2.4.2 Differences in sensory, perceptual and motor function
As we saw in our discussion of cognitive explanations, there has been longstanding debate over the possible contribution of perceptual problems to dyslexia. Subjectively, many children and adults with dyslexic difficulties do report ‘visual symptoms’ when trying to read. These include letters and words appearing to move or ‘blur’ on the page, particular difficulties with small, crowded print, and complaints of ‘glare’ or other kinds of visual discomfort (see Figure 5).
Experimental studies now provide evidence of some perceptual difficulties in dyslexia for tasks involving the processing of rapidly changing information, such as the perception of flicker or motion (Stein, 1994). Such difficulties in processing rapid visual information implicate the magnocellular visual system (Stein and Walsh, 1997). Furthermore, neuroanatomical abnormalities relating to this visual pathway have been reported in the brains of dyslexic people post-mortem (Livingstone et al., 1991). The magnocellular system is particularly important for the control of eye movements and visual attention.
Similar difficulties in processing rapidly presented auditory information have also been observed in people with dyslexia. Some have argued that this is evidence of general difficulties with rapid auditory perception, which would account for the difficulties in acquiring phonological awareness in dyslexia (Tallal et al., 1997). However, an alternative explanation that has increasing support suggests that the phonological awareness deficit is the result of a specific problem with speech sounds only, perhaps associated with difficulties in speech perception (Mody et al., 1997).
Attention has also turned to the possible role of the cerebellum in dyslexia. This brain structure is important for motor coordination and planning, but is now recognised to play an important role in cognitive development, particularly in the automatisation of skills and ‘rote’ learning (i.e. learning facts ‘off by heart’, like multiplication tables). Brain imaging studies using positron emission tomography (PET) have shown differences in the activity of the cerebellum in dyslexic versus non-dyslexic adults during motor learning tasks (Nicholson et at, 1999). In our discussion of cognitive explanations we noted that an ‘automatisation’ deficit could help to explain a wide range of features of dyslexic functioning, including (but not confined to) phonological deficits. Furthermore, because the cerebellum is known to act as a ‘timing’ device, a ‘cerebellar deficit’ theory is also highly compatible with the idea of problems in very rapid sensory processing (the ‘magnocellular’ hypothesis).
If you recall our discussion of Frith's model (see Figure 2), we emphasised that variability at the behavioural or the cognitive level (e.g. phonological or visual problems) need not rule out some single underlying cause at the biological level. It is perfectly possible that microscopic differences in brain architecture could have different effects according to the particular brain areas affected.
(Source: adapted from Martini et al., 2000, Figure 15–9 (a), p.395)
Box 9: Definitions
Ectopia: A collection of misplaced cells.
Cerebellum: A part of the brain (situated underneath the rear cerebral cortex) involved with motor and balance functions, and recently shown to be involved in the automatisation of many cognitive skills.
Magnocellular visual system: A visual sub-system specialised for processing information that changes very rapidly over time, characterised by large cells with fast responses. (Strictly, this refers to a specific sub-cortical pathway from retina to primary visual cortex, but it can also include further cortical areas to which these cells project.)