Steinmetz et al., 1989; Wada et al., 1975; Witelson & Pallie, 1973). Of interest, however, this pattern of asymmetry is not commonly found among individuals with dyslexia. A preponderance of the research has, in fact, suggested that dyslexic individuals tend to have either a longer right planum temporale relative to that in the left (i.e., rightward asymmetry) or plana of equal length in both hemispheres (i.e., symmetrical plana; Galaburda, 1989; Galaburda & Kemper, 1979; Galaburda et al., 1985; Humphreys et al., 1990; Hynd et al., 1990; Larsen et al., 1990; Morgan, 1996), although not all studies have supported this conclusion (Leonard et al., 1993; Schultz et al., 1994).
The relationship between planum temporale asymmetry and auditory/language lateralization has been examined more recently in studies using normal populations and nondyslexic clinical populations. For instance, Jäncke and Steinmetz (1993) explored the relationship between planum temporale asymmetry and auditory lateralization in a group of 52 normal healthy volunteers. In their study, the entire (folded) cortical surface of the planum temporale was measured from the first transverse sulcus of Heschl anteriorly to the termination of the Sylvian fissure, including the cortex in the posterior descending Sylvian ramus (but excluding the posterior ascending ramus). Of interest, they found no significant relationships between measurements of planum temporale asymmetry and performance on four different verbal dichotic listening tasks. Further, participants with a right ear advantage did not differ significantly from those with a left ear advantage in the average degree of planum temporale asymmetry. The researchers interpreted these findings as evidence that dichotic ear asymmetry does not arise from information processing at auditory cortical levels. Their proposal is commensurate with the belief that there is an anatomical substrate for language lateralization. However, even if leftward planum temporale asymmetry were found to be the anatomical basis for left hemisphere lateralization of language, it is yet to be demonstrated that abnormal language lateralization is a crucial contributing component in dyslexia (Hynd & Willis, 1988; Morgan & Hynd, 1998; Satz, 1976).
There is evidence to suggest that atypical rightward asymmetry or symmetry of the planum temporale is associated with weaker verbal comprehension (Morgan & Hynd, 1998; Semrud-Clikeman et al., 1991), poor phonological processing (Larsen et al., 1990; Semrud-Clikeman et al., 1991), and deficits in expressive language (Morgan & Hynd, 1998; Semrud-Clikeman et al., 1991) in dyslexic and nondyslexic populations. To explain these differences, Steinmetz and Galaburda (1991) suggest the possibility that the plana may be reorganized as a byproduct of cortical dysgenesis that was disrupted because of early trauma. Moreover, Galaburda (1989) reported that dyslexic individuals tend to have more neurons in the language cortex, particularly in the right hemisphere, because of reduced neuronal loss. A large number of right planum neurons may survive because of an increased number of synaptic targets. In fact, Galaburda et al. (1985) hypothesized that some of these extra neurons are ectopic neurons in the left hemisphere. This theory helps explain why both focal dysplasias and symmetrical plana temporale are found at postmortem in dyslexic individuals. Moreover, such a theory lends credence to the assertion that anomalous rightward asymmetry or symmetry of the planum temporale may be associated with dyslexia, and therefore, related to deficient linguistic processing (Hynd & Semrud-Clikeman, 1989; Morgan & Hynd, 1998). Such a theory makes sense, as for decades neuropsychologists have made the assumption that cognitive deficiencies in specific areas, such as reading and language skills, might be the result of anomalous or weak patterns of lateralization (Orton, 1937). However, if one attributes cognitive and linguistic deficits to deficient cerebral lateralization, then one must still determine whether aberrant lateralization is the result of developmental delay or basic structural differences in brain organization (Obrzut, 1988).
In order to examine the possible relationship between plana reorganization (rightward asymmetry or symmetry) and associated processing deficits, this study used a measure that is sensitive to variation in auditory processing, the dichotic listening task. When different digits, words, or consonant-vowel pairs are simultaneously received by each ear, individuals in the normal population tend to report more stimuli correctly from the right ear, thus reflecting a normal right-ear advantage (REA). While both ears are connected bilaterally to the primary auditory cortex through ipsilateral and contralateral pathways, the right ear effect for linguistic stimuli is produced because contralateral pathways are prepotent (Gaddes & Edgell, 1994). If one word set is heard clearly and the other is only poorly understood or not understood at all, a lesion involving the auditory system on the contralateral side may be suspected (Eslinger & Damasio, 1988; Lezak, 1995). Based on this reasoning, most normal right-handed children should, and in fact do, perceive stimuli presented to the right ear more accurately than they perceive stimuli presented to the left ear (Gaddes & Edgell, 1994; Obrzut, 1988).
In their laboratory, Strauss et al. (1987) examined 90 patients with epilepsy, all of whom were administered both the Wada amytal test and dichotic test, to assess the validity of the dichotic listening test for detecting cerebral language dominance. Of interest, agreement between these two methods of assessment (right ear advantage or REA-left cerebral speech, and vice versa) was found in 83% of the cases (Strauss et al., 1987). While patients with left hemispheric speech dominance demonstrated a right ear advantage (REA), those with right hemispheric speech dominance failed to exhibit a consistent ear advantage. Gaddes and Edgell (1994) also point out that Geffen et al. (1978) indicated that they were able to classify accurately 97% of their cases when using a different method of report. Overall, these findings suggest that the dichotic listening technique yields a high enough level of predictive validity to be used as an experimental measure of language lateralization with a reasonable level of confidence (Gaddes & Edgell, 1994).
After having compiled the results of earlier studies, Kimura (1961, 1967) developed a structural model of dichotic listening based on four tenets. More specifically, Kimura's theory asserts that (1) the left cerebral cortex is dominant for language, (2) the contralateral hemisphere perceives auditory input more accurately than the ipsilateral hemisphere, (3) contralateral input suppresses ipsilateral perception, and (4) input from the nondominant hemisphere must decussate and travel through the corpus callosum in order to be processed by the dominant cerebral hemisphere (Bradshaw & Nettleton, 1988). Therefore, Kimura's model (1961, 1967), which stresses contralateral inhibition of dichotic stimuli, implies that deficits in the normal right ear advantage may be a result of a dysfunctional left central language area (Cohen et al., 1992).
Conversely, Bradshaw (1986) and colleagues (Bradshaw & Nettleton, 1988; Pierson et al., 1983) proposed a more functional view which emphasizes temporal–spatial relationships in dichotic perception. Research investigating these models suggests that dichotic asymmetries result from selective attention to the ear of entry rather than from hemispatial perception (Anderson & Hugdahl, 1987; Bryden et al., 1983; Hugdahl & Anderson, 1986).
Based on these models and on studies using the dichotic listening paradigm, several hypotheses have been made regarding the relationship between cerebral laterality and learning disabilities. Some of the most widely supported hypotheses have been the incomplete dominance hypothesis, the maturational lag hypothesis, and the attentional bias hypothesis (Obrzut, 1988).
The incomplete dominance hypothesis proposes that ear preference for verbal or nonverbal material is based on the functional specialization of the contralateral hemisphere (Obrzut, 1988). Although there is some support for the incomplete dominance hypothesis (Sparrow & Satz 1970; Witelson & Rabinovitch, 1972; Zurif & Carson, 1970), the data are weak. It is more likely that language is lateralized in the left hemisphere in both dyslexics and normals (e.g., Leong, 1976; McKeever & Van Deventer, 1975; Witelson, 1976; Yeni-Komshian et al., 1975), but that the efficiency of this processing is less functional in learning disabled children than in others (Obrzut, 1988; Obrzut & Hynd, 1981).
Alternatively, Rourke and colleagues have proposed that this processing deficit may reflect a specific facet of a more generalized maturational lag (Rourke et al., 1983). According to the maturational lag hypothesis, differential control of language function between hemispheres should increase as the child matures (Rourke et al., 1983). For instance, Bakker and colleagues (Bakker, 1973; Bakker et al., 1973, 1976) as well as Satz and associates (Satz & Van Nostrand, 1972; Sparrow & Satz, 1970) contend that a relationship exists between developmental dyslexia and degree of lateralization, but argue that it is only found among older children. More specifically, Bakker and Satz propose a stage theory of development in which motor, perceptual, and linguistic/speech processes progressively lateralize to the left hemisphere as they become more crucial components in learning how to read. Within the context of this theory, younger dyslexics experience developmental delays in the areas of visual–motor and auditory–visual integration skills, while older dyslexics experience delays in more complex linguistic processing. Bryden and Saxby (1986) have pointed out that aside from a few studies which have supported the notion of an increasing REA (e.g., Larsen, 1984; Satz et al., 1975), a preponderance of the research indicates that there are no systematic increases in either the magnitude or incidence of an age-related REA in children between the ages of 5 and 14 (e.g., Berlin et al., 1973; Bryden & Allard, 1981; Geffen, 1976, 1978; Goodglass, 1973; Hynd & Obrzut, 1977; Saxby & Bryden, 1984). As Bryden (1982) proposes, the differences demonstrated between normal and dyslexic subjects could be due to differences in strategy selection. Finally, these studies also suggest that lateralized differences in language processes between normal and dyslexic children do not appear to be due to a developmental delay, but to possible attentional deficits in learning disabled children (Obrzut, 1988).
Such an attentional bias hypothesis posits that the REA is a function of selective arousal of the left hemisphere for verbal activity. More specifically, the expectation of a verbal task differentially activates the left hemisphere which in turn triggers a shift in attentional resources to the right side of space. Conversely, the expectation of a nonverbal task would activate the right hemisphere and direct one's attention to the left (Obrzut, 1988). Based on experiments using forced attention paradigms, it seems that children with learning disabilities do not suffer from poor or incomplete hemispheric specialization for language, but that an attentionally induced, decreased right ear advantage (REA) is seen in children with learning disabilities (e.g., Hugdahl et al., 1995; Kershner & Graham, 1995; Obrzut et al., 1981). An attentionally induced, increased right ear advantage has also been seen (e.g., Kershner & Graham, 1995).
In other words, support has been found for excessive activation of the right hemisphere (i.e., failure of the left hemisphere to engage or inhibit the right hemisphere) and for excessive activation of the left hemisphere (i.e., failure of the right hemisphere to engage or inhibit the left hemisphere) in learning disabled children depending on the order in which these conditions are administered (Kershner & Graham, 1995). Finally, some researchers have suggested that discrepancies in the dichotic listening literature with dyslexic subjects are a result of the fact that previous studies have failed to conceptualize individuals with dyslexia as a heterogenous group (Cohen et al., 1992; Lamm & Epstein, 1994; Obrzut, 1979).
Of interest, deficits in the normal right ear advantage have also been found in dichotic listening studies involving children with Attention-Deficit/Hyperactivity Disorder (ADHD). For instance, Prior et al. (1985) found that children with ADHD did not demonstrate deficits in their ability to sustain attention, although signal detection analysis evidenced “lowered capacity in both focused and selective attention to targets” in these children (p. 289). More specifically, ADHD subjects yielded a higher number of false positives on the attended ear throughout the procedure and for the first half of the experiment on the unattended ear. In other words, they were less able to inhibit erroneous responding than the control children. Furthermore, “they appeared to be unable to overcome the perceptual asymmetry involved in the left ear focus condition to the extent that normal controls were” (p. 301).
Similarly, Pearson et al. (1991) discovered that, relative to normal control children, subjects with ADHD evidenced deficits “at first orienting their attention to one relevant ear, as indicated by their greater omission and intrusion error rates … and that they have a longer latency of reorientation of auditory attention—if indeed they ever do reorient” (p. 488). The authors suggest that this “pattern of detecting fewer targets on the relevant channel and more distractors on the irrelevant channel is consistent with a divided attention strategy in which both channels are monitored instead of focusing on one” (p. 488) and cite Sergeant and Scholten's 1985 study indicating that this “decrement has been linked to weak self-regulatory processes and resource strategy deficits” (p. 489).
This current study is unique in that it examined the lengths of the temporal and parietal banks of the plana in both hemispheres, along with the corresponding interhemispheric (temporal bank, parietal bank, and total planum) and intrahemispheric (left and right hemisphere) asymmetry variables, in relation to dichotic listening performance in children with dyslexia. Morgan and Hynd (1998) point out that this is important because few studies (e.g., Leonard et al., 1993; Morgan, 1996) have differentiated between the temporal and parietal banks of the planum temporale or have regarded intrahemispheric coefficients of asymmetry when comparing dyslexics to other children. Further, we chose to compare the dichotic listening performance of children with dyslexia to a group of children diagnosed with ADHD given that children with ADHD have been found to exhibit deficits in the normal right ear advantage on dichotic listening tasks similar to children with dyslexia. Finally, unlike other studies (e.g., Bakker et al., 1973; Hynd & Obrzut, 1981; Jäncke & Steinmetz, 1993; Obrzut et al., 1980; Satz et al., 1971; Witelson & Rabinovitch, 1972) this study employed both free recall and directed-attention dichotic listening paradigms for bases of comparison. When the results of the free recall paradigm are compared with the results of the directed-attention paradigm, it is possible to control for the effects of attentional biases in auditory processing between the groups.
Thus, the overall goal of this study was to determine the relationship between planum temporale measurements and ear preference in dichotic listening performance. However, because the recent literature has suggested that there are no group differences in regard to planum asymmetry or symmetry and dichotic listening measures (e.g., Jäncke & Steinmetz, 1993), it was hypothesized that there would be no significant group differences in planum morphology, but that the free recall format would distinguish these groups, as learning disabled children are more likely to exhibit variability in selective attention in regard to auditory processing (Hugdahl et al., 1995; Kershner & Graham, 1995; Obrzut, 1988; Obrzut et al., 1981). In addition, it was predicted that results of the forced attention tasks would be related to the degree of planum asymmetry, possibly independent of group (“pure” dyslexic vs. dyslexic with comorbid ADHD vs. ADHD), given that learning disabled children tend to exhibit attentional biases on dichotic listening tasks if asked to direct their attention toward the stimuli focused toward a targeted ear. Moreover, dichotic listening studies involving children diagnosed with ADHD have found that ADHD children do not demonstrate deficits in their ability to sustain attention, but have difficulty with focused and selective attention on forced attention tasks similar to children diagnosed with dyslexia (e.g., Pearson et al., 1991; Prior et al., 1985).
METHODS
Research Participants
The children in this study were selected from consecutive referrals to the Center for Clinical and Developmental Neuropsychology at the University of Georgia who participated in a research project funded by the National Institutes of Health (NIH). Children were recruited and referred through schools, physicians, organizations, and community announcements. The participants ranged in age from 8 to 12 years. Any child with borderline cognitive ability (Full Scale IQ < 75) or a reported neurological disorder (e.g., epilepsy, closed-head injury, etc.) was not included in the research data base. Children were not on medication (e.g., methylphenidate) at the time of their clinical assessment or during the time experimental procedures were administered. Subjects consisted of 19 dyslexic children (M age = 115 months, SD = 15 months; 15 males and 4 females), 10 of whom had a comorbid diagnosis of ADHD (M age =120 months, SD = 13 months, 8 males and 2 females), 23 ADHD children (M age = 119 months, SD = 14 months; 17 males and 6 females), and 12 diagnosed control children (M age = 119 months, SD = 16 months; 5 males and 7 females). Each subject was diagnosed according to prespecified criteria described below.
Procedures
Behavioral and cognitive assessment
Each child and at least 1 biological parent participated in a comprehensive neuropsychological evaluation. The parent(s) provided informed consent and the children gave oral assent for participation. The evaluation included measures of cognitive, linguistic, visuospatial, visual–motor, achievement, behavioral, and neuropsychological abilities.
Diagnostic procedures and criteria
The participants in the study were diagnosed by a licensed psychologist according to pre-specified criteria. In order to receive a diagnosis of dyslexia, the child had to have a 20-point discrepancy between cognitive ability (Full Scale IQ; Wechsler, 1991) and reading achievement as measured by the Wide Range Achievement Test–3 (WRAT–3; Wilkinson, 1993), Reading subtest, and the Woodcock Reading Mastery Test-Revised (WRMT–R; Woodcock, 1987), Word Attack and Passage Comprehension subtests. The 20-point Severe Discrepancy Formula was used in making a diagnosis of dyslexia, as it is the criteria mandated by the Georgia Department of Education for diagnosing a Specific Learning Disability (Georgia Department of Education, 1994). All of the dyslexic subjects were classified as a phonological or mixed phonological/semantic subtype. Initially, children with comorbid ADHD diagnoses were included among the dyslexic subjects because research (Hynd et al., 1995a) has indicated that children with learning disabilities and co-occurring psychopathology do not differ significantly on neuropsychological measures from reading disabled children without a secondary diagnosis. In addition, reading disabled children with and without comorbid ADHD diagnoses have been found to have similar deficits in phonological coding skills as compared to children with only ADHD diagnoses (Hynd et al., 1995a). Further analyses were then conducted within the dyslexic group, comparing the children with “pure” dyslexia to the children with dyslexia and comorbid ADHD to eliminate any confounding variables. Finally, the children with “pure” dyslexia were compared to the children with ADHD. Children who were classified as controls after the comprehensive evaluation had no diagnoses. Moreover, they had no reported history of learning or attentional problems, no significant achievement deficits, and no reported or observed educational or social–emotional problems.
Dichotic listening protocol
As a measure of auditory–phonetic processing, participants were given a synthetic consonant-vowel version of a dichotic listening task, using six stop consonants and the vowel /a/ (/BA/KA/DA/GA/TA/PA), developed by Hugdahl at the Kresge Hearing Laboratory, University of Bergen, Norway. Dichotic listening data were not collected for 8 of the 12 normal control children and for 3 of the 23 ADHD children. Both free-recall and forced-attention paradigms were used (e.g., Hugdahl & Anderson, 1986), given that learning disabled children may yield variable performance patterns due to attentional biases (Hynd et al., 1979). In addition, dichotic listening studies involving children diagnosed with ADHD have found that these children do not demonstrate deficits in their ability to sustain attention, but have difficulty with focused and selective attention on forced attention tasks similar to children diagnosed with dyslexia (e.g., Pearson et al., 1991; Prior et al., 1985).
When the results of the free recall paradigm were compared with the results of the forced-attention paradigm, it was possible to control for the effects of attentional biases in auditory processing among the groups. Participants listened to repeated presentations of 30 pairs of voiced, dichotically presented, pairs of CV-syllables and indicated after each trial which syllable they heard. The number of correctly reported CV syllables for each ear was recorded for all subjects. In order to adjust for individual differences in recall, a laterality index (LI) was calculated for each child for the free recall trial as follows: (right ear correct − left ear correct)/(right ear correct + left ear correct) (Mohr & Costa, 1985). A positive LI score reflected an overall right-ear advantage (REA), while a negative LI score reflected a left-ear advantage (LEA).
Test–retest reliability for this particular CV syllable dichotic listening paradigm has been shown to be adequate for adults. Specifically, Hugdahl and Hammar (1997) compared the test–retest reliability of this instrument under three different attentional instruction conditions, including a divided attention condition and 2 directed attention conditions on each test occasion, in a group of subjects (N = 16) ranging in age from 20 to 67 years who were tested twice, using a 2-week interval, and found test–retest correlations ranging from .61 to .86 for the three attentional conditions, with the smallest correlation coefficient for the divided attention condition (i.e., free recall). Although this study employed adult participants, prior research has indicated that the dichotic listening task yields no developmental changes or sex differences for verbal stimuli (e.g., Hahn, 1987; Hynd et al., 1979; Hynd & Obrzut, 1981; Obrzut et al., 1980; 1981), as previously discussed. However, the reliability of this procedure has not been explicitly established in children.
MRI protocol
MRI scans were obtained using a 0.6-T Health Images (Atlanta, Ga) scanner. The protocol included gapless, three-dimensional, sagittal plane slices measuring 3.1 mm in thickness (TR = 51 : TE = 10). The images were analyzed on a Power Tower Pro 250 McIntosh Operating System using NIH IMAGE (National Institute of Health, 1996. NIH IMAGE. V1.6. Public Domain Software developed at the U.S. National Institute of Health and available on the Internet at http://rsb.info.nih.gov/nih image.) The children were scanned and measurements were made at Athens Magnetic Imaging in Athens, Georgia. All MRI scans were read by a board certified neuroradiologist to screen for and identify children with medically significant conditions. All scans were read as within normal limits. The borders of the planum temporale were determined according to the procedures followed by Leonard et al. (1993). The anterior border of the temporal bank of the planum was defined by the posterior tip of the most anterior part of Heschl's sulcus. The posterior border of the temporal bank was designated by the bifurcation of the Sylvian fissure into a posterior ascending ramus (PAR) and a posterior descending ramus. The length of the temporal bank was measured as the direct distance from the posterior tip of the most anterior Heschl's sulcus to the Sylvian fissure bifurcation. The posterior border of the parietal bank was defined by the termination of the PAR. The length of the parietal bank was measured as the direct distance from the Sylvian fissure bifurcation to the termination of the PAR.
For each subject, the slice in which the boundaries of the planum temporale were most easily distinguished was selected as the “best-view” slice. Measurements were made on that slice. The rater (Morgan, 1996) then measured the planum on up to two slices medial to the “best-view” slice and up to two slices lateral to the “best-view” slice. Measurements were not made on the slices medial or lateral to the “best-view” slice if the boundaries of the planum temporale were not distinguishable. Consistent with the methods of Leonard et al. (1993), the average length of each planum segment was used in the analysis. The employment of this “average length” technique was endorsed by Leonard and associates, as it increased the reliability of the measurements.
Following Leonard et al. (1993), interhemispheric coefficients of asymmetry for the temporal bank, parietal bank, and total planum length were derived from the following formula: (L − R)/[(L + R)(0.5)]. Intrahemispheric coefficients/indices of asymmetry were derived for the left and right hemispheres from the following equation: [T − P]/[(T + P)(0.5)]. Length measurements were used rather than area measurements because Leonard et al. reported that the lateral section measurements were unreliable given that (1) the anterior border of the planum temporale cannot be defined because Heschl's sulcus disappears and (2) it is difficult to reliably trace gyral surfaces at the lateral cortical edge due to numerous minor convolutions.
However, in order to obtain information regarding hemispheric asymmetry distinct from the former measurements, the lengths of the left and right bank areas containing the planum temporale were determined. Again, measurements were made on the slices medial or lateral to the “best slice” view. The average length of the right and left banks was used in all data analyses to increase reliability.
Supratentorial length measurements were made at the midline in order to gather information regarding overall brain size. If groups were discovered to vary in overall brain size, the supratentorial length was used as a covariate to ensure that the results found in the study were a result of variation in the planum temporale, as opposed to gross brain morphology. The supratentorial region includes areas of the brain superior to the corpus callosum, cerebellum, pons, and medulla.
Inter-rater reliability correlations were then obtained for the left temporal bank, right temporal bank, left parietal bank, right parietal bank, and supratentorial length. In the present investigation as well as in a previous study (Morgan, 1996), Morgan found excellent inter-rater reliability: left temporal bank (r = .9814, p < .001); right temporal bank (r = .9813, p < .001); right parietal bank (r = .9905, p < .001); supratentorial length (r = .9993, p < .001).
RESULTS
The overall goal of this study was to determine the relationship between planum temporale asymmetry and ear advantage in dichotic listening performance. It was proposed that the free recall format should distinguish these groups because learning disabled children (dyslexics) are more likely to exhibit variability in selective attention in regard to auditory processing (e.g., Hynd et al., 1979). In addition, it was predicted that the forced attention tasks would be related to the degree of planum asymmetry, perhaps independent of group, given that learning disabled children, and possibly children with ADHD, tend to exhibit attentional biases on dichotic listening tasks if asked to direct their attention toward the stimuli administered toward a targeted ear (e.g., Obrzut et al., 1981). Dichotic listening studies involving children diagnosed with ADHD have found that ADHD children do not demonstrate deficits in their ability to sustain attention, but have difficulty with focused and selective attention on forced attention tasks similar to children diagnosed with dyslexia (e.g., Pearson et al., 1991; Prior et al., 1985).
Data Analyses
Group descriptive statistics
The results of a series of one-way analyses of variance (ANOVA) indicated that the groups did not differ significantly in regard to age, Wechsler Intelligence Scale for Children–Third Edition FSIQ (WISC–III; Wechsler, 1991), or supratentorial length (Morgan, 1996). Edinburgh data (Oldfield, 1971) were not collected for 2 of the normal control children. For the remaining subjects, right handedness was found in 18 of the 19 dyslexic subjects, 18 of the 23 ADHD subjects, and 8 of 10 normal subjects. Differences on handedness were not significant [χ2(2, N = 52) = 2.37, p > .05]. Group means and standard deviations for age, test variables, handedness, and supratentorial length are presented in Table 1. Because it was predicted that the effects should be independent of group classification, descriptive statistics for the total group, illustrated in Tables 2 and 3, was examined in lieu of studying the groups in isolation.
Age, intelligence variables, and supratentorial length for dyslexic, attention-deficit/hyperactivity disorder, and normal control children*
Note. ADHD = Attention-Deficit/Hyperactivity Disorder.
*Adapted from and modified from Morgan, 1996.
Brain measurements (in centimeters), intrahemispheric asymmetry coefficients, and interhemispheric asymmetry coefficients for the total group
Note. Positive interhemispheric asymmetry coefficients reflect left greater than right asymmetry. Positive intrahemispheric asymmetry coefficients reflect temporal bank greater than parietal bank asymmetry.
Dichotic listening, intelligence, and visual–spatial variables for the total group
Note. DL = Dichotic Listening
Chi-square analyses
Chi-square analyses using intrahemispheric and interhemispheric asymmetry ratings were conducted for the total group. Planum measurements were considered to be equivalent in length if the absolute value of the difference between them was less than .10 cm. Logically following, a subject was classified as having asymmetry if the two hemispheres differed by .10 cm or greater. This classification paradigm is consistent with the methods used by Hynd et al. (1990). Morgan and Hynd (1998) indicate that this method of dichotomizing subjects is preferred over the use of asymmetry coefficients, as asymmetry coefficients have not distinguished between groups in some research (e.g., Leonard et al., 1993), while the simple difference method has reliably distinguished between groups (e.g., Hynd et al., 1990). Further, because the groups in the present study were not found to differ on overall brain size, it was deemed appropriate not to use asymmetry coefficients, as the purpose of asymmetry coefficients is to control for variability in brain area (Steinmetz & Galaburda, 1991).
Findings indicated that significantly more individuals (88.9%, n = 48) evidenced a longer left hemisphere temporal bank than those children who evidenced either a longer left hemisphere parietal bank (5.6%, n = 3) or left hemisphere temporal and parietal banks of equal length [5.6%, n = 3; χ<sup>2</sup>(2, N = 54) = 75.00, p < .01]. Similar findings were observed for the right hemisphere. More specifically, significantly more individuals (79.6%, n = 43) evidenced a longer right hemisphere temporal bank than those children who evidenced either a longer right hemisphere parietal bank (13.0%, n = 7) or right hemisphere temporal and parietal banks of equal length [7.4%, n = 4; χ<sup>2</sup>(2, N = 54) = 52.33, p < .01].
Other significant findings included the observation that more children displayed a longer right parietal bank (57.4%, n = 31) than those subjects who displayed either a longer left parietal bank (37.0%, n = 20) or parietal banks of equal length [5.6%, n = 3; χ<sup>2</sup>(2, N = 54) = 22.11, p < .01]. Moreover, more children displayed a longer right total planum (50.0%, n = 27) than those subjects who displayed either a longer left total planum (38.9%, n = 21) or total planum measurements of equal length [11.1%, n = 6; χ<sup>2</sup>(2, N = 54) = 13.00, p < .01]. Significant differences were not found for the temporal bank [χ2(2, n = 54) = 5.44, p > .05]. Observed frequencies for the temporal bank are found in Table 4.
Frequencies of temporal bank asymmetry patterns for the total group
*p > .05.
Follow-up chi-square analyses comparing the entire dyslexic (n = 19), ADHD (n = 23), and normal (n = 12) diagnostic groups on the basis of plana measurements revealed that 12 of the dyslexic subjects (7 “pure” and 5 with comorbid ADHD), 7 of the ADHD subjects, and all 12 of the normal control subjects displayed rightward asymmetry of the parietal bank, while 2 of the dyslexic (both “pure”) and 1 of the ADHD subjects exhibited symmetrical parietal bank plana, and 5 of the dyslexic (2 “pure” and 3 with comorbid ADHD) and 15 of the ADHD subjects exhibited leftward asymmetry of the parietal bank, contingency coefficient = .50366, p < .05. Observed frequencies for parietal bank plana measurements by diagnostic group (entire dyslexic, ADHD, and normals) are presented in Table 5. Results were also significant when the entire dyslexic group was divided into two subgroups (those with “pure” dyslexia and those with comorbid ADHD) and compared to the ADHD and normal control subjects, contingency coefficient = .53433, p < .05. No other significant differences were found, even when the dyslexic group was divided into subgroups to eliminate any confounding variables.
Frequencies of parietal bank asymmetry patterns by diagnostic group
Note. χ2 Contingency coefficient = .50366, p > .05. ADHD = Attention-Deficit/ Hyperactivity Disorder.
Dichotic listening data were not collected for 8 of the 12 normal control children and for 3 of the 23 ADHD children. For the remaining 43 subjects, chi-square analysis revealed no significant differences in regard to ear advantage on the free recall dichotic listening task [χ2(2, N = 43) = 3.67, pÛ > .05]. Observed frequencies for ear advantage in the total group are presented in Table 6. Finally, in contrast to expectations, chi-square analyses comparing the entire dyslexic (n = 19) and ADHD (n = 20) groups failed to predict ear advantage on the free recall dichotic listening task on the basis of group identification, contingency coefficient = .1801, p > .05. Similarly, follow up chi-square analyses within the dyslexic group comparing the children with “pure” dyslexia (n = 9) to the children with dyslexia and comorbid ADHD (n = 10) did not predict ear advantage on the free recall task, contingency coefficient = .3113, p > .05, nor did follow up chi-square analyses comparing the children with “pure” dyslexia (n = 9) to the children with ADHD (n = 20), contingency coefficient = .2853, p > .05. Because data were collected for only four of the normal control subjects, they were not included in the latter analysis. Observed frequencies for ear advantage by diagnostic group (entire dyslexic vs. ADHD, “pure” dyslexic vs. dyslexic with comorbid ADHD, and “pure” dyslexic vs. ADHD) are presented in Tables 7, 8, and 9.
Frequencies of ear advantage patterns on the free recall dichotic listening task for the total group
*p > .05.
Frequencies of ear advantage patterns on the free recall dichotic listening task by diagnostic group
Note. χ2 Contingency coefficient = .1801, p > .05. ADHD = Attention-Deficit/ Hyperactivity Disorder.
Frequencies of ear advantage patterns on the free recall dichotic listening task by diagnostic group
Note. χ2 Contingency coefficient = .2853, p > .05. ADHD = Attention-Deficit/ Hyperactivity Disorder.
Frequencies of ear advantage patterns on the free recall dichotic listening task by diagnostic group
Note. χ2 Contingency coefficient = .3113, p > .05.
Regression analyses
Stepwise regression analysis was chosen over other forms of regression procedures given that past research in this area has yielded inconsistent results, making this study exploratory in nature. Results indicated that the total average length of the right bank (RBA) contributed the most to predicting the Dichotic Listening Index (B = −.4054) and accounted for approximately 16.4% of the variance. Similarly, the total average length of the right bank (RBA) contributed the most variance (20.6%) in predicting left ear advantage scores on the free recall dichotic listening task (B = .4537). Logically following, the total average length of the right bank (RBA) contributed most to predicting right ear advantage scores on the free recall dichotic listening task (B = −.3381), accounting for approximately 11.4% of the variance. No other variables entered significantly into the equation for any of these analyses.
Of interest, none of the variables entered significantly into the stepwise regression analyses when attempting to predict left or right ear advantage scores on the directed left dichotic listening task. Conversely, it was revealed that the total average length of the right bank (RBA) contributed most to predicting left ear advantage scores on the directed right dichotic listening task (B = .3231), accounting for approximately 10.4% of the variance. No other variables entered significantly into the equation. Finally, three variables: total average length of the right bank (RBA), total average length of the left bank (LBA), and left parietal bank length average (LPBA), collectively accounted for approximately 36.2% of the variance when predicting right ear advantage scores on the directed right dichotic listening task (B = −.7085, .4787, and .2875, respectively). Therefore, when examining the total population of subjects as a group, it seems that children who were able to shift their attention to the left or right upon request tended to have a smaller right bank and larger left bank length on average. Conversely, subjects with larger right bank lengths tended to shift their attention toward the left, regardless of which direction to which they were asked to direct their attention. In other words, it appears that, on average, children who possessed a relatively larger right bank length had more difficulty selectively shifting their attention than children who had a relatively smaller right and larger left bank length.
DISCUSSION
A preponderance of the research on patterns of planum temporale morphology in individuals with dyslexia has evidenced abnormal patterns of lateralization (i.e., rightward asymmetry or symmetry; e.g., Galaburda, 1989; Galaburda & Kemper, 1979; Galaburda et al., 1985; Humphreys et al., 1990; Hynd et al., 1990; Larsen et al., 1990; Morgan, 1996). Moreover, numerous studies yielded a positive relationship between the presence of dyslexia and/or language impairment and deficits in the normal right ear advantage found in dichotic listening paradigms (e.g., Hynd & Obrzut, 1981; Hynd et al., 1979; Mercure & Warren, 1978; Sparrow & Satz, 1970; Zurif & Carson, 1970). Such findings have appeared to indicate that there may be a relationship between planum temporale asymmetry and ear dominance in dichotic listening performance. However, because the recent research has failed to uncover group differences in regard to planum asymmetry and dichotic listening measures (e.g., Jäncke & Steinmetz, 1993), it was predicted that there would be no significant group differences in regard to planum morphology when comparing the ADHD group and entire group of dyslexic children in this present study. Instead, it was proposed that the free recall format should distinguish these groups, as learning disabled children are likely to exhibit variability in selective attention in regard to auditory processing (e.g., Hynd et al., 1979). Further, it was predicted that the directed attention tasks would be related to the degree of planum asymmetry, possibly independent of group (ADHD vs. dyslexic), given that learning disabled children tend to exhibit attentional biases on dichotic listening tasks if asked to direct their attention toward the stimuli administered toward a targeted ear (e.g., Obrzut et al., 1981). Of interest, dichotic listening studies involving children diagnosed with ADHD have found that children with ADHD have difficulty with focused and selective attention on forced attention tasks similar to children diagnosed with dyslexia (e.g., Pearson et al., 1991; Prior et al., 1985).
Planum Temporale Morphology
The results of the present study found that a majority of the children examined displayed a longer temporal than parietal bank in both the left and right hemispheres. This finding is consistent with the pattern observed in most “normal” individuals (Leonard et al., 1993). Also commensurate with the usual asymmetrical patterns found in most normal subjects, the parietal bank of a majority of the children in this study was longer in the right hemisphere than in the left hemisphere (Leonard et al., 1993). Follow-up chi-square analyses comparing the entire group of dyslexic, ADHD, and normal children on the basis of plana measurements revealed that all of the normal control subjects displayed rightward asymmetry of the parietal bank, while approximately two-thirds of the entire group of dyslexic subjects and less than one-third of the ADHD subjects displayed this pattern. Of the remaining participants, approximately 10% of the entire group of dyslexic children and 5% of the ADHD children exhibited symmetrical parietal bank plana, while about 25% of the entire group of dyslexic subjects and 60% of the ADHD subjects exhibited leftward asymmetry of the parietal bank. No other significant differences were found. Moreover, the typical left greater than right interhemispheric difference for the temporal bank was not found. Overall, a majority (61.1%) of the children exhibited abnormal planum temporale asymmetry. More specifically, 27 of the 54 subjects (50.0%) were found to have atypical rightward asymmetry, and an additional 6 (11.1%) were found to have symmetrical plana.
An important point that should be addressed in regard to these results is that Morgan (1996) did not find group differences in total planum or temporal bank interhemispheric asymmetry variables among the entire group of dyslexic, ADHD, and normal control subjects in this same population of children on the basis of abnormal (e.g., rightward asymmetrical/symmetrical) versus normal (e.g., leftward asymmetrical) planar patterns. However, she indicated that exploratory analyses revealed that children who performed more poorly on linguistic measures were typically characterized by rightward asymmetrical or symmetrical plana. For instance, stronger linguistic abilities were associated with a long left temporal bank and a long total left planum in ADHD and normal children, while weaker linguistic abilities were associated with a long right temporal bank in dyslexic children. In addition, leftward asymmetry of the temporal bank and of the total planum temporale was related to better linguistic abilities in all subjects, but especially in normal and ADHD subjects. Further, right hemisphere intrahemispheric asymmetry (i.e., temporal bank longer than parietal bank) was associated with weaker linguistic abilities in dyslexic subjects. Finally, regardless of diagnosis, subjects possessing weaker linguistic skills tended to have atypical asymmetry. Morgan suggests that possible interpretations for such findings may be that the left temporal bank is particularly important for linguistic skills in general, but the right temporal bank plays a distinct role in dyslexia. In accordance, planum morphology may be primarily related to receptive and expressive language and reading comprehension skills, and/or atypical asymmetry may be as intimately related to overall weak linguistic skills as it is to dyslexia. Morgan (1996) concluded that these results contradict the notion set forth by Steinmetz and Galaburda (1991), asserting that degree rather than direction of asymmetry may be most functionally significant, given that functional and anatomical asymmetries are on a continuum. Instead, she points out that “there may be so much variability in planum morphology, that group differences diminish if degree of asymmetry rather than direction of asymmetry is compared” (p. 66).
Furthermore, the curvature of the planum was not considered when the computer measured its length. Alternatively, the computer determined “the direct distance from the posterior tip of Heschl's sulcus to the bifurcation of the Sylvian fissure … for the temporal bank measurements” (Morgan, 1996). Consequently, a more direct method of measurement may have yielded different results. Moreover while the current study utilized the measurement techniques of Leonard et al. (1993) who marked the anterior border of the planum temporale at the posterior tip of Heschl's sulcus, it might have been more prudent to have demarcated the anterior portion of the planum temporale as the area where Heschl's sulcus encounters the horizontal portion of the planum temporale as outlined by Hynd et al. (1990). Including the length of Heschl's sulcus might have made “it more difficult to find group differences using small samples” in the same way that including “the parietal bank may reduce the extent of asymmetry (Steinmetz et al., 1991)” (Morgan, 1996, p. 76).
An additional concern is whether employing an average measurement technique or a “best-view” measurement is more reliable and accurate (Morgan, 1996). Of importance, six of forty-eight subjects would have received different classifications if the “best-view” slice value had been used instead of an average measurement value when categorizing the children into those with leftward asymmetry versus those with rightward asymmetry/symmetry of the plana.
A further, related consideration is whether lateral section measurements are as reliable as length measurements when gathering information regarding hemispheric asymmetry. As Morgan (1996) points out, lateral section measurements may be unreliable given that the anterior border of the planum temporale cannot be easily defined because Heschl's sulcus disappears, and it is difficult to trace gyral surfaces reliably at the lateral cortical edge due to numerous minor convolutions (Leonard et al., 1993).
A final concern in regard to the brain measurement methodology is that the present study only addresses the length and not the area of the planum. Because the planum is a unidimensional surface, other neuroimaging techniques that allow for area measurement of the planum might have been more appropriate in the present investigation. In other words, the results of this study, as well as the results of other research (e.g., Hynd et al., 1990; Larsen et al., 1990) are clearly influenced by the measurement technique utilized.
Dichotic Listening Performance
Chi-square analysis on data from the 20 ADHD and 19 dyslexic subjects revealed no significant differences in regard to ear advantage on the free recall dichotic listening task. Moreover, further analysis indicated that the free recall dichotic listening task failed to predict ear advantage on the basis of group identification, contrary to our expectations. However, it should be noted that other studies (e.g,. Obrzut et al., 1981) have yielded similar findings. Similarly, follow up chi-square analyses within the dyslexic group comparing the children with “pure” dyslexia (n = 9) to the children with dyslexia and comorbid ADHD (n = 10) did not predict ear advantage on the free recall task, nor did follow up chi-square analyses comparing the children with “pure” dyslexia (n = 9) to the children with ADHD (n = 20). Further, as previously mentioned, normal control subjects were not included in this latter analysis, as data were collected for only 4 control subjects. Consequently, a normal control group was not available for comparison purposes. It is speculated that the dichotic listening performance of individuals with dyslexia may not have differed significantly from the performance of those individuals with ADHD, because children with ADHD also tend to have difficulty with selective attention (e.g., Pearson et al., 1991; Prior et al., 1985).
Relationship of Free Recall Dichotic Listening Performance to Brain Measurements
Stepwise regression analyses revealed that the total average length of the right bank contributed the most to predicting the Dichotic Listening Index in that children with a larger right bank length tended to have negative scores on the Dichotic Listening Index, reflecting a left ear advantage. Logically following, subjects who displayed an atypical left ear advantage on the free recall dichotic listening task tended to have larger right bank lengths than those who displayed a typical right-ear advantage. Similarly, the total average length of the right bank contributed the most to predicting normal right ear advantage scores on the free recall dichotic listening task in that those subjects who tended to have relatively smaller right bank lengths were more likely to exhibit a right ear advantage.
Therefore, children with an atypical left ear advantage did not appear to differ from children with a normal right ear advantage in regard to the size of their left bank length, as Geschwind and Levitsky (1968) might have suggested, but instead, tended to have larger right bank lengths. Such a finding seems to suggest that a larger right bank length plays a role in altering the functions of the left hemisphere. The mechanism through which this occurs may be one similar to that proposed by Hynd and Obrzut (1981) which contends that stimuli perceived transcallosally by the left hemisphere in learning disabled children may be subject to inattention and suppression as a means of accommodation. Moreover, these findings support the theory set forth by Galaburda and his colleagues which posits that individuals with cortical trauma may tend to have more neurons in the language cortex, particularly in the right hemisphere, because of reduced neuronal loss. Galaburda et al. (1985) further hypothesized that some of these extra neurons may be ectopic in the left hemisphere, explaining why focal dysplasias as well as symmetrical plana temporale have been seen frequently in individuals with dyslexia. Galaburda et al. (1987) assert that this is the most likely explanation for the aberrant rightward asymmetry or symmetry found in individuals with dyslexia, as the plana do not develop until approximately the thirtieth week of gestation (Chi et al., 1977; Wada et al., 1975) which is approximately six to ten weeks after neuronal migration and production has ended and during a period of neuronal loss. Finally, participants with a right ear advantage did not differ significantly from those with a left ear advantage in the average degree or direction of planum temporale asymmetry, consistent with the findings of Jäncke and Steinmetz (1993).
Relationship of Directed Dichotic Listening Performance to Brain Measurements
Although the directed dichotic listening tasks were not related to degree of planum asymmetry, as predicted, results revealed that the total average length of the right bank contributed most to predicting left ear advantage scores on the directed right dichotic listening task. In addition, three variables: total average length of the right bank, total average length of the left bank, and left parietal bank length average, collectively accounted for most of the variance when predicting right ear advantage scores on the directed right dichotic listening task. None of the variables, however, were able to predict either left or right ear advantage scores on the directed left dichotic listening task. Therefore, when examining the total population of subjects as a group, it seems that children who were able to shift their attention to the right upon request tended to have a smaller right bank and larger left bank length on average. Conversely, subjects with larger right bank lengths tended to shift their attention toward the left, regardless of the direction to which they were asked to direct their attention. As formerly discussed, such an attentionally induced decreased right ear advantage is seen in children with learning disabilities (e.g., Hugdahl et al., 1995; Kershner & Graham, 1995; Obrzut et al., 1981). Dichotic listening studies involving children diagnosed with ADHD have found that children with ADHD also have difficulty with focused and selective attention on forced attention tasks similar to children diagnosed with dyslexia (e.g., Pearson et al., 1991; Prior et al., 1985).
Kinsbourne and Hiscock (1981) explain that in order for the left hemisphere to mediate linguistic information verbally, there must be a simultaneous suppression of the functions of the right hemisphere (Obrzut, 1988). The results of this study suggest that suppression of right hemisphere functioning may be intact in children with “normal” brain asymmetry, as children with a relatively smaller right bank length appeared to have difficulty attending to verbal information in an incompatible perceptual field. Conversely, children who possessed a relatively larger right bank length did not appear to have developed reciprocal inhibition, as they had a tendency to exhibit a left ear advantage, regardless of the direction in which they were asked to direct their attention. As previously mentioned, none of the variables examined were able to predict which participants would display either a left or right ear advantage on the directed left dichotic listening task.
Of interest, Kershner and colleagues (Kershner & Graham, 1995; Kershner & Morton, 1990) discovered that directing attention to the left ear first yielded a weaker REA in children with dyslexia. Conversely, when the dyslexic children's attention was first directed to the right ear, their REA was stronger and comparable to that of the younger fluent readers. Further, more dyslexic and younger control children produced a REA when attention was directed to the right ear first. Therefore, it appears that the learning-disabled children performed as if there were little interaction between their two hemispheres. To explain such findings, Kershner (1988) proposes that poor callosal transfer may prevent reciprocal inhibition, or alternatively, each hemisphere may have increased access to processing resources and/or may innervate processes in the other hemisphere. This hypothesis is in line with the ideas of Galaburda and colleagues who posit that individuals with cortical trauma may tend to have more neurons in the language cortex, particularly in the right hemisphere, because of reduced neuronal loss. Consequently, it may be that stimuli perceived transcallosally by the left hemisphere in learning disabled children (dyslexic) may be subject to inattention and suppression as a means of accommodation. Alternatively, Kinsbourne and Hiscock (1981) have proposed that in order for the left hemisphere to mediate linguistic information verbally, there must be a simultaneous suppression of the functions of the right hemisphere. Therefore, if order effects had been more carefully examined, the obtained results might have differed.
In closing, some final issues should be addressed. First, as is the case in most research, our study employed a sample of convenience. Consequently, a majority of the subjects in our experimental groups are male, while our control group is predominantly female. Although normal control subjects were not included in our dichotic listening analyses, approximately half of all verbal dichotic listening studies report a greater right ear advantage for males than for females (Springer & Deutsch, 1989). Moreover, some postmortem studies suggest that females may be less lateralized than males, and in turn, may be more likely to possess a longer right than left temporal plane (Springer & Deutsch, 1989). Therefore, gender differences may have influenced our findings.
There is also ongoing controversy regarding the criteria for diagnosing children with dyslexia. As previously addressed, the 20-point Severe Discrepancy Formula was used in making a diagnosis of dyslexia in this investigation, as it is the criteria mandated by the Georgia Department of Education for diagnosing a Specific Learning Disability (Georgia Department of Education, 1994). However, this method has been widely criticized for failing to take regression effects into account. Accordingly, it is possible that our results might have varied if different criteria had been employed.
Future research may productively focus on the role of the corpus callosum in auditory processing, as many studies have pointed to its integral role. For instance, Obrzut et al. (1981) noted that learning-disabled children appear to suffer a deficit in callosal functioning which impedes their ability to process written verbal information with both hemispheres simultaneously. Moreover, Obrzut et al. (1983) proposed that learning-disabled children do not seem to have developed reciprocal inhibition, as there does not appear to be an efficient exchange of information across the corpus callosum. As a result, the learning-disabled children divided or directed their attention to either ear. In regard to this notion, Kershner (1988) points to Levy's (1985) detailed analysis of corpus callosal functioning which suggests that one of the many functions of the callosum is to inhibit interhemispheric processes for which there are redundant programs in the other hemisphere. Therefore, whenever dyslexic children exhibit poorly lateralized performance, it may be the result of either poor callosal transfer that prevents reciprocal inhibition, or alternatively, each hemisphere may have increased access to processing resources and/or may innervate processes in the other hemisphere. Finally, Hugdahl et al. (1995) suggest that more callosal transfer must occur during information processing in left-handers and in dyslexics than in right-handers and in controls. This hypothesis may be reflected in the significant morphological variation found in the area of the corpus callosum in dyslexics (Hynd et al., 1995b). Therefore, additional research in this area may help pinpoint the neurological and perceptual substrates of the attentional biases exhibited by learning-disabled or dyslexic children on dichotic listening tasks.
ACKNOWLEDGMENTS
Supported in part by a grant (R01-HD26890-04) awarded to the second author (G.W.H.) from the National Institute of Child Health and Human Development (NICHHD), National Institute of Health. Appreciation is expressed to Debbie Eliopulos, R.T., and Edward S. Novey, M.D., for assistance in image acquisition and analysis. Planum measurements reported in this study were derived from Allison E. Morgan's doctoral dissertation completed at the University of Georgia.
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Age, intelligence variables, and supratentorial length for dyslexic, attention-deficit/hyperactivity disorder, and normal control children*Note. ADHD = Attention-Deficit/Hyperactivity Disorder.*Adapted from and modified from Morgan, 1996.
Brain measurements (in centimeters), intrahemispheric asymmetry coefficients, and interhemispheric asymmetry coefficients for the total groupNote. Positive interhemispheric asymmetry coefficients reflect left greater than right asymmetry. Positive intrahemispheric asymmetry coefficients reflect temporal bank greater than parietal bank asymmetry.
Dichotic listening, intelligence, and visual–spatial variables for the total groupNote. DL = Dichotic Listening
Frequencies of temporal bank asymmetry patterns for the total group*p > .05.
Frequencies of parietal bank asymmetry patterns by diagnostic groupNote. χ2 Contingency coefficient = .50366, p > .05. ADHD = Attention-Deficit/ Hyperactivity Disorder.
Frequencies of ear advantage patterns on the free recall dichotic listening task for the total group*p > .05.
Frequencies of ear advantage patterns on the free recall dichotic listening task by diagnostic groupNote. χ2 Contingency coefficient = .1801, p > .05. ADHD = Attention-Deficit/ Hyperactivity Disorder.
Frequencies of ear advantage patterns on the free recall dichotic listening task by diagnostic groupNote. χ2 Contingency coefficient = .2853, p > .05. ADHD = Attention-Deficit/ Hyperactivity Disorder.
Frequencies of ear advantage patterns on the free recall dichotic listening task by diagnostic groupNote. χ2 Contingency coefficient = .3113, p > .05.
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