TITLE:
Deficits in Magnocellular Pathway in Developmental Dyslexia: A Functional Magnetic Resonance Imaging-Electroencephalography Study
AUTHORS:
Hisako Yamamoto, Yosuke Kita, Tomoka Kobayashi, Hiroko Yamazaki, Makiko Kaga, Hideki Hoshino, Takashi Hanakawa, Hitoshi Yamamoto, Masumi Inagaki
KEYWORDS:
Developmental Dyslexia; Simultaneous Functional MRI-EEG; Visual Evoked Potential; Magnocellular Deficit Theory; Magnocellular Pathway
JOURNAL NAME:
Journal of Behavioral and Brain Science,
Vol.3 No.2,
May
22,
2013
ABSTRACT:
Background: Magnocellular deficit theory is among the different hypotheses that
have been proposed to explain the pathophysiology of developmental dyslexia (DD).
Dysfunction of the magnocellular system in DD has been investigated
using mainly visual evoked potentials (VEPs), particularly transient VEPs,
although recently abnormal steady-state VEPs have also been reported. The brain
regions responsible for the abnormal VEPs in DD have yet to be elucidated,
however. In this study, we performed functional magnetic resonance imaging and
electroencephalography (fMRI-EEG) simultaneously to elucidate the brain areas
that were found in a previous study to be activated through stimulation of the
magnocellular system, and then investigated the mechanism involved in the
dysfunction seen in DD.Methods: Subjects were 20 healthy individuals (TYP group; 13 men, 7 women;
mean ± standard deviation age, 26.3 ± 5.53 years) and 2 men with DD (aged 42 and 30 years).
Images of brain activity were acquired with 3-Tesla MRI while the viewing the
reversal of low-spatial frequency and low-contrast black-and-white sinusoidal
gratings. EEG was recorded concurrently to obtain steady-state VEPs.Results: Stimulus frequency-dependent
VEPs were observed in the posterior region of the brain in the TYP group;
however, VEP amplitudes in both DD patients were clearly smaller than those in
TYP. fMRI images revealed that both the primary and secondary visual cortices
were activated by black-and- white sinusoidal gratings in the TYP group,
whereas activity in the visual cortex overall was reduced in both DD patients.Conclusions: Present low
spatial and high reversal frequency visual stimuli activated the primary visual
cortex presumably through predominant activation of the magnocellular pathway.
This finding indicates that some cases of adult patients of DD involve
impairment of the visual magnocellular system.