speed deficit but little phonological awareness deficit, and one with both deficits (Wolf &

Bowers, 1999). Obviously, the second and the third groups of children are important in

this theory because the two subgroups with only a naming-speed deficit and both deficits

may not be well supported if reading intervention, which is only based on phonological

awareness approaches, is provided (Wolf & Bowers, 1999). Wolf and Bowers (1999)

mentioned that researchers have found that about a fifth of dyslexic children mainly

suffered from naming-speed deficit and not from a phonological awareness deficit. More

interestingly, Lovett, Steinbach, and Frijters (2000) found that more than half of the

participants suffered from both deficits. Furthermore, extensive data replicated the

existence of these three subtypes of reading disabilities in several language systems (e.g.,

German, Dutch, and Finnish), and they provided various evidence on the existence on the

subtypes based on double deficit hypothesis (e.g., Deeney, Gidney, Wolf, & Morris,

1999). It is interesting that speed of processing and semantic fluency are included in

some of the recently produced tests for dyslexic children.

 Beyond a double deficit hypothesis, some researchers even argue for a triple

deficit hypothesis. For example, Badian (1997) purported evidence for a triple deficit

hypotheses implying that orthographic factors involving visual skills should also be

considered. The author found that there was a significant difference between the RD

group and the non-RD group on an orthography test, the Jordan Left-Right Reversal test,

which is a visual perception test.

A Visual Deficit Hypothesis

 Another hypothesis is that problems in reading acquisition are linked to deficient

visual perception. More than a century ago, James Hinshelwood, an ophthalmologist

practicing in Scotland, called his patients with RD "congenital word blindness" and