FEATURES: p24 11 Aug
01
#38 Infinite sensation
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
Alison Motluk
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
For more science news see http://www.newscientist.com
____________________________________________________________
Copyright 2001 New Scientist, Reed Business InformationFEATURES:
p24 11 Aug 01
#38 Infinite sensation
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
Alison Motluk
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
For more science news see http://www.newscientist.com
____________________________________________________________
Copyright 2001 New Scientist, Reed Business Information
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