It is a conference dedicated to studies on objective tests performed during the diagnostics of the auditory pathway in cochlear implant users. It is organized every 2 years. This year its organizers were Prof. Karen Gordon and Prof. Blake Papsin from the SickKids Hospital. SickKids Hospital is one of the worlds’ best known pediatric clinics maintaining a program of treatment of hearing loss with auditory implants. For many years it has been conducting a program of basic and clinical studies using brain function imaging techniques. This year conference focused on studies of auditory pathway with these techniques. More than 200 scientists from 20 countries presented results of their research. Assoc. Prof. Artur Lorens, Head of Implants and Auditory Perception Department, spoke with ‘Słyszę’ about the latest reports from Toronto.
Słyszę: Why imaging studies are so important part of this year conference?
Assoc. Prof. Artur Lorens: Because the way we think about objective studies has changed. Until today we focused on objective studies commonly used in diagnostics, such as registration of action potential of the auditory nerve, stapedius muscle reflex, or evoked potential in brainstem. These are all peripheral measurements, which means that they relate to the inner ear and auditory nerve (or brainstem impulses), not the higher levels of the auditory pathways. For many years these methods have been perfected, as we thought that in order to improve level of speech understanding in patients with total or partial deafness we need to program the cochlear implant system in such way that that the pattern on excitation comprised of millions of neuronal impulses generated through electric stimulation in thousands of nerve fibers would be as similar as possible to the excitation pattern of the auditory nerve caused by sound in normally working inner ear. In order to properly fit a sound processor (external part of the cochlear implant system), we need to confirm very precisely (i.e. using objective methods) activations occurring in cochlea and auditory nerve. In time it turned out however that although we can perform more and more precise objective measurements of peripheral auditory pathway and stimulate the inner ear better and better, some patients still do not achieve as good results as we would expect. Effects achieved after cochlear implantation are very diverse – some people make huge progress in hearing and speech understanding, while others hear better, but still have significant difficulties with speech understanding. This happens even though according to the objective measurements their auditory nerves receive equally good stimulation. These observations led to the revolution in thinking – experts came to the conclusion that we cannot focus exclusively on the peripheries of the auditory system, because results of therapy are in major degree dependent on brain.
S.: Can imaging studies help explaining why some implanted patients achieve excellent results while others perform worse than expected?
A.L.: Yes. At the conference in Toronto researchers from the Institut Arthur Vernes in Paris presented new results of imaging studies implying that the effects of rehabilitation may be related to the newly discovered so called ‘phonological loop’, specific neuronal pathway in the brain participating in speech processing. Researchers used fMRI pictures to follow the brain activity of subjects who had visual contact with text (decoded speech signal visually). It turned out that similar parts of brain participated in decoding of visual and auditory information. There are two roads on which this information may flow to the speech analysis center – a shorter and a longer one. The longer road, called dorsal phonological root, is related to the phonological processes, while the shorter one, called ventral semantic root, only to semantics (decoding of the meaning). When a normally hearing person, reading a text, ‘hears’ the words in her head, it means that the phonological root is activated. Sometimes though reading a text we activate a shorter road – we see letters and automatically, without thinking, understand the meaning of words and sentences. Such very fast reading, sometimes called photographic, is activated when the brain uses ‘shortcut’ bypassing the phonological root. Some people have a natural tendency for such fast reading, researchers suppose it may be genetically transmitted trait. Usually the phonological root is activated when we hear something. In case of hearing loss it is untrained, patients are increasingly apt to use only the shorter, sematic road. We observe correlation between hearing and speech understanding scores after cochlear implantation and activations of these two areas. People, who while reading activate phonological root even though they cannot hear achieve significantly better scores than these patients who activate semantic root.
S.: So these people should theoretically benefit more from an implant?
A.L.: Yes. In turn these patients, whose brains have a natural tendency to read fast, bypassing the phonological root, may achieve less benefit after cochlear implantation. An additional hypothesis is that patients who are better at lip reading may have more benefit from an implant, because lip reading prevents deactivation of the phonological root. We match the movements of lips to the sound and thus in a way mentally ‘hear’ the words. This theory is in complete opposition to earlier belief that lip reading blocks the auditory channel, thus people who are good at lip reading should achieve worse scores in hearing and understanding after cochlear implantation. Today we know that this reasoning was fallacious, at least in case of the postlingually deaf people who developed speech before becoming deaf.
S.: What may change in the field hearing rehabilitation in the nearest future due to these imaging studies?
A.L.: More and more frequently we speak not of rehabilitation but neuro-rehabilitation, i.e. therapy of hearing taking advantage of brain plasticity. We are not sure yet, how that could be accomplished. There are many different groups of patients, each group may have differently developed auditory regions of the brain and different potential for changes that may be caused taking advantage of the individual range of neuroplasticity. New imaging methods will enable identifying these differences and to answer a question – how, allowing for these differences, we could program a speech processor to allow the auditory cortex maximizing the utility of information provided by implant.
S.: Did any results of these studies find their way into clinical practice?
A.L.: At the Toronto conference there were presented many studies on impact of unilateral stimulation on changes of location of cortical structures. They point to a conclusion that before the 4th year of life (so called critical period) stimulation with a single implant causes radical changes in child’s auditory cortex. Cortex on the side opposite to the implanted ear becomes strongly dominating. Even when later we activate, though implantation, hearing in the other ear, information originating from it well mostly go to the dominant cortex, not, as we would like it, to the opposite cortex. It is an argument for implanting children who are deaf from birth with two implants simultaneously or in quick sequence. This would help avoiding too strong dominance of auditory cortex in one cerebral hemisphere. It is worth underlining that it is a pathological effect. It hinders normal bilateral processing of sound stimuli necessary for understanding of speech in noise of localization of the sound source. First studies of such domination in children were presented at the conference. They were possible thanks to a newly developing method of brain imaging using the near-infrared spectroscopy. Earlier, existence of such effect had been demonstrated only by Prof. Andrej Kral from Hannover, who performed studies on cats. He was the guest of honor of the Toronto conference. Other studies investigated whether a hearing aid worn in the ear opposite to the implanted one may prevent pathological auditory cortex domination. It turned out that no. Most implanted children have too small residual hearing for hearing aid to have any effect. In patients with residual hearing, brain receives much more auditory information from the implant than from the hearing aid. This preponderance is so strong that in children who are wearing hearing aids, the auditory cortex receiving signals from implant dominates as distinctly as in children who are not wearing any contralateral hearing aid.
S.: Imaging studies shed new light on many hearing-related processes. Problem is that these methods are invasive. Are scientists trying to solve this dilemma?
A.L.: Indeed, PET requires administration of an isotope preparation, while FMRI produces very strong magnetic field, which significantly limits its application in brain imaging for people using implants, due to the magnet which is inside the implant. This is why scientists are researching new imaging methods. During the conference Prof. Rene Giffort from the Vanderbilt University presented a novel method of near-infrared spectroscopy. It registers brain activity in an non-invasive way, through changes in flow of oxygenated blood. Very promising is also a new EEG measurement, using innovative technology which allows not only to register the presence of cortical impulses, but also to determine the area where they originate. It is possible that at the next conference in Budapest, the scientists will be able to present the results of studies with implanted patients, such as we cannot perform with present invasive methods.