15 October 2010

Impaired hearing and impaired long-term memory go together. Hearing researchers at Linköping University have a theory on why a hearing impairment affects long-term memory.

One common explanation of impaired memory in older people is that both impaired hearing and sight weakens the functions of the brain. Other theories indicate generally impaired information acquisition and increased intellectual exertion as possible causes. 

“We wanted to look specifically at what impaired hearing means for memory,” says Jerker Rönnberg, professor of psychology and head of the Swedish Institute for Disability Research (IHV).

Linköping researchers were given access to Betula, a large database for a broad long-term study that follows 3,000 people and different aspects of their ageing. From this database they sorted out 160 hearing-impaired people, all of whom use hearing aids. The various memory functions among the hearing-impaired were tested.

“We used five types of test that measure different functions,” Professor Rönnberg tells us.

Impaired hearing co-varies with impaired long-term memory

It dealt with both short-term and long-term memory, which in turn were divided up into semantic and episodic memory. To measure episodic short- and long-term memory, the research subjects were encouraged, for example, to perform various actions immediately – like removing their shoes – and to verbally reproduce the challenge. Or to reproduce lists of words, which can also be divided into short- and long-term components during reproduction.

Semantic long-term memory was tested using a word comprehension test and what is known as a ‘fluency test’. In the latter, the research subjects have to, for example, say as many words as they can that start with a certain letter. All the tests have been tried out and are well-established.

The degree of hearing impairment was also measured, as was vision.

The results were unambiguous: impaired hearing co-varies with impaired long-term memory. Short-term memory manages better. Impaired vision, on the other hand, produced no effect on memory. Visual episodic long-term memory also manages well. Facial recognition, for example, is not impaired.

Possible explanation for connection

The model Professor Rönnberg and his colleagues worked out in order to demonstrate how hearing interacts with various memory systems – the ELU model – contains a possible explanation for this connection, he says. ELU stands for Ease of Language Understanding.

The ELU model demonstrates that when someone immediately perceives what is being said, without disturbances, semantic long-term memory is activated – the word is matched with the ‘lexicon’ stored there. This happens at lightning speed, and without anyone being conscious of it.

If, on the other hand, someone doesn’t hear immediately what is being said, short-term (or ‘working’) memory must be engaged in order to try to analyse what was actually said and what it means.

“This means that episodic long-term memory is not used as much when someone has poor hearing,” Professor Rönnberg says, “while short-term memory has to work too much. And that, in turn, can lead to impaired episodic long-term memory. We know, of course, that functions that are not used degenerate.”

Age plays a role

One evident question is then whether impaired long-term memory is simply dependent on advancing age, and not on hearing impairment in and of itself.

“Yes,” Professor Rönnberg says, “the researchers’ analysis, called a structural equation model – in which the effects of different factors are weighed together at the same time – demonstrates that age also plays a role. But age alone cannot explain all of the memory loss. Impaired hearing also comes into play.”

It is also worth noting that a hearing aid cannot fully compensate, memory-wise, for a loss of hearing. All the research subjects wore hearing aids.

In a future study, the researchers will now try to make clear what effect, if any, a hearing aid has on memory. In the Betula group, hearing aid users will be matched with non-users, and any long-term memory impairment will be tested.

Anika Agebjörn 2010-10-15

Researchers - Cognitive hearing science