The Neural Correlates of
Consciousness
NRS 495
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Neuroscience Seminar
Christopher
DiMattina
, PhD
Outline
•
Introducing consciousness
•
Neural substrates of consciousness
•
Neural correlates of consciousness
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Introducing Consciousness
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Definitions and Science
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In everyday usage,
consciousness
means a variety of things
–
Not being in a coma, deep sleep, anesthesia
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Being aware of feelings, thoughts, perceptions
–
Dreaming
•
One might be tempted to find a precise definition before
embarking on scientific study
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The history of science shows this would be a huge mistake
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Greek Astronomy
•
Greeks thought there were two
realms following different laws
•
Newton showed that one can
explain astronomy and motion
on earth with one set of laws
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Modern definitions
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What is a “gene” precisely?
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Many things make it hard to define
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Removal of introns
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Post
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translational editing of proteins
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Some genes have regulatory roles
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Lack of precise definition does not
impede molecular biologists
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Definitions are quite often inexact
“I shall not attempt today to further
define pornography. But I know it
when I see it.”
-
Justice Potter Stewart
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Eliminative
Materialsm
•
Our folk psychology will eventually be replaced by a mature
cognitive neurobiology
•
Psychological terms like ‘attention’, ‘memory’, etc… will
eventually be supplanted as we learn more about how the
brain actually works
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Francis Crick
•
There should be some difference in neural activity when a
stimulus is presented and the subject is aware of it then when
the stimulus is presented and the subject is unaware
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Philosophers
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G
roup into two camps on the issue of consciousness
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Reductionists
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anti
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Reductionists
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The Hard Problem
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Many anti
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reductionist philosophers argue that explaining
consciousness in terms of brain states ultimately leaves
something out
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Qualia, experience, “what it is like”, etc…
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Logical possibility of
Zombies
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Neuroscience
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Consciousness is ultimately a problem for neuroscience
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Experiments
(behavioral and biological) and
n
eural models
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Bringing neuroscience concepts and discoveries to bear on
philosophical debates
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Neural correlates of visual awareness
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Crick and Koch make the problem tractable by focusing their
efforts on finding the
minimum set of neural events
which is
needed to give rise to a visual percept
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Neural correlates of consciousness (
NCC
)
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Neurocomputational
approach to
consciousness
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How might we proceed
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List some properties of consciousness
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Find what brain states correlate with those properties
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Consciousness
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Involves short
-
term memory
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Independent of sensory inputs
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Steerable attention
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Disappears in sleep
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Reappears in dreaming
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Single, unified experience
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Thalamo
-
cortical loops
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Properties of
Thalamo
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cortical loop
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Forms a recurrent neural
network
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This can account for several
known properties
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Short term memory
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Sensory independence
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Widespread awareness
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Pieces of the puzzle: 40 Hz
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ILN neurons tend to oscillate intrinsically at 40 Hz
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Cortical regions also oscillate at 40 Hz during wakefulness
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These cortical oscillations disappear in deep sleep when ILN
neurons are inactive
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Oscillations reappear in REM sleep
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ILN
l
esions abolish consciousness
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Lesion left ILN, one neglects right visual field
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Lesion ILN entirely, one looses consciousness
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A Neural hypothesis
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I have just outlined a simple, testable hypothesis of
consciousness which accounts for our list of known properties
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Of course it is not the whole story since one can get spatial
neglect with cortical damage (Posterior parietal)
•
Also, the relevant recurrent interactions may be within the
cortex, with ILN just functioning as a timekeeper
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Neural correlates of consciousness
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Crick and Koch
•
Neural correlates of consciousness (
NCC
)
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Focus on visual awareness
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V1 and visual awareness
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Crick and Koch hypothesize that V1 activity does not
correspond directly to visual awareness
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V1 activity independent of perception
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Patients with parietal cortex damage (extinction)
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Activity same in V1 whether stimulus is perceived or extinguished
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Vuilleumier
et al. (2001)
Blink
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We blink constantly yet do not perceive interruption of vision
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V1 neuron responses do get interrupted
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Adaptation
•
Subjects adapt to grating they
cannot consciously perceive
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Similar results for gratings that
they perceive as uniform fields
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Imagery
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Vivid mental imagery possible without V1 [Bridge et al, J.
Neurol. 2011]
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Binocular Rivalry
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The vast majority (~80%) of V1 neurons follow the physical
stimulus, not the percept
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Other lines of evidence
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Color:
Color
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tuned V1 neurons track a flickering
iso
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luminant
two color stimulus subjects perceive as uniform color
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Anatomical:
V1 does not project to prefrontal cortex as do
ventral and parietal areas
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Alternative viewpoints on V1
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Other experiments have suggested V1 may play a role in visual
awareness (Tong 2003)
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Binocular Rivalry
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Feedback from MT to V1 needed for
motion perception
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TMS stimulation of MT gives motion percept
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Stimulating V1 shortly after MT abolishes motion percept
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Ventral stream + awareness
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Several lines of evidence suggest neural correlates of visual
awareness in the ventral stream
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Binocular rivalry in monkeys
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Sheinberg and
Logothetis
(1997)
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Rivalry in people
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Attending different stimuli activates
different brain regions
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FFA and PPA
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Imagery neurons
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Krieman
et al. (2000) showed that medial temporal neurons
selective for visual images exhibit the same selectivity during
vision and imagery
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Activity in higher cortical areas may
not by itself be enough
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Neurons in IT can respond to objects even when the animal is
under anesthetic
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One can obtain responses in human extra
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striate cortex to
stimuli which are not perceived due to masking
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May get activation of higher ventral visual areas to unseen or
extinguished stimuli in subject with parietal lesion
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Frontal and Parietal areas
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Damage is associated with visual neglect
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Awareness in visual extinction is associated with co
-
variation
of activity between visual cortical areas and undamaged
parietal and prefrontal cortex
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Percepts of bi
-
stable figures
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Reversals:
E
nhanced activation of frontal and parietal areas
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Percepts of bi
-
stable figures
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Lumer
,
Friston
and Rees (1998, Science)
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Frontal and parietal areas differentially activated during rivalry and
non
-
rivalry conditions
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Co
-
variation of activity
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Frontal and parietal areas showed more co
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variation of
activity with visual areas during binocular rivalry than
stable viewing
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Summary
•
One hypothesis is that the NCC involve a variety of areas
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Areas must interact in both top
-
down and bottom
-
up manner
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Crick & Koch propose NCC may be activity in ventral stream
which interacts with activity in frontal and parietal areas
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