# Consciousness Studies/The Neuroscience Of Consciousness#Topological mapping and cortical columns

*\"All parts of the brain may well be involved in normal conscious
processes but the indispensable substratum of consciousness lies outside
the cerebral cortex, probably in the diencephalon\" Penfield 1937.*

*\"The brain stem-thalamocortical axis supports the state, but not the
detailed contents of consciousness, which are produced by cortex\" Baars
et al 1998.*

## Introduction

*It is recommended that readers review ../The Philosophical
Problem/ before reading the
sections on the neuroscience of consciousness.*

One of the most exciting discoveries of neuroscience is that nearly all
of the brain performs functions that are not part of conscious
experience. In everyday life we are usually unaware of breathing or
heartbeats yet there are parts of the brain dedicated to these
functions. When we pick up a pencil we have no experience of the fine
control of individual muscles yet large areas of cortex and cerebellum
implement this. Things do not appear as greyscale and then have the
colour poured into them although this strange colour addition is done in
the visual cortex. Most of the brain is non-conscious but how is the
\"ghost in the machine\", the mind, created by and linked into the
mostly non-conscious brain?

Although most of the processes in the brain are non-conscious there can
be little doubt that the output of sensory processes contribute to
experience. For example, although we do not experience the process of
adding colour to visual data in cortical area V4 we do experience
coloured forms and although we have little inkling of the hugely complex
creation of words in the temporal/frontal lobes we do experience verbal
thoughts. Our experience is an integrated output of most of the brain
processes that deal with sensation as well as dreams, thoughts and
emotions. But how and where does this experience occur?

The signals that compose phenomenal consciousness have not been
elucidated. Perhaps the least likely signals for this role are
electrical impulses in nerve fibres because they are distributed
unevenly in time and space and can even be absent for relatively long
periods. Furthermore, electrical impulses across the membranes of
neurons have an all or nothing character; they cannot be easily
superimposed on one another. There are many other possibilities however,
such as: the electrical fields on the dendrites of neurons, the fields
of chemicals spreading out from synapses, the radio-frequency emissions
of action potentials, events in the microtubules in cells, the
depolarisations of glia, the varying fields measured by EEG devices, the
quantum superposition of brain states etc\...

Phenomenal consciousness could exist in the dendritic field of ten
neurons receiving 100,000 synapses or as an oscillation of fields over
the whole brain. The substrate of phenomenal consciousness could be
staring us in the face as a state of the whole brain or be like a needle
in a haystack, lurking in a tiny region of brain, unsuspected and
undiscovered.

Given that there is no widely accepted theory of phenomenal
consciousness Crick (1994) and Crick and Koch (1998) approached the
problem of the location of the substrate of consciousness by proposing
that scientists search for the **Neural Correlates of Consciousness**.
These neural correlates consist of events in the brain that accompany
events in conscious experience.

References:

Crick, F. (1994). The Astonishing Hypothesis. New York: Scribners.

Crick, F. & Koch, C. (1998).Consciousness and Neuroscience. Cerebral
Cortex, 8:97-107, 1998
<http://www.klab.caltech.edu/~koch/crick-koch-cc-97.html>

## Neuroanatomy

### General layout of the CNS

The Central Nervous System (CNS) consists of the spinal cord, the brain
and the retina.

The CNS consists of two major groups of active cells, the **neurons**
and the **glia**. The neurons conduct short impulses of electricity
along their membranes called **\'action potentials** and encode data as
frequency modulated signals (i.e.: different intensities of stimulation
are converted into different rates of firing). The glia modify the
connections between neurons and can respond to neuron activity by a
change of voltage across their membranes. Glia also have many other
roles such as sustaining neurons and providing electrical insulation.

Neurons have three principal parts: the **cell body**, the **dendrites**
and the **axon**. Impulses flow from the cell body to the axon. The axon
can be over a metre long and bundles of axons form **nerve fibres**.
Where an axon makes contact with the dendrites or cell body of another
neuron there is a special sort of junction called a **synapse**.
Transmission of data across synapses is usually mediated by chemical
signals.

Areas of the brain where there are many cell bodies have a beige/grey
tinge and are called **grey matter**. Areas that contain mainly nerve
fibres are called **white matter**. Masses of grey matter outside of the
surface of the cerebral cortex or the cerebellum are called **nuclei**.

The brain is of central interest in consciousness studies because
consciousness persists even when the spinal cord is sectioned at the
neck.

The brain can be divided into five distinct divisions or *\'vesicles* on
the basis of embryological development. These are the myelencephalon,
metencephalon, mesencephalon, diencephalon and telencephalon (See the
illustration below).

![](Constuddivis.png "Constuddivis.png")

:   Myelencephalon: Medulla oblongata.
:   Metencephalon: pons and cerebellum.
:   Mesencephalon: midbrain (tectum containing the superior colliculus
    and inferior colliculus, red nucleus, substantia nigra, cerebellar
    peduncles.
:   Diencephalon: thalamus, epithalamus, hypothalamus, subthalamus.
:   Telencephalon: corpus striatum, cerebral hemispheres.

These divisions tend to obscure the physical anatomy of the brain which
looks like a rod of spinal cord with a swelling at the top due to the
thalamus and corpus striatum. Around the top of the rod is a globe of
deeply indented cerebral cortex and at the back there is the puckered
mass of cerebellum. The physical anatomy is shown in greater detail in
the illustration below where the thalamus and corpus striatum have been
splayed out to show more detail.

![](Constudoverbrain_-_2.png "Constudoverbrain_-_2.png")

The thalamus is a complex organ with numerous nuclei. These are listed
below:

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The location of these nuclei is shown in the illustration below:

![](constudthal.gif "constudthal.gif")

The cerebral hemispheres consist of a thin layer of nerve cell bodies on
the surface (the cerebral cortex) with a mass of white, interconnecting
fibres below (the cerebral medulla). Each hemisphere is divided into
four principle **lobes** as shown in the illustration below:

![](Main_brain_lobes.gif "Main_brain_lobes.gif")

The cortex is a set of interconnected processors. The general layout of
the cortex with the location of the processors is shown in the
illustration below:

![](Constudproc.png "Constudproc.png")

The pathways in the brain tend to preserve the **topography** of the
sense organs so that particular groups of cells on the retina, cochlear
or body have corresponding groups of cells in the thalamus or cortex.
The retina is said to have a topological mapping onto the thalamus so
that the projection of the optic nerve is said to be **retinotopic**.

Nerve fibres that go to a part of the brain are called **afferents** and
fibres that come from a part of the brain are called **efferents**.

The cortex and thalamus/striatum are intimately linked by millions of
connecting fibres and there is also a direct connection from the motor
cortex to the spinal cord.

### Sensory pathways

Information from the sense organs travels along the appropriate sensory
nerve (optic, auditory, spinal etc.) and once in the brain is divided
into three principal paths that connect either with the thalamus, the
cerebellum or the reticular formation.

There are thalamic nuclei for each broad type of sensation and these
have reciprocal connections with specific areas of cortex that deal with
the appropriate mode of sensation. The large mass of nerve fibres that
mediate the connection between the thalamus and cortex are known as the
thalamo-cortical and cortico-thalamic tracts. There tend to be more
sensory nerve fibres returning from the cortex to the thalamus than
connect from the thalamus to the cortex so it is difficult to determine
whether the cortex is the destination of sensory data or a region that
supplies extra processing power to thalamic nuclei.

The cerebellum mediates reflex control of complex movements and receives
input from most of the sense organs.

The reticular formation is a group of loosely distributed neurons in the
medulla, pons and mesencephalon. It receives a large amount of autonomic
input and also input from all the sense organs. The intralaminar nuclei
of the thalamus are the principal destination of reticular output to
higher centres. In the most primitive vertebrates the reticular
formation performs most of the higher control functions of the animal.
The reticular formation is implicated in the maintenance of sleep-wake
cycles and activates the higher centres. This activity has attracted the
label **ascending reticular activating system** (ARAS) to describe how
the activity of higher centres is controlled by reticular input. This
title is unfortunate from the point of view of consciousness studies
because it implies that conscious experience is a result of activating
the cortex when it could be due to turning on or off particular systems
all the way from the reticular formation to the cortex. Destruction of
the reticular formation leads to coma.

### Motor and output pathways

Motor control of the body below the skull is accomplished by three
principle routes.

The motor cortex of the frontal lobes and related cortex in the parietal
lobes can control movement directly via nerves known as the
cortico-spinal tract (also called the pyramidal tract). The activity of
the motor cortex is modified and controlled by a loop that passes
through the corpus striatum, the substantia nigra and the subthalamic
nucleus and returns to the cortex. These controlling nuclei are, along
with the amygdala, known as the **basal ganglia**.

The cerebellum and the corpus striatum provide complex reflex control of
the body through nerves that travel through the red nucleus and form the
rubro-spinal tract.

The vestibular nucleus, which processes signals related to balance and
posture, has direct connections with the periphery via the
vestibulo-spinal tract.

Apart from the routes for controlling motor activity there are also
other outputs from the brain, for instance the autonomic nervous system
is intimately linked with the reticular formation which has areas that
control blood pressure, respiratory rhythm etc.

## Topological mapping and cortical columns

!Sensorimotor
homunculus{width="400"}The
cerebral cortex has a highly convoluted surface that provides a large
area of tissue. The parts of the cortex that are used for motor and
sensory functions are organised so that different areas correspond to
different zones of the body. This **topological** organisation is shown
classically by a drawing of the *sensorimotor homunculus* such as that
shown on the right.

Within a given area of the cortex there are further subdivisions. For
example, the occipital cortex corresponds to the eyes of the
sensorimotor homunculus and it is further organised so that areas of the
retina have corresponding areas on the cortex. This mapping of the
layout of the retina onto the cortex is known as **topological
mapping**. It results in a corresponding mapping of the receptive field
of the eye onto the cortex. The mapping is like an image on the surface
of the brain tissue and the visual scene that is presented to a subject
can be recovered by using fMRI along with computer analysis (Miyawaki
*et al.* 2008).

The human cortex is fairly deep, containing 100-200 neurons from the
surface to the white matter. It is divided into six histological and
functional layers. These layers can be further subdivided. In 1957
Mountcastle used microelectrode measurements to show that activity of
small zones of cortex about 0.1 to 1 mm in diameter corresponded to
particular points in the receptive field. These functional columns of
cortical tissue are called **cortical columns**.

![](Corticalcol.png "Corticalcol.png")

The diagram above shows the organisation of **ocular dominance
columns**. Each column represents a particular part of the receptive
field of a single eye. The columns for left and right eyes are linked
together in lines. The lines of ocular dominance form a pattern like a
fingerprint on the surface of the cortex.

The same part of cortex can have overlapping columns for different
functions. For instance there are columns that react to particular
orientations of edges at particular places in the visual field. These
columns tend to be located together on the cortex forming a **pinwheel**
of columns that cover all orientations at a particular receptive field
position.

![](Corticalorient.GIF "Corticalorient.GIF")

There are also topologically arranged columns for colour, spatial
frequency etc.

Yoichi Miyawaki, Hajime Uchida, Okito Yamashita, Masa-aki Sato, Yusuke
Morito, Hiroki C. Tanabe, Norihiro Sadato and Yukiyasu Kamitani. (2008).
Visual Image Reconstruction from Human Brain Activity using a
Combination of Multiscale Local Image Decoders. Neuron, Volume 60, Issue
5, 10 December 2008, Pages 915-929

**Modules**

-   The neurophysiology of sensation and
    perception
-   The cortex and
    thalamus
-   Rivalries and
    synchronisation

