Brain: Language is not central to speech
Language is amodal. That is:
"The human brain does not discriminate between the hands and the tongue. People discriminate, but not our biological human brain." -- Dr. Laura-Ann Petitto (Gallaudet Today, Spring 2012, p. 17)
"Language is not mouth stuff – it’s brain stuff.” – Dr. William Stokoe, the father of ASL linguistics.
This Gallaudet's video below explains all important information in short on the brain, language, modality (signing and speech), language acquisition, and myths and facts on cochlear implants.
These common assumptions in quotes below are not correct:
"Speech is essential for human language acquisition." No, language is (in either signing or speech).
"Speech is essential for the human brain to have typical brain organization and processing of human language." No, tissues in the brain for language are not processing sounds, but patterns.
"Early exposure to sign language will hurt a child's acquisition of spoken language." The fact is that children who are exposed to ASL early in life are stronger in the acquisition of spoken English (a benefit of bilingualism).
"Exposing a child with a cochlear implant to sign language will hurt (auditory) tissue development." Research shows that sign language exposure early in life with a cochlear implant doesn't hurt the development of the (auditory) tissue. Using a signed language does not hinder nor delay speech development.
Language in the brain
Most of us are familiar with the fact that the left region of brain is activated when a language is used.
English or any spoken language is a vocal-aural language, whereas ASL or any signed language is a visual-spatial language. Though these modalities are different, languages (signed and spoken) are no different in term of natural human language.
Activities in the left brain. Researchers studied Group #1 of hearing subjects who are prelingually (native) English speakers and who have never learned ASL, and Group #2 of prelingually (native) bilingual hearing subjects who have learned both English and ASL from birth.
Both groups #1 and #2 show activation in the left brain only, when processing English sentences.
On the other hand, when processing ASL sentences, prelingually (native) hearing signers (and also true for prelingually or native Deaf signers) show the same cerebral activity in the left brain as hearing people!
This suggests that the regions of the left brain, which is responsible for language, is not central to sound-speech, as it had been previously believed to be.
In addition to the left brain, viewing or processing ASL sentences also activated the regions of the right brain for the visual-spatial domain used in language (just like Asian reading characters).
Interesting, the study shows that the region called "angular gyrus" (AG) in the right brain showed activity found only in prelingually signers, not found in postlingually signers. (Newman) This result suggests that there is a critical period for language processing in the right hemisphere.
Aphasia: If Wernicke's area in the left brain were damaged in an accident (receptive aphasia), both signer and vocal speaker alike would have a difficult time comprehending a language, but would have no problem with expression.
On the other hand, if Broca's area in the left brain were damaged (expressive aphasia), both signer and speaker similarly would struggle with expression, but they could understand words/signs normally.
If whoever's right brain were damaged, she/he would have trouble drawing task as well as responding to spatial tasks. If there is a damage to a signer's right brain, it would not affect any language, but she/he signer may have difficulty describing a room using spatial relations (spatial-visual processing); instead she/he would sign "CHAIR UNDER TABLE ACROSS-FROM (or NEXT-TO) BED)" instead of "TABLE CL:B | CHAIR CL:2-under-table".
Linguistics studies show that language acquisition in signed language is on the same timeline as spoken language from babbling to one-word stage to pronoun reveral errors at about 20 months.
"Sign language is the equal of speech, lending itself equally to the rigorous and the poetic, to philosophical analysis or to making love." -- Oliver Sacks.
Petitto, Dr. Laura Ann. Untitled. Montreal: McGill University media. October 28, 1997.
Newman, A.J., Bavelier, D., Corina, D., Jezzard, P. and Neville, H.J. A critical period for right hemisphere recruitment in American Sign Language processing. Nature Neuroscience, 5:76-80, 2002.
Suggestions for further reading
Emmorey, K., & McCullough, S. (2008). "The bimodal bilingual brain: Effects of sign language experience", Brain and Language , doi:10.1016/j.bandl.2008.03.005
Karen Emmorey. "Sign Language and the Brain". Language, Cognition, and the Brain: Insights From Sign Language Research. Chapter 9: pp 271-314.
Lane, Harlan, et al. (1996) A Journey into the Deaf-World. DawnSignPress. Pp 101-104.
Related posts: Sight and the brain of Deaf people.