2.2.2 ‘Visual deficit’ hypotheses
Samuel Orton was one of the earliest and most influential researchers into dyslexia, although he used the term strephosymbolia – literally meaning ‘twisted symbols’. He noticed that children with specific reading difficulties often wrote letters back to front, confused letters such as ‘b’ and ‘d’, and would swap the position of letters within a word during spelling (e.g. ‘was’ might be written ‘saw’). From these and other observations, he suggested that their reading difficulties might reflect some kind of visual processing impairment involving incomplete specialisation between the left and right sides of the brain. It is worth noting that the left hemisphere of the brain is specialised for processing language.
As we saw in Section 1.4, the original observations by Hinshelwood about what he called congenital word blindness also emphasised a visual-perceptual contribution. Much early research was therefore focused on trying to identify perceptual factors that could contribute to dyslexia. Visual deficit explanations fell out of favour during the 1970s and 1980s when psychologists increasingly adopted a phonological deficit model of dyslexia, arguing that reading difficulties reflect primary problems with language processing. While the phonological deficit explanation is still popular and widely researched, there has been a resurgence of interest in the idea that there may be an underlying visual deficit that could explain difficulties in learning visual-phonological correspondences (see Everatt, 1999; Whiteley and Smith, 2001).
More recently, the evidence for visual-sensory processing deficits in dyslexia has become robust. The challenge now is to determine whether these visual-perceptual problems affect the development of visual processing required for fluent and skilled reading, and if so how. Seymour (1986) has re-emphasised the obvious point that the cognitive systems specifically required for written language (as opposed to spoken language) are actually in the visual domain. He and others have shown that the reading performance of many dyslexic people reflects weaknesses in visual processing that can occur independently of phonological difficulties.
It has been claimed that phonological deficits are more common than visual deficits in dyslexia, and the fact that many dyslexic people show superior visual-spatial abilities is cited as supporting evidence. The trouble with this argument is that the psychological tests used to assess visuo-spatial abilities do not actually measure the same kinds of visual processing that Seymour refers to, which is more perceptual in nature. In fact, mild visual disturbances are consistently found in up to 70 per cent of people with dyslexia, and more importantly, these typically co-occur with phonological problems (Lovegrove, 1991). It has even been suggested that both types of problem might have a common cause.
As we have already seen, it is misleading to think either that visual-perceptual and phonological problems must be mutually exclusive, or that all people with specific reading difficulties are the same. What is more, variation in the ‘clinical’ picture of dyslexia (at either the behavioural or the cognitive level of Frith's model) does not in fact rule out some common underlying ‘cause’ at the biological level. The complex interactions between biology and environment mean that the same biological ‘problem’ can result in different cognitive and behavioural consequences for different people.