Suhana Shereef

Journal of Undergraduate Research
Volume 2, Issue 3 - December 2000

Cognitive-Linguistic Deficits in Down's Syndrome

ABSTRACT

Down's Syndrome (DS) is caused by a genetic anomaly that results in an extra chromosome. Individuals with DS experience various developmental delays. This paper reviews the existing literature on how language development is affected in individuals with DS in terms of memory impairment, cerebral specialization, speech perception, hearing impairment, and neurological evidence.

INTRODUCTION

There is a vast amount of research that has investigated the cognitive-linguistic deficits in DS. Memory impairments, atypical cerebral organization, and impaired hearing are just a few of the sources for their language delay.

Memory Impairments

Working memory is defined as the temporary maintenance of information that is necessary to perform certain cognitive tasks (Jarrold et al., 1999). Baddeley and Hitch (1974, cited in Jarrold et al.) claim that verbal and visuo-spatial material is maintained separately within the working memory of normally functioning individuals. These subsystems are called the phonological loop and the visuo-spatial sketchpad respectively. Jarrold et al. (1999) cited three pieces of evidence in support of this claim. One, the maintenance of verbal information in short-term memory is disrupted by simultaneous verbal tasks, but not by visuo-spatial tasks. Similarly, short-term memory for visuo-spatial information is affected by visual or spatial secondary tasks but not by verbal distractions (Brandimonte et al., 1992, Logie et al., 1990, cited in Jarrold et al., 1999). Two, brain-imaging studies indicated that different neural areas are involved in short-term storage of verbal and visuo-spatial information (Awh et al., 1996; Jonides et al.,1993; Paulesu et al., 1993; Petrides et al., 1993; Smith et al., 1996, cited in Jarrold et al., 1999) . Verbal short-term memory processing appears to be localized in the left hemisphere while visuo-spatial short-term memory appears to be linked with the right hemisphere. And three, there are reported cases of patients with selective loss of either verbal or visuo-spatial short-term memory following right or left hemisphere lesions (Hanley et al., 1991; Shallice and Vallar, 1990; Shallice and Warrington, 1970; Vallar and Baddeley, 1984, cited in Jarrold et al., 1999).

There is evidence to indicate that these working memory subsystems are affected in individuals with certain genetically based conditions. In fact, individuals with DS were hypothesized to have a phonological loop deficit (Das and Mishra, 1995; Bird and Chapman, 1994; Varnhagen et al., 1987, cited in Jarrold et al., 1999). Contrastingly, another genetically based condition known as William's Syndrome (WS) has been associated with a deficit in visuo-spatial short-term memory. Wang and Bellugi (1994, cited in Jarrold et al., 1999), for example, found that the WS group was superior in their Digit span (numbers of digits recalled) tasks while the DS group was superior in their Corsi span (number of spatial locations recalled) tasks. Findings from several studies (e.g. Gathercole and Baddeley, 1989; Gathercole and Baddeley, 1990; Gathercole et al., 1992; Papagno et al., 1991, cited in Jarrold et al. 1999) suggest a relation between verbal short-term memory skills and subsequent vocabulary acquisition. This hypothesis is supported by the evidence that individuals with DS do have delayed verbal abilities compared to non-verbal abilities (Gunn and Crombie, 1996, cited in Jarrold et al., 1999).

Cerebral Specialization

Elliot et al. (1987, cited in Weeks et al., 1995) claims that DS demonstrates a dissociation between the speech production (left hemisphere) system, and the speech perception (right hemisphere) system. A partial loss of information resulting from inter-hemispheric transmission was thought to be responsible for the difficulties the DS subjects have with tasks involving both speech perception and control of complex movement. Similar findings were found in other studies (Weeks et al., 1995; Elliot et al., 1990).

Moreover, a study on cerebral dominance for speech production examined mouth asymmetry in the DS population (Heath and Digby, 1999) and found that the right side of the mouth opens wider and sooner than the left suggesting greater contra-lateral projections from left hemisphere (LH) that innervate right side of body. Therefore the innervatory patterns of speech systems in LH is directly accessible to the right facial muscles. This is referred to as right mouth asymmetry or RMA (Van Gelder and Van Gelder, 1990 as cited in Heath and Digby, 1999). Previous studies have shown that in right-handed, non-handicapped subjects, (for example, Wolf and Goodale, 1987, cited in Heath and Digby, 1999) the right side of the mouth opens wider and sooner than the left. Therefore the DS group seems to display the same pattern of RMA as the non-handicapped group. According to Heath and Digby (1999), these results suggest that cerebral laterization for DS individuals are the same as it is for non-DS individuals thus supporting Elliot et al.'s (1987) biological dissociation model.

Speech Perception

Several researchers have investigated speech perception ability among DS individuals. Researchers (Lezak, 1983; Newcomer and Hammill, 1988; Sattler, 1990, cited in Marcell et al., 1995) found sentence imitation (SI) to be a very versatile test as it taps into verbal knowledge, comprehension, expressive syntax, short-term auditory memory, attention and left hemisphere functioning. Thus, Marcell et al (1995) studied sentence imitation in persons with DS and a control group of moderately intellectual disabled (ID). They found that the DS population began sentence imitation more slowly and initiated sentences less accurately. The absence of a DS-ID response time difference in orally defining vocabulary items suggested that the slowness with SI tasks was not tied to a general slowness in planning or initiating oral responses. Grammar was deliberately kept simple, therefore poor SI in DS was probably due to a reduced ability to accurately process and/ or precisely reproduce small amounts of verbal information in short-term memory. The DS group (adolescents and young adults) had trouble with SI from low to high information loads, i.e. sentences of three or more words. Results suggest that their difficulty with SI is more related to reproduction difficulty than with a lack of initial understanding or inability to store key words in sentences. Poor SI was associated with low expressive language skill, slow processing of single spoken words, poor acoustic reflex and reduced ability to discriminate speech. Individuals who required more effort to just identify input had reduced retention capacity. In general it was found that SI difficulties in DS are accompanied by problems in the area of hearing and auditory cognition. Therefore this study suggests that delayed language in DS could be due to difficulties in perceiving speech accurately.

Hearing Impairment

There is a high incidence of hearing impairments in DS. Otitis Media (OM), a middle ear infection, resulting in conductive hearing loss (CHL) is commonly seen in DS children. According to a recent study done on a young gerbil by Tucci et al. (1999), CHL leads to reduced synaptic activity in the central auditory system. This is a significant finding since excitatory synaptic activity is necessary for normal development and maintenance of structure and function of central auditory pathways. It is suggested that a diversity of auditory input may be required for normal maturation of synapses. Thus, limited auditory experience can lead to greater receptive field with poorer frequency resolution and more importantly presence of inappropriate input during development (Tucci et al.,1999). However according to Kumin et al., (1994) hearing impairment on its own cannot be generalized to the majority of this population so it is not a sufficient explanation for the significant difference in rate of language acquisition. Therefore a high incidence of hearing loss in DS cannot singly be responsible for their delayed linguistic ability (Kernan and Sabsay, 1996).

Neurological Evidence

Neuropsychological investigations by Azari et al (1994) found more language impairment than other cognitive functions in individuals with DS. Regional cerebral metabolic rates for glucose (rCMRglc) indicate brain functional activity. Correlations between normalized regional metabolic rates (ratios of regional to whole brain metabolism) can identify known functional interactions between brain regions (Horwitz, 1990 and Horwitz et al, 1992, cited in Azari et al., 1994). Therefore smaller correlations involving frontal and parietal regions reflected attentional deficits in DS and reduced correlations involving Broca's area reflected language deficits (Horwitz et al., 1990, cited in Azari et al., 1994). Atypical rCMRglc in the primary language areas may be caused by disruption in neural pathways significant to language. Since Broca's area is localized for language production (Geschwind, 1970, cited in Azari et al., 1994) and the left temporal cortex contains essential language regions (Ojeman, 1991, cited in Azari et al., 1994), decreased functional interactions in these areas produces greater language deficits versus non-language (Haxby, 1989, cited in Azari et al., 1994). These atypical functional interactions in the left hemisphere are congruent to the hypothesis of right hemisphere dominance for language. Therefore, this study suggests some significant physiological evidence to explain language deficits seen in DS.

CONCLUSIONS

Further research is needed to understand the role of CHL and short-term auditory memory deficits in language acquisition. It is worth noting that DS individuals' language acquisition has the same pattern of development as non-DS individuals except on a more delayed course. So the same phonemes that are acquired later in typical language development may be even more delayed in DS because of the limitations on their auditory memory. Also the more phonemes there are to master the longer it takes for children to build their phonetic inventory. There are certain standard categories of phonemes in a language like stops, nasals, fricatives, affricates, liquids and glides. These categories are based on their manner of articulation. For example, fricatives are phonemes that are produced by forcing air out a narrow constriction in the vocal tract like s, z, f, and v. Therefore DS individuals may find it more difficult to master these more complicated sequence of oral-motor movements.

In addition, the presence of hearing impairment may also affect the pattern of phoneme acquisition. They may have increased difficulty in discriminating between phonemes that sound similar, like voiced and voiceless pairs, or phonemes with higher frequency acoustic energy like fricatives (eg. s, z, f and v in English). Therefore DS individuals with hearing impairment may be at a disadvantage to receive auditory input depending on the different acoustic dimensions involved.

In conclusion, cerebral specialization, auditory memory impairments and the way it affects verbal and motor functions and dissociation of speech perception and speech production are all significant deviations from typical language acquisition. These differences affect individuals to varying degrees. Experts strongly advocate that we treat each individual without preconceived ideas of their capabilities or limitations. It is important to acknowledge the great degree of variability among DS and thus we must be careful about generalizing findings based on a sample population.

REFERENCES

Azari, N. P., Horwitz, B., Giacometti, K. D., & Schapiro, M. B. 1994. Abnormal pattern of cerebral glucose metabolic rates involving language areas in young adults with Down syndrome. Brain and Language, 46, 1-20.

Bird, E. K. R., Chapman, R. S. 1994. Sequential recall in individuals with Down syndrome. Journal of Speech and Hearing Research, 37, 1369-1380.

Heath, M., Elliot, D. 1999. Cerebral specialization for speech production in persons with Down syndrome. Brain and Language, 69, 193-211.

Jarrold, C., Baddeley, A.D., Hewes, A.K., 1999. Genetically dissociated components of working memory: evidence from Down's and William's syndrome. Neuropsychologia, 37, 637-651.

Kernan, K. T., Sabsay, S. 1996. Linguistic and cognitive ability of adults with Down syndrome and mental retardation of unknown etiology. Journal of communication Disorders, 29, 401-422.

Kumin, L., Councill, C., Goodman, M. 1994. A Longitudinal Study of the Emergences of Phonemes in Children with Down's Syndrome. Journal of Communication Disorders, 27, 295.

Marcel, M. M., Ridgeway, M. M., Sewell, D. H., Whelan, M. L. 1995. Journal of Intellectual Disability Research, 39,
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Tucci, D. L. (MD), Cant, N. B. (PhD), Durham, D. (PhD), 1999, Conductive hearing loss results in a decrease in the central auditory system activity in the young gerbil. Laryngoscope, 109, 1359-1371.

Weeks, D. J., Chua, R., Elliot, D., Lyons, J. and Pollock, B. J. 1995. Cerebral specialization for receptive language in individuals with Down's syndrome. Australian Journal of Psychology, 47(3), 137-140.

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