Transcript A framework for consciousness F.Crick C.Koch Nature neuroscience

Exploring the human mind: the perspective of
natural sciences
7th International interdisciplinary seminar
Ponte di Legno, Italy 31-12-2003
A framework for consciousness
Crick & Koch, Nature neuroscience, 2003
AD Manazza, MD
General strategy
Consciousness:
-self-consciousness
- emotions
-aspects of reality
(redness of red,
houseness of house)
They define the front-head
approach to the problem of
qualia “fruitless” and, in the
mean time, they statue that
no one ever explained it
successfully, although
without demonstrating it.
General strategy
They study:
They try to correlate the
neural correlates of one,
single conscious
perception: the minimal
set of neuronal events
that gives rise to a
specific aspect of a
perception (NCC).
-Alert macaque: single
neurons for a long
time
-Humans: visual
psychology
The authors’ framework
• A good f. is one that
• It’s a useful guide for
sounds reasonably
further studies.
plausible, relative to
available scientific
data. It often contains
unstated aassumptions.
A preamble on the cortex
• The visual system
is highly evolved,
learnes from
experience, has got
a complex
epigenetic control.
• Neurons higher in
the hierarchy will
respond to larger
and more complex
features than lower
neurons (receptive
field).
projection
neuron
reception
A preamble on the cortex
• Both receptive and
projective field are
dynamic and can be
modified with
experience
• Neurons detect most
common input
correlations and
modify themselves to
respond to these
signals
A preamble on the cortex
• We thus have a semihierarchical structure
of the visual cortex
• Many, different stimuli
can be merged or
separately analaysed
Correlation Ib
Correlation Ia – correlation Ia
Stimulus
stimulus
stimulus
stimulus
1-The homunculus
• The front of the brain
is “staring at” the
back, where sensory
data arrive;
• “ To be conscious of a
thought” is quite odd a
concept, not
previously shown or
demonstrated, thus the
two
sentences
are
not
• We’re not conscious of
easily understandable.
our thoughts, but only
of their sensory
representation;
2- the “zombie” mode
Sensory stimuli generate: • This is a brand new
concept of reflex,
-rapid acting responses,
lacking anatomical
stereotyped “cortical
and physiological
reflexes”;
explanation, and
invading the
-slow acting responses,
philosophical field.
requiring full
consciousness.
2b- the “zombie” mode
• “Cortical reflexes”
use dorsal parietal
stream, their stimuli
move forward
• Conscious activity
uses ventral parietal
stream, to and fro
the parietal cortex
and it depends on
different visual
areas.
3- Coalitions
• Transient networks of
connected neurons
elicit or inhibit their
neighbours activity;
• Networks may vary in
size & functions,
though using the same
neurons.
4- Representation
• Explicit and conscious
representation need
certain areas activity;
• E.g. achromatopsia,
prosopoagnosia,
achinetopsia
• Single visual columns • Observed human
send signals to
damages are usually
different parietal areas.
large, and monkeys
aren't usually
interactive 
5- Higher levels first
• The first signal
travels rapidly to the
frontal cortex, then
backwards; the
resulting back
signal may elicit
attention.
6- Neuron interconnections
There may be different
kind of inputs:
-Driving
-Modulating
This is actually a
working hypothesis.
7- Snapshots
The vision process is,
somehow, the result of
snapshots: the “digital”
7b- Snapshots
vision.
Snapshot half-life is not
measurable.
7b- Snapshots
Consciousness level is
reached when neural
networks fire above a
certain threshold;
Ca++levels may fall
down slowly, and
therefore maintain
network activity.
[Ca++]
time
8- attention and binding
• Attention: top-down or
bottom-up, it requires
a biasing activity
between different
signals
• Thalamus could be the
anatomical seat
8b- attention and binding
Binding means putting
together different aspects
of an object: its colour,
shape, movement,…
If it’s a novel
association, it also
requires previous nodes
collaboration
9- Styles of firing
• Synchronized firing
ν
• Burst of firing
• Signal oscillation
• First arrived, best served
time
10- Penumbra
• P. consists of the
• How does the brain
effects of the neural
know what the firing
network as a whole: its
represents?
firing will influence a
cohort of peripheral
• Confusion between
neurons, and these
brain and human being
ones should help solve
the problem of
meaning
General remarks
“Zombie modes show
that not all motor outputs
from the cortex are
carried out consciously.
Consciousness depends
on certain coalitions that
rest on the properties of
very elaborate neural
networks.”
It’s just a bit simplifying.
General remarks-2
“Discovering the
temporal sequence of
such activities, will help
us to move from
correlation to causation”.
POST HOC, ERGO
PROPTER HOC
Future experiments
• Visual illusions and binocular rivalry
• Anatomy of cortex areas, their genetic markers, cells
interconnections
• Fast time / small groups / simultaneous / ≠areas
• Seizure onset studies on humans (epilepsy), or
neurosurgical interventions
Omissions
•Thalamus role
•Basal ganglia role
•Brain stem activity
•Frontal cortex hierarchy
• Is consciousness
peculiar to men or to
their nervous systems?