Different forms of dementia may lead to similar changes in brain function
Researchers have found that in two different forms of dementia, there are some common changes to a network of brain regions, suggesting that different brain disorders might lead to similar changes in brain network activity.
There are many different types of dementia resulting from degeneration of different brain regions. But new research suggests that different forms of dementia may cause similar changes in brain networks.
Researchers from the University of Cambridge, supported by a Wellcome Trust grant, investigated two disorders in which the brain tissue is rapidly destroyed: frontotemporal dementia and progressive supranuclear palsy. Frontotemporal dementia is one of the most common degenerative dementias in younger adults, causing changes in behaviour, reasoning and personality. Progressive supranuclear palsy is rarer and patients show different symptoms: the main impairment is in balance and movement, with some milder cognitive problems.
The brain regions affected in these two disorders are largely different, but both cause damage to the prefrontal cortex (the region at the front of the brain). When the brain is damaged, it can reorganise its functional networks, so that relatively healthy areas are used instead. The researchers found that prefrontal damage can cause the brain to reorganise itself in a similar way for these two very different disorders.
They asked patients to listen to a sequence of sounds that contained many regular sounds and some unexpected ‘oddball’ sounds. Even thought the participants weren’t asked to do anything, just listen, this unpredictable change led to an electrical signal from several brain regions that can be measured outside the skull by MEG (magnetoencephalography, a technique that records the magnetic fields produced by electrical currents in the brain).
Though listening to sounds seems like a simple task, being able to detect a change is an important brain function and is considered an index of more complex cognitive abilities. To detect an unexpected sound, the brain must remember the regular sounds and predict what the next sound should be. An oddball sound violates these predictions and leads to a specific response in a network of different brain regions.
The researchers found that patients with both disorders had a reduced electrical response to the oddball sounds compared with healthy people. This was not surprising given their cognitive impairments. There was no difference between the patients and those in a healthy control group when listening to predictable tones, indicating that it is the higher-level predictive processes of the brain that had been damaged, not the simple hearing process.
What was more interesting was that the patient groups used a different network of brain regions to detect the oddball sound change compared to healthy people, and that patients with different disorders both used a similar expanded network, which recruited more brain regions to detect the odd-one-out.
This may seem counterintuitive; you would expect that to better process a sound you would use more brain regions. But in fact patients with neurodegenerative disorders recruited more brain areas than healthy people (especially towards the back of the brain) in this task. It’s a classic case of ‘too many cooks’ – usually, using more brain regions causes the brain to work less efficiently.
Scientists are unsure why degeneration can cause more regions to be recruited, but Dr Laura Hughes, first author of the study, has two possible explanations, “it may be because the extra regions are trying to compensate for the frontal lobe damage or because the frontal lobe can no longer inhibit these regions from joining the network”.
Since the prefrontal cortex is damaged in both these diseases, the researchers think that this is the likely cause of the brain reorganisation observed in this study. This suggests that when one brain region is affected by degeneration, other brain regions remote from the damage can also be affected. Since schizophrenia, multiple sclerosis and Parkinson’s disease also involve dysfunction in the prefrontal cortex, the findings could help understand a wider range of diseases.
- Hughes LE, Ghosh BCP, & Rowe JB (2013). Reorganisation of brain networks in frontotemporal dementia and progressive supranuclear palsy. Neuroimage: Clinical, 2, 459-468. PMCID: PMC3708296