Critical Period for Cross-Modal Plasticity in Blind Humans: A Functional MRI Study Norihiro Sadato, Tomohisa Okada, Manabu Honda, & Yoshiharu Yonekura Sepehr Nassiri

Introduction Braille Reading Vs. Visual Letter Identification Primary Visual Cortex (V1) activated in congenitally blind subjects during tactile discrimination tasks Transcranial magnetic stimulation (TMS) induces disruption of cortical function during identification of Braille letters in early-onset blind subjects (<10 years old) but not in sighted subjects reading Roman letters

Introduction Tactile processing pathways linked to the secondary somatosensory area (SII) reroute to the ventral occipital cortical regions blind subjects Plasticity in the brain allows for additional processing of tactile information in the visual cortical areas

Introduction Reorganization of brain function may differ in early- onset and late-onset blind subjects Braille reading activates V1 in early-onset but not late-onset blind subjects Effect of age at onset of blindness on plasticity in neural substrates for tactile discrimination not fully known

Purpose To clarify the critical period of the plastic change due to visual deprivation

Method Subjects: 15 Blind (9 16 years old) Blind due to eye/early optic nerve dysfunction 8 sighted volunteers No history of neurological/psychiatric illness No neurological deficits except blindness in blind subjects

Method Tactile Tasks Braille Discrimination Task: 6 task & 6 rest periods (30s ea) Stimuli presented using plastic rail with pairs of two- dot Braille characters Responses recorded by pressing a button via microcomputer

Method Cues Sighted Subjects: presented on a screen Cue circle=fix eye position (control eye movement) Yellow=Position both hands Red=Examiner moved rail to subject's finger pad Green=Subject response Blind Subjects: Touch subject's left toe every 6s

Method Response Pair-wise characters same=left index finger Pair-wise characters different=left middle finger Rest period Response (sighted subject's only): Red & Green cues presented alternatively Red=No stimulus presented Green=subject pushed buttons with index and middle fingers alternatively Why? Enable correction for effects of cue and response movement

Method 3.0 Tesla Functional Magnetic Resonance Imaging (fMRI) used to measure change of cerebral blood flow

Results Task Performance (percent correct responses) Early-onset blind group (80.7±12.4%) Late-onset blind group (57.8±14.9%) Sighted group (59.2±12.6%) P=0.0002, One-way ANOVA

Results V1 activated in early-onset blind, and inhibited in late- onset blind subjects Discrimination Vs. Rest Period Task performance significantly correlated with activity of V1 [F(1,13)=8.319, p=0.0128]

Red=Activation Blue=Inhibition

Discussion Results show critical period from Birth-16 years of age for reorganization of the V1 to function during tactile discrimination tasks Early-onset Blindness(<16 years old) had increased activity in V1during tactile discrimination task Late-onset Blindness (>16 years old) had decreased activity in V1 during the same task

Discussion Task Design Previous studies involved active exploration making it difficult to determine whether activity measured is sensory or motor Present study excluded effects of motor control Results still consistent with previous studies confirming posterior activation in blind subjects is due to sensory not motor processes

Discussion Activation of V1 dependent on age at onset of blindness Visual association cortex was not dependent on age In blind subjects tactile shape discrimination processing expands to visual association cortex In early-onset blind subjects V1 is also recruited resulting in better performance on shape discrimination

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