Abnormal Psychology

Childhood Disorders 645

Brain Systems

In most forms of dyslexia, the brain systems involved in auditory processing do not

function as they should (Marshall et al., 2008; Ramus et al., 2003). For example,

one study used electrodes placed on the scalp to examine the brain waves of infants

while they listened to syllables coming out of a speaker. Eight years later, the chil-

dren’s reading abilities were assessed, and the children classifi ed as dyslexic, poor

readers, or normal readers. The children who were classifi ed as dyslexic at 8 years

old had brain-wave patterns in infancy (while they listened to spoken syllables) that

were different from those of the children whose reading ability was classifi ed as

normal, which suggests that the children with dyslexia were born with processing

problems in the auditory system (Molfese, 2000). Further research has suggested

that these brain-wave differences continue at least through the fi rst 4 years of life

(Espy et al. 2004).

The results of many neuroimaging studies have converged to identify a set of brain

areas that is disrupted in people who have dyslexia (Shaywitz, Lyon, & Shaywitz,

2006). First, two rear areas in the left hemisphere are not as strongly activated during

reading tasks in people with dyslexia as they are in people who read normally. One of

these areas, at the junction of the parietal and temporal lobes, appears to be involved

in converting visual input to sounds (Friedman, Ween, & Albert, 1993). The other

area, at the junction of the parietal and occipital lobes, appears to be used to recognize

whole words, based on their visual forms (Cao et al., 2006; McCandliss, Cohen, &

Dehaene, 2003). Moreover, these areas are not activated normally even in young chil-

dren with dyslexia, and thus the malfunction observed in adults cannot be a result

of not reading properly over the course of many years but probably contributes to

reading disorder (Shaywitz et al., 2002). Second, two other brain areas (the bottom

part of the frontal lobe and the right occipital-temporal region) are more activated in

people with a reading disorder than in people who read normally. These areas appear

to be used in carrying out compensatory strategies, which rely on stored information

instead of the usual vision-sound conversion process.

Consistent with the neuroimaging results, researchers have also reported struc-

tural differences between the brains of people with dyslexia and normal readers.

Compared to people who read normally, people with dyslexia have reduced gray

matter (which includes the cell bodies of neurons) in the temporal lobes, particu-

larly the left temporal lobe (Vinckenbosch, Robichon, & Eliez, 2005), and portions

of their frontal lobes are relatively large (Vinckenbosch, Robichon, & Eliez, 2005;

Zadina et al., 2006). Moreover, people who have relatively large occipital lobes

(which are specialized for vision) tend to read better than those with smaller occipi-

tal lobes (Zadina et al., 2006); this difference may suggest that at least some people

with dyslexia have impaired visual abilities. (Fine et al., 2007).

However, the precise brain areas involved in dyslexia are infl uenced by culture,

as manifested by the language spoken in a society. Specifi cally, part of the left fron-

tal lobe is impaired in dyslexic children who speak Chinese, instead of the areas

just discussed (which were assessed in English-speaking children). Chinese writing

does not depend on an alphabet, but instead requires memorizing specifi c characters

that correspond to words (Siok et al., 2004). The frontal lobes are involved in using

stored information to help register current stimuli, and such processing may play a

large role in reading Chinese characters.

At least some forms of dyslexia also appear to reflect a specific problem in

processing visual stimuli—independent of problems in connecting those stimuli to

sounds—which may be why some people with dyslexia reverse letters when they

write (Vidyasagar, 2005). Thus, fi ndings regarding brain systems suggest that there

are probably different forms of dyslexia, with different underlying causes.

Genetics

Reading disorder, and possibly the other learning disorders, are moderately to

highly heritable (Hawke, Wadsworth, & DeFries, 2006; Schulte-Körne, 2001), and

at least four specifi c genes are thought to affect the development of these disorders

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