The Neural Correlates of

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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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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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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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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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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