Scandinavian Journal of Psychology, 1992, 33, 238-246

Hemispheric asymmetry effects in children studied by dichotic listening and visual half-field testing

SVEN-AKE CHRISTIANSON', JOUKO SAISA', KENNETH HUGDAHL3, and ARVE ASBJ0RNSEN3 'Department of Psychology, University of Stockholm, Stockholm, Sweden 2Department of Psychology, Umeri University, Umeri, Sweden 'Department of Bio fogicai and Medical Psychology, University of Bergen, Bergen, Norway

Christianson, S.-A., Slisl, J., Hugdahl, K. & A. Asbjsrnsen. (1992). Hemispheric asym- metry effects in children studied by dichotic listening and visual half-field testing. Scundina- vian Journal of Psychology, 33, 238-246.

Dichotic listening (DL) and visual half-field (VHF) testing were used to study hemisphere asymmetry in a developmental perspective. Five-, 8-, and 11-year-old children were pre- sented lists of fused words using a DL technique in Experiment 1, and 8- and 11-year-old children were presented pictures of common objects using a VHF technique in Experiment 2. In both experiments, measures of identification, free recall, and recognition of the wordslpictures were employed. The results revealed effects of ear input (right-ear advantage) and half-field presentation (right visual half-field advantage) for all age groups, although the magnitude of this lateralization effect differed between the three memory measures. The results are discussed in relation to developmental aspects of language laterality, and in relation to the clinical utility of non-invasive lateralization techniques.

Keywordr: Children, lateralization, memory, language, dichotic listening, visual half-field testing.

Swn-Ake Chrhtianson, Department of Psychology, University of Stockholm, S - 106 91 Stockholm, Sweden

A critical issue for the understanding of language and brain-laterality relations is whether Iaterality develops across ages. This idea was introduced by Lenneberg in 1967, and it was assumed that there should be increasing asymmetry with age. This has, however, not been found in most studies using the dichotic listening (DL) technique (see e.g., Hiscock & Decter, 1988; see also Bryden, 1988 for a review). In considering previous DL studies, there are two factors that might be of special importance for the lack of relation between ear asymmetry and the development of linguistic competence. First, the role of attentional bias (i.e., directed attention to either the left or the right ear), and second, the role of linguistic difficulty for the test items. The role of attentional bias was recently studied by Asbjnrrnsen et al. (1991) using dichotic presentations of consonant-vowel (0 syllables. Five-, 8-, and 1 1-year-old children were tested under instructions to attend to either the right or the left ear. The results showed a weak overall right-ear advantage (REA), but no age effect in asymmetry. This outcome is in line with previous research, which has shown a less clear REA in children compared with adults (e.g., Hiscock & Decter, 1988; see also Bryden, 1988), but no difference in REA magnitude across age levels. Another finding in the study by Asbjmsen et al. was that the older children (8- and 11-years old) did not show a REA across all test trials. When the test trials were divided and analyzed for the &st and the second half, a REA was obtained for the second, but not the first half'of the trials. This result suggests that a practice effect is involved in DL for children.

One aspect that might have been critical to the weak REA and the practice effect obtained in the A s b j m e n er al. study, is the use of CV-syllables. Since CV syllables tap phonetic

.

Scand J F‘sychol 33 (1992) Hemisphere asymmetry in children 239

rather than semantic discriminations between ears, possible developmental laterality effects may be obscured by non-lateralized features related to CV-syllables. It is known that dichotic CV-syllables stimuli do not promote a strong hemispatial response bias (Hiscock & MacKay, 1983, and that CV-stimuli may involve nonlinguistic processes in the right hemisphere for their discrimination (Kerschner & Morton, 1990; Molfese & Be&, 1988). It is therefore conceivable that a more semantically demanding test situation would generate a heightened level of unilateral left hemisphere activation, and thus produce a more pronounced lateraliza- tion effect, especially for the older compared with the younger children. This could be tested by using real words instead of CV-syllables in the DL test. Furthermore, the use of a visual verbal test, with no phonetic features, should be a good test of the “phonetic explanation” for the absence of a strong laterality effect in young children in DL. Finally, employing a memory strategy in the DL-test would further involve cognitive semantic word features, rather than sensory phonetic. Both free recall, identification, and recognition memory were studied.

In order to test this hypothesis, two experiments were conducted. For each experiment a different lateralization technique was used. In the first experiment, a DL technique was used to demonstrate lateral differences for verbal stimuli (three-letter words). If lexical meaning requires a heightened level of left hemisphere engagement, a general REA would be expected (see Bradshaw & Nettleton, 1983 for a review). In the second experiment, a visual half-field (VHF) technique was used to demonstrate lateraliiation effects for visual stimuli (pictures depicting common objects that could be translated easily into verbal as well as visual codes). In line with the assumption made for Experiment 1, a general right VHF advantage would be expected (see Beaumont, 1982 for a review of the VHF-technique).

EXPERIMENT 1

In Experiment 1, lists of fused words were used as stimulus items. Six word pairs were presented in each list, and the 5-, 8-, and 1 1-year-old children were instructed to report both words in each pair. It was predicted that a REA effect should be observed, and particularly for the older children if the developmental hypothesis is correct. It was furthermore predicted that an eventual practice effect should be more easily observed in the younger children, because of their less developed language skills.

Method Subjects. A total of 88 children from Umel, Sweden were tested. Twenty-one children were 5-years

old (13 boys and 8 girls); 31 children were 8-years old (14 boys and 17 girls); and 36 were 11-yean old (19 boys and 17 girls). All children were right-handed as determined from Montreal Neurological Institute (MNQ handedness inventory. The handedness questionnaire was filled out by the experimenter in collaboration with the children. The children were tested individually at school and their participation was voluntary. The parents had to give their written permission before the testing. None of the children had a history of concussion or psychiatric illness, nor did they receive any medication at the time of testing.

Stimuli and appuratus. The DL test was composed of three word-pair lists with six word pairs in each list ( a total of 36 words). The words were common Swedish three-letter nouns (consonant-vowel-con- sonant) and the two last letters were always identical in each word, for example “BIL-PIL” (car-arrow). Thus, the two words within each word pair differed only with respect to the first letter. The DL-stimuli used were thus similar to the “fusing-technique” developed by Wexler 8c Halwes (1983) which has proved to be a sensitive DL-technique for left hemisphere laterality effects. The words were furthermore chosen so as to be easily recognizable in all three age groups. Each pair was only presented once and was randomly assigned to one of the three lists.

The dichotic tape was prepared on a PDP 11/45 computer system and with A/D and D/A converters. The words were read by a male voice and digitized at a minimum of 10KHz sampling rate.

240 S - A . Chrhtiamon et al. Sfand J Psycho1 33 (1992)

Synchronization was then achieved by visually displaying the words on. an 80 monitor and aligning each word pair for effect onset synchronization. Each word pair was then fed to a NAGRA IV tape-recorder over the D/A converters and multiplexer for recording. Procedure. The three word-pair lists were presented to the child by means of headphones and a SONY

stereo tape recorder at a rate of one word pair every Sscconds. Before presentation, the child w a s instructed that two words would always be presented simultaneously; one word to the left ear and one word to the right ear. The child was asked to report both words as accurately as possible from each word pair and also to try to remember the words. If he/she only managed to identify one word from a word pair, he or she should then report and remember that word. No instruction was given concerning allocation of attention between the ears. The experimenter marked the answers continuously from each trial throughout the experiment.

After the presentation of the last word pair of each list, the child was asked to recall as many words as possible from the list just presented. Following recall of the third list, there was a 5 min filler interval during which questions were asked about the child’s hand preference and then a recognition test was presented for all the 36 words presented. In this recognition test, the child was presented with the words from the thm lists, intermingled with an equal number of distractors. Five minutes were allowed for responding on the recognition test.

After having completed the dichotic listening task, the 5-year-old children were asked some informal questions and were then fully debriefed. The 8- and 1 1-year-old children were tested one week later with the VHF technique, reported in Experiment 2 of this study.

Results and dFrcussion

The reported data relate to three different dependent variables: percentage of correctly identified, recalled, or recognized words. When scoring the data by items, it was found that one word in each of six word pairs

( # 2, 6, 8 , 9 , 15, and 16) was always identified and reported, irrespective of ear input. These six word pairs were accordingly excluded from all data analysis.

The mean percentages of correctly identified words for the three age groups are presented in the left-hand columns of Table 1. The data of Table 1 suggest a main effect of age, a main effect of ear input, but hardly any effect of sex, and seemingly no interaction. A 2 (age group, 5-, 8-, vs. 11-years old) x 2 (gender) x 2 (right vs. left ear input) analysis of variance (ANOVA), with the last factor treated as a within subject variable, were carried out. There was a main effect of age, F (2,82) = 16.08, p < 0.001. Tukey comparisons showed significant differences between the 5-year-olds and the 8-year-olds (p cO.Ol), and the 5-year-olds and the 11-year-olds (p < 0.01), but not between the 8-year-olds and the 1 1-year-olds (p > 0.05). The ANOVA also showed a significant main effect of ear input, F (1,82) = 5.01, p < 0.05. Neither the main effect of sex, nor any of the interactions reached statistical significance (p > 0.05).

The recall data for the three age groups are presented in the middle columns of Table 1. As can be seen from this table, the performance for the 11-year-olds (boys and girls) are somewhat higher than that of the 8-year-olds, who in turn performed better than the 5-year-olds. A 2 x 2 x 2 ANOVA was carried out, which showed a significant main effect of age, F (2,82) = 25.45, p < 0.001. Tukey comparisons showed significant differences between all three age groups (p < 0.01). There was also a marginally significant main effect of ear input, F (1,82) = 2.92, p < 0.10. The main effect of sex did not reach statistical significance, nor did any of the interactions (p > 0.05).

The mean scores for recognition of the words are presented in the right-hand columns of Table 1. The data are corrected for guessing by means of a correction formula: (Hits-False alarmll-False alarm) x Hits. Inspection of the recognition data suggests the same age effect as were obtained in the previous two measures. A 2 x 2 x 2 ANOVA carried out on these data confirmed a significant main effect of age, F (2,81) = 4.74, p < 0.05. Tukey comparisons showed significant differences between all three age groups (p c 0.01). There were no

Scand J Psycho1 33 (1992) Hemisphere asymmetry in children 241

Table 1. Percent correct in identification, free recall, and recognition of words presented in the left (L Ear) or right (R Ear) for I, 8; and 1 1 -year-old boys and girls

Identification Free Recall Recognition

L Ear R Ear L Ear R Ear L Ear R Ear

Boys 5 Boys 8 Boys 11 x Girls 5 Girls 8 Girls 11 x Total x

39.74 43.45 50.44 44.54

41.67 41.67 44.61 42.65

43.60

41.67 11.54 53.57 13.69 54.39 21.93 49.88 15.72

43.75 13.54 52.94 14.71 53.43 24.51 50.04 17.59

49.96 16.66

10.26 21.43 25.44 19.04

13.54 22.55 29.41 21.83

20.44

37.3 42.8 41.4 46.0 48.0 46.9 42.23 45.23

34.8 33.9 37.3 41.7 46.1 47.3 39.40 40.97

40.82 43.10

Table 2. Percent correct in identificntion, free recall, and recognition of pictures presented in the left (L VHF) and the right visual harfjeld (R VHF) for 8- and 11-year-old children

Identification Free Recall Recognition

LVHF RVHF LVHF RVHF LVHF RVHF

8-year-olds 67.20 78.14 27.06 27.24 46.86 53.17 1 I-year-olds 78.27 86.60 34.97 39.22 60.37 72.39

x 72.74 82.37 31.02 33.23 53.62 62.78

significant main effects of ear input or sex, nor did any of the interactions reach statistical significance ( p > 0.05).

In general, it can be concluded that the DL data obtained for words were discriminative with respect to ear input and between the three age groups. However, no developmental effects in hemisphere asymmetry were found between the groups. Considering the three dependent measures used, a REA was obtained in the identification measure for all three age groups, but not in the other two, more specific, memory measures (free recall and recogni- tion). This would then lend support to the suggestion that a left hemisphere effect for verbal material is obtained even in quite young children when the stimuli used involve semantically meaningful information. However, the fact that the REA effect was lost in the recall and recognition measures, especially for the 5-year-olds, suggests that the lateralization effect is of perceptual nature with little involvement of specfic memory strategies.

The results of Experiment 1 were followed-up in the second experiment (Experiment 2), using a visual presentation mode (i.e., the VHF technique) with similar bilateral presentations as in Experiment 1.

EXPERIMENT 2 Besides DL, VHF testing has been used extensively during the last decades to study laterality of language (see Beaumont, 1982; Bradshaw & Nettleton, 1983 for reviews). The logic behind the VHF technique is essentially the same as for the DL. That is, each hemisphere is accessed by ipsilateral fibers from the temporal hemiretina of one eye and by contralateral fibers from

242 S.-A. Christianson et al. Scand J F%ychol 33 (1992)

the nasal half of the retina of the other eye. Thus, given an adequate eye-fixation control, any image presented exclusively in the left half of the visual field will initially project only to the right hemisphere and vice versa for the right visual field.

Essentially, research utilizing the VHF technique has reported a right visual-field advan- tage (RVFA) for both unilaterally and bilaterally presented verbal stimulus material, although a RVFA is commonly found to be more pronounced with bilateral presentation (McKeever, 1986). It is possible that bilateral presentation constitutes a competing situation for the brain and that this extra load for the cognitive system makes assessment of hemisphere asymmetry more differentiating.

In line with the assumptions made in Experiment 1, it was argued that a more demanding task should be more differentiating .with respect to development of laterality, thus showing a more pronounced RVFA for older children compared with younger children. Furthermore, it was predicted that older children would show higher memory accuracy, especially for items presented in the right VHF.

Method Sibjects. A total of 65 children participated in Experiment 2. The subjects were the same 8- and

11-year-old children who were tested in Experiment 1. (Due to restrictions in the apparatus to control for eye movements, the Syear-old children were excluded from this test).

Stimulur. A total of 42 stimulus pictures were prepared. The pictures showed black and white outlines of simple objects that could be named easily by all subjects (e.g. a whale, a stove, a cat, etc.). The pictures were presented in pairs (one picture in each VHF) and divided into four lists: three pairs were presented in List 1 (training List) and six pairs were presented in List 2, 3, and 4, respectively. The pictures were rotated in the four lists and counterbalanced for the right VHF and the left VHF. The visual stimuli could easily be named by aU subjects.

Apparutus. Stimulus presentations in the right and the left VHFs were effected by means of a projection unit and an eye-movement detector. This device is described in detail in Lindahl et ul. (1988), and only a condensed description will be presented here.

The projection unit was composed of a modified wide-angle zooming slide projector, equipped with a Hunter electronic shutter. The focusing screen was vertically divided at the midline into two equally sized fields. A fixation point; a permanently illuminated red light emitting diode (LED), was located at the center of the screen. T'he picture pairs were projected on the s c r m in the range of 3-8 degms of visual angle to the right and left of the LED fixation point.

Eye movements were detected using a ocular limbus (the boundary between the iris and the sclera) tracking technique. The right eye of the child was illuminated with infra-red (IR) light, and IR sensitive detectors, located in an adjustable stand, 12mm in front of, and just below the eye, were used to measure the reflected IR light from the limbus area. A control board, containing circuitry for automatic control of stimulus presentation, received an eye-movement logic signal and a stimulus onset signal. Thus, the stimulus slide could not be presented unless the eye of the child were centered on the fixation point, which triggered the shutter to expose a slide with a picture pair.

Procedure. Upon arrival to the laboratory, the child was seated 1 m in front of the projection screen. A chinrest, a forehead rest, and a head restraint were used to prevent head movements. In order to calibrate the eye-movement detector, the child was asked to look at the red LED in the middle of the screen, and then at four red LEDs at 3 and 8 degrea of visual angle horizontal to the right and left of the fixation point.

Each trial was initiated by the Experimenter by forwarding a stimulus slide including two pictures (one picture in each VHF). Before the presentation of the first pair of pictures, the child was instructed to (a) immediately name the pictures he/she had just sMn on the screen, and (b) to remember the pictures for a subsequent memory test. Each trial began when the Experimenter pressed a button to initiate presentation of the stimuli. When the child looked steadily on the fixation point the shutter opened and exposed the slide for 150 ms. After the child had responded to the slide, the experimenter advanced the slide tray, made necessary calibrations of the eye-movement detector, and then presented

.the next picture pair. Three picture pairs were presented as training trials. After these pre-trials, three lists with six pairs of pictures in each list, were presented. After presentation of the last picture pair in each list, the child was released from the head restraint, and given a free recall test of the pictures. In

Scand J Psycho1 33 (1992) Hemisphere asymmetry in children 243

this test, the child was asked to recall aloud, and in any order, as many of the pictures as possible that had been presented in the previous list. Three minutes were allowed for this recall test. Next, the child was given a recognition test of the pictures. Twelve copies of the pictures from the list, intermingled with an equal number of new pictures (distractors), were presented in random order on a paper sheet. The child was asked to point out 12 pictures, which helshe recognized from the preceding list of pictures. Time for this recognition test was three minutes. Thereafter, the child was told to re-position his/her head in the head fixation stand, and the experimenter calibrated the eye-movement detector, and then initiated the presentation of the next list of picture pairs. The three picture lists, with recall and recognition tests after each list, were presented continuously. Following the recognition test of the third experimental list, the child was asked some informal questions and was then debriefed. Total time for the experimental procedure was approximately 20 minutes.

Results and discussion The results of identification, recall, and recognition of pictures presented in the left or the right VHF for the 8- and the 1 1-year-old children are summarized in Table 2. Inspection of the identification data presented in the left-hand columns of Table 2, suggests that more pictures were identified from the right VHF, and that the 11 -year-olds were somewhat better than the 8-year-olds in identifying the pictures. That is, a left hemisphere advantage was obtained also to VHF presentations.

A 2 (8-years old vs. 1 1-years old) x 2 (left vs. right VHF) ANOVA, with the second factor treated as a within-subject variable, confirmed the above impressions of the data. There was a statistically significant main effect of age, F (1,63) = 12.83, p < 0.001. There was further- more a significant main effect of VHF, F ( 1,63) = 18.63, p < 0.001. The VHF effect showed better identification of pictures presented in the right VHF. The interaction did not reach statistical significance.

The recall scores are presented in the middle columns of Table 2. These data indicate higher scores for the 11-year-old children compared with the 8-year-olds. The data also suggest that pictures presented in the right VHF were somewhat better recalled than those presented in the left VHF.

The same kind of 2 x 2 mixed factorial ANOVA as was used for the identification data confirmed a significant main effect of age, F (1,63) = 22.83, p <0.001. The main effect of VHF and the interaction did not reach statistical significance.

The recognition data are shown in the right-hand columns of Table 2. These data are correct for guessing according to the formula: (Hits-False alarm/l -False alarm) x Hits. Again, the most notable effects were overall right VHF superiority and better performance for the 11 -year-old children, but no interaction. A 2 x 2 mixed factorial ANOVA confirmed these impressions, by showing a signiticant main effect of age, F (1, 63) = 12.62, p c 0.001, and a significant main effect of VHF, F (1,63) = 9.23, p < 0.01. In addition, separate ANOVAs including sex as a factor, were conducted for each of the

three measures (identification, recall, and recognition). None of these analyses showed any statistically significant effects of sex.

GENERAL DISCUSSION The results from Experiments 1 and 2 seem to warrant reasonably straightforward conclu- sions. First, with respect to the three age groups, the 11 -year-olds performed on average at a higher level than the 5- and the 8-year-olds, and the 8-year-olds performed better than the 5-year-olds.

Second, left hemisphere laterahation effects (i.e., REA or RVFA) were obtained for the identification measure in both methods used. It is encouraging that the VHF data for pictures revealed similar lateralization effects and differences between age groups as the DL

244 S.-k. Christianson et al. Scand J Psycho1 33 (1992)

data obtained for words. This finding suggests that both techniques may tap the same underlying mechanism for the functional differences that were seen between the hemispheres. The obtained effects were however small, perhaps not warranting too strong a conclusion.

It is also important to note that lateralization effects were observed not only in the identification tests, but also in the recall (DL) and recognition (VHF) tests (i.e., for the 8- and the 11-year-olds), suggesting that the present techniques are sensitive to differences in memory processing between the two hemispheres. The fact that a left hemisphere effect was observed not only for identification and recognition, but also for recall suggests that the stimuli activated retrieval processes in the brain that surely involves an active memory search. However, while recall yielded a left hemisphere advantage in the DL test, recognition yielded a left hemisphere advantage in the VHF test. This pattern of results suggests that the two laterality tests tap different aspects of hemisphere specialization, or, alternatively, that lateralization effects are more distinct in identification than in recall and recognition. In comparison with the relatively weak REA observed in the non-forced condition of the

Asbjmsen er al. (1991) study, the present results demonstrate a lateralization effect for the identification measure in all age groups, although this effect was less robust for the 5-year-olds. This difference between studies might be explained by the fact that the stimulus items in the present study were semantically more meaningful (and perhaps more motivating for the children), thus generating a heightened level of left hemisphere activation. This is an important finding since Moscovitch (1979) has argued that laterality effects only occur at later, higher order, stages in an information processing perspectives. This may be even more important when studying children. Thus, having only phonetically activating stimuli as in the Asbjmsen et al. (1991) study, may not have been enough to activate other structures and processes beyond sensory matching on a phonetic level.

Further, it has been argued (e.g.. Hugdahl & Andersson, 1986; Obrzut & Hynd, 1986) that children have a less efficient attentional control in the DL situation than adults, and that children fluctuate more between trials. I t may be then, that a semantically more demanding stimulus material, which at the same time is more attention catching, is more efficient in the control of attention in children. Thus, when words are presented instead of CV-syllables, it is assumed that attentional resources of children are guided by neurolinguistic factors to a larger extent, and more pronounced asymmetry effects should accordingly be observed to words than to CV-syllables.

Similar to the results of the Asbjmsen et al. (1991) study, neither of the two experiments in the present study showed any significant interactions between age and ear input/VHF. This outcome suggests either that the test procedures used are insensitive to such effects, or that developmental effects of hemisphere asymmetry are more evident at an earlier age than at five years of age. From previous research, we know that effects of functional asymmetry have been observed in 3- and Cyear-olds (e.g., Hiscock & Decter, 1988), and also that asymmetry effects have been shown already at infant level (Witelson & Pallie, 1973; see also Young et al., 1983 for a review). Presumably, the development of hemisphere asymmetry is a continuous process, and we believe that developmental effects of neurolinguistic compe- tence can be studied over age (at least up to school age) if more optimal stimulus materials and test procedures are used. Unfortunately most studies in this area of research have used a wide variety of test procedures, which has made it difficult to compare findings across studies.

Given that the REA and the RVFA demonstrated in the DL and the VHF test, respectively, reflects a left hemisphere dominance for language functions, these techniques could be potentially valuable for clinical practice. The clinical applicability of non-invasive lateraliza- tion methods in assessing language, memory, and emotional functions in adults has been

Scand J Rychol 33 (1992) Hemisphere asymmetry in children 245

shown, for example, in connection with brain surgery.(see e.g., Christianson et al., 1987, 1989; Ststi et al., 1990; see also Hugdahl et al., 1990~1, b), and can thus be used to complement invasive techniques (e.g., Sodium Amytal testing, see e.g., Christianson et af., 1990; Rausch, 1987). It should be noted, however, that the present study did not employ brain lesioned subjects, thus the clinical utility has not been proven, and should there- fore be read cautiously until further research has been made. A recent study by Hiscock & Hiscock (1990) on hemisphere differences in children with congenital hemiplegia sug- gests, however, that both techniques may have an interesting, hitherto neglected, clinical application.

This research was supported by Grant 89/9 from the Bank of Sweden Tercentenary Foundation to Sven-he Christianson, and by grants from the Nordic Council for Research in the Social Sciences to Kenneth Hugdahl, Sven-Kke Christianson, Stem Larsen, and Heikki Lyytinen. The stimulus material used in Experiment 1 was constructed by Sven-Ake Christianson in collaboration with AB Consonant, Voice Communication & Phonetic Engineering, Uppsala, Sweden. We thank Lena Hagberg for her help with data collection. We also thank Jerker RBnnberg and one anonymous reviewer for their helpful suggestions for revision.

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