“People deaf from birth may be able to reassign the area of their brain used for hearing to boost their sight” reported BBC News. It said that although deaf people often report that their other senses are improved, it has not been clear how the brain does this.
The study behind this news story compared the vision of three cats that had been deaf from birth, and three hearing cats. The researchers found that the deaf cats had better peripheral vision and movement detection than the hearing cats. They also found that these improvements appeared to be caused by changes in brain function in areas that were normally associated with hearing.
These results further the understanding of how the brain may compensate for the loss of a sense. Further research is needed to determine whether similar changes occur in the brains of humans who are deaf from an early age. However, such research may be difficult to perform, as it would need to use non-invasive brain imaging techniques.
Where did the story come from?
The study was carried out by researchers from the University of Western Ontario in Canada and other universities in the US and Germany. It was funded by the Canadian Institutes of Health Research, Natural Sciences and the Engineering Research Council of Canada, Deutsche Forschungsgemeinschaft and the US National Institutes of Health. The study was published in the peer-reviewed journal Nature Neuroscience.
BBC News coverage of this study was accurate and balanced.
What kind of research was this?
This was animal study in cats looking at whether the brain may compensate for deafness by improving vision. The researchers report that when the brain loses one sense, it often compensates by improving the other intact senses. For example, some studies have found that deaf people have better visual function. It has been suggested that this may be because the part of the brain normally involved in hearing (the auditory cortex) is recruited to perform visual tasks. However, this has never been proven.
What did the research involve?
The researchers compared the visual abilities of three deaf cats and three non-deaf cats. The deaf cats had been deaf from birth, and their deafness was confirmed using a standard test. The cats were also given seven different tests of their vision, including:
- placing them in an open space to see if they could accurately find and approach red LED lights placed at different positions in their visual fields, from central areas to peripheral areas of vision
- tests of motion detection, and testing of the ability to detect different directions of motion; to detect different speeds of motion; tell apart a grey grid pattern from a uniformly grey field; tell apart lines that were aligned perfectly from those which were not; and to differentiate between a vertical line and a non-vertical line.
The researchers also looked at the role of the auditory cortex in any differences in vision. They also investigated whether they could pinpoint the parts of the auditory cortex that were performing specific visual functions. They did this by implanting a ‘cryoloop’ (stainless steel tubes) into the cats’ brains after their initial training and visual testing was complete. They used the cryoloop to temporarily cool different areas of the auditory cortex, which stopped those individual areas from functioning properly, thus enabling the researchers to see what effect this had on vision.
What were the basic results?
The deaf cats had better peripheral vision and better motion detection than the hearing cats. The deaf cats did not differ from hearing cats in their ability to detect different directions of motion or different speeds of motion, nor did they differ on the other visual tests performed.
The researchers found that they could identify the specific areas within the auditory cortex that were performing the improved visual functions. The area involved in the improved peripheral vision was found to be the posterior auditory field, an area that is usually involved in locating where sounds are coming from. They found that the area at the top of the auditory cortex (called the dorsal zone) was responsible for the improved motion detection. Inhibiting the auditory cortex of the deaf cats (by cooling) was found to have no effect their other, unenhanced visual abilities.
How did the researchers interpret the results?
The researchers conclude that better vision in deaf people is a result of a reorganisation of the part of the brain involved in hearing (the auditory cortex). They say that they could identify the exact areas of the reorganised cortex that were responsible for the individual improved visual functions.
This research has shown that cats that are deaf from birth can compensate for this by developing better vision than hearing cats. These improvements in vision appear to be due to the hearing parts of the brain taking on new visual functions.
These results further our understanding of how the brain may compensate for the loss of a sense. However, the study cannot tell us whether similar changes would occur in cats who become deaf in adulthood or, more importantly, whether similar changes occur in the brains of humans. Research aimed at determining whether similar changes occur in humans would need to use non-invasive brain imaging techniques.