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).
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!
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:
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.
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:
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.
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?
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:
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.
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 🙂