Digby Elliot, Department of Kinesiology, McMaster University, Hamilton, Ontario
Daniel J. Weeks and Romeo Chua, School of Kinesiology, Simon Fraser University, Burnaby, B.C.
2nd International Conference "Motor Control in Down Syndrome"
Chicago, IL. November 19-20, 1994
Brazilian International Journal of Adapted Physical Education Research, 1(1), 167-8 (1994).
|Reprinted with the permission of the authors|
Over the past fifteen years, a number of studies have been conducted to determine how the karyotype associated with Down syndrome influences the development of cerebral specialization. The premise has been that atypical patterns of brain organization may be responsible for some of the specific and general information processing difficulties experienced by these people. The majority of the studies have employed non-invasive neurobehavioral techniques developed to examine laterally in the intact brain (1).
The initial studies were designed to examine cerebral specialization for speech and language. Investigators employing dichotic listening techniques have found that persons with Down syndrome display either no ear advantage(2) or irregular left ear superiority(3) for the perception of speech sounds. Hartley (1982) proposed that reversed cerebral dominance for speech and language may be responsible for some of the specific sequential language problems associated with Down syndrome (4). Specifically, when performing language tasks, persons with Down syndrome may be relying on right hemisphere systems which are non-optimal for the type of sequential processing required. Research from our laboratory, however, indicates that the brain-behavior relations in these people are more complicated.
Our initial studies on laterality and Down syndrome were designed to determine if these individuals exhibit the same manual asymmetries for tasks such as rapid finger-tapping and finger-sequencing as right-handed persons without Down syndrome. These studies revealed that adults with Down syndrome generally displayed the same pattern of lateral advantage as everyone else. Specifically, they exhibited greater temporal consistency with the right hand than the left hand suggesting left hemisphere superiority for the parametrization and timing of muscular forces (5). These results, along with two studies in which we examined interlimb transfer of training (6), suggest that persons with Down syndrome are left hemisphere specialized for the organization and control of sequential movement.
Since the neural structures that provide the basis for manual asymmetries have been proposed to underlie left hemisphere specialization for speech production, we decided to examine lateralization for speech production in persons with Down syndrome. Employing a dual task paradigm, we investigated the influence of concurrent speech on rapid finger-tapping. Contrary to the model of reversed cerebral specialization proposed by Hartley (1982) (7), we found that speech production disrupted right hand performance more than left hand performance. This asymmetric dual task effect is thought to reflect within-hemisphere interference between the cerebral areas controlling the production of speech and the organization of finger movements.(8) This study, as well as a recent replication (9), suggests that, while persons with Down syndrome may depend on the right hemisphere for the perception of speech, their left cerebral hemisphere seems to play a more important role in speech production. Based on the dichotic listening studies and our work on asymmetries in oral and manual motor control, we (10) developed a model of cerebral organization for persons with Down syndrome. The model has as its main feature the disconnection of cerebral systems responsible for speech perception (right hemisphere) from those responsible for movement organization (left hemisphere), including the organization of speech movements. This separation leads to a loss of information due to interhemispheric transmission. Thus, persons with Down syndrome are expected to exhibit specific information processing difficulties in tasks that require both the perception of speech, and the organization and control of limb and oral movement. Our model of cerebral organization in persons with Down syndrome is depicted in Figure 1 (below).
Support for our model comes from studies in which we have shown that, relative to other mentally handicapped people, adults with Down syndrome have difficulty performing limb and oral movements on the basis of verbal instruction (11). This verbal-motor difficulty becomes more pronounced as the complexity of the movement sequence is increased, and does not appear to be due to a simple language comprehension problem. These movement difficulties appear to be specific to verbal-motor behavior, since subjects with Down syndrome perform the same movements slightly better than control subjects following a demonstration (12). Recent work designed to identify the specific locus of the difficulty suggests that persons with Down syndrome take longer to prepare movements on the basis of verbal information(13). As well, preliminary work indicates that this information processing problem also affects the ability of adults with Down syndrome to learn a new motor task (14).
Not only have we found group differences in verbal- motor ability, but it also appears that persons with Down syndrome who display a stronger left ear (right hemisphere) advantage for speech sounds evidence more pronounced verbal-motor difficulties (15). Presumably this is due to the greater functional separation of speech perception and movement organization systems. Interestingly, within a group of adults with Down syndrome, a left ear dichotic listening advantage also predicted poorer performance on the Raven's Coloured Progressive Matrices (16). This finding suggests that there is another price to pay for the development of right hemisphere receptive language function; that is, the disruption of the visual-spatial abilities required to perform this type of complex task. In some of our most re- cent work, we have attempted to extend our understanding of brain organization in persons with Down syndrome to cerebral specialization for visual-spatial function (17).The majority of this research was funded by the Ontario Mental Health Foundation.
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In a companion paper (Elliott, Weeks, & Chua, 1994), we discussed how our earlier research prompted us to forward a model of cerebral lateralization for the DS population that has as its primary feature the dissociation of the functional subsystems subserving speech perception and the organization of complex movement. The behavioral consequence of the model is manifested as difficulty in performing tasks that require both speech perception and the organization and control of movement. In support of the model, we nave shown that, relative to other mentally handicapped individuals, adults with DS exhibit greater difficulty performing limb and oral movements on the basis of verbal instruction. Further, the deficit is apparently specific to verbal-motor behavior as individuals with DS perform marginally superior to control subjects in response to a demonstration (Elliott, Weeks, & Gray, 1990). Moreover, we have shown that these performance deficits also may influence motor skill acquisition by DS persons as well (Elliott, Gray, Weeks, 1991). Finally, an evaluation of individual differences revealed that persons with DS who display a stronger left ear (right hemisphere) advantage for speech sounds also display more pronounced verbal-motor difficulties (Elliott & Weeks, 1993). It may be that the development of right hemisphere receptive language in DS persons contributes to the disruption of visual-spatial abilities.
In order to further articulate our model, we (Elliott, Pollock, Chua, & Weeks, in press; Weeks, Chua, Elliott, Lyons, & Pollock, in preparation) have recently considered cerebral specialization for spatial and receptive language processing in the DS population. For tasks requiring spatial processing, the results suggested that, like people who are not disabled, most persons with DS are right hemisphere specialized for the processing of spatial information. Examination of receptive language processing revealed that atypical cerebral specialization for receptive language in the DS group may extend beyond the auditory modality (Weeks, Elliott, Chua, Lyons & Pollock, 1994). In sum, our work together with the dichotic listening studies has led us to the position that atypical cerebral organization of function in the DS group is confined to speech perception.
We propose two new directions for our research designed to bring closure to many of the outstanding implications of our model. One direction is to determine if the disconnection between functional systems predicted by our model impacts upon dynamic, coordinative actions requiring visual-motor or auditory-motor integration. The emerging perspective of Dynamical Systems will be used to further examine the effective coupling of perception to action (i.e., speech perception and movement production) in persons with DS. The second direction is to employ electrophysiological measures (EEG and MEG) to examine the active neural systems in the brain that underlie the performance of verbal and motor tasks by persons with DS. By extending our observations beyond the behavioral level to the level of the neural systems, the internal and external validity of our model will be strengthened by obtaining more "direct" evidence.
The long term goal for our model is to render it sufficiently detailed to motivate the development of sound and effective strategies for circumventing, or at least reducing, the behavioral challenges characteristic of the DS population.
This research has been funded by the Ontario Mental Health Foundation, the British Columbia Health Research Foundation, and the Natural Sciences and Engineering Research Council of Canada.