Harvard University University of Toronto Beth Israel Hospital Laboratory for Magnetic Stimulation
OBJECTIVE: To evaluate the cortical activation patterns associated with light touch sensory stimulation in early blind subjects using blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI).
BACKGROUND: Neuroimaging studies have documented activation of visual cortex in early blind subjects reading Braille or performing other spatial acuity tasks. This occipital activation seems necessary for Braille reading . It is unknown whether there is a correlation between sensory acuity or dominant reading finger and occipital activation.
METHODS: We studied seven blind subjects with average onset of peripheral blindness at 1.2 years (range 0 to 6 years) and no neurologic disorders. All were proficient Braille readers since age 4 to 14 years (mean 6.5 years). All imaging studies were performed on a 1.5 Tesla Seimens Magnetom EPI System using a standard quadrature head coil. Subjects were positioned supine in the MR scanner with their head supported to restrict head motion. Sensory stimulation was provided with a wooden brush to the distal pad of one finger: right index, left index, right middle or left middle finger. Imaging parameters for the fMRI studies were as follows: TR 2ms; TE 64ms; TD 45; matrix 128X128; FOV 320. Repetitive stimulus sets containing control and stimulus conditions were presented in a counterbalanced order. Image analysis was done using the Advanced Visual Systems software platform. A motion correction network was run to register all fMRI data into a signal intensity map of a reference BOLD volume. A correlation analysis was then done to compare control and stimulus condition sets. The reference function for this analysis was a standard boxcar function. All fMRI was data was superimposed on high resolution T1 weighted MR images for anatomical correlation. Analysis of cortical regions including sensory and occipito-parietal areas was then completed. A quantification of the number of active pixels and an analysis of the active pixels r values was done for each subject.
RESULTS: fMRI activation maps demonstrated activation of the primary and secondary sensory and visual cortical regions in all subjects. There was an increase in the degree of primary and secondary visual cortex activation when the sensory stimulus was applied to the subjectís dominant or preferred Braille reading finger (Figure 1). An increase in primary and secondary sensory cortex activation correlated with the subjectís lower sensory acuity. This increase in active pixels was seen in both the ipsilateral and contralateral hemispheres (Figure 1).
CONCLUSIONS: This study expands the information about cross-modal plasticity in early blind subjects. The results suggest that the visual cortex is more involved in the processing of tactile information from the dominant Braille reading finger as compared to other digits. In contrast, activity in the primary and secondary sensory cortex appears to be inversely related to tactile acuity.