No abstract

Contrast sensitivity was determined for dyslexic and normal readers. When testing with temporally ramped (i.e. stimuli with gradual temporal onsets and offsets) gratings of 0.6, 4.0, and 12.0 cycles/deg, we found no difference in contrast sensitivity between dyslexic readers and controls. Using 12.0 cycles/deg gratings with transient (i.e. abrupt) onsets and offsets, we found that dyslexic individuals had, compared to controls, markedly inferior contrast sensitivity at the shortest stimulus durations (i.e. 17, 34, and 102 ms). This deficit may reflect more sluggish temporal summation. There was no difference in sensitivity to 0.6 cycles/deg gratings with transient onsets and offsets. Under these conditions, the two groups showed a consistent and equal increase in sensitivity relative to the ramped baseline condition at 0.6 cycles/deg at the longer stimulus durations. This demonstrates that dyslexic readers have no deficit in their ability to detect stimulus transients, a finding which appears to be inconsistent with a transient system deficit. That detection of the low-frequency stimuli was mediated by the transient system is further indicated by the fact that these stimuli were more susceptible to forward masking than were the high-frequency stimuli. The effects of masking of both high and low spatial-frequency stimuli were about equal for dyslexic readers and controls. This is not in agreement with the transient system deficit theory, according to which one would expect there to be less masking of high spatial-frequency stimuli in the case of dyslexic readers.

Speech perception was investigated in a carefully selected group of adult subjects with familial dyslexia. Perception of three synthetic speech continua was studied: /a/-//, in which steady-state spectral cues distinguished the vowel stimuli; /ba/-/da/, in which rapidly changing spectral cues were varied; and /sta/-/sa/, in which a temporal cue, silence duration, was systematically varied. These three continua, which differed with respect to the nature of the acoustic cues discriminating between pairs, were used to assess subjects’ abilities to use steady state, dynamic, and temporal cues. Dyslexic and normal readers participated in one identification and two discrimination tasks for each continuum. Results suggest that dyslexic readers required greater silence duration than normal readers to shift their perception from /sa/ to /sta/. In addition, although the dyslexic subjects were able to label and discriminate the synthetic speech continua, they did not necessarily use the acoustic cues in the same manner as normal readers, and their overall performance was generally less accurate.

No abstract

In addition to providing information on the inheritance of dyslexia, the present study of eleven three-generation families has provided a unique opportunity to compare affected and unaffected family members at all ages. The data presented here are based on pedigree information, a questionnaire administered to all participating family members in relation to sex ratio, handedness, the severity of dyslexia by sex, pre- and perinatal complications, medical complications, years of education and earning ability, and a battery of standardized tests to define the presence or absence of dyslexia. The pattern of inheritance was consistent with the postulated autosomal dominant mode of inheritance and penetrance was found to be > 90 percent. Of 73 individuals determined to have a gene leading to dyslexia, seven were classified as obligate carriers and six as compensated adults who had no current symptoms or diagnostic evidence of dyslexia. The sex ratio (1.06) was not different from the expected ratio of 1.04. Left-handedness, major pre- and perinatal complications, and autoimmune disorders and allergy were not more common in dyslexics than non-dyslexics. The number of years of education and average income were similar in affected and unaffected family members. Compensated adults and obligate carriers were similar to unaffected family members in each of these parameters.