Auditory processing deficits in dyslexia: task or stimulus related?

The nature of the fundamental deficit underlying reading disability is the subject of a long-standing debate. We previously found that dyslexics with additional learning difficulties (D-LDs) perform poorly in simple auditory tasks. We now tried to determine whether these deficits relate to stimulus or task complexity. We found that the degree of impairment was dependent on task rather than stimulus complexity. D-LDs could adequately detect and identify mild frequency changes in simple pure tones and minimal phonemic changes in complex speech sounds when task required only simple same-different discriminations. However, when task required the identification of the direction of frequency change or the ordinal position of a repeated tonal or speech stimulus, D-LDs' performance substantially deteriorated. This behavioral pattern suggests that D-LDs suffer from a similar type of deficits when processing speech and nonspeech sounds. In both cases, the extent of difficulties is determined by the structure of the task rather than by stimulus composition or complexity.