The Cerebellar Deficit Hypothesis

Nicolson et al. (1995) argued that "A search for the underlying cause of deficits in balance, in motor skill and in automatisation would generally point strongly to the cerebellum" (p. 43). In this paper, they report research based on previous findings with patients who had damage to the cerebellum. Ivry and Keele (1989) had found that in comparison with Parkinson's disease patients, cerebellar patients were impaired on a task requiring them to judge which of two auditorily presented intervals of time was the longer. Nicolson et al. gave the same task to dyslexic children and found them to be impaired relative to age and IQ matched control children. Like cerebellar patients, the dyslexics were not impaired on a comparable loudness-estimation task.

In a replication of their earlier work, Fawcett and Nicolson (1999) tested a new sample of dyslexic children on a battery of tests which have been associated with cerebellar damage. The dyslexics were drawn from a school for dyslexics and two schools with large dyslexic units. The "cerebellar" tests involved assessment of postural ability (degree of sway or movement in response to a gentle push in the back), arm shake (degree of arm movement when the wrist was passively shaken by the experimenter, a measure of muscle tone), and speed of toe tapping. Other tests administered were of accuracy in phonemic segmentation, repetition of nonsense words, and speed of picture naming. The dyslexics were said to be particularly impaired on the "cerebellar" tests and it was concluded therefore that the cerebellum is a key structure involved in dyslexia. A summary of the cerebellar hypothesis was presented by Nicolson, Fawcett, and Dean (2001).

In a PET imaging study, Nicolson et al. (1999) measured brain activation levels while six adult dyslexic subjects and six control subjects performed motor tasks. The tasks involved carrying out a sequence of finger movements with the right hand with the eyes closed. In one condition, the sequence was highly overlearned; in another condition, participants learned a new sequence.

Compared with a resting condition, controls showed greater activation than dyslexics in the right cerebellum during performance of both the overlearned task and during acquisition of the novel sequence. This was taken as supporting the cerebellar deficit hypothesis.

Rae et al. (2002) used MRI with 11 adult male dyslexics and 9 controls. Among controls there was a significant left-right asymmetry in the proportion of grey matter (reflecting cell bodies) to total cerebellar volume. Among the dyslexics, there was no such asymmetry. The authors saw their finding of no cerebellar asymmetry in dyslexics as consistent with previous reports of symmetrical temporal plana in dyslexia. In a previous paper, this group claimed to find evidence of a biochemical asymmetry in the cerebellum of dyslexic men (Rae et al., 1998). Although they interpreted their magnetic resonance spectroscopy findings in terms of "an altered pattern of cell density in the cerebellum of a dyslexic individual" (p. 1852), they did not directly measure cell size. Recently, Finch, Nicolson, and Fawcett (2002) have reported that there are relatively fewer small cells and more large cells in the cerebellum of "dyslexic" brains (and in a part of the auditory pathway) as compared with control brains. They did not find any significant cerebellar asymmetry.

The brains examined by Finch et al. (2002) were the four male brains reported on by Galaburda et al. (1985) in relation to asymmetry of the PT (and to certain sub-cortical features—see below). This places a heavy interpretative load on the brains of only a few individuals, yet there are reasons to doubt whether these specimens can be considered representative of dyslexic brains in general (Beaton, 1997). In addition, certain other aspects of the Finch et al. (2002) study are less than ideal. For further discussion of this paper, and of the cerebellar deficit hypothesis in general, the reader is referred to Beaton (2002), Bishop (2002), and to the commentaries which follow the paper by Nicolson et al. (2001).

Given that dyslexics exhibit certain difficulties in the sphere of motor activity, the question has been posed as to whether remediation of motor deficits will have any impact on reading. McPhillips, Hepper, and Mulhern (2000) recently examined the effects of movement training with 3 groups each of 20 dyslexic children. One group was trained by asking them to repeat movements associated with what is known as the asymmetrical tonic neck reflex (ATNR). This reflex is elicited in neonates by a sideways turning of the head when the infant is lying on its back. It consists of extension of the arm and leg on the side to which the head turns and flexion of the upper and lower limb on the opposite side of the body. Normally the ATNR is not seen after the age of about 6 months and its persistence beyond this time is associated with central nervous system abnormality. In the children of this study, a persistent ATNR was assessed by asking them to stand upright with their feet together while an experimenter slowly turned the head one way or the other. Positive indicators of this reflex were said to include extending arms in the same direction as the head turn, dropping the arms, or swaying and loss of balance.

The study by McPhillips et al. was intended to see whether the ATNR could be inhibited by a training regime involving rehearsal and voluntary repetition of the movements involved in the TNR and other primary reflexes. The rationale was that, during infancy, the occurrence of the reflex might contribute in some way to its eventual inhibition. The experimental group was provided with a regime of specific movements to carry out each day for about 10 min for a period of 12 months. A placebo control group was also given a sequence of movements to carry out but these were not based on the primary reflexes. A control group of children were given no specific instructions. The results seem to indicate that the experimental group made significantly greater gains in reading than either of the other two groups which did not differ between themselves. It will be of great interest to see whether the results prove replicable by other groups of workers (see Reynolds et al., 2003), especially since ideas as to why movement "therapy" should enhance reading ability are somewhat vague. The conclusion of McPhillips et al. was simply that "persistent primary reflexes may have a critical role in early neurological maturation which, in turn, has repercussions for later reading development" (p. 540).

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Adult Dyslexia

Adult Dyslexia

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