Can false memories tap into hidden brain networks in people with Alzheimer’s disease?
Study suggests false memories are automatically created in adults with moderate Alzheimer’s disease.
Alzheimer’s disease is thought to be the most common form of dementia. A progressive condition, the earliest, typical symptoms are minor memory problems, which worsen gradually over years to become severe and accompanied by other symptoms of brain impairment.
People at the stage of moderate Alzheimer’s disease typically have significant problems with their explicit memory, such as with their awareness of whether or not they have seen or heard something before.
A recent study from academics at City, University of London and the University of Hertfordshire suggests that whilst people at this stage of the disease may have impaired explicit memory, what remains intact are many of the associative networks of brain cells (neurons) involved in the individual's memory of facts and things. It suggests they can subconsciously use these networks to enhance their performance on problem-solving tasks. This finding is important, as it offers a new insight into how implicit memory (memory of which the person is unaware), may be spared in Alzheimer’s disease.
Building on previous research
The current study replicated and built on findings from previous studies involving participants with mild Alzheimer’s disease and healthy older adult volunteers. These studies suggested that in a word recognition task involving highly associated information (the DRM paradigm), both groups were able to subconsciously create a false memory of a key word. This was after being presented with a list of words read out to them which were associated with that key word, but with the key word crucially excluded.
For example, a list read out could include the words ‘table’, ‘sit’, and ’legs’, but not the associated key word ‘chair’. When read out a second list of words, including words from the first list; unrelated ‘foil’ words; and the key word ‘chair’, participants often falsely remembered the word ‘chair’ as having been present in the first list. It was found that these falsely remembered key words ‘primed’ participants’ responses on a subsequent problem-solving task (called a CRAT test), improving their performance.
CRATs are problems involving the presentation of three unrelated words, for example, ‘walk’, ‘beauty’, ‘over’, and the participant has to come up with the fourth associated word to create a compound word; in this example, the answer is the word ‘sleep’ to form the compund words 'sleepwalk, 'beauty sleep' and 'sleepover'. If the word ‘sleep’ had been a falsely remembered key word from the prior recognition (DRM) task, participants performed better on the CRAT than if the word ‘sleep’ had been a truly remembered word, or not falsely recognised during that task. This priming effect points to the power and saliency of the false memories even in memory-impaired individuals.
The current study consisted of two experiments. In the first experiment, the DRM word-recognition task followed by CRAT tests from previous study were replicated in 30 participants with mild Alzheimer’s disease and 30 healthy, older adult volunteers. However, an additional 30 participants with moderate Alzheimer’s disease took part as well.
The second experiment involved a completely separate sample of participants to the first experiment, but again consisting of a total of 90 participants across the same three groups. Participants undertook a DRM task followed this time by a more complicated problem-solving task than the CRAT, known as an analogical-reasoning task. In this second task, participants were presented with analogical-reasoning problems in the format of ‘a is to b as c is to ?’ For example, for the problem, ‘hat is to head as sock is to _____’, the answer is ‘foot’ which may have been present in the prior DRM task as a false memory, a true memory or not appearing at all.
What the current study tells us
As had been suspected by the researchers, in both experiments the participants with moderate Alzheimer’s disease were only able to respond to the DRM task by saying that the list of words read out to them were new to them. Unlike the participants with mild Alzheimer’s disease and the older healthy adults, they had no explicit memory of any of the words read out from the list.
To address this problem, the reaction time of responses to tasks was recorded for all groups of participants across both experiments. It was found that although slower than participants with mild Alzheimer’s disease, who were again slower than older healthy adult volunteers, participants with moderate Alzheimer’s disease mirrored the relative reaction times of the other two groups, which correlated well with those groups’ choice of response. As such, reaction times were used as a surrogate marker for the responses chosen by the moderate Alzheimer's disease group, albeit at a subconscious level.
Using this technique, the study suggests that ‘probable false memories’ were elicited and could be identified in the moderate Alzheimer’s disease group during the DRM tasks. Furthermore, for each subsequent problem-solving task for this group (the CRAT test in the first experiment, and the analogical reasoning task in the second experiment), improved performance was noted under the condition where a ‘probable false memory’ had been identified in the DRM task, and used to prime the response to the problem-solving task.
The authors say this strongly suggests that such ‘probable false memories’, are actual false memories comparable to those more directly observed in mild Alzheimer’s disease participants and healthy older adult volunteers.
Commenting on the study, first author Mark L Howe, Professor of Psychology at City, University of London said:
These results show that although people with moderate levels of Alzheimer’s do not exhibit explicit episodic memory for lists they have studied, the time taken to respond to items on the memory test parallel those of healthy older adults and can be used as a proxy measure for what is in their memory. These latency measures also show that they do form false memories like other adults and that these false memories can be used to prime performance on subsequent problem-solving tasks. Thus, although people with moderate levels of Alzheimer’s disease may not be aware of the contents of their memory, their memory still functions in a manner which is similar to other older adults.
The study was published in the journal, Cortex.
Find out more
Visit the Department of Psychology, City, University of London webpage.