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Why Can Some Blind People Process Speech Far Faster Than Sighted Persons?

Functional brain imaging has revealed that some blind people's brains rewire themselves, giving them extraordinary auditory comprehension



Photo by David Benbennick, courtesy Wikimedia Commons

SAN DIEGO—Books fly from the shelf as Superman fans the pages in a blur devouring the information at blinding speed. Superhuman mental powers, including his extraordinary sense of hearing and blazing speed-reading, are as vital to Superman as his bullet-beating velocity and steel-bending strength. But it seems Superman isn't the only being with the gift of quickness. Neuroscientists reported in November at the Society for Neuroscience's annual meeting in San Diego that they have found an interesting group of real individuals with such superhuman mental abilities—blind people. Moreover, functional brain imaging now reveals how they achieve their extraordinary cerebral feats.

A popular notion is that blind people sharpen their remaining senses to compensate for lost vision. Blind musicians, such as Stevie Wonder and Ray Charles, may excel in music because of their highly developed sense of hearing. Researchers from the Hertie Institute for Clinical Brain Research at the University of Tübingen in Germany have found scientific support for this belief. Blind people can easily comprehend speech that is sped up far beyond the maximum rate that sighted people can understand. When we speak rapidly we are verbalizing at about six syllables per second. That hyperactive radio announcer spewing fine print at the end of a commercial jabbers at 10 syllables per second, the absolute limit of comprehension for sighted people. Blind people, however, can comprehend speech sped up to 25 syllables per second. Human beings cannot talk this fast. The scientists had to use a computerized synthesizer to generate speech at this speed. "It sounds like noise," Ingo Hertrich, one of the scientists involved in the research told me. "I can't understand anything…maybe it sounds like some strange foreign language spoken very rapidly." (To hear what speech at 16 syllables per second sounds like, listen to a sample recording the scientists used in their experiments.)

Hertrich and his colleagues Hermann Ackermann and Susanne Dietrich wanted to find out what was going on inside the brains of blind people that gives them this "superpower" to understand speech at ultrafast rates.  Examining brain regions activated by blind and sighted people while they listened to ultrafast speech and laid inside a (functional magnetic imaging, or MRI) brain scanner revealed that in blind people the part of the cerebral cortex that normally responds to vision was responding to speech.

No wonder blind people seem to have superhuman powers of high-speed listening comprehension. Normally, this brain region, situated at the back of the skull and called V1, only responds to light. Vision is such an important sense for humans that a huge portion of the brain is devoted to visual processing—far more gray matter than is dedicated to any other sense. In blind people all this brain power would go to waste, but somehow an unsighted person's brain rewires itself to connect auditory regions of the brain to the visual cortex.

Ackermann explained that the age at which a person loses sight is likely to be critical in rewiring brain regions controlling hearing to the region that normally processes vision. In people who are born blind the visual cortex is completely unresponsive to any auditory or visual stimulation. This region of the brain becomes functionally disconnected because visual input is necessary early in life to wire up visual brain circuitry properly. Younger people who lose sight after these connections formed, however, are able to reroute them to process auditory information after becoming blind. On the other hand, people who lose sight late in life are also less able to rewire their brains, because the critical period during which visual experience can influence this process is limited to earlier years in life. (All the subjects in this study had lost their sight between two and 15 years of age.)

But how do brain regions connected to the ears get rewired to brain regions that are normally connected to the eyes? The fact is that most of our senses have some interacting circuitry between them, which is called cross modality. There are some connections between the brain's auditory and visual regions, because the two senses must work together. Seeing a person's lips move helps comprehension of speech. We also need to orient our visual and auditory attention to the same events and to the same place in space, so there is an exchange of information between the auditory and visual cortices. Nerves from muscles that control our eye movements, for example, connect to the brain's hearing centers as well. These connections between visual and auditory regions of the brain become strengthened after losing sight. Also, some regions of cerebral cortex that border visual and auditory cortices—the left fusiform gyrus, for example—expand territory in blind people to make use of the idle circuitry in visual cortex.

Interestingly, the researchers found that blind people only use the right visual cortex for understanding ultrafast speech. Ackermann suspects that this may be because the right brain is specialized for processing low-frequency information, which is typical of speech, but this theory is still unproved. What blind people might use the left visual cortex for is something the group is investigating and hopes to report at next year's meeting.

The main interest of the researchers is in brain stroke. By investigating how the blind brain rewires itself to compensate for lost function, the researchers hope to discover new information that can be helpful to patients recovering from stroke. But Ackermann also stresses that an important outcome of this research is the help it can provide the blind. Whereas it is always better to be sighted than not, people who have lost vision do have certain extraordinary abilities that can give them advantages over sighted people. He finds that blind people are able to turn up the rate of text-to-speech converting computer programs to read three books in the time it would take a sighted person to read one. This extraordinary ability will benefit blind people in processing large amounts of written information in textbooks for study at school, and perhaps open new job opportunities to exploit their high-speed reading skills for translation or other auditory comprehension at blazing speeds that to Lois Lane and the rest of us mere mortals sounds like babble.

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