Learning Disabilities

There are many different subtypes of learning disabilities. Byron Rourke, writing in 1993, reported three major groupings: (1) reading/spelling, (2) arithmetic, and (3) reading/spelling/arithmetic. Larry Silver, also writing in 1993, suggested a model that includes input disabilities (visual/perceptual, auditory/perceptual, and sensory integrative), integrative disabilities (sequencing, abstraction, and organization), memory disabilities, and output disabilities (language and motor). Reading/spelling disabilities are by far the most prevalent form, with such disabilities estimated to comprise from 5 percent up to 17 percent of the child and adolescent population. Estimates for the occurrence of disorders of written expression range from 2 percent to 8 percent. Although Learning disabilities, such as dyslexia, can be caused by premature birth or asphyxia at a younger age. In other cases, the direct cause of a learning disability is not known. (Ellen B. Senisi/Photo Researchers, Inc.) the prevalence of arithmetic LDs ranges from 1 percent to 6 percent, it is not clear whether weak mathematics performance is due to the quality of instruction or an actual LD.

Nonverbal LDs are often overlooked, occur less frequently than reading disorders, and are characterized by problems in arithmetic computation, graphomotor skills, reading comprehension, math reasoning, science, complex concept formation, visual memory, and social-behavioral skills; these are often found in children with white-matter disorders, and are assumed to be more right-hemisphere-based. As of the late 1990s, a classification schema (based on reading disability/dyslexia research) was applied to all achievement domains included in federal and state definitions of LD. Three major types of LDs were identified: specific language impairment, specific reading disability/dyslexia, and specific math disability.

The area of greatest knowledge is reading disorders. These fall into two main groupings: phonological (dysphonetic) and orthographic (dyseidetic). The former is more prevalent and is characterized by deficits in decoding and word analysis, with guesses made based on the initial letter of the word and misspellings being phonetically inaccurate. Shaywitz wrote in 1998 that a deficit in basic phonemic awareness (inability to segment phonemes [the smallest unit of sound] into phonological units) is the underlying cause in virtually all cases of dyslexia. The orthographic reading disability subtype involves an inability to develop a memory for the whole word (gestalt), with visuospatial reversals occurring (e.g., "was" read as "saw") and misspellings being phonetically accurate. There also is a mixed reading disorder, which consists of characteristics found in both types of deficit. The major new finding is that reading disabilities are more strongly associated with auditory rather than visual deficits.