Bethesda, MD (February 3, 2002) -- The federal government records that more than 365,000 Americans under the age of 65 suffer from a severe loss of sight that merits assistance. Among the challenges faced by the suddenly blind is learning Braille, the international system of writing and printing by means of reading raised dots corresponding to letters, numbers, and punctuation.
How can a person who has become sightless learn Braille, allowing access to the printed word? For most, hard work and determination plays a major role. However, new research has demonstrated that the brain compensates for loss of vision, and works to assist the sightless individual in learning Braille.
Background
The ability read Braille depends on remarkable adaptations that connect the somatosensory (sensation relating to the body's superficial and deep parts as contrasted to specialized senses such as sight system to language). Now, a group of St. Louis researchers have hypothesized that the pattern of cortical activations in blind individuals reading Braille would reflect these adaptations. Activations in visual (occipital-temporal), frontal-language, and somatosensory cortex in blind individuals reading Braille have been examined for evidence of differences relative to previously reported studies of sighted subjects reading print or receiving tactile stimulation.
The Study
The authors of the study, "Adaptive Changes in Early and Late Blind: A fMRI Study of Braille Reading," are H. Burton, A. Z. Snyder, T. E. Conturo, E. Akbudak, J. M. Ollinger and M. E. Raichle all from the Washington University School of Medicine, St. Louis, MO. Their findings appear in the January, 2002 edition of the Journal of Neurophysiology.
In their study, the investigators:
· Examined a theory that blind individuals use areas of the cerebral cortex normally reserved for vision during Braille reading and other nonvisual tasks involving touch discrimination.
· Studied the association between active foci (the point at which the light rays meet after passing through a convex lens) in occipital cortex of blind individuals and the multiple visual areas of sighted. Finding evidence of activity circumscribed to anatomically distinct portions of visual cortex in blind individuals might suggest functional specialization like that attributed to corresponding regions in sighted individuals.
· Examined whether changes occurred in the activated extent or components of the somatosensory system because of the intense dependence on tactile perceptions when reading by touch. A hypothesis presented stated that expansion of the representation for the Braille reading finger in the somatosensory cortex given prior evidence of remarkable plasticity in this cortex
Methodology
Sixteen blind, proficient Braille readers from St. Louis were paid volunteers. A detailed neurologic history was obtained from each subject using a standardized questionnaire, and only neurologically normal (excepting visual function) subjects were scanned. The nine subjects (four females, five males, average age of 44.78) were early-blind having no sight at birth or by five years of age. Seven (four females, 3 male; average age 49.14) were late blind, having lost sight after an average age of 12.7 years); one subject retained sufficient vision to read large print.
This study emphasized image analysis (functional MRI) in individual subjects rather than pooled data. Group differences were examined by comparing magnitudes and spatial extent of activated regions first determined to be significant using the general linear model.
The control task was to read the nonlexical Braille string "######", and the major adaptive change was robust activation of visual cortex despite the complete absence of vision in all subjects. This included foci in peri-calcarine, lingual, cuneus and fusiform cortex, and in the lateral and superior occipital gyri encompassing primary, secondary, and higher tier visual areas previously identified in sighted subjects.
Results
Among the findings the researchers observed were the fact that:
· Subjects who never had vision differed from late blind subjects in showing even greater activity in occipital-temporal cortex, provisionally corresponding to V5/MT and V8.
· The early blind had stronger activation of occipital cortex located contralateral to the hand used for reading Braille. Responses in frontal and parietal cortex were nearly identical in both subject groups. There was no evidence of modifications in frontal cortex language areas (inferior frontal gyrus and dorsolateral prefrontal cortex).
· Surprisingly, there was also no evidence of an adaptive expansion of the somatosensory or primary motor cortex dedicated to the Braille reading finger(s).
Conclusions
The authors confirmed earlier reports of visual cortex activation by Braille reading. This finding is counter to preconceived theories that (1) in neuroanatomical connections, particularly those involving the thalamus, this cortex is visual and that (2) the loss of vision presumably renders the affected region inoperable.
In this study, blind subjects showed clear, well localized activation of both lower and higher tier visual areas. The lower tier responses in large measure correspond to visual cortex subregions in sighted individuals. These findings indicate a need to reexamine the contribution of occipital cortex to reading. One possibility is that blind Braille readers use occipital cortex in a novel way, unlike the use of this cortex for early vision in sighted people. Another possibility is to suggest that mechanisms and connections within occipital cortex are necessary for encoding orthography (visual or touch) into information used by language areas in frontal, and possibly temporoparietal, cortex.
The activity in occipital cortex of blind individuals is not a totally novel adaptation; rather, it represents a function already present in sighted individuals, i.e., recoding information for language areas. Loss of vision does not remove this role from occipital cortex, which accounts for the fact that this region is active in blind individuals.
The research suggests that minute movement strategies used when touching Braille cells selectively might increase activity in medial, frontal premotor areas. Braille reading obviously is at least in part a somatomotor task. However, the contribution of primary somatomotor cortex evidently can be balanced by a suitably implemented low-level control task. Potential differences in the reading strategies of early and late blind individuals possibly explains the distinctions in activated temporo-occipital foci including MT/V5, V8 and BA 21. The verb generation task used in this study matches semantic and phonological features extensively studied in sighted individuals.
The study also suggests that word recognition precedes the semantic processing in the verb generation task, and the possibility that touching Braille cells involves object recognition. Thus, word and object encoding foci previously described in sighted subjects in dorsolateral prefrontal cortex and ventral occipital temporal cortex are correspondingly engaged in blind individuals during Braille reading.
The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.
Contact: Donna Krupa: 703.527.7357
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Journal
Journal of Neurophysiology