unexpected in relation to age and other cognitive and academic abilities; they are not the result of generalized developmental disability or sensory impairment. Dyslexia is manifested by variable difficulty with different forms of language, often including, in addition to problems reading, a conspicuous problem with acquiring proficiency in writing and spelling. (Lyon, 1995a, p. 9)
It will be interesting to see if phonemic awareness research will be a factor in shaping future definitions of learning disabilities or federal regulations pertaining to identification of learning disabilities.
Biological Causes of Learning Disabilities
Since the 1960s, most definitions of learning disabilities have made reference to a neurological basis for learning disabilities. However, it was not until the 1980s and especially the 1990s that evidence began to accumulate to support a biological basis for learning disabilities. Researchers have used two different sources of evidence to support the conclusion that learning disabilities may be the result of neurological dysfunction: postmortem studies and neuroimaging studies. Furthermore, evidence has begun to mount that hereditary factors are implicated in many cases of learning disabilities.
Postmortem studies. Albert Galaburda and Norman Geschwind and their colleagues (Galaburda & Kemper, 1979; Galaburda, Menard, & Rosen, 1994; Galaburda, Sherman, Rosen, Aboitz, & Geschwind, 1985; Geschwind & Levitsky, 1968; Humphreys, Kaufmann, & Galaburda, 1990) made postmortem comparisons between the brains of people with and without dyslexia. When they first started this research, it was difficult to assess its reliability because the number of cases was so small. By the 1990s, however, they had accumulated data on more than a dozen cases, and their results were demonstrating a consistent pattern. In most brains of the nondyslexic group, the left planum temporale (a section of the left temporal lobe, including a large segment of Wernicke’s area) is larger than the planum temporale in the right temporal lobe. The left and right planum temporales in the brains of those with dyslexia, in contrast, are the same size or the planum temporale in the right hemisphere is larger than the one in the left hemisphere.
Neuroimaging studies. Using magnetic resonance imaging (MRIs) and computerized axial tomography (CAT) scans, researchers have found the same symmetry or reversed symmetry for the planum temporales of adults with dyslexia (Hynd & Semrud-Clikeman, 1989; Kusch et al., 1993; Larsen, Hoien, Lundberg, & Odegaard, 1990). Studies of brain metabolism, using positron emission tomography (PET) scans and fMRIs, have also begun to reveal differences between individuals with and without dyslexia (Flowers, 1993; Flowers, Wood, & Naylor, 1991; Gross-Glenn et al., 1991; Hagman et al., 1992; Shaywitz et al., 1998). Again, the left hemisphere appears to be the locus of the abnormal functioning, with some of the evidence pointing to Wernicke’s area.
Hereditary factors. The 1990s also witnessed an increase in evidence pointing to the hereditary nature of learning disabilities. Researchers have found that about 40% of first-degree relatives of children with reading disabilities have reading disabilities themselves (Pennington, 1990). An approximately equal degree of familiality has also been found for speech and language disorders (Beichtman, Hood, & Inglis, 1992; Lewis, 1992) and spelling disorders (Schulte-Korne, Deimel, Muller, Gutenbrunner, & Remschmidt, 1996). Furthermore, studies of heritability, comparing monozygotic versus dizygotic twins, have found a high degree of concordance for reading disabilities (DeFries, Gillis, & Wadsworth, 1993), speech and language disorders (Lewis & Thompson, 1992), and oral reading ability (Reynolds et al., 1996).
Concern over Identification Procedures
At least two issues related to identification have occupied the learning disabilities literature at the end of the twentieth century. The first pertains to the use of the discrepancy between achievement and intellectual potential; the second is the issue of over-representation of minorities in the learning disabilities category.
Discrepancy between achievement and intellectual potential. By the 1990s, the majority of states had adopted a discrepancy between achievement and intellectual potential as part of their identification procedures (Frankenberger & Franzaglio, 1991). However, during this same time period many learning