Quantifying the mind-body gap
http://www.the-scientist.com/2005/9/12/14/1
The Inchoate Science of Consciousness
New approaches could help quantify the mind-body gap
By Christof Koch
A new scientific field is being born, one that seeks to understand which organisms have
subjective states, what purpose theymight serve, and how distinct states of consciousness
come about. Here, the Holy Grail is to provide a satisfactory, quantitative account for
why select states of complicated, neuronal networks go hand-in-hand with experiences such
as seeing blue, feeling pain, smelling a dog that's just come in from the rain, or of
simply being.
Philosophers call these feelings and sensations that constitute the elements of
consciousness "qualia." In contrast, most brain states are not directly associated with conscious
sensations: We have almost no access to the structures that give rise to speech, to depth
perception or color vision, to the rapid sequence of sensory-motor transformations
necessary to play soccer, climb a rock wall, or return a tennis ball, let alone those
influencing perspiration, heart rate, or the action of our immune systems. Unlike qualia, these
proceed in blankness. Where is the difference between the two?
THE PROBLEM WITH MIND
The body-mind conundrum traces back to Socrates, Plato, and Aristotle. Yet during the
past 2,300 years, progress on these questions has been almost imperceptible. Centuries
passed before people realized that the brain, rather than, say, the heart or liver, has the
most intimate relationship with the mind. Only towards the latter part of the 19th century
did it become apparent that the cerebral cortex is not just a homogeneous, reactive
tissue, but instead comprises different parts performing distinct functions. That nervous
systems are made up of discrete, complex, and interconnected nerve cells constituted another
great advance at the end of the 19th century. In the 20th century, technology began to
deliver reliable and inexpensive methods to record, store, and analyze electrical activity
from individual neurons in anesthetized and, later on, in awake animals – even in
people.1 In the closing decades of the millennium, technologies to peer safely inside the living
human brain, and
witness it in action, became widely accessible to the research community.
Today, there is a newfound optimism among philosophers, scientists, clinicians, and other
scholars, that science can successfully tackle the mystery of how brain matter expresses
subjective feelings. This was evident at the ninth annual meeting of the Association for
the Scientific Study of Consciousness (ASSC9) this past June.
APPROACHING THE CHASM
Many of the foundational, philosophical debates raging in the 1980s and 1990s have
subsided – to the extent that philosophers ever agree on anything besides the importance of
studying philosophy. Thus, a scientific program focusing on the neuronal correlates of
consciousness (NCC) provides the best avenue for progress. As defined by Francis Crick and
myself, the NCC are the minimal set of neuronal mechanisms or events jointly sufficient for
a specific conscious percept or experience.2
Two broad scientific strategies could be discerned in the ASSC9 presentations. One seeks
to isolate the conditions necessary for any conscious experience to occur at all. The
collection of more than three dozen heterogeneous midbrain nuclei, known as the
mesencephalic reticular formations, are needed, as are other midline structures, in particular the
intralaminar nuclei of the thalamus. A common theme is imaging the hemodynamic brain
activity of patients hovering on the borderline between coma and wakefulness (see story page
17). Such research has immediate practical consequences for thousands of people and should
be vigorously pursued. Yet from a scientific standpoint this paradigm suffers from a lack
of control, a lack of reversibility, and a lack of specificity.
In the second general experimental strategy, aimed largely at addressing these
shortcomings, many experimentalists are tracking the footprints of consciousness by manipulating a
specific state of consciousness. Visual perception is the most popular choice, as
scientists have learned to manipulate what a subject sees with considerable precision. Not
unlike a magician distracting his audience with a beautiful, bikini-clad assistant,
psychologists can manipulate the attention of their subjects such that, while they may be looking
at a stimulus with one or even both eyes, they do not see it.
© 2005 Nature Publishing Group
FLASH SUPPRESSION:
While viewing a stationary gray image with the left eye and colored Mondrian patterns
flashing every 100 ms in the right eye, subjects are instructed to fixate on the central
cross and press a button when the gray figure starts to become visible. Suppression of the
gray image was more than 10 times longer than in binocular rivalry using the same stimulus
but with stationary Mondrian pattern. (From N. Tuschiya, C. Koch, Nat Neurosci,
8:1096–101, 2005.)
At ASSC9, Nao Tsuchiya, a graduate student in my lab, demonstrated a technique (flash
suppression) that allows him to project a picture into one eye while hiding it – for minutes
at a time – from consciousness by rapidly flashing a series of salient images into the
other eye.3 His work was awarded best prize for a student presentation. Other techniques
(e.g., binocular suppression or binocular rivalry) rely on a Necker cube-like bistability,
where the image can be seen in one of two distinct ways. The subject's mind flips back
and forth between these views. Unlike naïve realism, which assumes a simple one-to-one
mapping between the external world and its representation in the privacy of one's head, these
illusions demonstrate that the link can be multifaceted, depending on the viewing history
and other factors.
Using magnetic resonance imaging to measure blood-oxygen-level-dependent (BOLD) signals
provides insight into the responses of subjects seeing these illusions. Some parts of the
visual system, in particular the ones closest to the eyes, will only reflect processing
of the physical image. Others will be influenced by the subject's perception. A consistent
finding of many such experiments is that primary visual cortex (V1), where the tract of
fibers from the visual periphery terminate, is already clearly modulated by visual
consciousness. That is, the amplitude of the BOLD signal in V1 is not only driven by the
physical image, but also by what the subject perceives. This is surprising and at odds with
single-cell recordings from macaques. Trained to pull levers or move their eyes to signal
what they are seeing, the spiking activity of one or a handful of neurons in early visual
cortex is indifferent to their perception. It is only in higher regions of visual cortex
that cells follow
the monkey's percept.
DEVELOPING A THEORY OF MIND
The Achilles' heel of all such experiments is that they correlate a state of
consciousness with one or more brain states. Ultimately, science must move beyond mere observations
to causal explanations. It is a good first step to note that phenomenal state X correlates
with some sort of activity in population NX. Much more is needed, however. What happens
to perception if NX is perturbed, for instance, by applying a brief magnetic field from
outside the skull using transcranial magnetic stimulation? What happens to the percept X if
NX is transiently and delicately turned off and then back on again?
Intervening in the human brain is beset with grave practical and ethical difficulties.
This is where animal experiments, done with compassion and care, come into their own.
Molecular biology will be essential to any future science of consciousness. One of the high
points of ASSC9 was Jean-Pierre Changeux's masterful demonstration of how genetic tools can
be exploited. His team directly injected a modified lentivirus, containing the β2
nicotinic acetylcholine receptor, into the ventral tegmental area (VTA) in the midbrain of mice
engineered to lack β2 receptors throughout their bodies.4 Strikingly, the reintroduction
of one specie of molecule into a single brain region rescued certain complex exploratory
and social behaviors. While the β2 knockout animals move rapidly through a novel terrain
with little exploration, animals in which nicotinic transmission has been restored in the
VTA show more adaptive behavior that, if observed in humans, would be associated with
planning and consciousness.
Christof Koch
Another step forward is the first, tentative appearance of theories. By and large,
musings on consciousness have either been motivated by philosophical and logical considerations
or by empirical observations. Giulio Tononi put forth a theoretical formulation of
consciousness at ASSC9.5 He starts with phenomenology – each of us can experience an almost
infinite number of distinct conscious states and each of these experiences is unified – and
proceeds to ask what type of networks can generate a large number of distinct states,
each of which is integrated. Using the well-defined concept of mutual information, Tononi
defines Φ, the amount of causally effective information that can be integrated among
various subset of any network, and averts that Φ corresponds to consciousness. His theory
offers a principled framework for understanding which types of network architecture maximize Φ
(think cortex versus cerebellum), how consciousness waxes and wanes with non-REM and REM
sleep, how it varies across phylogeny and ontogeny, and how to build conscious machines.
In principle, although not yet in practice, his theory allows consciousness to be
quantified, a great boon.
These are heady times for neuroscientists. Our growing ability to monitor the brain's
activity at the cellular level with unprecedented precision and breadth, and precisely
manipulate these networks opens the stunning possibility that the quest to understand the
oldest of all epistemological problems will come to an end in our lifetime.
Christof Koch is Lois and Victor Troendle Professor of Cognitive and Behavioral Biology
and executive officer of Computation and Neural Systems, California Institute of
Technology. His group investigates the neuronal basis of visual awareness and consciousness as
well as the biophysical mechanisms underlying neuronal computation.
He can be contacted at koch@klab.caltech.edu.
References
1. RQ Quiroga et al, "Invariant visual representation by single neurons in the human
brain," Nature 435: 1102-7. [Publisher Full Text] June 23, 2005
2. C Koch The Quest for Consciousness: A Neurobiological Approach Roberts Publishing:
Denver CO 2004.
3. N Tsuchiya, C Koch "Continuous flash suppression reduces negative afterimages," Nat
Neurosci 2005, 8: 1096-1101. [PubMed Abstract][Publisher Full Text]
4. U Maskos et al, "Nicotine reinforcement and cognition restored by targeted expression
of nicotinic receptors," Nature 2005, 436: 103-7. [PubMed Abstract][Publisher Full Text]
5. G Tononi "An information integration theory of consciousness," BMC Neurosci 2004, 5:
42. [PubMed Abstract] [BioMed Central Full Text] [PubMed Central Full Text]
The Inchoate Science of Consciousness
New approaches could help quantify the mind-body gap
By Christof Koch
A new scientific field is being born, one that seeks to understand which organisms have
subjective states, what purpose theymight serve, and how distinct states of consciousness
come about. Here, the Holy Grail is to provide a satisfactory, quantitative account for
why select states of complicated, neuronal networks go hand-in-hand with experiences such
as seeing blue, feeling pain, smelling a dog that's just come in from the rain, or of
simply being.
Philosophers call these feelings and sensations that constitute the elements of
consciousness "qualia." In contrast, most brain states are not directly associated with conscious
sensations: We have almost no access to the structures that give rise to speech, to depth
perception or color vision, to the rapid sequence of sensory-motor transformations
necessary to play soccer, climb a rock wall, or return a tennis ball, let alone those
influencing perspiration, heart rate, or the action of our immune systems. Unlike qualia, these
proceed in blankness. Where is the difference between the two?
THE PROBLEM WITH MIND
The body-mind conundrum traces back to Socrates, Plato, and Aristotle. Yet during the
past 2,300 years, progress on these questions has been almost imperceptible. Centuries
passed before people realized that the brain, rather than, say, the heart or liver, has the
most intimate relationship with the mind. Only towards the latter part of the 19th century
did it become apparent that the cerebral cortex is not just a homogeneous, reactive
tissue, but instead comprises different parts performing distinct functions. That nervous
systems are made up of discrete, complex, and interconnected nerve cells constituted another
great advance at the end of the 19th century. In the 20th century, technology began to
deliver reliable and inexpensive methods to record, store, and analyze electrical activity
from individual neurons in anesthetized and, later on, in awake animals – even in
people.1 In the closing decades of the millennium, technologies to peer safely inside the living
human brain, and
witness it in action, became widely accessible to the research community.
Today, there is a newfound optimism among philosophers, scientists, clinicians, and other
scholars, that science can successfully tackle the mystery of how brain matter expresses
subjective feelings. This was evident at the ninth annual meeting of the Association for
the Scientific Study of Consciousness (ASSC9) this past June.
APPROACHING THE CHASM
Many of the foundational, philosophical debates raging in the 1980s and 1990s have
subsided – to the extent that philosophers ever agree on anything besides the importance of
studying philosophy. Thus, a scientific program focusing on the neuronal correlates of
consciousness (NCC) provides the best avenue for progress. As defined by Francis Crick and
myself, the NCC are the minimal set of neuronal mechanisms or events jointly sufficient for
a specific conscious percept or experience.2
Two broad scientific strategies could be discerned in the ASSC9 presentations. One seeks
to isolate the conditions necessary for any conscious experience to occur at all. The
collection of more than three dozen heterogeneous midbrain nuclei, known as the
mesencephalic reticular formations, are needed, as are other midline structures, in particular the
intralaminar nuclei of the thalamus. A common theme is imaging the hemodynamic brain
activity of patients hovering on the borderline between coma and wakefulness (see story page
17). Such research has immediate practical consequences for thousands of people and should
be vigorously pursued. Yet from a scientific standpoint this paradigm suffers from a lack
of control, a lack of reversibility, and a lack of specificity.
In the second general experimental strategy, aimed largely at addressing these
shortcomings, many experimentalists are tracking the footprints of consciousness by manipulating a
specific state of consciousness. Visual perception is the most popular choice, as
scientists have learned to manipulate what a subject sees with considerable precision. Not
unlike a magician distracting his audience with a beautiful, bikini-clad assistant,
psychologists can manipulate the attention of their subjects such that, while they may be looking
at a stimulus with one or even both eyes, they do not see it.
© 2005 Nature Publishing Group
FLASH SUPPRESSION:
While viewing a stationary gray image with the left eye and colored Mondrian patterns
flashing every 100 ms in the right eye, subjects are instructed to fixate on the central
cross and press a button when the gray figure starts to become visible. Suppression of the
gray image was more than 10 times longer than in binocular rivalry using the same stimulus
but with stationary Mondrian pattern. (From N. Tuschiya, C. Koch, Nat Neurosci,
8:1096–101, 2005.)
At ASSC9, Nao Tsuchiya, a graduate student in my lab, demonstrated a technique (flash
suppression) that allows him to project a picture into one eye while hiding it – for minutes
at a time – from consciousness by rapidly flashing a series of salient images into the
other eye.3 His work was awarded best prize for a student presentation. Other techniques
(e.g., binocular suppression or binocular rivalry) rely on a Necker cube-like bistability,
where the image can be seen in one of two distinct ways. The subject's mind flips back
and forth between these views. Unlike naïve realism, which assumes a simple one-to-one
mapping between the external world and its representation in the privacy of one's head, these
illusions demonstrate that the link can be multifaceted, depending on the viewing history
and other factors.
Using magnetic resonance imaging to measure blood-oxygen-level-dependent (BOLD) signals
provides insight into the responses of subjects seeing these illusions. Some parts of the
visual system, in particular the ones closest to the eyes, will only reflect processing
of the physical image. Others will be influenced by the subject's perception. A consistent
finding of many such experiments is that primary visual cortex (V1), where the tract of
fibers from the visual periphery terminate, is already clearly modulated by visual
consciousness. That is, the amplitude of the BOLD signal in V1 is not only driven by the
physical image, but also by what the subject perceives. This is surprising and at odds with
single-cell recordings from macaques. Trained to pull levers or move their eyes to signal
what they are seeing, the spiking activity of one or a handful of neurons in early visual
cortex is indifferent to their perception. It is only in higher regions of visual cortex
that cells follow
the monkey's percept.
DEVELOPING A THEORY OF MIND
The Achilles' heel of all such experiments is that they correlate a state of
consciousness with one or more brain states. Ultimately, science must move beyond mere observations
to causal explanations. It is a good first step to note that phenomenal state X correlates
with some sort of activity in population NX. Much more is needed, however. What happens
to perception if NX is perturbed, for instance, by applying a brief magnetic field from
outside the skull using transcranial magnetic stimulation? What happens to the percept X if
NX is transiently and delicately turned off and then back on again?
Intervening in the human brain is beset with grave practical and ethical difficulties.
This is where animal experiments, done with compassion and care, come into their own.
Molecular biology will be essential to any future science of consciousness. One of the high
points of ASSC9 was Jean-Pierre Changeux's masterful demonstration of how genetic tools can
be exploited. His team directly injected a modified lentivirus, containing the β2
nicotinic acetylcholine receptor, into the ventral tegmental area (VTA) in the midbrain of mice
engineered to lack β2 receptors throughout their bodies.4 Strikingly, the reintroduction
of one specie of molecule into a single brain region rescued certain complex exploratory
and social behaviors. While the β2 knockout animals move rapidly through a novel terrain
with little exploration, animals in which nicotinic transmission has been restored in the
VTA show more adaptive behavior that, if observed in humans, would be associated with
planning and consciousness.
Christof Koch
Another step forward is the first, tentative appearance of theories. By and large,
musings on consciousness have either been motivated by philosophical and logical considerations
or by empirical observations. Giulio Tononi put forth a theoretical formulation of
consciousness at ASSC9.5 He starts with phenomenology – each of us can experience an almost
infinite number of distinct conscious states and each of these experiences is unified – and
proceeds to ask what type of networks can generate a large number of distinct states,
each of which is integrated. Using the well-defined concept of mutual information, Tononi
defines Φ, the amount of causally effective information that can be integrated among
various subset of any network, and averts that Φ corresponds to consciousness. His theory
offers a principled framework for understanding which types of network architecture maximize Φ
(think cortex versus cerebellum), how consciousness waxes and wanes with non-REM and REM
sleep, how it varies across phylogeny and ontogeny, and how to build conscious machines.
In principle, although not yet in practice, his theory allows consciousness to be
quantified, a great boon.
These are heady times for neuroscientists. Our growing ability to monitor the brain's
activity at the cellular level with unprecedented precision and breadth, and precisely
manipulate these networks opens the stunning possibility that the quest to understand the
oldest of all epistemological problems will come to an end in our lifetime.
Christof Koch is Lois and Victor Troendle Professor of Cognitive and Behavioral Biology
and executive officer of Computation and Neural Systems, California Institute of
Technology. His group investigates the neuronal basis of visual awareness and consciousness as
well as the biophysical mechanisms underlying neuronal computation.
He can be contacted at koch@klab.caltech.edu.
References
1. RQ Quiroga et al, "Invariant visual representation by single neurons in the human
brain," Nature 435: 1102-7. [Publisher Full Text] June 23, 2005
2. C Koch The Quest for Consciousness: A Neurobiological Approach Roberts Publishing:
Denver CO 2004.
3. N Tsuchiya, C Koch "Continuous flash suppression reduces negative afterimages," Nat
Neurosci 2005, 8: 1096-1101. [PubMed Abstract][Publisher Full Text]
4. U Maskos et al, "Nicotine reinforcement and cognition restored by targeted expression
of nicotinic receptors," Nature 2005, 436: 103-7. [PubMed Abstract][Publisher Full Text]
5. G Tononi "An information integration theory of consciousness," BMC Neurosci 2004, 5:
42. [PubMed Abstract] [BioMed Central Full Text] [PubMed Central Full Text]
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