by Dean Burnett
3 S. M. Ebenholtz, M. M. Cohen and B. J. Linder, ‘The possible role of nystagmus in motion sickness: A hypothesis’, Aviation, Space, and Environmental Medicine, 1994, 65(11), pp. 1032–5
4 R. Wrangham, Catching Fire: How Cooking Made Us Human, Basic Books, 2009
5 ‘Two Shakes-a-Day Diet Plan – Lose weight and keep it off’, http://www.nutritionexpress.com/article+index/diet+weight+loss/diet+plans+tips/showarticle.aspx?id=1904 (accessed September 2015)
6 M. Mosley, ‘The second brain in our stomachs’, http://www.bbc.co.uk/news/health-18779997 (accessed September 2015)
7 A. D. Milner and M. A. Goodale, The Visual Brain in Action, Oxford University Press, (Oxford Psychology Series no. 27), 1995
8 R. M. Weiler, ‘Olfaction and taste’, Journal of Health Education, 1999, 30(1), pp. 52–3
9 T. C. Adam and E. S. Epel, ‘Stress, eating and the reward system’, Physiology & Behavior, 2007, 91(4), pp. 449–58
10 S. Iwanir et al., ‘The microarchitecture of C. elegans behavior during lethargus: Homeostatic bout dynamics, a typical body posture, and regulation by a central neuron’, Sleep, 2013, 36(3), p. 385
11 A. Rechtschaffen et al., ‘Physiological correlates of prolonged sleep deprivation in rats’, Science, 1983, 221(4606), pp. 182–4
12 G. Tononi and C. Cirelli, ‘Perchance to prune’, Scientific American, 2013, 309(2), pp. 34–9
13 N. Gujar et al., ‘Sleep deprivation amplifies reactivity of brain reward networks, biasing the appraisal of positive emotional experiences’, Journal of Neuroscience, 2011, 31(12), pp. 4466–74
14 J. M. Siegel, ‘Sleep viewed as a state of adaptive inactivity’, Nature Reviews Neuroscience, 2009, 10(10), pp. 747–53
15 C. M. Worthman and M. K. Melby, ‘Toward a comparative developmental ecology of human sleep’, in M. A. Carskadon (ed.), Adolescent Sleep Patterns, Cambridge University Press, 2002, pp. 69–117
16 S. Daan, B. M. Barnes and A. M. Strijkstra, ‘Warming up for sleep? – Ground squirrels sleep during arousals from hibernation’, Neuroscience Letters, 1991, 128(2), pp. 265–8
17 J. Lipton and S. Kothare, ‘Sleep and Its Disorders in Childhood’, in A. E. Elzouki (ed.), Textbook of Clinical Pediatrics, Springer, 2012, pp. 3363–77
18 P. L. Brooks and J. H. Peever, ‘Identification of the transmitter and receptor mechanisms responsible for REM sleep paralysis’, Journal of Neuroscience, 2012, 32(29), pp. 9785–95
19 H. S. Driver and C. M. Shapiro, ‘ABC of sleep disorders. Parasomnias’, British Medical Journal, 1993, 306(6882), pp. 921–4
20 ‘5 Other Disastrous Accidents Related To Sleep Deprivation’, http://www.huffingtonpost.com/2013/12/03/sleep-deprivation-accidents-disasters_n_4380349.html (accessed September 2015)
21 M. Steriade, Thalamus, Wiley Online Library, [1997], 2003
22 M. Davis, ‘The role of the amygdala in fear and anxiety’ Annual Review of Neuroscience, 1992, 15(1), pp. 353–75
23 A. S. Jansen et al., ‘Central command neurons of the sympathetic nervous system: Basis of the fight-or-flight response’, Science, 1995, 270(5236), pp. 644–6
24 J. P. Henry, ‘Neuroendocrine patterns of emotional response’, in R. Plutchik and H. Kellerman (eds), Emotion: Theory, Research and Experience, vol. 3: Biological Foundations of Emotion, Academic Press, 1986, pp. 37–60
25 F. E. R. Simons, X. Gu and K. J. Simons, ‘Epinephrine absorption in adults: Intramuscular versus subcutaneous injection’, Journal of Allergy and Clinical Immunology, 2001, 108(5), pp. 871–3
* It’s not exactly a one-way relationship either. The brain doesn’t just influence the food we eat; it seems the food we eat does (or did) have considerable influence over how our brains work.4 There’s evidence to suggest that the discovery of cooking meant humans could suddenly obtain a great deal more nourishment from food. Perhaps an early human tripped and dropped his mammoth steak into the communal campfire. The determined primitive maybe got a stick and hooked his steak out, only to find it was suddenly more palatable and appetising. Raw food being cooked means it’s easier to eat and digest. The long and dense molecules in it are broken down or denatured, allowing our teeth, stomachs and intestines to get better nourishment from our food. This seemingly led to a rapid expansion in brain development. The human brain is an incredibly demanding organ when it comes to bodily resources, but cooking food allowed us to meet its needs. Enhanced brain development meant we got smarter, and invented better ways of hunting, and methods of farming and agriculture and so on. Food gave us bigger brains, and bigger brains gave us more food, forming a literal feedback.
† This is a joke. For now.
2
The gift of memory (keep the receipt)
The human memory system, and its strange features
The word ‘memory’ is often heard these days, but in the technological sense. Computer ‘memory’ is an everyday concept that we all understand – a storage space for information. Phone memory, iPod memory, even a USB flash drive is referred to as a ‘memory stick’. There’s not much simpler than a stick. So you could forgive people for thinking that computer memory and human memory are roughly the same in terms of how they work. Information goes in, the brain records it, and you access it when you need it. Right?
Wrong. Data and info are put into the memory of a computer, where they remain until needed, at which point they are retrieved, barring some technical fault, in exactly the same state in which they were first stored. So far, so logical.
But imagine a computer that decided some information in its memory was more important than other information, for reasons that were never made clear. Or a computer that filed information in a manner that didn’t make any logical sense, meaning you had to search through random folders and drives trying to find the most basic data. Or a computer that kept opening your more personal and embarrassing files, like the ones containing all your erotic Care Bears fan fiction, without being asked, and at random times. Or a computer that decided it didn’t really like the information you’ve stored, so altered it for you to suit its preferences.
Imagine a computer that did all these things, all the time. Such a device would be flung out of your office window less than half an hour after being switched on, for an urgent and terminal meeting with the concrete car park three storeys below.
But your brain does all these things with your memory, and all the time. Whereas with computers you can buy a newer model or take a malfunctioning one back to the shop and scream at the salesperson who recommended it, we’re basically stuck with our brain. You can’t even turn it off and on again to reboot the system (sleep doesn’t count, as we saw earlier).
This is just one example of why ‘the brain is like a computer’ is something you should say to many modern neuroscientists, if you enjoy watching people twitch due to barely suppressed frustration. This is because it’s a very simplistic and misleading comparison, and the memory system is a perfect illustration of this. This chapter looks at some of the more baffling and intriguing properties of the brain’s memory system. I would have described them as ‘memorable’, but there’s no way to guarantee that, given how convoluted the memory system can be.
Why did I just come in here?
(The divide between long-term and short-term memory)
We’ve all done it, at some time or other. You’re doing something in one room, when it suddenly occurs to you that you need to go to a different room to get something. Along the way, something distracts you – a tune on the radio, someone saying something amusing as you pass, or suddenly figuring out a plot twist in a TV show that’s been bugging you for months. Whatever it is, you reach your destination and suddenly have no idea why you decided to go there. It’s frustrating, it’s annoying, it’s time-wasting; it’s one of the many quirks thrown up by the surprisingly complex way the brain processes memory.
The most familiar division in human memory for most people is that between short-term memory and long-term memory. These differ considerably, but are still interdepende
nt. Both are appropriately named; short-term memories last about a minute max., whereas long-term memories can and do stay with you your whole life. Anyone referring to something they recall from a day or even a few hours ago as ‘short-term memory’ is incorrect; that’s long-term memory.
Short-term memory doesn’t last long, but it deals with actual conscious manipulation of information; the things we’re currently thinking about, in essence. We can think about them because they’re in our short-term memory; that’s what it’s for. Long-term memory provides copious data to aid our thinking, but it’s short-term memory that actually does the thinking. (For this reason, some neuroscientists prefer to say ‘working’ memory, which is essentially short-term memory plus a few extra processes, as we’ll see later.)
It will surprise many to find that the capacity of short-term memory is so small. Current research suggests the average short-term memory can hold a maximum of four ‘items’ at any one time.1 If someone is given a list of words to remember, they should be able to remember only four words. This is based on numerous experiments where people were made to recall words or items from a previously shown list and on average could recall only four with any certainty. For many years, the capacity was believed to be seven, plus or minus two. This was labelled as the ‘magic number’ or ‘Miller’s law’ as it was derived from 1950s experiments by George Miller.2 However, refinements and reassessment of legitimate recall and experimental methods have since provided data to show the actual capacity is more like four items.
The use of the vague term ‘item’ isn’t just poor research on my part (well, not just that); what actually counts as an item in short-term memory varies considerably. Humans have developed strategies to get around limited short-term-memory capacity and maximise available storage space. One of these is a process called ‘chunking’, where a person groups things together into a single item, or ‘chunk’, to better utilise their short-term memory capacity.3 If you were asked to remember the words ‘smells’, ‘mum’, ‘cheese’, ‘of’, and ‘your’, that would be five items. However, if you asked to remember the phrase ‘Your mum smells of cheese’, that would be one item, and a possible fight with the experimenter.
In contrast, we don’t know the upper limit of the long-term-memory capacity as nobody has lived long enough to fill it, but it’s obscenely capacious. So why is short-term memory so restricted? Partly because it’s constantly in use. We’re experiencing and thinking about things at every waking moment (and some sleeping ones), which means information is coming and going at an alarmingly speedy rate. This isn’t somewhere that’s going to lend itself well to long-term storage, which requires stability and order – it would be like leaving all your boxes and files in the entrance of a busy airport.
Another factor is that short-term memories don’t have a ‘physical’ basis; short-term memories are stored in specific patterns of activity in neurons. To clarify: ‘neuron’ is the official name for brain cells, or ‘nerve’ cells, and they are the basis for the whole nervous system. Each one is essentially a very small biological processor, capable of receiving and generating information in the form of electrical activity across the cell membranes that give it structure, as well as forming complex connections with other neurons. So short-term memory is based on neuronal activity in the dedicated regions responsible, such as the dorsolateral prefrontal cortex in the frontal lobe.4 We know from brain scanning that a lot of the more sophisticated, ‘thinking’, stuff goes on in the frontal lobe.
Storing information in patterns of neuronal activity is a bit tricky. It’s a bit like writing a shopping list in the foam on your cappuccino; it’s technically possible, as the foam will retain the shapes of words for a few moments, but it’s not got any longevity, and hence can’t be used for storage in any practical sense. Short-term memory is for rapid processing and manipulation, and with the constant influx of information anything unimportant is ignored, and quickly overwritten or allowed to fade away.
This isn’t a foolproof system. Quite often, important stuff gets bumped out of short-term memory before it can be properly dealt with, which can lead to the ‘Why did I just come in here?’ scenario. Also, short-term memory can become overtaxed, unable to focus on anything specific while being bombarded with new information and demands. Ever seen someone amid some hubbub (such as a children’s party, or a frantic work meeting) with everyone clamouring to be heard, suddenly declare, ‘I can’t think with all this going on!’? They’re speaking very literally; their short-term memory isn’t equipped to cope with that workload.
Obvious question: if the short-term memory where we do our thinking has such a small capacity, how the hell do we get anything done? Why aren’t we all sitting around trying and failing to count the fingers on one hand? Luckily, short-term memory is linked to long-term memory, which takes a lot of pressure off.
Take a professional translator; someone listening to long detailed speech in one language and translating it into another, in real time. Surely this is more than short-term memory can cope with? Actually, it isn’t. If you were asking someone to translate a language in real time while actually learning the language, then, yes, that would be a big ask. But for the translator the words and structure of the languages are already stored in long-term memory (the brain even has regions specifically dedicated to language, like Broca’s and Wernicke’s areas, as we’ll see later). Short-term memory has to deal with the order of the words and the meaning of the sentences, but this is something it can do, especially with practice. And this short-term/long-term interaction is the same for everyone; you don’t have to learn what a sandwich is every time you want a sandwich, but you can forget that you wanted one by the time you get to the kitchen.
There are several ways information can end up as long-term memory. At a conscious level, we can ensure that short-term memories end up as long-term memories by rehearsing the relevant information, such as a phone number of someone important. We repeat it to ourselves to ensure we can remember it. This is necessary because, rather than patterns of brief activity like short-term memories, long-term memories are based on new connections between neurons, supported by synapses, formation of which can be spurred on by doing something like repeating specific things you want to remember.
Neurons conduct signals, known as ‘action potentials’, along their length in order to transmit information from the body to the brain or vice versa, like electricity along a surprisingly squidgy cable. Typically, many neurons in a chain make up a nerve and conduct signals from one point to another, so signals have to travel from one neuron to the next in order to get anywhere. The link between two neurons (or possibly more) is a synapse. It’s not a direct physical connection; it’s actually a very narrow gap between the end of one neuron and the beginning of another (although many neurons have multiple beginning and end points, just to keep things confusing). When an action potential arrives at a synapse, the first neuron in the chain squirts chemicals known as neurotransmitters into the synapse. These travel across the synapse and interact with the membrane of the other neuron via receptors. Once a neurotransmitter interacts with a receptor, it induces another action potential in this neuron, which travels along to the next synapse, and so on. There are many different types of neurotransmitter, as we’ll see later; they underpin practically all the activity of the brain, and each type of neurotransmitter has specific roles and functions. They also have specific receptors that recognise and interact with them, much like security doors that will open only if presented with the right key, password, fingerprint or retinal scan.
Synapses are believed to be where the real information is ‘held’ in the brain; just as a certain sequence of 1s and 0s on a hard drive represents a specific file, so a specific collection of synapses in a specific place represents a memory, which we experience when these synapses are activated. So these synapses are the physical form for specific memories. Just like certain patterns of ink on paper become, when you look at them, words that make sense i
n a language you recognise, similarly, when a specific synapse (or several synapses) becomes active, the brain interprets this as a memory.
This creation of new long-term memories by forming these synapses is called ‘encoding’; the process where the memory is actually stored in the brain.
Encoding is something the brain can do fairly quickly, but not immediately, hence short-term memory relies on less permanent but more rapid patterns of activity to store information. It doesn’t form new synapses; it just triggers a bunch of essentially multipurpose ones. Rehearsing something in short-term memory keeps it ‘active’ long enough to give the long-term memory time to encode it.
But this ‘rehearsing something until I remember it’ method isn’t the only way we remember things, and we clearly don’t do it for everything we can remember. We don’t need to. There’s strong evidence to suggest that nearly everything we experience is stored in the long-term memory in some form.
All of the information from our senses and the associated emotional and cognitive aspects is relayed to the hippocampus in the temporal lobe. The hippocampus is a highly active brain region that is constantly combining the never-ending streams of sensory information into ‘individual’ memories. According to a great wealth of experimental evidence, the hippocampus is the place that the actual encoding happens. People with a damaged hippocampus can’t seem to encode new memories; those who have constantly to learn and remember new information have surprisingly large hippocampi (like taxi drivers having enlarged hippocampal regions that process spatial memory and navigation, as we’ll see later), suggesting greater dependence and activity. Some experiments have even ‘tagged’ newly formed memories (a complex process involving injecting detectable versions of proteins used in neuronal formation) and found that they are concentrated at the hippocampus.5 This isn’t even including all the newer scanning experiments that can be used to investigate hippocampal activity in real time.