Infinite sensation
11 Aug 01
Could we have an ever-changing variety of senses instead of just the basic
five?
Alison Motluk asks if it's time to rethink how our brains work
EVERYONE knows there are five basic senses. But try separating them one
from the other
in your daily life and suddenly they don't feel so distinct.
Eat a banana, for instance, and try to taste it without smelling it and
experiencing that
banana-y texture on your tongue. Can you really just taste, or must you
sometimes
taste-smell-feel? Try talking to your lover. Listen to what is said without
watching the
mouth move or feeling the caress of a hand. Can you simply hear, or is
there always an
element of hear-see-touch? Even on the phone, can you hear a voice without
imagining a
face? Hard, isn't it?
The prevailing view of the brain still holds that there are five separate
senses that feed
into five distinct brain regions preordained to handle one and only one
sense. The
yellowness of the banana skin, the texture of its flesh, its smell and
taste-each of these
elements is parcelled up and analysed in isolation. Some theories of consciousness
suggest that these dedicated brain areas somehow stamp each sense with
a unique
"feeling". Then, the theory goes, the brain pastes the fragments back together,
calls on
memory to give it a name and recall what it's for and, voila, a banana.
But perhaps it's time for a radical rethink of how the brain works. Tasks
we've long
assumed were handled by only one sense turn out to be the domain of two
or three. And
when we are deprived of a sense, the brain responds-in a matter of days
or even
hours-by reallocating unused capacity and turning the remaining senses
to more
imaginative use. All this begs the questions: are the senses really so
segregated? Are
they separate at all? Indeed, is it possible that our senses are continuously
developing
and merging so that each one of us has our own private view of the world?
It might be a big shift in thinking, but it began with a simple finding-the
discovery of
"multisensory" neurons. These are brain cells that react to many senses
all at once
instead of just to one. No one knows how many of these neurons there are-maybe
they
are just a rare, elite corps. But perhaps there are no true vision, hearing
or touch areas
dedicated to a single sense, after all. Perhaps all the neurons in our
brains are
multisensory-and we mistakenly label them "visual" or "auditory" simply
because they
prefer one sense over the others.
That's the view of Alvaro Pascual-Leone at Harvard University. He made
a splash five
years ago when he showed that people who were born blind use the visual
cortex when
they read Braille. He wondered if rather than lie idle, parts of the brain
meant for seeing
just started helping out with touching. His more recent work has convinced
him that not
only blind people but everyone has the capacity to swap senses if they
have to. He thinks
that the brain is much more versatile than most researchers would have
us believe.
To test the idea, Pascual-Leone blindfolded healthy, sighted volunteers
for five days
running, taught them Braille and watched how their brains responded. He
even fitted their
blindfolds with photographic paper-just to be sure volunteers weren't tampering
with
them. Before, during and after the blindfolding, they had a series of brain
scans while they
were set different tactile and auditory tasks-feeling either Braille characters
or brush
strokes on their fingertips and listening to tones or word fragments. Before
the blindfolding
began, the "visual" areas were not switched on by the touching and hearing
tasks. But
as the week wore on the visual regions became more and more involved in
routine
touching and hearing.
If a person isn't seeing, Pascual-Leone found, parts of the "visual" cortex
are roped in to
help out in tasks involving other senses. In fact, the newly recruited
regions soon become
indispensable. When he tried temporarily disrupting the workings of the
visual areas, using
a technique called trans-cranial magnetic stimulation, or TMS, the blindfolded
volunteers
found it hard to read their Braille.
Taking the blindfolds off for just a day, though, was enough to undo the
changes;
suddenly touching and hearing tasks no longer triggered visual areas, even
though
volunteers were blindfolded again briefly for the scan. "Removing the blindfold
and being
exposed to the seeing world for 12 to 24 hours is sufficient to revert
all changes induced
by the five days of blindfolding," says Pascual-Leone.
What was astonishing was how quickly the brain seemed able to recruit new
areas and
equally effortlessly reverse that process. It was far too quick to be the
result of new
connections forming from scratch reasoned Pascual-Leone. "It must be assumed,"
he
says, "that tactile and auditory input into the 'visual cortex' is present
in all of us and
can be unmasked if behaviourally desirable."
Pascual-Leone now feels the brain is not organised into "visual" and "auditory"
and
"tactile" regions at all. Instead he thinks it is split into units that
have specific jobs to do
or particular problems to solve-calculating distance, for example, or timing
intervals.
These problem-solving units simply use the best information available.
Sometimes they
may prefer certain senses to others, based on how suitable they are for
the assigned
computation, and sometimes they may use more than one, if that helps. Vision,
for
instance, might be the preferred way to judge distances. But if you can't
see, hearing or
touch can certainly fill in.
The preference of a particular problem-solving unit for a specific sense
may explain the
notion of sense-specific regions, he says. Just because an area tends to
call on vision
doesn't mean it can't process other senses, only that it may not bother
if its first choice
sense is on hand. This may have tricked neuroscientists into thinking that
the brain is
structured in parallel, segregated systems processing different types of
sensory signals,
says Pascual-Leone.
There is some good evidence that the brain can mix up the senses to solve
particular
problems. One of the main benefits of sensory integration may be better
clarity and
detection, says Barry Stein, at Wake Forest University in Winston-Salem,
North Carolina,
one of the first researchers to identify the brain's multisensory capabilities.
Even weak
signals should be taken seriously if they're picked up by more than one
sense.
We are, for example, much more sensitive to a chemical when we combine
smell and
taste. Pamela Dalton, at the Monell Chemical Senses Center in Philadelphia,
asked 10
people to smell benzaldehyde, a cherry-almond odour that has no taste,
and to taste
saccharin, a sweetener that has no smell. Before each testing session,
she worked out
the point where each volunteer could no longer detect each substance and
prepared even
weaker samples. Then she asked them to slosh the solution around in their
mouths and
sniff the odour at the same time. Combining taste and smell made both substances
much
more apparent, she found. "Ten minutes before, they hadn't been able to
detect it," says
Dalton.
A brain combining senses can also make better sense of ambiguous information.
David
Lewkowicz at the New York State Institute for Basic Research in Developmental
Disabilities on Staten Island shows this nicely with a visual image of
two balls moving from
opposite sides of a screen, merging briefly in the centre, then continuing
along their merry
ways (see "Brain Games"). But when a beep sounds at the moment the two
balls merge,
what you see changes completely. Now, instead of passing through each other
and
continuing along the same trajectory, the two balls bounce off each other
and return to
the side they came from.
Combining hearing with vision can lead us to draw different conclusions
about what
we've seen too. A single flash of light, can appear to be two flashes when
it coincides
with two beeps, says Ladan Shams and her colleagues at Caltech in Pasadena.
Even when
we know there is just one flash, we can't help perceiving it as two. Apparently
the brain
won't let us draw contradictory conclusions from two different senses.
Increasingly, scientists are discovering that even everyday activities
may actually make
use of more than one sense. Consider the task of running your fingers over
a pattern of
raised ridges and deciding in what direction they are running. What sense
do you call
upon? Most of us would guess the obvious: touch. But a group at Emory University
in
Atlanta has demonstrated that in perfectly normal people parts of the "visual"
brain are
also essential for perceiving touch.
They started by scanning people's brains to see what regions were activated
when they
were trying to decide the orientation of some grating patterns on a touch
pad. They
found that a part of the brain that's involved in recognising objects by
sight was active
while people felt the gratings, even though they couldn't see them. "What
excited us was
what our subjects told us," says Krish Sathian, a lead member of the team.
"When they
were doing the tactile task, they were actually visualising in their mind's
eye the
orientation of the grating."
Did visual imagery just provide a convenient aid, or was it essential to
the task? To find
out, they used the TMS technique to disrupt the activity in the "visual"
region the
volunteers had been using. Suddenly, their volunteers could no longer tell
the direction of
the pattern.
The researchers concluded last year in the journal Nature (vol 401, p 587)
that the
"visual" cortex is closely involved in certain tactile tasks. They claimed
it was the first
time that visual processing was shown to be instrumental in ordinary tactile
perception.
But Sathian admits that the activated region may not really be visual at
all. It could be a
part of the brain that helps us visualise what's being touched. "We certainly
can't rule out
that what we're seeing is multimodal processing in an area previously thought
to be just
visual," he says.
Pascual-Leone's bold interpretation, that the brain is organised by task
rather than by
individual sense, is by no means the accepted one. Even most scientists
who study
multisensory processing consider it extreme. "At least some areas are exclusively
unisensory," says Sathian. There's very clearly a primary visual cortex
with strong inputs
from the eye, he says, and a primary somatosensory cortex getting information
from the
body. But that's not to say that the map of the brain is static-far from
it. New
multisensory areas are being found all the time. "The boundaries are being
pushed back,"
says Sathian, "just not pushed back all the way."
Those boundaries were seriously tested by an experiment that involved "rewiring"
the
brains of ferrets. The findings called into question the well-guarded notion
that certain
brain areas can only dedicate themselves to certain tasks. They suggest
that, although
the brain may tend to develop in a particular way, with vision processed
at the back of
the head and hearing on the sides, it doesn't have to be that way.
A group at MIT in Boston wanted to know how much they could override innate
developmental pathways. "If we put the retina into the auditory cortex,
will it see?" asks
Sarah Pallas, a member of the team, now at Georgia State University in
Atlanta. The
researchers surgically rearranged one brain hemisphere in a handful of
newborn ferrets, so
that the nerves from the retina, which normally go to the visual thalamus
and then on to
the visual cortex, now connected to the auditory thalamus and eventually
to the
auditory cortex.
To their surprise, they found that the auditory cortex on the rewired side
arranged itself
like a visual cortex: the cells showed selectivity for orientation and
motion, and they
encoded a two-dimensional map of visual space. The rewired animals also
seemed to
behave perfectly normally. Using only the untouched hemisphere the researchers
trained
the animals to go to a food spout on one side of a test room if they heard
a sound and
one on the other if they saw a light. Amazingly, even after the visual
cortex on the
healthy side was completely destroyed, the animals found their way to the
food.
"We were able to turn the auditory cortex into a visual cortex," says Pallas.
"Maybe they
couldn't recognise their grandmother with that, but they certainly could
detect light." In
fact, the young ferrets seemed so normal that the researchers had to mark
them to tell
them apart from their siblings.
The experiment revealed just how multimodal the brain may be. The amazing
rewired
auditory cortex was not only seeing-it was hearing at the same time, Pallas
told a
meeting of multisensory scientists in New York last autumn. Though the
finding has not
yet been published, she said that preliminary testing showed that the rewired
auditory
cortex was responding well to sound.
What's more, the study shows that what goes into the brain can have a lot
of influence
on how it's ultimately organised. Although some parts of the brain may
be predisposed to
become one thing or another, the rewiring shows they aren't predetermined.
"Sensory
inputs can influence the regional identity of the cortex," says Pallas.
But how far does this go? We can fairly assume that people deprived of
sight early on will
have their brains wired up differently from people who see. But what about
someone who
has been nearsighted since birth-could that person have a quite a different
brain from
someone who's experienced the world through sharper eyes? Is someone born
into the
high rises of Hong Kong wired up differently from a person growing up in
the Gobi desert?
Pascual-Leone thinks that, both at the functional and the anatomical level,
our brains are
quite unique. "Blind people are not experiencing the world like a sighted
person with eyes
closed," he says, "but rather, they have a dramatically different world
representation and
hence consciousness." Indeed, maybe each of us has our own very personal
take on the
world, sensed by our own unique brain.
Alas, we only know how it feels to be ourselves, so it's impossible to
know. And we can't
ask those ferrets whether they were really seeing, or somehow hearing the
light. It
makes you wonder all over again about bananas-is the divine yellow fruit
the very same to
you as it is to me? Probably not.
Brain games
Balls seem to bounce when a sound is added
http://neuro.caltech.edu/scheier/BouncingIllusion/BouncingIllusion.html
Can your brain detect a single flash if two beeps sound?
http://neuro.caltech.edu/~lshams/demo.html
Find out what you perceive when you hear a voice say: "My bab pop me poo
brive" and you see a mouth say: "My gag kot me koo grive"
http://mambo.ucsc.edu/~course/dad.mov
The McGurk effect:
http://www.media.uio.no/personer/arntm/McGurk_english.html
Alison Motluk
From New Scientist magazine, vol 171 issue 2303, 11/08/2001, page 24
© Copyright New Scientist, RBI Limited 2001