Psychology: How many senses do we have?
By Christian Jarrett
Scarlett Johansson in the film Lucy (Columbia
Pictures)
We often talk of having five senses as a
universal truth. In reality, there may be more
– or fewer – depending on the way you look
at the question. Christian Jarrett explains the
controversy.
Some myths about the brain, such as the idea
we only use 10% of our grey matter, are
notorious, especially among neuroscientists.
These myths crop up every now and then
(look at the premise of the Lucy movie this
summer), but they are quickly shot down by
those in the know.
In contrast to these enduring stories, other
misconceptions are stealthier and slip
beneath the radar unrecognised. One of these
is the idea that the human brain is served by
five senses. This belief is so ingrained that
even the scientifically literate will treat it as
taken-for-granted common knowledge.
Perhaps it is due to the idea’s noble origins.
The principle of five basic human senses is
often traced back to Aristotle’s De Anima (On
the Soul), in which he devotes a separate
chapter to vision, hearing, touch, smell and
taste. Today, the five senses are considered
such an elementary truth that it is sometimes
used as a point of consensus before writers
embark on more mysterious or contentious
topics. “What do we actually mean by
reality?” asked the author of a recent article in
New Scientist magazine. “A straightforward
answer is that it means everything that
appears to our five senses.”
Incoming information
If only it were that simple. Simply defining
what we mean by a “sense” leads you down a
slippery slope into philosophy. One,
somewhat vague, definition might argue that
a human sense is simply a unique way for the
brain to receive information about the world
and the body. If that is the case, then we can
claim with confidence that there are certainly
more than five human senses.
First consider the senses that relate to the
position of our bodies. Close your eyes, and
then touch your right forefinger to your left
elbow tip. Easy? How did you do it?
Somehow you knew where the end of your
finger was and you also knew the position of
your left elbow. This sense is known as
proprioception and it’s the awareness we
have of where each of our body parts is
located in space. Proprioception is possible
thanks to receptors in our muscles known as
spindles, which tell the brain about the current
length and stretch of the muscles.
Now imagine you are blindfolded and I tilted
you forwards slowly. You’d immediately have
a sensation of how your body’s position was
changing in relation to gravity. This is thanks
to the fluid-filled vestibular system in your
inner ear, which helps us keep balance. This
system also gives us our experience of
acceleration through space, and it links up
with the eyes, making it possible to cancel
out our own motion. If you wiggle your head
around while reading, for example, you’ll see
that it makes little difference to your ability to
read and stay focused on the words.
There are also numerous senses providing us
with information about the inner state of our
bodies. The most obvious of these are hunger
and thirst, inner body pain, and the need to
empty the bladder or bowel. Less obvious and
less available to conscious awareness are
incoming signals about blood pressure, the
pH level of the cerebrospinal fluid, plus many
more.
Some might take that definition further, to
argue that the senses should be defined by
the types of receptors we have; a different
sensor means a different sense. If that were
the case, then even well-known senses
quickly split into different varieties. For
instance, if you closed your eyes and I
surprised you with an ice cube down your
back, you’d experience a shock of cold. This
sensation would be distinct from the mere
touch of a plastic cube, say. Alongside
temperature-sensitive receptors, packed in
our skin we also have receptors dedicated to
mechanical pressure, pain (known as
nociceptors) and itch (pruritic receptors).
Using the same logic, however, taste can be
divided into sweet, sour, salty and bitter and
potentially “ umami ”, which is activated by
monosodium glutamate and is associated
with a “meaty” flavour. Splitting the senses in
this way doesn’t feel like the most intuitive
way of dealing with the question, however,
and it becomes even more absurd if we turn
to smell: humans have over 1,000 distinct
olfactory receptors tuned to different odorous
molecules. Should each one be counted as a
different sense?
At the other extreme, you could restrict our
definition of discrete sense to the physical
categories of incoming information. We can
simplify the human senses down to just three
– mechanical (which takes in touch, hearing
and proprioception); chemical (including
taste, smell and internal senses); and light.
Yet another way of approaching this issue is
to think not about the category of incoming
information or the perceptual experience, but
about how incoming sensory information is
used. A great example is the human capacity
for echo-location. Human echo-location
works by a person emitting a clicking sound
with the tongue and listening for how it
rebounds off the immediate environment. In
the USA there is even a remarkable team of
blind cyclists – Team Bat – led by Daniel
Kisch, who use echo-location to go mountain
biking (see www.worldaccessfortheblind.org
for videos). This ability depends on the
traditional sense of hearing, but the
perceptual experience and function is more
akin to vision. You don’t need to be blind to
try it; even sighted people can learn to “see in
the dark ” using echo-location. For these
reasons, some consider it a separate sense.
As you can see, there is no single, logical way
to define the senses. In some ways, it might
make little sense to draw divisions between
them at all – considering that they often seem
to blend together; the colour of food – and
even the sounds of a restaurant – can
influence taste, for instance. Understanding
these relationships is important when
studying conditions like synaesthesia and
could even shed light on consciousness itself.
But whichever way you look at it, five is a
pretty arbitrary and meaningless number – a
glaring “myth” of the brain that needs further
recognition. Indeed, once you start thinking
about all the different kinds of information
reaching the human brain, you might even
find that you develop a brand new sense – a
radar-like sensitivity to some of the other
misconceptions regarding the way the brain
experiences the world. You might have once
called it a “sixth sense” – but you know better
now, don’t you?
This article is based on a chapter from
Christian Jarrett’s new book Great Myths of
the Brain (Wiley).
source:bbcnews