FMRI contrast response in the calcarine sulcus: No difference between dyslexic and control groups

Purpose: To test the hypothesis that dys lexia is associated with a magnocellular visual pathway deficit. Magnocellular neurons have higher contrast sensitivity than parvocellular neurons, especially at low mean luminances, low spatial frequencies, and high temporal frequencies. Thus, cortical lesponses to such stimuli should be lower in subjects with dyslexia than in control ubjects. Methods: Stimuli were 8 Hz counterphase modulating 0.4 cyc/deg sinew ave gratings. We measured functional MRI response as a function of stimulus contrast and mean luminance (95 and 5 cd/m2) in 6 adults with a history of development! dyslexia (3 males) and 5 control subjects (3 males). Responses were averaged tiroughout each subject's calcarine sulcus. A T2-weighted spiral pulse sequence was used to acquire functional images ( l .5T; Interleaves; Slices=8;TR=750ms;TE=40ns; FA=70deg; Vox=l.lxl.Ix4mm). Results: The dyslexic and control group respon'-es as a function of contrast did not differ for either high or low mean luminances. In both groups, responses increased monotonically with increasing contrast, and the high mean luminance stimulus evoked about twice the response as the low mean luminance stimulus. Conclusions: No differences were observed between dyslexic and control group averaged responses in the calcarine sulcus using stimuli effective at exciting magnoceliular neurons.