Hamilton et al. (2000)

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Current revision as of 10:06, 28 April 2008

Alexia for Braille following bilateral occipital stroke in an early blind woman (Hamilton et al. 2000) Tom Makin

Introduction Recent findings using functional imaging and neurophysiologic studies in early blind subjects have suggested that activation of the occipital cortex is associated with the reading of Braille. The study involves a woman who was blind from birth who sustained bilateral occipital damage following an ischemic stroke. Prior to the stroke, the women was an exceptional Braille reader and worked for the national radio station for blind people in Spain. After the stroke occurred, she was no longer able to read Braille but her somatosensory perception appeared to be otherwise functioning normally. This case study adds credence to the growing amounts of evidence for the recruitment of striate and prestriate cortex for Braille reading in early blind subjects. Studies have shown that an increase in regional cerebral blood flow in the striate and prestriate cortex in congenitally and early blind subjects during tactile (touch) tasks, whereas normally sighted individuals showed decreased levels of regional cerebral blood flow. Studies have also shown that when repetitive transcranial magnetic stimulation is used to transiently disrupt the function of the occipital cortex, interference occurs with tactile reading of Braille in early blind subjects but not in sighted control subjects. Case Study The focus subject of the study was a 63 year old blind woman who was blind since birth due to retrolental fibroplasia (A disease of the eye that affects prematurely born babies). Apart from being blind, the subject did not manifest any other long-term effects normally associated with premature birth. The woman developed normally and successfully and by the age of 22 was working for the national radio station for the blind in Spain, having already completed a bachelor’s degree. She showed exceptional ability in reading Braille and was able to read 120-150 symbols per minute. When deciphering the Braille characters, she used both hands and several fingers. At the age of 52, the subject developed adult onset diabetes, which was controlled with a degree of success with diet and hypoglycemic agents. However, there was no evidence of diabetic peripheral neuropathy that might have caused difficulties with Braille reading. The subject suffered bilateral occipital strokes and became alexic for Braille. Soon after the onset of the stroke, the subject complained that her fingers felt like they were “Covered by thick gloves” and she was unable to extract enough information from the Braille characters to deduct any meaning from what she was reading. However, she was able to distinguish between the roughness of surfaces and she was able to identify items on a board. The patient’s impairment of tactile information processing was limited to her Braille reading skill, and she had no other abnormalities in brain functioning according to a neurological examination and neurophysiological studies. These findings strongly support the idea that in this early blind patient, the occipital cortex was responsible for the decoding of spatial and tactile information required for Braille reading skill. Evidence indicates that cortico-cortico connections mediate the tactile perception of Braille in blind readers. Studies carried out on monkeys suggest that cortico-cortico networks which are normally reserved for visual processing of object and pattern discrimination may result in haptic (touch) discriminations in blind Braille readers. The findings in this patients’ case document the important role played by the occipital cortex in tactile Braille reading, in some subjects at least. Conclusion/Take home messages Due to the lack of extensive testing on the patient, it was not possible to know for certain whether the patient’s predominant symptom was an inability to make sense of the Braille symbols she could feel or whether she had simply lost the tactile acuity that is required to detect Braille stimuli. However, due to the fact that she was able to detect other forms of tactile stimuli, it is possible that she suffered from a true alexia and that her perception of the stimuli was altered. As the patients’ other language abilities were still intact, it can be deduced that her language deficit was limited to perception of haptic symbols which lends support to previously reported cases of alexia in blind Braille readers. The findings from the case of this patient substantiate existing evidence that Braille reading in the early blind is an example of cross-modal plasticity whereby the occipital cortex is recruited for difficult tactile tasks, which enhances the sensory discrimination of blind subjects and makes Braille reading possible. The findings of the study lend further credence to the notion that the occipital cortex can be recruited to promote the function of tactile information processing among early blind subjects.