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A group of poor readers classified as dyslexic by age/IQ discrepancy criteria (n = 42) were contrasted with two clinic control groups: 56 adequate-for-age readers with attention deficit disorder (ADD) and 21 poor-for-age readers not meeting the IQ discrepancy criterion (slow/borderline group). The children (33 girls, 86 boys) ranged in age from 7.5 years to 12 years. Variables chosen for study included simple and complex phonological processing, speech rate, continuous naming speed, running memory span, serial memory span, and mental addition. Evidence is presented that the two poor reader groups are distinguishable. Unlike the dyslexic group, the slow/borderline group did not differ from the ADD group on three key measures: simple auditory phonological sensitivity, continuous naming speed, and running memory span. Stepwise regression to predict word list reading level showed that once age and verbal IQ were removed (51% of variance), these three key measures accounted for an additional 22% of the variance (R = 0.86, R2 = 0.73). The single best predictor of word list reading level was nonsense word list reading level, which was explained by the same set of five variables that explained real word reading (R = 0.77, R2 = .60). Severity of attentional problems was not linearly related to reading skill in this clinic sample.

Within a large (N = 182) heterogeneous sample of clinic-referred children with DSM-III-diagnosed attention deficit disorder (ADD), three behavioral subgroups were identified via cluster analysis of teacher ratings: 40% of the children had ADD with hyperactivity (ADDH), 30% had ADD with hyperactivity and aggressivity (ADDHA), and 31% had ADD without hyperactivity or aggressivity. Proportionally more girls were in the ADD-only subgroup. Over half the sample (n = 94) were poor readers, with 82 meeting discrepancy criteria for specific reading disability (RD). Proportionately more boys than girls met the RD criteria (9.2:1.0), whereas the sex ratio of males to females for the whole sample was 5.1 to 1.0. Focusing just on white males, the three behavioral subgroups were significantly different on convergent validity measures, such as other teacher ratings, parent ratings, and interview-elicited ratings of externalizing behavior, but were not different on such divergent validity measures as IQ and achievement scores, self-ratings, and laboratory performance tasks. Boys in the ADD sample who did not meet criteria for RD had significantly higher IQs than those who did, but subgroups with and without RD still differed significantly on WRAT-R reading and spelling scores with IQ covaried out. Both groups with and without RD could be differentiated from a control group on laboratory measures of sustained attention and impulse control. Methylphenidate benefited all subgroups equally, whether RD or not, and whether given a low (0.3 mg/kg) or high (0.6 mg/kg) dose.

ERPs were recorded from four groups of children: reading disabled, attentional deficit disorder with and without hyperactivity, and normal controls. Subjects pressed a button to a low probability nonsense syllable (target, p= .168) and ignored all other events, which included a high probability nonsense syllable (nontarget, p= .664) and either low probability (category, p= .168) symbols (Block 1) or 3‐letter words (Block 2). The amplitudes of several late ERP components and the latency of the P3 component were examined. The overall amplitude of P3 was significantly smaller in all clinical groups than in controls, but the difference in P3 amplitude between targets and nontargets was smaller only in the two attentional deficit groups. Reading disabled children had smaller P3 and Pc components to words than to symbols, while controls had equivalent values. The N2 component had a different scalp distribution for words and symbols, but did not differentiate reading disabled children from controls. P3 latency was significantly longer in the three clinical groups than in controls, but only the attentional deficit groups showed an increase in P3 latency across blocks of the task. The results are discussed within the framework of recent cognitive models dealing with attentional processes.

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