We have all heard the amazing things the brain can do when deprived of one of the senses – stories about blind people with incredible hearing or deaf people with amazing visual skills. That is because the part of the brain responsible for hearing reorganizes itself in the deaf to take in visual information (and vice versa in the blind). A new study of deaf people explores how that reorganization occurs and how it is affected by use of hearing aids.
The new work builds on previous research showing that visual motion could activate auditory regions of the brain in deaf people. “Our work differs from previous studies in that we followed up on this result, first by looking at the functional connectivity of the reorganized auditory region, and then by looking at the role of hearing aid use,” says Martha Shiell of McGill University.
Shiell and colleagues recruited a group of deaf people who varied in their degree of hearing loss and in the duration of their hearing-aid use to compare functional changes in their brains to those of hearing people. The researchers showed the participants a visual stimulus that was either moving or staying still to isolate in the fMRI the areas of the brain specific to motion. They wanted to see how varying degrees of hearing affected how brain processed this visual motion.
They found that in the deaf participants, an auditory area of the brain – the posterior superior temporal gyrus (STG) – responded to visual motion instead of auditory information, as it did for the hearing participants. And in the deaf, the STG was more connected to areas of the brain that process visual information. Furthermore, using a hearing aid for longer periods of time reduced the extent to which this reorganization occurred.
Shiell spoke with CNS about these important results, just published in the Journal of Cognitive Neuroscience, and what they mean for deaf people, as well as the general population.
CNS: What is cross-modal reorganization?
Shiell: The term “cross-modal reorganization” describes a phenomenon where a region of the brain changes in its sensitivity to information from different sensory systems. This phenomenon is usually studied in the context of sensory deprivation, such as blindness or deafness, where the parts of the brain that would normally process the missing sense reorganize to process input from other senses instead. For example, as in our study, an area of the brain that normally responds to auditory information changes in deaf people, who don’t use auditory information, so that it responds increasingly to visual information.
CNS: How did you personally become interested in studying cross-modal reorganization?
Shiell: I started studying cross-modal reorganization in deaf people during my undergraduate degree, when I did an independent study project with Dr. Melvyn Goodale and Dr. Jacqueline Snow at Western University. At the time I thought it was a great model to understand plasticity in humans.
Since coming to McGill and working with Dr. Robert Zatorre, I’ve realized that it is also a powerful tool to understand normal brain function: We can learn a lot about the way that the brain is organized by looking at how it changes itself under different circumstances. These changes are surprisingly specific; if we can identify these specifics, then we can deduce what that region is supposed to accomplish under normal circumstances.
CNS: How did you identify the subjects for your study?
Shiell: We looked for participants who were early deaf – that is, deaf since birth or within the first year of life – and with no cochlear implants. We wanted some variability in the characteristics of our participants because we were interested in how cross-modal reorganization might vary under different circumstances, so we recruited people who varied in their degree of residual hearing, either from differences in hearing loss (from severe to profound deafness) or from their hearing aid use.
This is unlike most previous research, which focused on people who are congenitally and profoundly deaf. While the latter group is ideal for establishing strong effects, we need to examine cross-modal reorganization under varying circumstances in order to fully understand the mechanisms that support it.
CNS: What was the significance of the activity you found in the posterior superior temporal gyrus and its connectivity?
Shiell: In deaf people, the posterior superior temporal gyrus responded to visual motion stimuli, while in hearing people, this same area responded to auditory stimuli. This is evidence for cross-modal plasticity after deafness.
In deaf people more than in hearing, the activity within the posterior superior temporal gyrus correlated with activity in primary visual cortex, demonstrating that the reorganized auditory cortex is increasingly interacting with the visual system. This is relevant because it illustrates that cross-modal reorganization affected not only auditory areas, but also how auditory areas interacted with the rest of the brain. Identifying this network was relevant for understanding the mechanisms that support cross-modal plasticity, and will help us generate hypotheses on what is going on in the brain after deafness.
CNS: What are the implications of your study for use of hearing aids?
Shiell: The results of our study suggest that, in profoundly deaf people, there is a relationship between hearing aid use and the degree of cross-modal reorganization that occurs in auditory regions. Because we’ve measured correlation, we don’t know which factor is driving this relationship, but we can say that a long-term hearing aid user probably has less cross-modal reorganization than a short-term hearing aid user. This information can be helpful for someone who is considering a cochlear implant, because we know that the effectiveness of cochlear implants varies according to how much reorganization has occurred.
CNS: How does the effectiveness vary? Is less reorganization better for the cochlear implants?
Shiell: The current hypothesis is that reorganization after deafness inhibits rehabilitation through cochlear implants. This is based on the theory that if the auditory cortex reorganizes to respond to non-auditory input during deafness, then it is no longer available to respond to auditory input when hearing is restored.
CNS: What were you most excited to find in your results?
Shiell: The most exciting result was the functional connectivity between the reorganized auditory region and primary visual cortex. Notably, this occurred for an area of primary visual cortex that processes the peripheral visual field, which is also where deaf people tend to show an advantage in visual tasks compared to hearing people. This finding may be a clue for understanding the anatomical changes that occur in deaf people that support their enhanced visual abilities.
We also had a surprising result, which was the relationship between hearing aid use and cross-modal reorganization – we didn’t expect that hearing aids would be relevant for people with profound deafness, who by definition have very little residual hearing. This result, together with reports from the participants themselves, show that hearing aids can be useful for at least some profoundly deaf participants, and that their use interacts with cross-modal reorganization. Now that we have identified that this relationship exists, we need to do more research to understand it.
CNS: What is the significance of your findings for the general population?
Shiell: The brain is plastic – it can adapt to maximize its effectiveness in different sensory environments – but it does so in only a very specific way, that can be influenced by many external factors. The story of neuroplasticity is a complex one, that we are only beginning to understand.
CNS: What’s next for this work? What do you ultimately hope to accomplish?
Shiell: We are now using MRI to look at brain anatomy in deaf people, in order to find anatomical changes that may support the cross-modal reorganization that we have observed. Our guiding goal is to understand what occurs in the auditory regions of deaf people so that these areas process visual information, and why those specific changes occur and not others. Ultimately, this will help us understand the principles of brain organization and plasticity.
-Lisa M.P. Munoz
The paper, “Reorganization of Auditory Cortex in Early-deaf People: Functional Connectivity and Relationship to Hearing Aid Use” by Martha M. Shiell, François Champoux, and Robert J. Zatorre, was published online on July 7, 2014, in the Journal of Cognitive Neuroscience.
