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.'

Patient H.M.

Today’s blog post is about one of the most studied individuals in psychology and neuroscience. By studying him, scientists were able to massively expand their knowledge of how to human brain is structured, and how different abilities (or “functions”) are located in different cortical areas.
H.M. was born in 1928, and was 10 when he first started having epileptic seizures. These were extremely debilitating, and although several medications were tested, none had any affect. When H.M. was 27, a pioneering neuroscientist called William Scoville worked out where the seizures in H.M.’s brain were coming from, and decided that the best way to stop them was to operate, and cut out the parts of the brain that was responsible.
In terms of removing his seizures, this surgery was largely successful. However it came at a cost: H.M. could no longer form new long term memories, nor remember anything from X years before his operation. The image below is a scan taken of H.M.’s brain, and shows the lesions made during surgery, and how this differs from a normal brain scan.
memory-hm-anatomy2
As you can see from this image, H.M. was left with extensive damage to the central parts of his brain – this area is known as the medial temporal lobe. By analysing the brain damage, neuroscientists were able to make inferences about where certain brain functions are located. As H.M.’s memory was impaired, but other cognitive functions such as language were not, the medial temporal lobe was identified as being important in the formation of long term memories.
Perhaps unsurprisingly, things aren’t as simple as this. H.M.’s long term memory was affected, so severely that he was unable to remember things that happened a few minutes ago. However his short term memory was intact, with a normal digit span (a string of numbers that you can keep in your mind at once) of 7 +/- 2. Therefore, the structures damaged can’t be involved in short term memory.
There are also distinctions that can be made within long term memory. This can be divided into 3 different types of memory: semantic, which is general knowledge about the world; episodic, which are memories about ourselves and our lives; and procedural, which are learned physical movements e.g. riding a bike. Only H.M.’s episodic and semantic memory were damaged, which shows that our procedural memory must be located elsewhere. Other brain areas such as the cerebellum have been identified as involved in this. Not only was his procedural memory intact, he could also improve it by practicing new movements over time. The image below shows a mirror drawing task, where participants have to trace an image by only looking at its reflection in a mirror.
starmirror
H.M.’s performance improved each time he did this task, even though he had no recollection of ever doing the task before! This illustrates nicely the different between the automatic, learned ‘procedural’ memories, and the episodic memories about previous experiences.
One last question remains – why did H.M. lose his episodic memories from the years before the operation, but not ones from when he was much younger? There are several theories for this, with one being that older memories are ‘consolidated’ into the rest of the cortex – only newer memories remain in medial temporal structures such as the hippocampus. Therefore, when this area of brain was destroyed, so were the newer long term memories.
When H.M. died in 2008 aged 82, scientists were able to reveal his real name – Henry Molaison. He made a massive contribution to the field of neuroscience, and is thought to have been one of the most tested patients in medical history.
For those interested in reading more about H.M., I would recommend this article, written while he was still alive: Corkin, S. (2002). What’s new with the amnesic patient HM?. Nature Reviews Neuroscience3(2), 153-160.
And finally, I’ll leave you with a quote from H.M. himself, when he was asked “Are you happy?”
“Yes. Well the way I figure it is, what they find out about me helps them to help other people. And that’s more important.”

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.

 

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?

Memory

Memories: everyone has them, and probably takes them for granted. You think that you can always rely on them, although it does let you down occasionally (ever walked into a room and forgot why you went in there?!). So how reliable actually is it? Here’s my explanation of human memory.

You can separate memory into two different types: short term memory and long term memory. In this post, I’m going to focus on long term memory, just because I think it’s more interesting.

Here is a diagram explaining how human long term memory can be broken down into various different subsystems:

http://nphs-ib-psych.wikispaces.com/Explain+how+biological+factors+may+affect+one+cognitive+process.+(Elizabeth)

As you can see – there is more to long term memory than you’d expect. The two types of long term memory are declarative (memories you can vocalise) and non-declarative (memories you’d fine hard to talk about or explain). Examples of declarative memories include semantic memories (e.g. facts) and episodic memories (personal memories). These differ from non-declarative memories such as procedural memories (e.g. how to ride a bike) or emotional memories (e.g. feeling happy). These sub-systems of memory help researchers to classify memory deficits and measure how they are learnt.

Hopefully, you now know a bit more about the things which make up our long term memory. However, there is more to its different components than the memories they contain – these different types of memory are stored in different areas of the brain. Although the range of locations in the brain used to store memories can get quite confusing, I’ll aim to make it as simple as possible.

The hippocampus is a structure in the middle of the temporal lobe on both sides of the brain, and is vital for the storage and recollection of episodic and some semantic memories. This is known because of research on case studies, such as patient H.M. whose hippocampus was removed to cure his violent epilepsy. He was left unable to form new declarative memories (anterograde amnesia) and his declarative memories from the years just before his operation had also disappeared (retrograde amnesia). However, he was still able to carry out motor functions such as walking, in other words, his procedural memory was still intact. This therefore shows that procedural memories must be stored in another area of the brain.

http://morphonix.com/software/education/science/brain/game/specimens/hippocampus.html

As well as the hippocampus, it is also believed that the structures around it such as the mammillary bodies, fornix and perirhinal cortex are also important for remembering. The perirhinal cortex is thought to be especially important for semantic memory. The frontal lobe is also thought to play a part in memory, especially for memories which have been rehearsed several times, and perhaps date back years. It is thought that stronger traces are developed as each memory is recalled over and over, which means that it becomes less dependent on the hippocampus.

As for procedural memory, this is thought to be reliant on several structures in the brain that are involved in motor-learning, such as the basal ganglia and the cerebellum (the structure at the back of the brain). The amygdala are next to the hippocampus and are involved in processing emotional memory, something which is also done by the frontal cortex.

I hope you now understand human long term memory a bit better than you did before you read this post! As you can see, it relies upon many structures in the brain: there is no one area responsible for our memories. Check back for future posts about memory impairments, and how reliable our memory actually is 🙂