Brain power: Can a duck have four legs? This was the question addressed by Prof Matthew Lambon Ralph in his BA Darwin Award lecture at the Festival of Science in Norwich yesterday.
Prof Lambon Ralph has been investigating how the brain learns the meanings of words and how this knowledge can deteriorate in certain types of dementia.
Semantic dementia is the second most common form of dementia after Alzheimer's disease in people under 65.
In this progressive disease, there is a significant loss of tissue in the left temporal lobe of the brain, specifically the "temporal pole", the area of brain we would touch if we could put our finger all the way through the ear.
As the disease progresses, people have increasing difficulties remembering and using familiar words. The mistakes they make are particularly intriguing.
"It is not like going through the [ brain's] dictionary and deleting entire entries," Prof Lambon Ralph said. Instead, meanings become fuzzier and it becomes more difficult to distinguish between similar items, for example, different types of animal.
In one experiment, patients with semantic dementia were asked to copy drawings of familiar objects, such as a duck. After some delay, the volunteers were asked to draw the same object from memory.
When copying familiar objects, patients had no difficulty, demonstrating that their vision and drawing abilities were fine. However, when asked to draw from memory, strange things started to happen.
After a delay of one minute, one patient drew the "duck" with eyebrows and a mouth, turkey feathers and four legs.
"With a delay, the patients have to rely on meaning to create the drawing," he explained. The four-legged duck demonstrates that patients are losing key features of what "duck" means. Not all information is lost though.
"Patients confuse items with similar meanings, such as ducks and other animals, but wouldn't confuse dissimilar objects, such as animals and cars," he said.
The loss of meaning is evident not just in drawing, but in speech and writing. It affects the person's ability to identify objects, such as a lemon, by smell or touch.
"This problem is very, very frustrating for patients. They may forget family names and have difficulty asking for things they want, such as a glass of water," he said.
Studies with patient populations give scientists important clues about the areas of the brain involved in storing information about word meanings. To isolate the specific brain regions involved, Prof Lambon Ralph has been using transcranial magnetic stimulation (TMS) to explore brain function in healthy volunteers.
In TMS, a coil is placed over a volunteer's skull and a magnetic pulse is transmitted to the brain area of interest. The pulse stimulates that brain region and temporarily wears it out, in much the same way that repeated press-ups would cause muscle fatigue in the arms.
This fatigue temporally disrupts brain function, though function is fully restored after five to 10 minutes, he said.
Twelve healthy volunteers took part in the study. The findings showed that when TMS was applied to the temporal pole region of the brain, the volunteers were 10 per cent slower in their ability to name familiar objects and understand familiar words.
"These findings mimic the patient results, albeit at a milder level," he said.
Prof Lambon Ralph suggested that understanding how the brain retains information would help speech and language therapists devise better treatment programmes for individuals with semantic dementia.
"One-third of my research group are speech and language therapists," he said. Their future research will include studies of how best to help patients relearn and retain words that are important for communicating with friends and loved ones.