Re-Entraining the Brain: Exploring and Exploiting Oscillatory Models of Speech Perception in Wernicke's Aphasia

To understand spoken language we need to hear it at the ear and then analyse the sounds and extract the words in the brain. This process is known as speech perception. It is only after accurate speech perception that we can analyse the grammar and meaning of those words.

Strokes which damage regions supporting speech perception cause a severely disabling impairment in understanding spoken language. This stops people from connecting with their friends and family and cuts people off from work and leisure activities. Our long-term goal is to improve treatments for these individuals, allowing them to lead a more complete and fulfilling life.

One prominent theory of speech perception argues that when we hear sounds our brain cells start to fire in time (synchronise) with the rhythm of the sound, and this helps us to work out what we are hearing. This is not unlike tapping along to the rhythm of a song you are listening to. Brain activity synchronises with the pattern of syllables in speech and helps us to identify the words we are hearing.

We hypothesise that speech perception impairments in people with strokes may arise from a failure of this synchronisation. To test this, we will measure brain synchronisation to speech in people with speech perception problems after stroke, a condition known as Wernicke's aphasia. We will compare the abilities of people without strokes and of people with strokes in different parts of their brain to see if problems are specific to Wernicke's aphasia.

We will also compare the degree of brain synchronisation to the degree of comprehension impairment on language tests and in daily life to understand the impact on people. Surprisingly, speech perception is sometimes accurate in Wernicke's aphasia, even though some speech perception brain structures are lost. We will investigate which parts of the brain are responding and how different parts of the brain work together when speech perception is more accurate and when it is inaccurate.

This is important because we want to help the brain to work in the most effective way more often. We can use this information to try and improve speech perception using electrical stimulation.

Research in people without strokes shows that listening to simple rhythmic sounds can change the way the brain synchronises with the sounds that follow. This also happens when the brain is stimulated with a very small electrical current applied to the scalp, and this has been shown to help people hear sounds and understand speech better. We hypothesise that we can use these methods to improve speech perception and language comprehension in people with Wernicke's aphasia.

Our pilot data suggests that this is true - when we played rhythmic sounds before speech, people with Wernicke's aphasia were better able to hear the difference between the speech sounds. This is promising because this ability is crucial for understanding speech. We will also explore why rhythmic sounds improve speech perception by measuring brain activity to see whether the brain responds more "typically" when we try to enhance synchronisation.

The last study in the fellowship will use electrical current stimulation to help to synchronise brain activity to speech. We will use our previous results to work out which parts of the brain respond most effectively to speech and stimulate those areas, then measure whether people are better able to understand spoken sentences with electrical stimulation.

If we find evidence for the hypotheses in this fellowship, we will have identified new ways to treat speech perception and language comprehension impairments after stroke, by enhancing brain synchronisation. We will also have found new ways of measuring how our therapies are working, by looking at the effects on synchronised brain activity.