The Neural Correlates of

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The Neural Correlates of
Consciousness

NRS 495


Neuroscience Seminar

Christopher
DiMattina
, PhD

Outline


Introducing consciousness


Neural substrates of consciousness


Neural correlates of consciousness

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Fall 2012

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Introducing Consciousness

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Definitions and Science


In everyday usage,
consciousness

means a variety of things


Not being in a coma, deep sleep, anesthesia


Being aware of feelings, thoughts, perceptions


Dreaming


One might be tempted to find a precise definition before
embarking on scientific study


The history of science shows this would be a huge mistake

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Grinnell College
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Fall 2012

4

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


What is a “gene” precisely?


Many things make it hard to define


Removal of introns


Post
-
translational editing of proteins


Some genes have regulatory roles


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


G
roup into two camps on the issue of consciousness


Reductionists


anti
-
Reductionists





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10

The Hard Problem


Many anti
-
reductionist philosophers argue that explaining
consciousness in terms of brain states ultimately leaves
something out


Qualia, experience, “what it is like”, etc…


Logical possibility of
Zombies

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Neuroscience


Consciousness is ultimately a problem for neuroscience


Experiments

(behavioral and biological) and
n
eural models


Bringing neuroscience concepts and discoveries to bear on
philosophical debates

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Neural correlates of visual awareness


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


Neural correlates of consciousness (
NCC
)

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Neurocomputational

approach to
consciousness

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How might we proceed


List some properties of consciousness


Find what brain states correlate with those properties

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Consciousness


Involves short
-
term memory


Independent of sensory inputs


Steerable attention


Disappears in sleep


Reappears in dreaming


Single, unified experience

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Thalamo
-
cortical loops

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Properties of
Thalamo
-
cortical loop


Forms a recurrent neural
network


This can account for several
known properties


Short term memory


Sensory independence


Widespread awareness


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18

Pieces of the puzzle: 40 Hz


ILN neurons tend to oscillate intrinsically at 40 Hz


Cortical regions also oscillate at 40 Hz during wakefulness


These cortical oscillations disappear in deep sleep when ILN
neurons are inactive


Oscillations reappear in REM sleep

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ILN
l
esions abolish consciousness


Lesion left ILN, one neglects right visual field


Lesion ILN entirely, one looses consciousness

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A Neural hypothesis


I have just outlined a simple, testable hypothesis of
consciousness which accounts for our list of known properties


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
)


Focus on visual awareness

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V1 and visual awareness


Crick and Koch hypothesize that V1 activity does not
correspond directly to visual awareness

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V1 activity independent of perception


Patients with parietal cortex damage (extinction)


Activity same in V1 whether stimulus is perceived or extinguished

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Vuilleumier

et al. (2001)

Blink


We blink constantly yet do not perceive interruption of vision


V1 neuron responses do get interrupted

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Adaptation


Subjects adapt to grating they
cannot consciously perceive



Similar results for gratings that
they perceive as uniform fields

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Imagery


Vivid mental imagery possible without V1 [Bridge et al, J.
Neurol. 2011]

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Binocular Rivalry


The vast majority (~80%) of V1 neurons follow the physical
stimulus, not the percept

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Other lines of evidence


Color:
Color
-
tuned V1 neurons track a flickering
iso
-
luminant

two color stimulus subjects perceive as uniform color


Anatomical:

V1 does not project to prefrontal cortex as do
ventral and parietal areas

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Alternative viewpoints on V1


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


TMS stimulation of MT gives motion percept


Stimulating V1 shortly after MT abolishes motion percept

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Ventral stream + awareness


Several lines of evidence suggest neural correlates of visual
awareness in the ventral stream

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Binocular rivalry in monkeys


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


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


Neurons in IT can respond to objects even when the animal is
under anesthetic


One can obtain responses in human extra
-
striate cortex to
stimuli which are not perceived due to masking


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


Damage is associated with visual neglect


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


Reversals:
E
nhanced activation of frontal and parietal areas

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Percepts of bi
-
stable figures


Lumer
,
Friston

and Rees (1998, Science)


Frontal and parietal areas differentially activated during rivalry and
non
-
rivalry conditions

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Co
-
variation of activity


Frontal and parietal areas showed more co
-
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


Areas must interact in both top
-
down and bottom
-
up manner


Crick & Koch propose NCC may be activity in ventral stream
which interacts with activity in frontal and parietal areas

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Fall 2012

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