Why do we dream?

Have you ever stopped to wonder why we dream at night? From sweet dreams to recurring nightmares, our mind is rarely silent – regardless of whether we can really remember their content in the morning.

Sometimes, we find our dreams are linked to things going on in our lives right now, worries about future events or strong memories from the past. This therefore seems to suggest that dreams are in some way linked to our memory, but exactly how, no one seemed sure.

Recent research has investigated the role of dreams and REM sleep (the phase of deep sleep) in the consolidation of long term memory. Consolidation just means the process whereby our memories move from short term to long term storage. In our long term memory, memories are stored for recall. Rehearsal (thinking about) these long term memories briefly involves short term processing, and this rehearsal strengthens the storage of these memories. Dreams may play a part in this consolidation and rehearsal process.

To find out more about REM sleep and our sleep cycle then why not read my previous post here.

151112_DRIFT_Dream-Research.jpg.CROP.promo-xlarge2

Photo by clownbusiness/Shutterstock, with additional illustration by Lisa Larson-Walker

As I mentioned early, our dreams can have similarities to events which have taken place in our lives. Some research has focused on investigating the content of our dreams and found that the events which tend to be included in our dreams are ones which are rated as more emotional, although not more stressful, than those not incorporated (Malinoski & Horton, 2014). This suggests that REM sleep might help to process emotional memories. Further evidence to support this hypothesis is that levels of REM sleep are lower in people with depression (Cartwright, 1983) and PTSD (Ross et al, 1989).

However, although these dreams can contain elements of real life, they are often distorted: it is rare for the complete memory to be ‘played out’ in our dream. It is been suggested that this is because during sleep we cannot access full episodic memories (memories of events) but instead just traces of them.  This has been hypothesised to be due to reduced hippocampus (the part of our brain involved in memory processing) activity during REM sleep (Buzsàki, 1996). The fact that our dreams can contain strange events or impossibilities is thought to be due to a lack of activity in the prefrontal cortex – the area involved in attention and logic (Stickgold et al, 2001).

In addition to consolidating episodic memories another proposed function of our dreams is to enhance learning of procedural tasks (Smith et al, 1996). Studies in rats have found increased levels of REM sleep after procedural learning, and that less REM sleep resulted in poorer memory (Smith et al, 1985).

Whilst REM sleep and our dreams may be useful for certain types of memory consolidation, it doesn’t mean that this is the only way consolidation takes place, or that it is needed to consolidate every type of memory (Stickgold et al, 2001). The authors of this review hypothesize that dreaming enables the brain “to identify and evaluate novel cortical associations in the light of emotions… during REM”. To put it simply, when we dream our brain is working on processing new memories, learning procedures, and our emotions to events.

 

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Perfect Memory Syndrome

Can you imagine being able to remember every single day of your life? This is the case for people with highly superior autobiographical memory (HSAM) – an extremely rare condition which affects fewer than 100 people in the world.

In contrast to the majority of us, who can probably recall some details about what we’ve been doing on specific days for the last fortnight or so, people with HSAM can do this for years, and some even right back to when they were a baby.

The first recorded case of HSAM was in a woman called Jill Price in 2000, by memory specialist Dr James McGaugh at the University of California. Jill could remember every day of her life in detail, back until she was 14 years old. She knows what happened on any given date and what day of the week it was, right down to specific details like sounds and smell. She believes her extraordinary memory was triggered by her and her family moving to a different part of the USA when she was 8 – she was anxious about forgetting things about her old life and after this period, found her memory had changed.

However, just because people with HSAM can remember every detail about what has happened in their lives, this doesn’t mean that they have a superior memory when it comes to other types of information. For a quick recap – our long term memories are divided into 3 main groups: episodic – personal information about us e.g. memories of what we did for our birthday last year, or our experience of school when we were little; semantic – facts e.g. knowing the year London held the Olympics or the capital city of Spain. The third category is procedural memory, which is memory for actions e.g. how to ride a bike (for more information see this blog post). People with HSAM have extraordinary episodic memory, but they perform similarly to the general population on tests which involve the other two – they have no greater capacity to remember facts or memorise large amounts of information than we do. Another study has shown that they are more susceptible than control participants to a task which aims to plant false memories (Patihis et al, 2013) – so their memory is still as unreliable as ours.

How people with HSAM encode memories has also been tested, and the authors of the study (Leport et al, 2017) concluded that they seem to create memories in exactly the same way as the general population. This, added to the results of the false memory test seems to suggest that there isn’t something special about the way memories of people with HSAM are made which means they can remember more. The current hypothesis is that it is something in between encoding and retrieval which makes their memory so special.

The brain structure of people with HSAM has been investigated using fMRI, with images showing that people with the condition have differences to the parahippocampal gyrus, anterior insula and temporal gyrus. (LePort et al, 2012). Previous research has shown that these areas are involved in autobiographical memory, so this result perhaps isn’t surprising. There was also evidence of improved coherence in the white matter tract which connects the two hemispheres, suggesting a superior ability to transfer information between different parts of the brain. However, this study alone is not enough to show whether these differences were caused by the advanced memory capabilities of these participants, or whether they are a result of them remembering so much information.

Although having perfect memory might seem to be an advantage, people will this condition can often struggle with the sheer amount of information they can remember. Memories are often described as intrusive, popping up when they see anything which reminds them of something in the past. Jill Price says that she perceives a ‘split screen’, with the present happening on the left, and a constant stream of memories on the right. Having the ‘perfect memory’ might be more trouble than it’s worth.

 

'Memory stick.'

Déjà vu

I’m sure you’ve all experienced that feeling where you find yourself thinking that things you are currently experiencing have happened before. Déjà vu (meaning ‘already seen’) can feel kind of creepy, but why does it happen?

Déjà vu has been reported to occur in about 60-80% of the healthy population (e.g. Adachi et al, 2003), but is also thought to be linked to temporal lobe epilepsy (Stevens, 1990). There have been several different theories about why this occurs, including the two sides of the brain not functioning together, a sense of familiarity to one part of an experience being mistakenly applied to it all, a problem with how we perceive the timescale of an event, so that something which is happening at the moment is viewed as happening long ago, or a problem with processing sensory information, so that it is processed and reviewed at the same time (see review by Wild, 2005 for a full list).

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There have also been several attempts to use neuroanatomy to explain déjà vu. Brázdil et al (2012) compared the brains of healthy participants who did or did not experience déjà vu using an imaging technique called source-based morphometry to measure the amount of grey matter (neurons) in different cortical areas. They found a correlation in certain subcortical areas of the brain (the hippocampus, STS, insula cortices, basal ganglia, and thalami) between lower amount of grey matter and an increase in déjà vu experienced. Several of these structures are in the mesial temporal lobe, which could therefore explain the link between increased déjà vu in patients with temporal lobe epilepsy.

Work to establish the anatomical basis of déjà vu in patients with temporal lobe epilepsy has also suggested that these mesial areas of the temporal lobe are involved. Bancaud et al (1994) studied the anatomical basis of déjà vu using electrodes in epileptic patients prior to surgery which were placed in the temporal lobe, the hippocampus, and the amygdala (you may remember from previous posts that the hippocampus is a structure important for memory, whilst the amygdala is thought to be involved in emotional processing).  They found that déjà vu could be induced by stimulating all of these areas, but that it was 10 times more likely to occur if stimulation was in the hippocampus or amygdala, suggesting that these areas are key to experiencing déjà vu.

As well as occurring in epilepsy, déjà vu is a feature of other psychiatric disorders including schizophrenia, anxiety disorders (like PTSD), depression, and dissociative disorders. There have also been reported cases of constant déjà vu, with sufferers constantly feeling as though their current experiences have happened before. For example, one case study of a 23 year old male was reported by Wells et al 2014, who concluded that it was caused by his severe anxiety and tendency of depersonalisation. This patient did not show a memory deficit, although other cases of persistent déjà vu have been reported amongst elderly patients with dementia.

One of the things I find interesting about déjà vu is that it is a feature of several psychiatric disorders as well as something which occurs in most of the healthy population. It doesn’t seem that psychiatrists are entirely sure about why is occurs in some people but not others, and like with several other areas of psychology – more research is needed to be sure of it’s true course. Thanks for reading this week’s post, I’ll try to be back soon with more new material!

Smell and Memory

I’m sure this has happened to you before – you’re walking down the street and you smell something that takes you back to a holiday, or a time when you were younger. It could be the smell of a sweet shop or someone’s perfume, and you are taken straight back to a moment from years ago. But why are smells so linked to memories?

A simple answer is that this link is due to how the brain is organised. Our sense of smell is triggered by a molecule that enters our nose and binds to the hair-like projections (cilia) on neurons at the top of your nasal passage. These neurons project to a part of the brain called the olfactory bulb, which run along the front of the brain, at the bottom. This structure is thought to be involved in interpreting these signals and processing information about smells.

What’s interesting about the olfactory bulb is that it’s the one part of the brain responsible for our senses that has projections to and from the areas of our brain responsible for memory and emotion – the hippocampus and amygdala. You can see this from the image below:

HowSmellWorks01

 

This explains why smells can trigger memories and emotions. The hippocampus is responsible for our episodic memories in particular – personal memories about our lives, which is why it is this type of memory activated by smell. One theory about why these connections exist between the hippocampus and the olfactory bulb is that they enable us to recognise smells from previous experience.

Studies have shown that using smells to trigger memories can be more effective than cuing them with words. For example, Maylor et al (2002) asked young and old adults to recall autobiographical memories associated with 6 cue words. They were then shown the same words and were asked to recall new memories, and for half of these words the appropriate smell was presented too. The researchers found that for both age groups, the participants recalled twice as many memories when the smell was presented too, showing the large impact of smell and memory recall.

 

Brain Plasticity

Although you might think that the structure of your brain is formed before you are born and does not change, this actually isn’t the case. As we grow and learn, the brain is constantly making new connections and pathways between different areas. It used to be believed that anything which had not been developed by a ‘critical period’ during childhood would be lost, with little change after this time, although we now know this is not true.

For example, our different skills and experiences can help to shape our brain. This has been particularly studied using musicians, as extensive practice and repetition of certain fine-tuned motor actions can result in more of the motor cortex being involved in directing the actions of the hand and fingers.

Pascual-Leone et al (1995) found that novices learning to play a simple exercise on a piano over 5 days showed an increase in size in the cortical areas involved in the movement of the fingers. Schlaugh (2001) carried out fMRI to compare the size of the intrasulcal length (part of the motor cortex) in professional musicians and controls, and found it was much longer for musicians in the right hemisphere (which controls the left hand). This is shown in the image below, taken from this paper.

music

It is through the process of brain plasticity that new memories are formed. Motor memories such as becoming more accomplished at music are one type of memory which alter the brain structure, but our personal memories also change our brain. This occurs through the process of Long-term Potentiation (LTP), which is the process of connections between cells at synapses strengthened. It mainly occurs in the hippocampus and other cortical areas responsible for our long term memories. This process is illustrated by the image below:

Long-Term-Potentiation1

Brain plasticity is also encouraged in treatment and rehabilitation from brain injury. For example, after a stroke it has been found that giving excitatory stimulation to the damaged areas can improve function (e.g. improving language function – Szaflarski et al, 2011). Just by encouraging movement in people who have had a stroke can also help them to regain function of limbs on their impaired side.

Thank you for reading and don’t forget to check back next week for another post!

 

Nostalgia

Following on from the post about my dissertation.. now on to the next thing that’s been taking up quite a lot of my time this year – my research project. For this part of my degree, I had to carry out my own experiment on something which had not been shown before, and analyse the results.

I chose to look into nostalgic memories, and in particular, do we feel nostalgic from reading someone else’s nostalgic memories? It seemed like there was a bit of a gap in the research that had been done so far: although we know the functions of nostalgia (e.g. self esteem) and features of nostalgic memories (e.g. loved ones), not much work had been done on ways of making people feel nostalgic.

To start, I should probably make clear what nostalgia is: it is defined as a
“sentimental longing or wistful affection for a period in the past”

The problem I identified with existing research was that most experiments manipulated nostalgia by asking participants to write down a nostalgic or ordinary memory, reading words that describe nostalgia and writing a memory based on them, or by listening to a nostalgic or ordinary song. These methods are fine if you then want to find out about effects of nostalgia, but are they equally effective in inducing nostalgia?
Nostalgia is also a very social emotion, and a study has shown that thinking of a nostalgic memory involving an out-group member leads to fewer feelings of prejudice towards that person (Turner et al, 2012). If nostalgia is a social emotion, then I hypothesised that reading someone else’s nostalgic memories could cause you to feel nostalgic. There had only been one study which found that nostalgia could be induced by reading someone else’s old love letters, or looking at their old photos. I decided to present participants with an actual nostalgic (or ordinary) memory narrative from someone else, and told participants the memory was from someone similar or dissimilar to them in age (to see whether similarity affected results). I compared how effective this method was with a previous method of making people feel nostalgic – giving participants a list of words that described nostalgia, or were more general, and asked them to write a memory based on these features.

My hypotheses:
1. That reading the nostalgic memory would make people feel nostalgic (compared to reading an ordinary memory).
2. That participants who were told the nostalgic memory was from someone similar to them would feel more nostalgic than those who were told it was from someone different.
3. That this method would be effective, but more nostalgia would be induced from writing your own memory in the comparison condition.

Similarity:
The reason I thought that similarity would effect results was because of principles shown in social psychology – it has been found that when people are categorised into groups, they will automatically perceive themselves as being more similar to other in-group members, and therefore more dissimilar from out-group members (Tajfel & Turner, 1979). Therefore, the perceived similarity of the reader to the person who wrote the memory could increase the amount of nostalgia transferred to the reader of the narrative. I called this a similarity-based “transfer effect” of nostalgia.

Design of Experiment:
– 121 participants, all between the ages of 16 and 24 (so I could manipulate similarity by age)
– Participants split into 6 groups: nostalgia similar, nostalgia dissimilar, ordinary similar, ordinary dissimilar, central features (write memory) and peripheral features (write memory).
– At the top of the memory narratives, a sentence explained this was ‘an actual memory from someone aged 20 (similar condition) or 60 (dissimilar condition).
– After participants had either read the memory, or read the features and written their own memory, they then completed a questionnaire to assess how nostalgic they felt.

What I found:

Hypothesis 1: Reading someone else’s nostalgic memory did make people feel more nostalgic than those who read an ordinary memory.

Hypothesis 2: Similarity had an effect on the amount of nostalgia people felt: participants in the nostalgia similar condition felt more nostalgia than participants in the nostalgia dissimilar condition – EVEN THOUGH the memory was the same, the age of writer differed.This is shown by the graph below.

r2

Hypothesis 3: The participants who wrote their own memory based on central features of nostalgia felt more nostalgic than participants who had read a nostalgic memory. However, there was no difference between the central and peripheral conditions, which differed from the original study (Hepper et al, 2012), who found peripheral features did not induce nostalgia. The graph below shows the results for this, and hypothesis 1: more nostalgia felt by participants who read someone else’s nostalgic memory than someone’s ordinary memory.

r1

The results of my experiment are the first to show that people can be made to feel nostalgic by reading someone else’s nostalgic memory, and that the amount of nostalgia felt depends on how similar the writer of the memory is to the reader. Therefore, it sets the basis for more research to be done on different ways of manipulating similarity and other ways of inducing nostalgia.

Hope you found this interesting and let me know if you’ve got any questions – I know this is a really complicated experiment!

Is human memory reliable?

Here’s a memory fact for you: the human memory is NOT like a video camera.

This would imply that the visual information we receive in our eyes is encoded and stored, without any further processing, to just be recalled in exactly the same way we first perceived it. This is not the case.

One theory states that our long term memory is Reconstructive. This means that abstract principals about the input material are stored and the memory is then reconstructed according to these principals during recall. An experiment to show this was done way back in 1932 by Bartlett, who showed a bias in picture recall to real life objects, when the pictures were actually abstract. Here is an example of some of the stimuli and recall images used in his experiment:

http://www.softdevlabs.com/personal/Eyewitness_Testimony/Eyewitness_Testimony.html

As you can see, participants’ memory of the original object was skewed towards whichever real life object they thought it looked like. This are simple shapes, but yet the participants were unable to recall them accurately.

The interference theory provides strong evidence that human memory isn’t reliable as it shows that our memories can be altered by previous learning (proactive interference) or by new learning (retroactive interference). There are several examples of this is real life: for example, when you get a new phone you find it difficult to type as well as you did on your old one (proactive), and if later for some reason you try to type on your old phone again you will also find that difficult, as you are used to the new one (retroactive).

Isurin & McDonald (2001) carried out an experiment using bilingual participants to assess the effects of retroactive interference. They presented participants with a picture and corresponding word in either Russian or Hebrew, and then gave half the participants the same pictures but with the words in another language. Recall was then tested, and it was found that it became worse the more learning trials they had using words in the 2nd language. They concluded that these results showed retroactive interference, and hypothesised that this could be why many bilingual individuals forget their first language if it is not used.

More evidence that human memory is not reliable come from the fact that you can have ‘false memories’ – memories you are certain are true but did not actually happen. One of the most famous studies to show this was carried out by Roediger & McDermott (1995). They gave participants lists of words with something in common and found that 40% of the time participants recalled this theme as one of the words, even if it was not presented in the list. For example: participants were given sky, wet, cloud, puddle etc, and recalled the word rain. When they asked participants why they did this after the experiment, nearly 3/4 of them said that they had a strong memory of the word being in the original list (not just a feeling that they had seen it somewhere). This shows how easily memory can be manipulated.

While these may seem like pretty trivial examples of incorrect memories, this topic has some really important real-life applications, such as eye-witness testimonies to a crime. Elizabeth Loftus is a leader in this field, and had done many experiments to show the optimum conditions for eyewitness recall. Misleading questions are a type of retroactive interference which can alter an eyewitness’ memory.

In her well known experiment, she found participants were more likely to overestimate speed if they were asked ‘how fast were the cars moving when they smashed into each other?’ when referring to a video, rather than if the word ‘smashed’ was replaced with ‘bumped’. This shows that interviewers must be extremely careful about the questions they ask eyewitnesses.

http://www.simplypsychology.org/loftus-palmer.html

Another experiment by Loftus illustrated ‘weapon focus’: this is the impairment of memory caused by the eyewitness focusing on a weapon and less on other details. they asked participants to watch one of two videos, one containing a gun and one without. They found that participants had worse memory of the scene in the video containing the gun, suggesting that weapon focus had occurred.

Unfortunately, there is also evidence that racial/social stereotypes can also affect a persons’ memory of an event. Lindholm & Christianson (1998) found that Swedish students were more likely to identify an immigrant, rather than a Swede from a line up when asked to identify the person carried out a crime in a video they were shown.

So, the question is: do you trust your memory?