My PhD research – semantic dementia
One year ago
today yesterday (oops – I didn’t manage to finish and post this in time yesterday) I had my viva, which is like a defense, for my PhD. I can’t believe how much has happened in a year, but I thought it was fitting to talk a little bit about my own research in a few upcoming posts, and I hope it will be interesting for other people to read. My PhD could be split into two halves in terms of the topics which I researched, and I will talk about these two topics in the coming days, but I decided that the patient group I was working with really needed its own post since the disorder these people suffer from (semantic dementia) forms the basis of all of my research. Therefore, this is what I will talk about today.
In a previous post, I talked a bit about different types of memory. I won’t go into as much detail here, but I will briefly mention episodic and semantic memory (the distinction between semantic and episodic memory was introduced by: Tulving, 1972). Episodic memory is our memory of events, or episodes which have occurred. This includes remembering what we did last night, what we ate for breakfast, what we did on a family trip growing up, or perhaps that we got a bicycle for our fifth birthday. Semantic memory, on the other hand, is remembering what a birthday is, what a trip is, what a bicycle is, etc. I think of semantic memory as the distillation of all of our episodic memories about a topic and all of the factual knowledge we have learned about it. Therefore, your semantic memory (conceptual knowledge) about about any given topic extracts the general trends which are related to this topic from your own experience and forms a concept which encompasses your knowledge. Semantic memories are therefore partially based on our own personal experiences, but we can also extract semantic information from seeing things on TV, hearing about other people’s experiences, reading about a topic, etc., so people’s conceptual knowledge tends to overlap to a large degree in spite of some individual differences.
Semantic dementia (SD) is a disorder which leads to a gradual, relatively selective break-down of semantic memory (Warrington, 1975; Bozeat et al., 2000; Lambon Ralph & Patterson, 2008). People with SD remember what they had for dinner last night and what they did on a family trip growing up, as their episodic memory is intact, but if you ask people with SD what broccoli is (which may have been part of their dinner last night), they probably won’t be able to tell you.
Memory break-down does not happen all at once. The break-down of concepts is gradual and structured (Rogers et al., 2004), but once a concept is lost, people have not just forgotten the name. They have actually lost the meaning. Therefore, although people with SD are equally good at discriminating between concepts – in other words, their sensory skills are intact (e.g., Snowden et al., 2001; Luzzi et al., 2007; Piwnica-Worms et al., 2010) so they can tell things apart from one another, they are impaired at relating concepts to their corresponding sounds (e.g., matching the sound of a siren to a picture of an ambulance) tastes, smells, etc. These and other studies have shown that people with SD are worse than control participants at recognizing concepts no matter which sensory modality the items are presented in (Bozeat et al., 2000; Lambon Ralph & Patterson, 2008; Luzzi et al., 2007; Piwnica-Worms et al., 2010)
Interestingly, people with SD are generally still able to appropriately use and interact with the items which they use on a regular basis because the ongoing interaction seems to maintain an episodic memory even if the conceptual knowledge is lost (Bozeat et al., 2002). I think perhaps the most illustrative example (Bozeat et al., 2002) shows that if someone with SD had forgotten what a kettle was, then when given an unfamiliar kettle, they were not sure how to use it, even though they made tea using their own kettle every day (this study was conducted in the UK where tea is a staple). This highlights the interaction between episodic and semantic memory, and again shows that conceptual knowledge is really being lost. I personally encountered a striking example of this – one of the people with SD who I worked with for my PhD was a lady who drank green tea each day. When I would visit her, she was always a bit embarrassed because she only knew how to make green tea, but not black tea. The process for making these two types of tea is virtually identical, but she had lost the concept of black tea and therefore was not sure how to make it for visitors. Luckily, I was quite happy with water, but it was incredible as a researcher to see such a real-life example of this phenomenon which I had read about.
Talking with people who have SD is very interesting because their speech sounds normal. They do not make speech errors, they speak fluently without extra pauses, and their intonation sounds normal. Therefore, if you met someone with SD in a shop or on a bus, you probably wouldn’t know that anything was wrong. However, people with SD tend to use very common words (words which occur frequently in everyday language) and more general terms (perhaps vegetable instead of broccoli, or even food instead of vegetable). This is because people with SD remember more frequent, more general concepts and words longer than they remember less frequent, more specific concepts and words (Rogers et al., 2004).
In terms of their behavior, people with SD can become slightly dis-inhibited due to the brain damage which occurs. For example, they may say things which are slightly inappropriate – generally just giving slightly more information than you might expect about a personal topic – or they may laugh at times which may seem inappropriate, or they may eat foods which seem strange. However, the level of dis-inhibition varies, as it depends on the particular pattern of brain damage which occurs and the level of severity. People with SD often become very rigid in their behavior. They often follow a very strict schedule and do things in a very particular way. I am not entirely sure if this is a result of behavioural changes caused by brain damage or whether the rigidity provides people with SD with a way to remember what they are doing when their conceptual knowledge is failing, but I am guessing it may be a combination of both of these things.
There are many other interesting things about SD and the loss of semantic knowledge, but I don’t want to focus completely on the cognitive impairments in SD at the expense of mentioning the impact it has on people’s lives. If you are interested in seeing more of the cognitive impairments, there is a video (not the best video but the only one which seems be available), you can watch here.
Semantic dementia is often left undiagnosed for a long time because it is quite rare and doctors are not always as aware of this disorder as they are of some of the more common forms of dementia. In addition, SD is not picked up on many neuropsychological tests, because episodic memory and other cognitive skills are still relatively intact. Living with an undiagnosed condition is often very difficult for people with SD, as they know that something is wrong but no one seems to believe them. However, being diagnosed with SD is also difficult because there are not yet any effective treatments, it is a progressive disorder, and it eventually leads to death. SD is considered an early-onset dementia, meaning that it is often occurs when people are relatively young (often when people are in their 60s). This means that people with SD are generally still very active and independent, and may still be working when they start having symptoms, so being diagnosed with SD can be particularly stressful. People with SD are often worried about how their disorder will impact their relationships with their families and friends, and they are often nervous about making mistakes or not being able to think of a word or a name when they are in social or work situations. This often leads them to isolate themselves from other people, and this can lead them to feel even worse. Depression seems to be common, perhaps partly due to changes in the brain, but probably at least partly due to the many changes in the person’s life and the worries that often accompany SD.
In spite of all of this, I have been so inspired by the people I have met who have SD, and by their families. The people I met through my PhD research were some of the warmest, most welcoming and uplifting people I have ever spoken with, and it was amazing to see how they coped with such a debilitating and frustrating disorder. I feel very lucky to have met each and every person who took part in my studies and I hope that my research may help advance our knowledge of SD in some small way.
We are now at my parents’ house in northern Wisconsin, after a wonderful few days staying with my brother and his wife in the Twin Cities. It feels great to be home with my parents and to relax, and we are looking forward to a couple of fun family weekends coming up. Here is one quick picture I took out the window – the snow isn’t very deep but it’s wonderful to have the ground white at least!
Bozeat, S., Lambon Ralph, M.A., Patterson, K., Garrard, P., & Hodges, J.R. (2000). Non-verbal semantic impairment in semantic dementia. Neuropsychologia, 38, 1207-1215.
Bozeat, S., Lambon Ralph, M.A., Patterson, K., & Hodges, J.R. (2002). The influence of personal familiarity and context on object use in semantic dementia. Neurocase, 8, 172-134.
Luzzi, S., Snowden, J.S., Neary, D., Coccia, M., Provinciali, L., & Lambon Ralph, M.A. (2007). Distinct patterns of olfactory impairment in Alzheimer’s disease, semantic dementia, frontotemporal dementia, and corticobasal degeneration. Neuropsychologia, 45, 1823-1831.
Piwnice-Worms, K.E., Omar, R., Hailstone, J.C., & Warren, J.D. (2010). Flavour processing in semantic dementia. Cortex, 46(6), 761-768.
Rogers, T.T., Lambon Ralph, M.A., Garrard, P., Bozeat, S., McClelland, J.L., Hodges, J.R., et al. (2004). The structure and deterioration of semantic memory: A neuropsychological and computational investigation. Psychological Review, 111, 205-235.
Snowden, J.S., Bathgate, D., Varma, A., Blackshaw, A., Gibbons, Z.C., & Neary, D. (2001). Distinct behavioural profiles in frontotemporal dementia and semantic dementia. Journal of Neurology Neurosurgery and Psychiatry, 70(3), 323-332.
Tulving, E. (1972). Episodic and semantic memory. In E. Tulving & W. Donaldson (Eds.), Organisation of memory. London: Academic Press.
Warrington, E.K. (1975). The selective impairment of semantic memory. Quarterly Journal of Experimental Psychology, 27 (4), 635-657.