Exploring the human mind: the perspective of natural sciences

7th International interdisciplinary seminar

Ponte di Legno, Italy 31-12-2003

AD Manazza, MD

A framework for consciousnessCrick & Koch, Nature neuroscience, 2003

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 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).

They study:

-Alert macaque: single neurons for a long time

-Humans: visual psychology

The authors’ framework

• A good f. is one that sounds reasonably plausible, relative to available scientific data. It often contains unstated aassumptions.

• It’s a useful guide for further studies.

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 semi-hierarchical 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;

• We’re not conscious of our thoughts, but only of their sensory representation;

• “ To be conscious of a thought” is quite odd a concept, not previously shown or demonstrated, thus the two sentences are not easily understandable.

2- the “zombie” mode

Sensory stimuli generate:-rapid acting responses, stereotyped “cortical reflexes”;

-slow acting responses, requiring full consciousness.

• This is a brand new concept of reflex, lacking anatomical and physiological explanation, and invading the philosophical field.

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;

• Single visual columns send signals to different parietal areas.

• E.g. achromatopsia, prosopoagnosia, achinetopsia

• Observed human damages are usually 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” vision.

Snapshot half-life is not measurable.

7b- Snapshots

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. time

[Ca++]

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

• Burst of firing

• Signal oscillation

• First arrived, best servedFirst arrived, best served time

ν

10- Penumbra

• P. consists of the effects of the neural network as a whole: its firing will influence a cohort of peripheral neurons, and these ones should help solve the problem of meaning

• How does the brain know what the firing represents?

• Confusion between brain and human being

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?