RESUMO
Purpose: We present a novel approach that allows the estimation of morphological features of axonal fibers from data acquired in vivo in humans. This approach allows the assessment of white matter microscopic properties non-invasively with improved specificity. Theory: The proposed approach is based on a biophysical model of Magnetic Resonance Imaging (MRI) data and of axonal conduction velocity estimates obtained with Electroencephalography (EEG). In a white matter tract of interest, these data depend on (1) the distribution of axonal radius [P(r)] and (2) the g-ratio of the individual axons that compose this tract [g(r)]. P(r) is assumed to follow a Gamma distribution with mode and scale parameters, M and θ, and g(r) is described by a power law with parameters α and ß. Methods: MRI and EEG data were recorded from 14 healthy volunteers. MRI data were collected with a 3T scanner. MRI-measured g-ratio maps were computed and sampled along the visual transcallosal tract. EEG data were recorded using a 128-lead system with a visual Poffenberg paradigm. The interhemispheric transfer time and axonal conduction velocity were computed from the EEG current density at the group level. Using the MRI and EEG measures and the proposed model, we estimated morphological properties of axons in the visual transcallosal tract. Results: The estimated interhemispheric transfer time was 11.72 ± 2.87 ms, leading to an average conduction velocity across subjects of 13.22 ± 1.18 m/s. Out of the 4 free parameters of the proposed model, we estimated θ - the width of the right tail of the axonal radius distribution - and ß - the scaling factor of the axonal g-ratio, a measure of fiber myelination. Across subjects, the parameter θ was 0.40 ± 0.07 µm and the parameter ß was 0.67 ± 0.02 µm-α. Conclusion: The estimates of axonal radius and myelination are consistent with histological findings, illustrating the feasibility of this approach. The proposed method allows the measurement of the distribution of axonal radius and myelination within a white matter tract, opening new avenues for the combined study of brain structure and function, and for in vivo histological studies of the human brain.
RESUMO
The visual system forms the basis of visual word decoding processes. Reading is a left-lateralized function. The interaction between the two hemispheres via the corpus callosum is required for successful reading. It is known that callosal function and morphology are affected in reading disorders. This study investigated the differences in callosal transfer speed of verbal and nonverbal stimuli in healthy university students. We hypothesized that if the callosal transfer has a role in slow reading, transfer speed would differ between slow and fast readers. Moreover, if the difference was affected by the type of stimulus, this will provide information about the level of neural processing at which the difference is based/aroused. Fifty-one participants were grouped as slow (n = 15, 8 female) and fast (n = 36, 22 female) readers. Three types of stimuli (word, legal pseudoword, and non-verbal grating) were presented from the right or left visual field. Latencies of the evoked potentials (N1) were used to measure interhemispheric transfer time. We found that slow readers have a slower right-to-left transfer speed at the parietal site, which is related to the visual word decoding process. The finding was similar to previous studies examining individuals with dyslexia. This difference was not seen with grating stimuli; we suggest that the difference originates at the orthographic visual lexical level rather than at earlier basic visual processing. We did not observe any effect of lexical and sublexical routes on the callosal transfer time because of evaluated time windows.