By Paddy Kamen
It seems like only yesterday that neuroscientists thought the brain was a fixed, machine-like entity, unable to change or learn much after childhood – that is, until the cell death and decline associated with aging.
Neuroplasticity – the ability of the brain to change in response to experience – was brought to the fore by American Paul Bach-y-Rita in the mid-1960s. It was many years, however, before this idea gained wide acceptance.
Bach-y-Rita determined that we see with our brain, not our eyes1. He was one of the first scientists to question localization (the idea that areas of the brain are hardwired for specific functions, and for those only). According to The Brain That Changes Itself, (a must-read book by Canadian Norman Doidge), Bach-y-Rita was working with a team, “that was studying how vision worked by measuring with electrodes the electrical discharge from the visual processing area of a cat’s brain. The team fully expected that when they showed the cat an image, the electrode in its visual processing area would set off an electric spike, showing it was processing that image. And it did. But when the cat’s paw was accidentally stroked, the visual area also fired, indicating that it was processing touch as well. And they found that the visual area was also active when the cat heard sounds.”2
The fact that the visual, auditory and sensory cortices all have similar processing structures was discovered by American neuroscientist Vernon Mountcastle 3, of Johns Hopkins University, and taken up by Bach-y-Rita, who published hundreds of articles and wrote several books on the subject.
In hindsight, the brain’s ability to use the real estate in one region (say the auditory cortex) when another region needs more than usual power seems like a ‘no brainer’. Surely it is not just 20th-century people who noted that those with visual impairment tended to develop keener senses of touch, hearing and smell, or that those without hearing tended to have keener vision. Indeed, in the 1820s French physician and anatomist Marie-Jean-Pierre Flourens showed that the brain could reorganize itself. Such is the nature of human learning – one step forward and two steps back. And yet the advent of modern neuro-imaging technologies have made such knowledge incontrovertible for we can now see, in real time, where and how specific areas of the brain respond to stimuli.
Efforts to understand the precise nature of the plasticity of the brain in general and the visual cortex in particular continue today. A recent publication in the Proceedings of the National Academy of Sciences is a case in point. Researchers from theUniversity ofMontreal’s Saint-Justine Hospital Research Centre, led by Dr. Olivier Collignon, recently published a study that compared the brain activity of people who can see with that of others who were born blind. The research was undertaken in collaboration with Dr. Franco Lepore of the Neuropsychology and Cognition Research Center (CERNEC) at theUniversity ofMontreal.
Collignon told Envision: seeing beyond magazine that the main objective of the research was to see if the organizational structure for visual processing evident in sighted people would be maintained in those who had never used these structures for vision. “We know that the occipital region is highly organized and processes different kinds of visual information in different areas,” explains Collignon. “There is one area for movement, another for face recognition and another for spatial orientation. Would we find such organization in the occipital cortex of the congenitally blind?”
The researchers worked with 11 individuals who were born blind and 11 who were sighted. Their brain activity was analyzed via MRI scanning while they listened to a series of tones. “The results demonstrate the brain’s amazing plasticity,” Collignon said. “We learned that when a blind person processes spatial sounds, they use the exact same region that sighted people use to make spatial distinctions with their eyes. So the region maintains the ability to process spatially but shifts to another sense modality if (it has been) deprived of vision since birth.”
Whereas in Collignon’s experiment the visual cortex was used to process sounds, another experiment on ferrets showed that the auditory cortex can rewire itself to process visual information. According to Doidge: “All reasonable doubt that the senses can be rewired was recently put to rest in one of the most amazing plasticity experiments of our time… Mriganka Sur, a neuroscientist, surgically rewired the brain of a very young ferret. Normally the optic nerves run from the eyes to the visual cortex, but Sur surgically redirected the optic nerves from the ferret’s visual cortex to its auditory cortex and discovered that the ferret learned to see. Using electrodes inserted into the ferret’s brain, Sur proved that when the ferret was seeing, the neurons in its auditory cortex were firing and doing (sic) the visual process. The auditory cortex… had rewired itself so that it had the structure of the visual cortex. Though the ferrets that had this surgery did not have 20/20 vision, they had about a third of that, or 20/60 – no worse than some people who wear eyeglasses.” 4
Other interesting research on the visual cortex has shown that restoring or instigating a person’s vision isn’t necessarily accompanied by the brain’s ability to process the information received by the eyes. British scientists Professor Richard Gregory and J.G. Wallace published a fascinating paper in 1963 on a patient known as S.B., who had been blind since shortly after birth and had his vision restored at age 52. According to a paper published on Professor Gregory’s website, S.B. found much of what he saw confusing and overwhelming. He changed from being an assertive and adventurous blind person into a sighted person with a profound lack of confidence, deriving almost no pleasure from his new sense of vision.5
Similarly, auditory cortices may have a limited ability to process new sound information when deaf people receive cochlear implants. Other research out of the University of Montreal by Dr. Lepore and his colleagues states:
“All studies agree that congenitally deaf children implanted early in age when plasticity is greatest, perform better in open-speech perception tests than those who are implanted later. Furthermore, adults who have been profoundly deaf since birth are usually incapable of understanding speech from CI (cochlear implant) stimulation. (Busby et al., 1993; Zwolan et al., 1996). 6”
In conclusion, the brain most definitely rewires itself in response to experience, (and it is well known that even into their senior years people can make new neuronal connections). Nevertheless, it appears to be the case that once cross-modal accommodation has been made, the brain may not be able to re-wire itself repeatedly, and that the earlier the rewiring is done, the more highly functioning it is apt to be.
