Results from cochlear implants can be extremely variable, having only little benefit for some patients, without any means of predicting failure based only on purely clinical factors, according to a release from the University of Geneva. Using data from brain imaging techniques that enable visualizing the brain’s activity, a neuroscientist at the University of Geneva (UNIGE) and a Parisian ENT surgeon have managed to decipher brain reorganization processes at work when people start to lose their hearing, and thus predict the success or failure of a cochlear implant among people who have become profoundly deaf in their adult life.

The results of this research were published in the journal Nature Communications.

Despite the technological advances, there are still some 5 to 10% of adult patients who have become deaf for whom cochlear implants remains stubbornly ineffective. Why? In order to answer this question crucial for clinical practice, Diane Lazard, an ENT surgeon at the Institut Vernes (Paris) and Anne–Lise Giraud, a neuroscientist in the UNIGE’s Faculty of Medicine, have sought to identify which brain factors might be linked to the success or failure of implants.

The two scientists have studied how the brain of a deaf person manages to represent the sound of the spoken word and its capacity of re–using these representations after a cochlear implant. “The test went like this,” explains Giraud. “We presented some visual stimuli to the subjects, in the form of written word, and asked them to determine whether two words, without the same orthographic ending, rhymed or not—for instance wait and gate. Subjects would then have to recourse to their memory of sounds and, using functional neuroimaging (fMRI) techniques, we observed the neural networks in action.” Whereas the researchers were expecting that the subjects would be slower and less accurate than those in a control group of people without any hearing difficulty, to their surprise, they found that certain deaf people completed the task quicker and more accurately than their normal–hearing counterparts. For “super–readers,” who appear to be able to handle written words quicker than those with no hearing impediment, the brain has opted to replace orality by written exchanges and has restructured itself accordingly, say the researchers. The brain circuits used by such “super–readers,” and which are situated in the right hemisphere, are organized differently and thus cochlear implants give poor results. The other deaf people, those who carried out the task at the same speed as the control subjects, remain anchored to orality and therefore gain more benefit from cochlear implants. Unlike the “super–readers,” the latter manage to master lip reading as deafness encroaches, and, therefore, maintain a central phonological organization very similar to that of normal–hearing people, which uses the left hemisphere of the brain. There are therefore two categories of subjects whose brain circuits function very differently.

This research points to the essential role played by the interactions between the auditory and visual systems in the success or failure of cochlear implants, say the researchers. Their outcome will depend on this cortical reorganization. For “super–readers,” the fact of having adapted to deafness by developing certain “supra–natural” visual capabilities constitutes a handicap for the use of implants.

The researchers also contend that such results also explain why it is so important to be able to equip congenitally deaf children during their first few months.