Your Memory Stinks!
A familiar scent triggers childhood memories for our
brain columnist, prompting him to wonder what is going on in his head.
The toy cupboard at my grandmother's house had a
particular smell. I cannot tell you what it was, but sometimes now, as an adult, I will catch a whiff of it. The smell brings with it
memories I thought were lost, memories of visits to my grandparents' house, of
my grandmother, and of playing with the toys from the toy cupboard. But why do
smells have this power to unlock forgotten memories?
Neuroscience is a lot like a detective story ˇV we have
to look for clues to reveal the cause. But before we examine the clues, what
background information do we have about the case?
Complex sense
What we know is that smell is the oldest sense, having
its origins in the rudimentary senses for chemicals in air and water ˇV senses
that even bacteria have. Before sight or hearing, before even touch, creatures
evolved to respond to chemicals around them.
Sight relies on four kinds of light sensors in the
human eye, cells known as receptors, which convert light into the
electrochemical language of our brain, and touch relies on different receptor types
for pressure (at least four of these), for heat, for cold and for pain, but
this pales into comparison for what is required for detecting smell. There are
at least 1,000 different smell receptor types, which regenerate throughout your
lifetime, and change according to what you are used to smelling. The result of
this complexity is that we are able discriminate many, many different kinds of
smells.
We do not, however, have names for all the smells we
can differentiate. Smell is perhaps the sense we are least used to talking
about. We are good at describing how things look, or telling how things
sounded, but with smells we are reduced to labelling
them according to things they are associated with
("smells like summer meadows" or "smells like wet dog", for
instance). An example of this ˇ§hard-to-talk-about-nessˇ¨
is that while we have names for colours which mean
nothing but the colour, such as ˇ§redˇ¨, we generally
only have names for smells which mean the thing that produces that smell, such
as ˇ§cedarˇ¨, ˇ§coconutˇ¨ or ˇ§fresh breadˇ¨.
Memory episodes
So now we have the background information, what are
the important clues? Well, first, the part of the brain that is responsible for
processing smells ˇV the ˇ§olfactory bulbˇ¨ ˇV is next to a part of the brain
called the hippocampus. This name means ˇ§seahorseˇ¨, and the hippocampus is
so-called because it is curled up like a seahorse, nested deep within the
brain, a convergence point for information arriving from all over the rest of
the cortex. Neuroscientists have identified the hippocampus as crucial for
creating new memories for events. People with damage to the hippocampus have
trouble remembering what has happened to them.
Although they can learn new skills, like riding a
bike, and new facts, like what someone is called, they do not create memories
of doing these things or having the experiences. This ˇ§episodic memoryˇ¨ is
precisely the kind of memory I have when I recall visits to my grandmother. And
the olfactory bulb, seat of smell in the brain, is conveniently placed just
next to the hippocampus, the primary brain nucleus for these memories.
Deep dive
Now, admittedly, this evidence is powerful, but
circumstantial. We have the suspect (smell) placed at the scene of the crime
(next to the hippocampus). But we are going to need more than circumstantial
evidence if the case is going to stand up the scientific court. I hope my next
piece of the evidence, a second clue from neuroscience, will convince you as to
why smells are so powerful in unlocking memories.
Smell is unique among the senses in that it enters
directly deep into the brain. If we look at the major pathways travelled by the
other senses, such as hearing and vision, they start at the sense organs ˇV that
is, the eyes or the ears ˇV and move to a relay station called the thalamus,
before passing on to the rest of the brain.
With smell the
situation is different. Rather than visiting the thalamic relay station on its
journey into the brain, smell information travels directly to the major site of
processing ˇV the olfactory bulb ˇV with nothing in between. We do not know what
stopping off at the thalamus does for the other senses, but it certainly means
that signals generated in the other senses are somehow ˇ§further awayˇ¨ from the
nexus of processing done in the brain.
Could this be
part of the reason why smells are both hard to put into words, but also able to
trigger deeply hidden memories? Memory research has shown that describing
things in words can aid memory, but it also reduces the emotion we feel about
the subject. When we come up with a story about our memories, we start
remembering the story as much as the raw experience.
So with my
grandmotherˇ¦s toy cupboard, that particular, unique,
smell was picked up by the complex smell receptors in my young nose. The smell
experience of the cupboard, which I have never found a name for, travelled
directly into my brain, lodging next to the part specialised
for encoding experiences. There it got entangled with the other memories of the
cupboard, untouched by language, difficult to think about on purpose, but still
lodged in my memory. Now, years later, the smell is not only enough to relive
that experience but it is also enough to pull out the rest of the memories
along with it.
Tom Stafford
13th
March 2012
http://www.bbc.com/future/story/20120312-why-can-smells-unlock-memories