Critical Period for Cross-Modal Plasticity in Blind Humans:

A Functional MRI StudyNorihiro 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 duringidentification of Braille letters in early-onset blind subjects (<10 years old) but not in sighted subjects reading Roman letters

IntroductionTactile 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, 6>16 years old)Blind due to eye/early optic nerve dysfunction8 sighted volunteersNo history of neurological/psychiatric illnessNo 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 charactersResponses recorded by pressing a button via microcomputer

Method

CuesSighted Subjects: presented on a screenCue circle=fix eye position (control eye movement)Yellow=Position both handsRed=Examiner moved rail to subject's finger padGreen=Subject responseBlind Subjects:Touch subject's left toe every 6s

Method

ResponsePair-wise characters same=left index fingerPair-wise characters different=left middle finger

Rest period Response (sighted subject's only):Red & Green cues presented alternativelyRed=No stimulus presentedGreen=subject pushed buttons with index and middle fingers alternativelyWhy? 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=ActivationBlue=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 DesignPrevious 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

Questions