[tri-med] FYI - Part 2 of an editorial on speech disorders

I am forwarding this article or URL for your information (FYI) as I believe
it may be of interest and is from a reliable source. As always, check the
information with your own doctor or health care professional before starting
or changing any treatments.

PART 2 of 2 (continued)
Brain, Vol. 125, No. 3, 449-451, March 2002
© 2002 Oxford University Press
Editorial

Towards the elucidation of the genetic and brain bases of
developmental speech and language disorders

Jenny Harasty1 and John R. Hodges2

One possible explanation for a bigger cortex in developmental
disorders is a lack of apoptosis (or programmed cell death) that
occurs in the normal developing brain. Such cellular pruning
presumably enhances the cortex's specialization and ensures that
appropriate cellular connections occur (Seldon, 1981). Perhaps a
larger cortical gyral volume in certain brain regions suggests that
this important developmental process did not occur leaving a more
haphazard cellular structure whose lack of form and structure impede
cortical functioning.

The results of the study by Watkins et al. (2002a) highlights the
importance of subcortical structures particularly the caudate nucleus
and putamen in language development. Interestingly, the dorsal part
of the caudate appeared to be particularly involved in the KE family,
a pattern of volume loss similar to that found in Huntington's
disease (Vonsattel et al., 1985). Furthermore, the volume of the
caudate nucleus was significantly correlated with the performance of
affected family members on tests of oral praxis and nonword
repetition but with a complex pattern: the greater the reduction on
the left, the poorer the performance on a test of oral praxis,
whereas the greater the reduction on the right, the better the
performance on a test of nonword repetition requiring complex
articulation. The authors are wisely cautious in their interpretation
of this pattern given the relatively small number of subjects
involved. One final comment relates to the bilateral nature of the
abnormalities which point to a very generalized defect in neural
development and tie in with the finding of their parallel behavioural
study discussed below.

The companion paper by Watkins et al. (2002b) explores, in some
detail, the behavioural consequences of the gene mutation and
resultant neural maldevelopment. Thirteen affected and 12 unaffected
members of the KE family were assessed using a comprehensive battery
of tests of general intellectual ability, receptive and expressive
language and praxis. Exactly the same battery was also given to a
group of 11 patients with aphasia resulting from left hemisphere
strokes which involved the opercular region. The findings provide an
important contrast to other studies of individuals with developmental
speech and language problems. Tallal and colleagues have argued
persuasively in favour of a core defect in temporal processing of
speech sounds, and moreover that specific temporal order re-training
can ameliorate the problem (Tallal et al., 1983). Gathercole and
Baddeley (1990) have proposed that a deficit in the phonological loop
component of working memory represents the key defect in some cases.
Others, such as Rice and Wexler (1986) have suggested that a deficit
in the development of the grammatical aspects of language
characterises specific developmental speech and language disorders.
Even the nature of the disorder in the KE family has been the topic
of considerable debate among different groups of investigators. The
first report of the KE family described affected members as suffering
from a 'severe form of developmental verbal apraxia' (Hurst et al.,
1990 ). Gopnik and colleagues have focused on the linguistic
impairments in affected individuals; in particular, their deficit in
the use of infectional morphosyntactic rules (e.g. changing word
endings to mark tense and number), which has been described as
selective (Gopnik and Goad, 1997).

One major finding of the Watkins et al. (2002b) study is that
affected members of the KE family have widespread deficits which
involve virtually all aspects of speech and language, as well as
aspects of non-verbal intelligence. Indeed, affected members and
patients with stroke-related aphasia had remarkably similar profiles
of impairment on the tests administered, except that the aphasia
group had less impairment on non-verbal tasks. Longitudinal test
scores available in a subset of younger affected individuals showed a
progressive decline in performance IQ. These findings suggest that 'a
developmental speech and language disorder could have detrimental
effects on various components of nonverbal intelligence, as well as
lexical development and familiarity with the articulation of common
word'. The finding of the present study also make untenable the prior
claims that the family has a specific deficit in morphosyntactic rule
usage. Watkins et al. (2002b) argue, instead, in favour of a core
deficit in sequencing and learning of verbal and nonverbal
associations, although the exact nature of this core deficit requires
clarification. From a practical viewpoint, affected and unaffected
family members were best discriminated on a test of nonword
repetition thus confirming the value of this simple test in screening
for developmental speech and language impairment (Gathercole and
Baddeley, 1990; Bishop et al., 1996). Whereas Gathercole and
Baddeley, who devised the test, have suggested that impairment in
nonword repetition is related to a specific deficit in the storage of
phonological information in working memory, the present authors
propose that the defect in the KE family reflects deficits not in
phonological memory per se, but rather in sequential articulation of
phonological units. It remains possible, if less parsimonious and
attractive, that the FOXP2 gene defect produces multiple independant
speech, language and cognitive impairments. It should also be
remembered that the study involved members of a single, and in many
ways, unique kindred and may be applicable to other individuals with
development speech and language disorders. As with other clinical
neuropsychological syndromes, it is highly likely that this
represents a heterogeneous disorder which has a number of different
underlying cognitive explanations.

As well as the specific implications, these landmark studies
illustrate the importance of a combined multi-disciplinary approach.
These two papers represent a triumph for international collaboration
and dogged determination on the part of the scientists and clinicians
involved to pursue both the cause and the wider implications of this
fascinating disorder. It is through the combination of genetic,
neuroanatomical and
cognitive analyses of this type that further advances are likely to
be made in this and other developmental and degenerative disorders of
the nervous system.


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