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Letter and speech sound association in early blind
Poster E11 in Poster Session E, Saturday, October 8, 3:15 - 5:00 pm EDT, Millennium Hall
Joanna Beck1, Gabriela Dzięgiel-Fivet1, Katarzyna Jednoróg1; 1Nencki Institute of Experimental Biology PAS
Braille is a tactile writing system derived from the Latin alphabet and used by the blind or visually impaired people. Similarly to the Latin alphabet, in Braille a specific symbol based on a pattern of dots is assigned to each speech sound and “only” the modality of reading is different. In all alphabetic languages, the critical step for reading development is learning letter and speech sound (LS) associations. However, tactile literacy acquisition in the blind lacks preceding pre-literacy symbolic and logographic stages, which might affect the process of forming LS associations. Additionally, changed reading modality could influence the neural basis of this process. In the sighted, LS integration is related to left superior temporal cortex activity. Whether the LS integration takes place in the blind brain and how similar this process is to the sighted population is still unknown. A study on blind adults suggested that they do not integrate audio-tactile syllables due to the mal-development of multisensory mechanisms, but a small sample and lack of sighted controls greatly limit these findings. To better understand LS integration, we tested 42 early blind subjects (9 to 61 years old) and compared their brain activity for implicitly processed letters, speech sounds, and congruent and incongruent LS pairs to a matched population of sighted controls. We focused on examining the supra-additive effect in each group (congruent LS > letters + speech sounds) and the interaction between group and the congruency effect (congruent > incongruent LS pairs). Behaviorally, we tested reading, verbal skills as well as the accuracy and reaction times for judging the congruency of LS associations. Behaviorally, the blind showed enhanced verbal abilities compared to the sighted and similarly accurate but slower reading. The groups did not differ in the performance on LS associations task and presented a similar relation between reaction times in this task and reading efficiency. At the brain level, we found supra-additive effects in both groups bilaterally in the superior temporal cortex and in visual motion sensitive cortex. In sighted subjects supra-additive effects were also present in the ventral occipito-temporal cortex, whereas in the blind in the somatosensory cortex bilaterally. Only in the blind population activation we found subadditivity effect (letters + speech sounds > congruent LS) in the left inferior frontal gyrus. An interaction between the group and the congruency effect was present in the superior temporal cortex, bilaterally. Blind group presented a congruency effect in the right hemisphere, and the sighted group presented an incongruency effect in both hemispheres. Our results, which identify basic sensory aspects (supra- and sub-additivity effect) as well as higher-level associative aspects of audio-visual integration ((in)congruency effect), show that both groups integrated letters and speech sounds, and the areas of integration are similar. The superior temporal cortex is involved in both audiovisual and audiotactile LS integration. Subtle differences in the direction of congruency effect and sub-additivity in the blind might reflect lower decoding efficiency for Braille than print reading or lower exposure to reading in the blind compared to the sighted group.
Topic Areas: Reading, Multisensory or Sensorimotor Integration