A UUC researcher is detecting how stroke patients process language and how brain damage alters access to the meaning of words, writes Courtenay Norbury
Survivors of stroke are often faced with severe difficulties communicating with family and friends. Now a Cork-based scientist is developing new methods to assess language understanding and measure language recovery in adults recovering from stroke.
Dr Catharine Pettigrew of University College Cork, has opened a new laboratory that will investigate how the brain responds to language after neurological damage.
The Irish Heart Foundation estimates that up to 10,000 adults in the Republic will experience stroke in any given year. Dr Pettigrew hopes that this research will lead to better rehabilitation methods.
She uses electro-encephalography (EEG) to measure the electrical activity of the brain as it responds to sounds and words. These signals are captured by placing a special device on the head of the participant being tested. The device can measure the tiny electrical signals given off when our brains respond to any stimulation.
"When we hear a sound, the neurons in the brain produce these event-related potentials (ERPs), which quickly travel to the scalp," explains Pettigrew.
"We measure these ERPs by asking participants to wear a cap, which is like a black swimming cap with little electrodes embedded in it. Participants listen to different sounds played through earphones and every sound gives off a brain response, which is captured by the electrodes in the cap," she says.
The electrodes are in turn connected to a computer developed by Compumedics Neuroscan, which analyses the electrical signal. The software changes the signal into a series of wave patterns and these patterns change in response to different sounds.
Pettigrew is particularly interested in changes in the wave form when a different sound is introduced. This sudden change is known as "mismatch negativity" (MMN).
"The MMN provides an objective measure of how the brain discriminates different sounds or words, which is essential to understanding language," Pettigrew explains.
Her original study was conducted at the University of Queensland in Australia. In this study, she compared individuals who had suffered damage to the language centres of the brain as a result of stroke, with healthy adult volunteers. All participants listened to repetitive streams of speech sounds, such as "duh, duh, duh". Occasionally, a known word such as "day" was inserted into the speech stream.
"In the healthy volunteers, we saw a large brain response to the word 'day' relative to the other sound. This indicated that the healthy volunteers recognised that 'day' was a word. The individuals with stroke damage had a much smaller MMN, which indicates that these individuals have some difficulty determining what is a real word of their language and what is not," she says.
This method is much more objective than traditional tests of language-understanding which depend on the participant listening and then responding to questions, she says.
"The real advantage of this technology over behavioural measures of language-processing is that it does not require the client to pay attention to the stimulus and it does not require an overt response from the client. In fact, they can watch videos while they are listening."
This is a real issue with more traditional language assessments after stroke. "Individuals who have had a stroke often fatigue quickly and have difficulty attending for long periods of time." They may also suffer from post-stroke damage to movement or the ability to speak.
"These additional impairments can make it difficult to gauge understanding accurately," she says.
Dr Pettigrew believes this technology could in future help speech-language therapists pinpoint the exact nature of the communication breakdown, aiding both clinical diagnosis and the ability to monitor language-recovery.
Dr Pettigrew is currently applying for funds to bring the equipment she needs to Cork, where she has been a lecturer in the department of speech and hearing sciences since 2004.
"This technology has the potential to increase our understanding of language functioning in a range of different pathological populations, including dementia and Parkinson's disease, as well as stroke," says Dr Pettigrew.
• Dr Courtenay Norbury is an experimental psychologist at Oxford University and a participant in the British Association for the Advancement of Science's media fellow programme