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Linguistic lateralization in adolescents with Down syndrome revealed by a dichotic monitoring test
Research in Developmental Disabilities 30 (2009) 219–228
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Research in Developmental Disabilities
Linguistic lateralization in adolescents with Down syndrome revealed by a dichotic monitoring test Hiroaki Shoji *, Natsuko Koizumi, Hisaki Ozaki Laboratory of Physiology, Faculty of Education, Ibaraki University, 2-1-1 Bunkyo, Mito 310-8512, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 March 2008 Accepted 21 March 2008
Linguistic lateralization in 10 adolescents with Down syndrome (average age: 15.7 years), 15 adolescents with intellectual disabilities of unknown etiology (average age: 17.8 years), 2 groups of children without disabilities (11 children, average age: 4.7 years; 10 children, average age: 8.5 years), and 14 adolescents without disabilities (average age: 18.7 years) was examined, using a dichotic monitoring test (DMT). Different Japanese words with 2 consonant– vowel syllables were presented to each ear simultaneously. Participants pressed a button when they heard the target word. The younger children without disabilities and the adolescents with intellectual disabilities exhibited a right-ear advantage, whereas the adolescents with Down syndrome showed the reverse pattern, i.e., a left-ear advantage. These results suggest that there is atypical linguistic lateralization in adolescents with Down syndrome. ß 2008 Elsevier Ltd All rights reserved.
Keywords: Dichotic listening Hemispheric dominancy Atypical linguistic lateralization Speech perception Adolescents with Down syndrome Adolescents with intellectual disabilities
1. Introduction The dichotic listening test (DLT) is a well-known neuropsychological tool for clarifying the hemispheric cerebral dominance of auditory information processing. In the DLT method, different auditory stimuli are presented simultaneously to each ear, and, in a free recall procedure, participants are requested to recall the perceived words orally. Kimura (1961a,b, 1967), who first applied this method to hemispheric lateralization in auditory processing, identified better recall from the right ear than from the left. Many DLT studies have also reported this right-ear advantage (REA) for linguistic stimuli, regardless of the age and gender of participants (Hugdahl, 2002; Hugdahl, Carlsson, & Eichele, 2001).
* Corresponding author. Tel.: +81 29 228 8291; fax: +81 29 228 8291. E-mail address: [email protected] (H. Shoji). 0891-4222/$ – see front matter ß 2008 Elsevier Ltd All rights reserved. doi:10.1016/j.ridd.2008.03.004
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A REA has been regarded as indicative of superior processing of linguistic stimuli in the left hemisphere, especially for speech sounds. When linguistic stimuli are presented dichotically, ipsilateral projections to the auditory cortices might be inhibited, and the contralateral projection might serve as the dominant pathway of the auditory information. This theory had been supported by observations of a high correlation between DLT and the Wada test, which paralyzes one hemisphere through administration of a sedative drug (Hugdahl, Carlsson, Uvebrant, & Lundervold, 1997). Because DLT enables non-invasive examination of cerebral dominance, it has been widely used in neuropsychological examinations. Various stimuli besides speech sounds have also been adapted to the DLT method for clarifying hemispheric specialization (e.g., emotional tone: Carmon & Nachshon, 1973; Ley & Bryden, 1982; environmental sounds: Kraft, 1981; Kraft, Harper, & Nickel, 1995; musical stimuli: Kimura, 1964). The findings of studies on lateralization have revealed that the left hemisphere is dominant for speech sounds, and the right, for emotional, environmental, and musical sounds. Within speech sounds, Studdert-Kennedy and Shankweiler (1970) demonstrated that a REA was prominently observed for consonants, but not for vowels. Furthermore, recent neurophysiological studies have revealed that the left superior temporal gyri are activated not only by speech sounds but also by tones with rapid frequency changes in a serial sequence (Celsis et al., 1999). These results suggest that detection of rapid sound changes might be specialized in the left hemisphere. Many studies using the DLT have examined lateralization in people with disabilities. Participants in such research have included people with Down syndrome (Hartley, 1981; Pipe, 1983; Zekulin-Hartley, 1981, 1982), autism (Hayashi, Takamura, Kohara, & Yamazaki, 1989; Prior & Bradshaw, 1979), intellectual disabilities of unknown etiology (Berman, 1971; Paquette, Tosoni, Lassonde, & Peretz, 1996), and learning disabilities (Obrzut, Bryden, Lange, & Bulman-Fleming, 2001). These studies aimed to explore atypical lateralization that might be cerebral evidence associated with participants’ poor linguistic abilities. Although a right-ear advantage has been reported in people with intellectual disabilities in some DLT studies (Hartley, 1985; Welsh, Elliott, & Simon, 2003; Zekulin-Hartley, 1981), most studies using the DLT have found that people with Down syndrome have a left-ear advantage (LEA) for linguistic stimuli, rather than the right-ear advantage typically found in children and adults without disabilities (Hartley, 1981; Pipe, 1983; Zekulin-Hartley, 1981, 1982). This atypical lateralization in people with Down syndrome might be correlated with their specific delay in language compared to their mental age. However, some previous studies have failed to find atypical lateralization in youth with Down syndrome (Bowler, Cufflin, & Kiernan, 1985; Sommers & Starkey, 1977). This discrepancy may be the result of methodological differences, such as the methodological problems in DLT pointed out by Bryden (1978, 1988). In the free recall procedure, in which participants are requested to recall orally the words perceived, they have to remember one of the words during vocalization of the other. Therefore, the condition in which they report the first word might be different from that in which they report the second (Bryden, 1978). On the other hand, in the dichotic monitoring test (DMT), participants are requested to respond when they notice a target word among presented words. Since, in this procedure, participants cannot predict which ear the target will be delivered to, they attend to the stimuli in both ears. In addition, a serial effect of verbal vocalization can be ruled out in the DMT. Therefore, hemispheric specialization could be assessed without possible confounding effects of attention, memory, or speech vocalization. Geffen and Caudrey (1981) reported that the DMT is quite reliable for the identification of cerebral laterality, and is highly correlated with the Wada test. Recently, cerebral activity during dichotic listening has been examined in humans using modern brain imaging techniques, for instance, positron emission tomography (PET: Hugdahl et al., 1999) and functional magnetic resonance imaging (fMRI: Ja¨ncke, Specht, Shah, & Hugdahl, 2003). In order to avoid artifacts due to vocalization, the DMT method has been adopted in these neurophysiological studies. Most previous studies have indicated that dichotic listening is associated with not only the temporal cortex but also the frontal cortex (e.g., Ja¨ncke & Shah, 2002; Thomsen, Rimol, Ersland, & Hugdahl, 2004). Therefore, neurophysiological studies using the DMT method might be expected to
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disclose the physiological mechanism of atypical linguistic lateralization in people with Down syndrome. However, evidence of atypical linguistic lateralization using the DMT method has not yet been reported in adolescents with Down syndrome. Some previous studies using dichotic listening tests with non-verbal responses have reported that participants with Down syndrome or intellectual disabilities of unknown etiology failed to exhibit laterality of speech perception (Bunn, Welsh, Simon, Howarth, & Elliott, 2003; Paquette et al., 1996). It is possible that methodological differences might alter the results of cerebral lateralization tests for people with Down syndrome or intellectual disabilities. The purpose of the present study is to examine linguistic lateralization in adolescents with Down syndrome and adolescents with intellectual disabilities, using the DMT method with a manual response. In addition, this study might provide some useful cross-cultural data on linguistic lateralization of Japanese children and adolescents. Ear advantage is affected by the degree of task difficulty, specifically, ear advantage is not observed if the task is too easy or too difficult. Such an effect is dependent upon the type of stimulus material used in the experiment (Obrzut, Boliek, & Obrzut, 1986). In order to clarify an appropriate age for the detection of linguistic lateralization, the participants in the present study included younger children and adolescents without disabilities. We assumed that a reliable right-ear advantage would be observed in children and youth without disabilities who were of an appropriate age. Given that atypical linguistic lateralization is observed only in people with Down syndrome and not in people with intellectual disabilities, the reversal pattern of ear advantage in people with Down syndrome might be derived from the biological or genetic background caused by Trisomy 21, rather than by intellectual disability. 2. Methods 2.1. Participants The five groups of participants in this study were (1) adolescents with Down syndrome, (2) adolescents with intellectual disabilities of unknown etiology, (3) younger children without disabilities whose chronological age corresponded to the mental age of the adolescents with Down syndrome, (4) older children without disabilities whose chronological age corresponded to the mental age of the adolescents with intellectual disabilities, and (5) adolescents without disabilities (see Table 1 for characteristics of the participants). Participants’ hand preference was determined by a 6item questionnaire that asked which hand was used when eating with chopsticks (or a spoon), when holding a toothbrush, when writing with a pencil, when drawing with a pen, and when cutting with scissors. Participants were asked to indicate their hand preference for each of those items (left hand, right hand, or either hand). If it was difficult for them to answer by their own, we asked their teachers or parents to complete the questionnaire. Even if a left-hand preference was indicated on only one of the items, such individuals were excluded from the study, so all the participants were right-handed. Auditory acuity was measured for each ear with a 1000 Hz pure tone from an audiometer (AA61BN, Rion). The auditory acuity of all participants was below 15 dB, and the difference in auditory acuity between their left and right ears was less than 10 dB. Prior to the experiment, we obtained informed consent for participation from each participant and/or from participants’ parents. Table 1 Mean chronological and mental age of the participants in each group Group
Number of participants
Gender
Chronological age
Mental age
Adolescents with Down syndrome Adolescents with intellectual disabilities Younger children without disabilities Older children without disabilities Adolescents without disabilities
10 15 11 10 14
7 males, 3 females 10 males, 5 females 7 males, 4 females 6 males, 4 females 4 males, 10 females
15.7 17.8 4.7 8.5 18.7
5.4 (1.4) 8.6 (1.4)
(1.6) (0.7) (0.2) (1.4) (0.4)
Mean chronological and mental age in years. Standard deviations in parentheses. Mental age was not assessed in the participants without disabilities.
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Fig. 1. Experimental situation and the paired CV syllables used in the dichotic monitoring test (DMT). Participants were asked to press a button when they heard /kagi/ (the target word, underlined in the list in the figure) presented to either ear.
2.2. Stimuli Seven Japanese words that had two consonant–vowel (CV) syllables starting with a plosive consonant (Fig. 1) were used, viz., /bara/ (rose), /kabe/ (wall), /kagi/ (key), /kane/ (bell), /kaze/ (wind), /tora/ (tiger), and /tori/ (bird). These words were paired; 12 word pairs were used as stimuli for the DMT (see Fig. 1). The stimuli were natural speech sounds vocalized by a female adult, digitized by the STIM system (Neuroscan, Inc.). When the recordings were made, we requested the speaker to utter the CV sounds in accord with the pitch and timing of a 1000 Hz pure tone repeated at intervals of 200 ms, in order to exclude physical differences between sounds as much as possible. The duration of the sounds was controlled by cosine tapering at 400 ms, including a 20-ms rise and fall. Although such a long duration of the CV sounds might result in a ceiling effect in the adolescents without disabilities, it was a necessary part of the method to obtain valid responses from the participants with Down syndrome and those with intellectual disabilities. Stimulus signals from the STIM system were delivered through headphones (AD-02, Rion) via an audiometer. We modified all waveforms with the STIM system, so that the VU peak value on the audiometer indicated 0 dB on the basis of a 1000-Hz pure tone. To avoid a discrepancy in auditory acuity between ears, the sound pressure level was adjusted for each participant to 40 dBSL over threshold. 2.3. Procedure Participants were seated on a chair in a quiet room. Prior to the experiment, we confirmed that each participant could identify all the two CV-syllable stimulus words in each ear. If a participant appeared to find it difficult to answer orally, the word could be selected from a word list. In the DMT, participants were asked to press a button when they perceived the target word (/kagi/) presented at either ear. The target word was the same in all blocks of trials, in order to minimize cognitive confusion in people with intellectual disabilities. In order to prevent false alarms that might occur if participants made the target discriminations hastily, participants were requested to take their time when pressing the button. Although we did not instruct the participants as to which hand to use for responding, almost all of them used their right hand. Each trial block consisted of 24 trials, because each of the 12 word pairs was reversed on each side. Four blocks of trials were run in one session, with a short rest between blocks. In order to equalize the physical properties of the receiver, the right and left headphones were exchanged after two blocks. In order to confirm that the participants understood the task, a practice trial was performed before the experiment. The experiment took about 30 min.
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2.4. Analysis The percentage of correct responses in each ear was obtained for each participant, and an angular transformation was done. The transformed values were compared statistically for significant effects by group and by ear side, using analysis of variance (ANOVA, repeated measures on the ear-side factor). Contrast tests were done by statistical post hoc comparisons (Tukey’s HSD tests). A laterality index was calculated by the following formula (Studdert-Kennedy & Shankweiler, 1970): laterality index = (R L)/(R + L) where R is the number of correct responses to the target at the right ear and L is the number of correct responses to the target at the left ear. The laterality index was compared statistically using ANOVA on the factor of groups. Contrast tests were done by statistical post hoc comparisons (Tukey’s HSD tests). 3. Results 3.1. Percentage of correct responses for each ear The mean hit rate for each ear is shown in Fig. 2. There was no significant difference in the factor of the ear side (F(1, 55) = 2.452, p = .123), but the main effect for group was significant (F(4, 55) = 14.874, p < .001). Since the interaction between group and ear side was significant (F(4, 55) = 3.539, p < .05), the main factor for ear side in each group was analyzed by one-way ANOVA. Accuracy in the right ear was significantly better than in the left in the younger children (F(1, 10) = 5.040, p < .05) and in the adolescents with intellectual disabilities (F(1, 14) = 4.797, p < .05), i.e., they showed the typical pattern of REA. In contrast, the participants with Down syndrome performed significantly better with targets presented at the left ear than with those presented at the right (F(1, 9) = 7.616, p < .05). This demonstrated that the participants with Down syndrome showed a LEA. However, a significant difference was not observed in the older children (F(1, 9) = 1.811, p = .211) and adolescents without disabilities (F(1, 13) = 0.518, p = .484). In those two groups, there were slightly more correct responses to right-ear presentation compared to left-ear presentation. In addition, differences between the groups in target detection at each ear were examined by oneway ANOVA. Regardless of side, significant differences appeared in the factor of group (left ear: F(4, 55) = 8.663, p < .001; right ear: F(4, 55) = 17.658, p < .001). In the left ear, multiple comparisons with Tukey’s HSD test revealed that the adolescents without disabilities were significantly more accurate than the younger children (p < .05), the participants with intellectual disabilities (p < .005), and the participants with Down syndrome (p < .001). There was also a significant difference between the accuracy of the adolescents with Down syndrome and the older children (p < .01). Accuracy in the right ear in the adolescents with Down syndrome was significantly worse than that in the right ear in the other groups (p < .001). There was also a significant difference in accuracy between the adolescents with intellectual disabilities and those without disabilities (p < .005).
Fig. 2. Percentage of correct responses in each ear. Error bars above the means indicate the standard error. Asterisks indicate a significant difference in an ANOVA (p < .05). DS = adolescents with Down syndrome; ID = adolescents with intellectual disabilities of unknown etiology; L = left ear; R = right ear.
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Fig. 3. Laterality index in each group. A positive score indicates a right-ear advantage (REA) and a negative score, a left-ear advantage (LEA). Error bars above the means indicate the standard error. Asterisks indicate a significant difference in a post hoc test (*p < .05; **p < .01; ***p < .001).
The percentage of false alarms was also calculated for each participant. The rate of false alarms was less than 8% in all groups. 3.2. Laterality index The laterality index for each group is shown in Fig. 3. A positive score indicates a REA, and a negative score, a LEA. A REA was observed in all groups except for the participants with Down syndrome, who showed a LEA. Since there was a significant difference between the groups in a oneway ANOVA (F(4, 55) = 6.789, p < .001), a post hoc test was performed between pairs of groups. A significant difference was observed between the laterality indices of the participants with Down syndrome and those of the participants in the other groups (younger children: p < .01; older children and adolescents without disabilities: p < .05; adolescents with intellectual disabilities: p < .001). 3.3. Ear advantage for each group of participants In order to classify ear advantage, the accuracy of each ear was compared for each participant. When the accuracy of the right ear exceeded that in the left ear, we classified it as a REA, and vice versa, as a LEA. If there was no difference between the ears, it was classified as no ear advantage (NEA). The total number of LEA, REA, and NEA was counted for each group (Table 2). Seven of the 10 participants with Down syndrome showed a LEA. However, only a few participants in the other groups demonstrated a LEA (2 of 11 younger children, 2 of 10 older children, 4 of 14 adolescents without disabilities, and 1 of 15 adolescents with intellectual disabilities). A x2-test with factors for ear advantage was performed for each group. The results showed that there was a significant difference in ear advantage for the younger children (x2(2) = 7.818, p < .05), the adolescents with intellectual disabilities (x2(2) = 8.400, p < .05), and the adolescents with Down syndrome (x2(2) = 6.200, p < .05). 4. Discussion 4.1. Developmental changes in laterality The DLT is a neuropsychological tool frequently used to clarify language lateralization. In the study of functional brain asymmetry, left hemispheric dominance during speech processing has been
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Table 2 Number of participants in each group with each type of ear advantage Group
Left-ear advantage (LEA)
Right-ear advantage (REA)
No ear advantage (NEA)
Adolescents with Down syndrome Adolescents with intellectual disabilities Younger children without disabilities Older children without disabilities Adolescents without disabilities
7 1 2 2 4
2 10 8 4 5
1 4 1 4 5
When accuracy in the right ear exceeded that in the left, we classified the individual as REA, and vice versa, as LEA. If there was no difference between the ears, it was classified as NEA.
extensively documented. In studies of developmental changes in laterality, although a few researchers have indicated that the degree of REA increases with age during the primary school years (cf. Larsen, 1984; Satz, Bakker, Teunissen, Goebel, & van der Vlugt, 1975), it is commonly held that functional brain asymmetry appears in an early stage of development (Obrzut, 1988). Kamptner, Kraft, and Harper (1984) reported that 2.5-year-old children already have a REA in speech perception. Hayashi (1982) also reported that a REA was observed in 3-year-old children examined by the DLT, using Japanese two-syllable words. The present results using the DMT method are in line with Hayashi’s (1982) previous findings, in that language processing in the younger children, i.e., the 4–5-year old, was performed dominantly in the left hemisphere. However, in the older children and the adolescents without disabilities, the proportion of participants with a REA decreased, and that of those with NEA increased. Similarly, Hayashi (1982) reported that linguistic laterality showed the highest intensity in 4-year-old children, and became obscured with increasing age. Even so, these findings do not necessarily mean that laterality becomes obscure with development, since many previous studies have shown that the left hemisphere is predominant in speech perception in 8–9-year-old children and in adults (e.g., Hugdahl, 2002; Hugdahl et al., 2001). In the present study, the older children and the adolescents without disabilities answered accurately about the dichotic words presented on each side. That is, the hit rates of the older children were above 89% on the right side, and the hit rates of the adolescents without disabilities were above 95% on both sides. Therefore, the NEA observed in the older children and the adolescents without disabilities might have been caused by a ceiling effect, rather than by a diminution of laterality. The present results suggest that the DMT task we used might be suitable for detecting linguistic lateralization in children around 5 years of age. 4.2. Linguistic lateralization in people with intellectual disabilities According to the hypothesis of an increasing REA with development, people with intellectual disabilities would show reduced lateralization. Many previous studies on the handedness have reported that people with intellectual disabilities are more likely to be left-handed or ambidextrous, compared to people without disabilities (e.g., Pipe, 1987; Soper, Satz, Orsini, Van Gorp, & Green, 1987). However, most previous DLT studies of people with intellectual disabilities have observed a REA in speech processing (for reviews, see Elliott, Weeks, & Chua, 1994; Pipe, 1988), and few have supported atypical cerebral lateralization. In the present study, we adopted the DMT for identification of linguistic lateralization. Our data confirmed that the participants with intellectual disabilities definitely exhibited a REA, which supports the results of previous DLT studies. In a similar experiment, Paquette et al. (1996) examined linguistic lateralization in people with intellectual disabilities, using French words with one CVsyllable as the dichotic monitoring stimuli. Paquette et al. (1996) compared the hit rate between ears, and reported that the participants with intellectual disabilities failed to exhibit laterality of speech perception. The discrepancy between the present results and Paquette et al.’s (1996) findings might be a result of differences in the method that participants used to respond. The response method was the same in both studies in that participants were asked beforehand to make a certain reaction when they noticed the target words. However, in Paquette et al.’s (1996) study, in which participants were asked to move
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a lever in one direction when the target word was heard and in the opposite direction when the target word was absent, the participants were asked to respond as soon as possible. In contrast, in the present study, we requested that the participants go at their own pace when they noticed the target stimulus. We adopted the participants’ pacing for their responses because we thought that adolescents with intellectual disabilities would not be able to judge the target words calmly if they were urged to respond rapidly. Such an additional cognitive load might complicate the results of the linguistic laterality measure in people with intellectual disabilities. In the context of the many previous findings on linguistic laterality as measured by the DLT, we suggest that people with intellectual disabilities typically have cerebral lateralization for speech processing, specifically a left hemisphere advantage. 4.3. Linguistic lateralization in people with Down syndrome Many DLT studies have revealed an atypical right cerebral lateralization of speech perception in people with Down syndrome. Using the DMT method, the present research also found a LEA in the participants with Down syndrome. However, as mentioned in the introduction, several previous studies have failed to find such a reversal pattern in people with Down syndrome (Bowler et al., 1985; Sommers & Starkey, 1977; Tannock, Kershner, & Oliver, 1984). For instance, Tannock et al. (1984), using a focused attention procedure as an alternative procedure, reported that people with Down syndrome were more likely to exhibit a REA for correct responses than a LEA. In the focused attention procedure with dichotic sounds, participants are instructed prior to the trial to focus their attention on a stimulus either from the left ear or from the right, and to recall the word heard on that side. However, some people with intellectual disabilities might find it difficult to continue to pay attention. Such an additional cognitive factor might have caused the ambiguous results for linguistic laterality. The other studies mentioned above (Bowler et al., 1985; Sommers & Starkey, 1977) used another DLT method, i.e., asking participants to point to pictures corresponding to the words they had heard, instead of making an oral response. This pointing procedure combines motor control with verbal information. According to Welsh et al. (2003), people with Down syndrome exhibited a LEA when tested with the traditional DLT method. When those participants were required to perform rapid movements with the right hand in the focused attention procedure, however, they showed a REA. According to their hypothesis, a functional dissociation occurs in right-handed people with Down syndrome, with the right hemisphere responsible for speech perception, and a left hemisphere advantage for motor control. As a result, when speech perception and motor organization are required in the same task, the left hemisphere might have more priority than the right, and integrated processing might be treated within the left hemisphere (Heath et al., 2005; Welsh et al., 2003). In previous studies that used a pointing response, participants were not instructed about response speed (rapid or self-paced), so far as we are able to determine. If some of the participants with Down syndrome had responded rapidly with their right hand, a laterality shift to the left hemisphere might have occurred. In the present study, because, in order to prevent rapid movements, we instructed participants to press the button at their own pace, the LEA in the adolescents with Down syndrome might have been disclosed by the DMT method. Recently, the relationship between functional asymmetry in speech perception and the anatomical structure of the cerebral cortex has been examined, using MRI. Several studies reported that the planum temporale (PT) in the upper posterior temporal lobe was larger in the left hemisphere than in the right hemisphere (Good et al., 2001; see Shapleske, Rossell, Woodruff, & David, 1999, for a review). However, the atypical asymmetry of the PT has not yet been observed in people with Down syndrome (Frangou et al., 1997; Pinter, Eliez, Schmitt, Capone, & Reiss, 2001). In addition, cerebral activity during dichotic listening in adults without disabilities has been examined by PET and fMRI (O’Leary et al., 1996; Thomsen et al., 2004). In almost all of these previous brain-imaging studies, the DMT method was adopted, because artifacts such as head movements are very likely in a DLT, since participants respond orally. Brain-imaging studies done with the DMT have revealed asymmetric cerebral activities not only in the temporal area, but also in several cortices. The left inferior frontal gyrus, including Broca’s area, was also activated in the DMT in which an oral response was not required (Ja¨ncke & Shah, 2002). People with Down syndrome have impaired spatial representational abilities but performed in a manner comparable to that observed in patients with left
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hemisphere lesions (Uecker, Mangan, Obrzut, & Nadel, 1993). This finding might indicate that the left hemisphere seems to be damaged more than the right hemisphere. Since some patients with left frontal lesions failed to exhibit a REA (Hugdahl, Bodner, Weiss, & Benke, 2003), asymmetric function in the frontal cortex might be associated with linguistic lateralization. Chein, Fissell, Jacobs, and Fiez (2002) reported that the left inferior frontal area was associated with verbal working memory. Although temporary maintenance of verbal information would be necessary to perform dichotic monitoring, previous studies have demonstrated that verbal short-term memory specifically is poor in people with Down syndrome (e.g., Jarrold & Baddeley, 2001). Future work using neurophysiological tools might reveal more about the correlation between the frontal cortices and atypical linguistic lateralization in people with Down syndrome. In conclusion, in the present study, using the DMT method, the children without disabilities and the adolescents with intellectual disabilities exhibited a REA, but the participants with Down syndrome showed a LEA. These findings suggest that the atypical linguistic lateralization in people with Down syndrome might be related to their specific auditory information processing, but not to their intellectual abilities. The DMT might be a suitable method for examining linguistic laterality in people with intellectual disabilities and those with Down syndrome.
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Contents lists available at ScienceDirect
Research in Developmental Disabilities
Linguistic lateralization in adolescents with Down syndrome revealed by a dichotic monitoring test Hiroaki Shoji *, Natsuko Koizumi, Hisaki Ozaki Laboratory of Physiology, Faculty of Education, Ibaraki University, 2-1-1 Bunkyo, Mito 310-8512, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 March 2008 Accepted 21 March 2008
Linguistic lateralization in 10 adolescents with Down syndrome (average age: 15.7 years), 15 adolescents with intellectual disabilities of unknown etiology (average age: 17.8 years), 2 groups of children without disabilities (11 children, average age: 4.7 years; 10 children, average age: 8.5 years), and 14 adolescents without disabilities (average age: 18.7 years) was examined, using a dichotic monitoring test (DMT). Different Japanese words with 2 consonant– vowel syllables were presented to each ear simultaneously. Participants pressed a button when they heard the target word. The younger children without disabilities and the adolescents with intellectual disabilities exhibited a right-ear advantage, whereas the adolescents with Down syndrome showed the reverse pattern, i.e., a left-ear advantage. These results suggest that there is atypical linguistic lateralization in adolescents with Down syndrome. ß 2008 Elsevier Ltd All rights reserved.
Keywords: Dichotic listening Hemispheric dominancy Atypical linguistic lateralization Speech perception Adolescents with Down syndrome Adolescents with intellectual disabilities
1. Introduction The dichotic listening test (DLT) is a well-known neuropsychological tool for clarifying the hemispheric cerebral dominance of auditory information processing. In the DLT method, different auditory stimuli are presented simultaneously to each ear, and, in a free recall procedure, participants are requested to recall the perceived words orally. Kimura (1961a,b, 1967), who first applied this method to hemispheric lateralization in auditory processing, identified better recall from the right ear than from the left. Many DLT studies have also reported this right-ear advantage (REA) for linguistic stimuli, regardless of the age and gender of participants (Hugdahl, 2002; Hugdahl, Carlsson, & Eichele, 2001).
* Corresponding author. Tel.: +81 29 228 8291; fax: +81 29 228 8291. E-mail address: [email protected] (H. Shoji). 0891-4222/$ – see front matter ß 2008 Elsevier Ltd All rights reserved. doi:10.1016/j.ridd.2008.03.004
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A REA has been regarded as indicative of superior processing of linguistic stimuli in the left hemisphere, especially for speech sounds. When linguistic stimuli are presented dichotically, ipsilateral projections to the auditory cortices might be inhibited, and the contralateral projection might serve as the dominant pathway of the auditory information. This theory had been supported by observations of a high correlation between DLT and the Wada test, which paralyzes one hemisphere through administration of a sedative drug (Hugdahl, Carlsson, Uvebrant, & Lundervold, 1997). Because DLT enables non-invasive examination of cerebral dominance, it has been widely used in neuropsychological examinations. Various stimuli besides speech sounds have also been adapted to the DLT method for clarifying hemispheric specialization (e.g., emotional tone: Carmon & Nachshon, 1973; Ley & Bryden, 1982; environmental sounds: Kraft, 1981; Kraft, Harper, & Nickel, 1995; musical stimuli: Kimura, 1964). The findings of studies on lateralization have revealed that the left hemisphere is dominant for speech sounds, and the right, for emotional, environmental, and musical sounds. Within speech sounds, Studdert-Kennedy and Shankweiler (1970) demonstrated that a REA was prominently observed for consonants, but not for vowels. Furthermore, recent neurophysiological studies have revealed that the left superior temporal gyri are activated not only by speech sounds but also by tones with rapid frequency changes in a serial sequence (Celsis et al., 1999). These results suggest that detection of rapid sound changes might be specialized in the left hemisphere. Many studies using the DLT have examined lateralization in people with disabilities. Participants in such research have included people with Down syndrome (Hartley, 1981; Pipe, 1983; Zekulin-Hartley, 1981, 1982), autism (Hayashi, Takamura, Kohara, & Yamazaki, 1989; Prior & Bradshaw, 1979), intellectual disabilities of unknown etiology (Berman, 1971; Paquette, Tosoni, Lassonde, & Peretz, 1996), and learning disabilities (Obrzut, Bryden, Lange, & Bulman-Fleming, 2001). These studies aimed to explore atypical lateralization that might be cerebral evidence associated with participants’ poor linguistic abilities. Although a right-ear advantage has been reported in people with intellectual disabilities in some DLT studies (Hartley, 1985; Welsh, Elliott, & Simon, 2003; Zekulin-Hartley, 1981), most studies using the DLT have found that people with Down syndrome have a left-ear advantage (LEA) for linguistic stimuli, rather than the right-ear advantage typically found in children and adults without disabilities (Hartley, 1981; Pipe, 1983; Zekulin-Hartley, 1981, 1982). This atypical lateralization in people with Down syndrome might be correlated with their specific delay in language compared to their mental age. However, some previous studies have failed to find atypical lateralization in youth with Down syndrome (Bowler, Cufflin, & Kiernan, 1985; Sommers & Starkey, 1977). This discrepancy may be the result of methodological differences, such as the methodological problems in DLT pointed out by Bryden (1978, 1988). In the free recall procedure, in which participants are requested to recall orally the words perceived, they have to remember one of the words during vocalization of the other. Therefore, the condition in which they report the first word might be different from that in which they report the second (Bryden, 1978). On the other hand, in the dichotic monitoring test (DMT), participants are requested to respond when they notice a target word among presented words. Since, in this procedure, participants cannot predict which ear the target will be delivered to, they attend to the stimuli in both ears. In addition, a serial effect of verbal vocalization can be ruled out in the DMT. Therefore, hemispheric specialization could be assessed without possible confounding effects of attention, memory, or speech vocalization. Geffen and Caudrey (1981) reported that the DMT is quite reliable for the identification of cerebral laterality, and is highly correlated with the Wada test. Recently, cerebral activity during dichotic listening has been examined in humans using modern brain imaging techniques, for instance, positron emission tomography (PET: Hugdahl et al., 1999) and functional magnetic resonance imaging (fMRI: Ja¨ncke, Specht, Shah, & Hugdahl, 2003). In order to avoid artifacts due to vocalization, the DMT method has been adopted in these neurophysiological studies. Most previous studies have indicated that dichotic listening is associated with not only the temporal cortex but also the frontal cortex (e.g., Ja¨ncke & Shah, 2002; Thomsen, Rimol, Ersland, & Hugdahl, 2004). Therefore, neurophysiological studies using the DMT method might be expected to
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disclose the physiological mechanism of atypical linguistic lateralization in people with Down syndrome. However, evidence of atypical linguistic lateralization using the DMT method has not yet been reported in adolescents with Down syndrome. Some previous studies using dichotic listening tests with non-verbal responses have reported that participants with Down syndrome or intellectual disabilities of unknown etiology failed to exhibit laterality of speech perception (Bunn, Welsh, Simon, Howarth, & Elliott, 2003; Paquette et al., 1996). It is possible that methodological differences might alter the results of cerebral lateralization tests for people with Down syndrome or intellectual disabilities. The purpose of the present study is to examine linguistic lateralization in adolescents with Down syndrome and adolescents with intellectual disabilities, using the DMT method with a manual response. In addition, this study might provide some useful cross-cultural data on linguistic lateralization of Japanese children and adolescents. Ear advantage is affected by the degree of task difficulty, specifically, ear advantage is not observed if the task is too easy or too difficult. Such an effect is dependent upon the type of stimulus material used in the experiment (Obrzut, Boliek, & Obrzut, 1986). In order to clarify an appropriate age for the detection of linguistic lateralization, the participants in the present study included younger children and adolescents without disabilities. We assumed that a reliable right-ear advantage would be observed in children and youth without disabilities who were of an appropriate age. Given that atypical linguistic lateralization is observed only in people with Down syndrome and not in people with intellectual disabilities, the reversal pattern of ear advantage in people with Down syndrome might be derived from the biological or genetic background caused by Trisomy 21, rather than by intellectual disability. 2. Methods 2.1. Participants The five groups of participants in this study were (1) adolescents with Down syndrome, (2) adolescents with intellectual disabilities of unknown etiology, (3) younger children without disabilities whose chronological age corresponded to the mental age of the adolescents with Down syndrome, (4) older children without disabilities whose chronological age corresponded to the mental age of the adolescents with intellectual disabilities, and (5) adolescents without disabilities (see Table 1 for characteristics of the participants). Participants’ hand preference was determined by a 6item questionnaire that asked which hand was used when eating with chopsticks (or a spoon), when holding a toothbrush, when writing with a pencil, when drawing with a pen, and when cutting with scissors. Participants were asked to indicate their hand preference for each of those items (left hand, right hand, or either hand). If it was difficult for them to answer by their own, we asked their teachers or parents to complete the questionnaire. Even if a left-hand preference was indicated on only one of the items, such individuals were excluded from the study, so all the participants were right-handed. Auditory acuity was measured for each ear with a 1000 Hz pure tone from an audiometer (AA61BN, Rion). The auditory acuity of all participants was below 15 dB, and the difference in auditory acuity between their left and right ears was less than 10 dB. Prior to the experiment, we obtained informed consent for participation from each participant and/or from participants’ parents. Table 1 Mean chronological and mental age of the participants in each group Group
Number of participants
Gender
Chronological age
Mental age
Adolescents with Down syndrome Adolescents with intellectual disabilities Younger children without disabilities Older children without disabilities Adolescents without disabilities
10 15 11 10 14
7 males, 3 females 10 males, 5 females 7 males, 4 females 6 males, 4 females 4 males, 10 females
15.7 17.8 4.7 8.5 18.7
5.4 (1.4) 8.6 (1.4)
(1.6) (0.7) (0.2) (1.4) (0.4)
Mean chronological and mental age in years. Standard deviations in parentheses. Mental age was not assessed in the participants without disabilities.
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Fig. 1. Experimental situation and the paired CV syllables used in the dichotic monitoring test (DMT). Participants were asked to press a button when they heard /kagi/ (the target word, underlined in the list in the figure) presented to either ear.
2.2. Stimuli Seven Japanese words that had two consonant–vowel (CV) syllables starting with a plosive consonant (Fig. 1) were used, viz., /bara/ (rose), /kabe/ (wall), /kagi/ (key), /kane/ (bell), /kaze/ (wind), /tora/ (tiger), and /tori/ (bird). These words were paired; 12 word pairs were used as stimuli for the DMT (see Fig. 1). The stimuli were natural speech sounds vocalized by a female adult, digitized by the STIM system (Neuroscan, Inc.). When the recordings were made, we requested the speaker to utter the CV sounds in accord with the pitch and timing of a 1000 Hz pure tone repeated at intervals of 200 ms, in order to exclude physical differences between sounds as much as possible. The duration of the sounds was controlled by cosine tapering at 400 ms, including a 20-ms rise and fall. Although such a long duration of the CV sounds might result in a ceiling effect in the adolescents without disabilities, it was a necessary part of the method to obtain valid responses from the participants with Down syndrome and those with intellectual disabilities. Stimulus signals from the STIM system were delivered through headphones (AD-02, Rion) via an audiometer. We modified all waveforms with the STIM system, so that the VU peak value on the audiometer indicated 0 dB on the basis of a 1000-Hz pure tone. To avoid a discrepancy in auditory acuity between ears, the sound pressure level was adjusted for each participant to 40 dBSL over threshold. 2.3. Procedure Participants were seated on a chair in a quiet room. Prior to the experiment, we confirmed that each participant could identify all the two CV-syllable stimulus words in each ear. If a participant appeared to find it difficult to answer orally, the word could be selected from a word list. In the DMT, participants were asked to press a button when they perceived the target word (/kagi/) presented at either ear. The target word was the same in all blocks of trials, in order to minimize cognitive confusion in people with intellectual disabilities. In order to prevent false alarms that might occur if participants made the target discriminations hastily, participants were requested to take their time when pressing the button. Although we did not instruct the participants as to which hand to use for responding, almost all of them used their right hand. Each trial block consisted of 24 trials, because each of the 12 word pairs was reversed on each side. Four blocks of trials were run in one session, with a short rest between blocks. In order to equalize the physical properties of the receiver, the right and left headphones were exchanged after two blocks. In order to confirm that the participants understood the task, a practice trial was performed before the experiment. The experiment took about 30 min.
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2.4. Analysis The percentage of correct responses in each ear was obtained for each participant, and an angular transformation was done. The transformed values were compared statistically for significant effects by group and by ear side, using analysis of variance (ANOVA, repeated measures on the ear-side factor). Contrast tests were done by statistical post hoc comparisons (Tukey’s HSD tests). A laterality index was calculated by the following formula (Studdert-Kennedy & Shankweiler, 1970): laterality index = (R L)/(R + L) where R is the number of correct responses to the target at the right ear and L is the number of correct responses to the target at the left ear. The laterality index was compared statistically using ANOVA on the factor of groups. Contrast tests were done by statistical post hoc comparisons (Tukey’s HSD tests). 3. Results 3.1. Percentage of correct responses for each ear The mean hit rate for each ear is shown in Fig. 2. There was no significant difference in the factor of the ear side (F(1, 55) = 2.452, p = .123), but the main effect for group was significant (F(4, 55) = 14.874, p < .001). Since the interaction between group and ear side was significant (F(4, 55) = 3.539, p < .05), the main factor for ear side in each group was analyzed by one-way ANOVA. Accuracy in the right ear was significantly better than in the left in the younger children (F(1, 10) = 5.040, p < .05) and in the adolescents with intellectual disabilities (F(1, 14) = 4.797, p < .05), i.e., they showed the typical pattern of REA. In contrast, the participants with Down syndrome performed significantly better with targets presented at the left ear than with those presented at the right (F(1, 9) = 7.616, p < .05). This demonstrated that the participants with Down syndrome showed a LEA. However, a significant difference was not observed in the older children (F(1, 9) = 1.811, p = .211) and adolescents without disabilities (F(1, 13) = 0.518, p = .484). In those two groups, there were slightly more correct responses to right-ear presentation compared to left-ear presentation. In addition, differences between the groups in target detection at each ear were examined by oneway ANOVA. Regardless of side, significant differences appeared in the factor of group (left ear: F(4, 55) = 8.663, p < .001; right ear: F(4, 55) = 17.658, p < .001). In the left ear, multiple comparisons with Tukey’s HSD test revealed that the adolescents without disabilities were significantly more accurate than the younger children (p < .05), the participants with intellectual disabilities (p < .005), and the participants with Down syndrome (p < .001). There was also a significant difference between the accuracy of the adolescents with Down syndrome and the older children (p < .01). Accuracy in the right ear in the adolescents with Down syndrome was significantly worse than that in the right ear in the other groups (p < .001). There was also a significant difference in accuracy between the adolescents with intellectual disabilities and those without disabilities (p < .005).
Fig. 2. Percentage of correct responses in each ear. Error bars above the means indicate the standard error. Asterisks indicate a significant difference in an ANOVA (p < .05). DS = adolescents with Down syndrome; ID = adolescents with intellectual disabilities of unknown etiology; L = left ear; R = right ear.
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Fig. 3. Laterality index in each group. A positive score indicates a right-ear advantage (REA) and a negative score, a left-ear advantage (LEA). Error bars above the means indicate the standard error. Asterisks indicate a significant difference in a post hoc test (*p < .05; **p < .01; ***p < .001).
The percentage of false alarms was also calculated for each participant. The rate of false alarms was less than 8% in all groups. 3.2. Laterality index The laterality index for each group is shown in Fig. 3. A positive score indicates a REA, and a negative score, a LEA. A REA was observed in all groups except for the participants with Down syndrome, who showed a LEA. Since there was a significant difference between the groups in a oneway ANOVA (F(4, 55) = 6.789, p < .001), a post hoc test was performed between pairs of groups. A significant difference was observed between the laterality indices of the participants with Down syndrome and those of the participants in the other groups (younger children: p < .01; older children and adolescents without disabilities: p < .05; adolescents with intellectual disabilities: p < .001). 3.3. Ear advantage for each group of participants In order to classify ear advantage, the accuracy of each ear was compared for each participant. When the accuracy of the right ear exceeded that in the left ear, we classified it as a REA, and vice versa, as a LEA. If there was no difference between the ears, it was classified as no ear advantage (NEA). The total number of LEA, REA, and NEA was counted for each group (Table 2). Seven of the 10 participants with Down syndrome showed a LEA. However, only a few participants in the other groups demonstrated a LEA (2 of 11 younger children, 2 of 10 older children, 4 of 14 adolescents without disabilities, and 1 of 15 adolescents with intellectual disabilities). A x2-test with factors for ear advantage was performed for each group. The results showed that there was a significant difference in ear advantage for the younger children (x2(2) = 7.818, p < .05), the adolescents with intellectual disabilities (x2(2) = 8.400, p < .05), and the adolescents with Down syndrome (x2(2) = 6.200, p < .05). 4. Discussion 4.1. Developmental changes in laterality The DLT is a neuropsychological tool frequently used to clarify language lateralization. In the study of functional brain asymmetry, left hemispheric dominance during speech processing has been
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Table 2 Number of participants in each group with each type of ear advantage Group
Left-ear advantage (LEA)
Right-ear advantage (REA)
No ear advantage (NEA)
Adolescents with Down syndrome Adolescents with intellectual disabilities Younger children without disabilities Older children without disabilities Adolescents without disabilities
7 1 2 2 4
2 10 8 4 5
1 4 1 4 5
When accuracy in the right ear exceeded that in the left, we classified the individual as REA, and vice versa, as LEA. If there was no difference between the ears, it was classified as NEA.
extensively documented. In studies of developmental changes in laterality, although a few researchers have indicated that the degree of REA increases with age during the primary school years (cf. Larsen, 1984; Satz, Bakker, Teunissen, Goebel, & van der Vlugt, 1975), it is commonly held that functional brain asymmetry appears in an early stage of development (Obrzut, 1988). Kamptner, Kraft, and Harper (1984) reported that 2.5-year-old children already have a REA in speech perception. Hayashi (1982) also reported that a REA was observed in 3-year-old children examined by the DLT, using Japanese two-syllable words. The present results using the DMT method are in line with Hayashi’s (1982) previous findings, in that language processing in the younger children, i.e., the 4–5-year old, was performed dominantly in the left hemisphere. However, in the older children and the adolescents without disabilities, the proportion of participants with a REA decreased, and that of those with NEA increased. Similarly, Hayashi (1982) reported that linguistic laterality showed the highest intensity in 4-year-old children, and became obscured with increasing age. Even so, these findings do not necessarily mean that laterality becomes obscure with development, since many previous studies have shown that the left hemisphere is predominant in speech perception in 8–9-year-old children and in adults (e.g., Hugdahl, 2002; Hugdahl et al., 2001). In the present study, the older children and the adolescents without disabilities answered accurately about the dichotic words presented on each side. That is, the hit rates of the older children were above 89% on the right side, and the hit rates of the adolescents without disabilities were above 95% on both sides. Therefore, the NEA observed in the older children and the adolescents without disabilities might have been caused by a ceiling effect, rather than by a diminution of laterality. The present results suggest that the DMT task we used might be suitable for detecting linguistic lateralization in children around 5 years of age. 4.2. Linguistic lateralization in people with intellectual disabilities According to the hypothesis of an increasing REA with development, people with intellectual disabilities would show reduced lateralization. Many previous studies on the handedness have reported that people with intellectual disabilities are more likely to be left-handed or ambidextrous, compared to people without disabilities (e.g., Pipe, 1987; Soper, Satz, Orsini, Van Gorp, & Green, 1987). However, most previous DLT studies of people with intellectual disabilities have observed a REA in speech processing (for reviews, see Elliott, Weeks, & Chua, 1994; Pipe, 1988), and few have supported atypical cerebral lateralization. In the present study, we adopted the DMT for identification of linguistic lateralization. Our data confirmed that the participants with intellectual disabilities definitely exhibited a REA, which supports the results of previous DLT studies. In a similar experiment, Paquette et al. (1996) examined linguistic lateralization in people with intellectual disabilities, using French words with one CVsyllable as the dichotic monitoring stimuli. Paquette et al. (1996) compared the hit rate between ears, and reported that the participants with intellectual disabilities failed to exhibit laterality of speech perception. The discrepancy between the present results and Paquette et al.’s (1996) findings might be a result of differences in the method that participants used to respond. The response method was the same in both studies in that participants were asked beforehand to make a certain reaction when they noticed the target words. However, in Paquette et al.’s (1996) study, in which participants were asked to move
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a lever in one direction when the target word was heard and in the opposite direction when the target word was absent, the participants were asked to respond as soon as possible. In contrast, in the present study, we requested that the participants go at their own pace when they noticed the target stimulus. We adopted the participants’ pacing for their responses because we thought that adolescents with intellectual disabilities would not be able to judge the target words calmly if they were urged to respond rapidly. Such an additional cognitive load might complicate the results of the linguistic laterality measure in people with intellectual disabilities. In the context of the many previous findings on linguistic laterality as measured by the DLT, we suggest that people with intellectual disabilities typically have cerebral lateralization for speech processing, specifically a left hemisphere advantage. 4.3. Linguistic lateralization in people with Down syndrome Many DLT studies have revealed an atypical right cerebral lateralization of speech perception in people with Down syndrome. Using the DMT method, the present research also found a LEA in the participants with Down syndrome. However, as mentioned in the introduction, several previous studies have failed to find such a reversal pattern in people with Down syndrome (Bowler et al., 1985; Sommers & Starkey, 1977; Tannock, Kershner, & Oliver, 1984). For instance, Tannock et al. (1984), using a focused attention procedure as an alternative procedure, reported that people with Down syndrome were more likely to exhibit a REA for correct responses than a LEA. In the focused attention procedure with dichotic sounds, participants are instructed prior to the trial to focus their attention on a stimulus either from the left ear or from the right, and to recall the word heard on that side. However, some people with intellectual disabilities might find it difficult to continue to pay attention. Such an additional cognitive factor might have caused the ambiguous results for linguistic laterality. The other studies mentioned above (Bowler et al., 1985; Sommers & Starkey, 1977) used another DLT method, i.e., asking participants to point to pictures corresponding to the words they had heard, instead of making an oral response. This pointing procedure combines motor control with verbal information. According to Welsh et al. (2003), people with Down syndrome exhibited a LEA when tested with the traditional DLT method. When those participants were required to perform rapid movements with the right hand in the focused attention procedure, however, they showed a REA. According to their hypothesis, a functional dissociation occurs in right-handed people with Down syndrome, with the right hemisphere responsible for speech perception, and a left hemisphere advantage for motor control. As a result, when speech perception and motor organization are required in the same task, the left hemisphere might have more priority than the right, and integrated processing might be treated within the left hemisphere (Heath et al., 2005; Welsh et al., 2003). In previous studies that used a pointing response, participants were not instructed about response speed (rapid or self-paced), so far as we are able to determine. If some of the participants with Down syndrome had responded rapidly with their right hand, a laterality shift to the left hemisphere might have occurred. In the present study, because, in order to prevent rapid movements, we instructed participants to press the button at their own pace, the LEA in the adolescents with Down syndrome might have been disclosed by the DMT method. Recently, the relationship between functional asymmetry in speech perception and the anatomical structure of the cerebral cortex has been examined, using MRI. Several studies reported that the planum temporale (PT) in the upper posterior temporal lobe was larger in the left hemisphere than in the right hemisphere (Good et al., 2001; see Shapleske, Rossell, Woodruff, & David, 1999, for a review). However, the atypical asymmetry of the PT has not yet been observed in people with Down syndrome (Frangou et al., 1997; Pinter, Eliez, Schmitt, Capone, & Reiss, 2001). In addition, cerebral activity during dichotic listening in adults without disabilities has been examined by PET and fMRI (O’Leary et al., 1996; Thomsen et al., 2004). In almost all of these previous brain-imaging studies, the DMT method was adopted, because artifacts such as head movements are very likely in a DLT, since participants respond orally. Brain-imaging studies done with the DMT have revealed asymmetric cerebral activities not only in the temporal area, but also in several cortices. The left inferior frontal gyrus, including Broca’s area, was also activated in the DMT in which an oral response was not required (Ja¨ncke & Shah, 2002). People with Down syndrome have impaired spatial representational abilities but performed in a manner comparable to that observed in patients with left
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hemisphere lesions (Uecker, Mangan, Obrzut, & Nadel, 1993). This finding might indicate that the left hemisphere seems to be damaged more than the right hemisphere. Since some patients with left frontal lesions failed to exhibit a REA (Hugdahl, Bodner, Weiss, & Benke, 2003), asymmetric function in the frontal cortex might be associated with linguistic lateralization. Chein, Fissell, Jacobs, and Fiez (2002) reported that the left inferior frontal area was associated with verbal working memory. Although temporary maintenance of verbal information would be necessary to perform dichotic monitoring, previous studies have demonstrated that verbal short-term memory specifically is poor in people with Down syndrome (e.g., Jarrold & Baddeley, 2001). Future work using neurophysiological tools might reveal more about the correlation between the frontal cortices and atypical linguistic lateralization in people with Down syndrome. In conclusion, in the present study, using the DMT method, the children without disabilities and the adolescents with intellectual disabilities exhibited a REA, but the participants with Down syndrome showed a LEA. These findings suggest that the atypical linguistic lateralization in people with Down syndrome might be related to their specific auditory information processing, but not to their intellectual abilities. The DMT might be a suitable method for examining linguistic laterality in people with intellectual disabilities and those with Down syndrome.
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