|The Sense of Self
interactions are involved in generating neuronal
oscillations across various parts of the cortex. In this sense,
the thalamus acts somewhat like the
conductor of an orchestra whose musicians are distributed throughout the cortex.
The conductor doesn’t play the music for the musicians, but instead co-ordinates
them and imposes a rhythm on them. Without the thalamus, the cortex could probably
display some isolated examples of synchrony but would not be able to bind the
various properties of a perception into a coherent concept .
metaphor also helps us to understand why it is pointless to look for any one seat
or centre of consciousness in the brain. The thalamic conductor might impose the
rhythm, but that would be meaningless unless the cortical musicians were playing
their sensory scores. It is the co-ordination of all these things that makes the
mental symphony - the object of consciousness - coherent.
The thalamus is very well positioned
to control the inputs to the cortex. Among the various thalamic nuclei, the reticular
nucleus is known to exert an inhibiting modulation on the other specific
sensory nuclei of the thalamus. The reticular nucleus thus helps to select the
sensory inputs that can reach the cortex, and hence enter consciousness.
circuits in this thalamic nucleus can favour one particular input over several
others. For example, this is what happens when a stimulus that has a strong meaning
for someone (such as the sound of their own name) manages to clear a path through
numerous other auditory stimuli and thus reach that person’s consciousness.
kind of activation, “from
the bottom up”, would be controlled by
the brainstem, the amygdala, or the systems associated with the perception of
pain. In contrast, activation “from
the top down” would be controlled by the executive functions
of the frontal cortex and, according to certain authors, would operate through
the anterior cingulate cortex.
Brodmann area 46,
located in the frontal cortex, is activated by a wide range of tasks and seems
well situated for co-ordinating conscious thoughts. In conjunction with all the
other areas of the brain, area 46 might help us to switch from one thought to
another by facilitating certain global activation patterns at the expense of others.
The particular content of a thought—“what
you have on your mind”— is associated with the content of working
memory: the temporary memory that you use for tasks such as doing
arithmetic in your head, or keeping your train of thought while formulating long
sentences or advancing complex arguments, or for assessing possible moves when
you are playing chess.
This working memory is often
described as being composed of a central
executive (identified with frontal area 46) and two main auxiliary
One of these auxiliary
systems is a visual/spatial form of memory that engages several areas in the right
hemisphere. This memory holds mental images,
the pictures that you can “see in your head” and that are so helpful
for solving spatial-configuration problems.
auxiliary memory system comprises an auditory form of memory, or “phonological
loop”. This is the locus of your inner discourse, that
small voice that you constantly use to talk to yourself and that activates the
in the left hemisphere that are used to decode language.
regardless of which of these two auxiliary systems is at work, the executive processor
in the frontal lobes is always activated.
Cognition and Emotions
Brains Default Network
To try to explain the complex role
of the frontal cortex more clearly, some authors use the metaphor
of an executive committee composed of five members, each of whom represents a
sub-committee of more posterior or subcortical areas in the brain.
first member is the Perceiver. Located mainly in the ventral-lateral
portion of the right frontal hemisphere, the Perceiver is the frontal extension
of the ventral
perceptual system and is focused on objects. The second member
is the Verbalizer. Dominant in the ventral-lateral portion of
the left prefrontal cortex, it is the frontal extension of the language
circuits. The third committee member is the Motivator.
Located in the ventral-medial region of the orbitofrontal cortex, it is the cortical
extension of subcortical pathways that include the amygdala
and represent the world in terms of emotional motivations. The fourth member,
the Attender, occupies the dorsal-medial portion of the frontal
cortex, as well as the anterior cingulate cortex. The Attender is the frontal
extension of a subcortical pathway involving the hippocampus.
The Attender represents the world and self in spatiotemporal co-ordinates and
can direct attention
to internal and external events. Lastly, the fifth member is the Co-ordinator
(or central executive - see preceding
sidebar). It is located in the dorsolateral region of the frontal cortex and is
the frontal extension of the dorsal
pathway. The Co-ordinator represents the world and self in body-centred
coordinates, which enables it to control willed movements and working
|CAN STATES OF CONSCIOUSNESS BE MAPPED IN THE BRAIN?
models of consciousness,
such as the global
workspace theory, assume that the contents of consciousness are widely distributed
in the brain. This assumption has been confirmed by many brain-imaging experiments,
in particular those
of Stanislas Dehaene and his collaborators. In these experiments, when the
amount of time that a word was projected onto a screen was extended just past
the threshold required for subjects to perceive it consciously, there was a major
increase in activity in their frontal, prefrontal, anterior cingulate, and parietal
Thus conscious sensory inputs would
appear to produce far more extensive brain activity than comparable unconscious
stimuli, and a sudden activation of the frontal and parietal lobes would appear
to be the typical signature of a conscious perception.
this perceptual consciousness, or as some would call it, primary
not the only form of consciousness. When we are trying to associate consciousness
with particular structures in the brain, we must therefore clearly define what
level of consciousness we are talking about. For example, the first condition
necessary for the brain to be able to process external sensory stimuli consciously
is that it must be in an appropriate state of alertness (for instance, awake
rather than asleep).
Starting from this premise, authors
such as Damasio distinguish a very primitive form of consciousness that he calls
and that is more like a moment-to-moment perception of the body’s internal
emotional state. This state is associated with activity of such brain structures
as the reticular formation, the hypothalamus,
and the somatosensory cortex.
formation is also associated with consciousness in the minimal sense
of wakefulness. Other structures involved in simply maintaining wakefulness include
the raphe nuclei and the locus coeruleus.
It should be noted here that the activity of the
reticular formation, like that of the primary sensory areas, seems to be necessary
but not sufficient for a more elaborate level of consciousness. This latter level
is attained with what several authors call primary
consciousness, meaning a waking state in which we are in relationship
with our environment “here and now”. On the basis of the research
done by Swedish neuroscientist Bjorn Merker, it seems that the
brainstem plays a more important role in primary consciousness than was formerly
Damasio calls this type of consciousness
“core consciousness” and says that it depends chiefly on the cingulate
cortex and on the intralaminar
nuclei of the thalamus. Indeed, experiments have
shown that bilateral destruction of the centromedial portion of the intralaminar
nuclei of the thalamus also eliminates consciousness, produces a coma,
or causes other states similar to brain death. In addition, this region of the
thalamus is one of the main sites acted upon by anaesthetics
and by antipsychotic drugs.
Models of consciousness
that attribute a role to the thalamus are no recent development. As far back as
Crick offered one of the first hypotheses about consciousness, the “thalamic
searchlight hypothesis”, according to which the thalamus controlled which
region of the cortex became the focal point for consciousness. A similar but more
sophisticated idea has recently been proposed by Rodolfo
Llinas. He hypothesizes that the oscillations of certain neurons in the thalamus
serve as a sort of basic rhythm with which the cortical oscillations of the various
sensory modalities synchronize themselves to form a unified image of the environment—somewhat
like an orchestra conductor who provides the beat for all the musicians to follow
(see sidebar). This is an original solution to the binding
is often compared to a railroad switching yard, because the signals from all of
the senses (except smell) must pass through it before they can reach the cortex.
The cortex also sends many connections back to the thalamus. Most of the nuclei
in the thalamus are considered “specific” because their neurons make
connections with relatively circumscribed areas in the cortex (for example, the
neurons of the lateral
geniculate nucleus project to the primary
The thalamus also has many “non-specific”
nuclei that send diffuse projections to wide areas of the cortex. The intralaminar
nuclei, located in the internal medullary lamina, are a good example
of non-specific thalamic nuclei.
To complete this overview of the thalamus,
we should note that only one of its nuclei, the reticular
nucleus, which wraps around the lateral portion of the thalamus,
does not send any projections directly to the cortex. It does, however, play a
role in the thalamocortical feedback loops, by receiving inputs from the cortex
and sending outputs to the dorsal nucleus of the thalamus.
loops have come to play an important role in practically all
of the neurobiological theories that attempt to explain the higher states
of consciousness, for which the lower levels of consciousness that we have been
discussing up to now are in a sense only the prerequisites. These higher levels
of human consciousness are known as reflexive consciousness and self-consciousness.
sense that “it is I who am perceiving”—is often presented as
a necessary condition for self-consciousness:
the feeling of being oneself and not someone else. This autobiographic dimension
of consciousness implies that we can form mental representations of conscious
experiences in the past or the future, and therefore requires the support of memory
and the higher functions that make abstract conceptualization and planning possible.
You would therefore expect that the areas of the brain
that are known to be involved in these functions, especially in the frontal
and parietal lobes, would be actively engaged in self-consciousness.
And that indeed has been found to be so in certain studies that addressed this
These higher levels of consciousness
also appear to involve other brain structures whose roles were long poorly understood,
partly because some of them are located deep in the brain, which made them hard
to access. Modern brain imaging techniques have now overcome this problem.
of these structures—the angular gyrus, the precuneus,
and the anterior cingulate cortex, which are often very active
in a conscious state of rest—may be part of a functional network that makes
The case of the precuneus,
which is the postero-medial portion of the parietal lobe, is especially revealing.
The conscious resting state is a state in which the subject’s eyes are generally
closed and the subject’s EEG
shows an alpha
rhythm, or in which the subject is passively looking at a simple target such
as a “+” sign. Among all the areas of the brain that are active during
this state, the precuneus is the one that shows the highest rate of neural activity.
But in contrast, the precuneus is known to be less active during tasks that make
no reference to the self. Some authors have therefore proposed that the activation
of the precuneus, and of the posterior cingulate cortex,
which is closely connected to it, is correlated with the feeling of selfhood and
the sense of being an “agent”.
Wheatley et al., 2007.
hypothesis is also consistent with studies that have shown decreased activity
in the postero-medial parietal cortex in many states of altered consciousness,
such as sleep, anaesthesia, or a vegetative state. Other studies have also shown
decreased activity in the precuneus and the posterior cingulate cortex when the
subject is under hypnosis, another altered state of consciousness.
the precuneus also seems to play a role in visual/spatial imagery. For example,
some experiments have shown that the precuneus is more active when subjects are
is engaged in motor
imagery of a finger movement than when they are actually performing this movement.
This again seems to indicate that people have a propensity to represent their
own bodies in space.
insula (also known as the insular cortex) is another region of
the brain that remained little understood for a long time because of its position
deep in the folds of the cortex. Also, because it was not associated with the
“higher” brain functions, it was of less interest to scientists who
were investigating consciousness.
But this indifference
gave way to intense interest after Antonio
Damasio conducted research on the insula and proposed that most of this structure
consists of somatic markers. Damasio hypothesized that this part of the cortex
maps the bodily states associated with our emotional experiences, thus giving
rise to conscious feelings. This hypothesis falls within the school of thought
known as embodied
cognition, according to which conscious rational thought cannot be separated
from emotions and their incarnation in the rest of the body.
Wheatley et al., 2007.
insula thus appears to provide an emotional context that is suitable for a given
sensory experience. The insula is also well positioned to integrate information
about the state of the body and to make this information available to higher-order
cognitive and emotional processes. For example, the insula receives homeostatic
sensory inputs via the thalamus and sends outputs to several structures associated
with the limbic system, such as the amygdala,
striatum, and the orbitofrontal cortex.
insula has also been convincingly shown to be associated with pain processes,
as well as with several basic emotions such as anger, fear,
disgust, joy, and sadness. Its most anterior portion is regarded as part of the
system. The insula also appears to be deeply involved in conscious desires,
such as the active search for food or drugs.
What is common to all of these states is that they affect the entire body profoundly—which
tends to strengthen the case for the insula’s likely role in the way we
represent our bodies to ourselves and in the subjective aspect of emotional experience.
Lastly, the insula in humans, and to a lesser extent
in the great apes, appears to incorporate two evolutionary innovations
that provide these species with a greater ability to read the state of
their own bodies than any other mammals.
anterior portion of the insula, and more particularly that of the insula in the
right hemisphere, is more developed in humans and great apes than in other animal
species. This greater development might enable more precise decoding of bodily
states—the capability that translates a bad odour, for example, into a feeling
of disgust, or the touch of a lover into a feeling of delight.
other major evolutionary modification in the insula is a type of neuron that is
found only in the great apes and in humans. These large,
elongated, cigar-shaped nerve cells are known as von Economo neurons (VENs).
VENs occur only in the insula and in the anterior cingulate cortex. These neurons
connect to various parts of the brain, which would be an essential attribute for
the higher functions attributed to these two brain structures.
|Now it is time
to say a few words about the anterior cingulate cortex,
which also acts as an important interface between emotion and cognition,
and more specifically in the conversion of feelings into intentions and actions.
This structure is involved in higher functions such as controlling one’s
own emotions, concentrating on solving problems, recognizing one’s own mistakes,
and making adaptive responses to changing conditions. All of these functions are
closely linked with our emotions.
Wheatley et al., 2007.
experimental subjects are pricked with a needle, the activity in their cingulate
cortex increases; this response is so clear-cut that the neurons in question are
often called the “pain neurons”. A fascinating side-note: in 1999,
William Hutchison and his colleagues at the University of Toronto
showed that these same neurons in the cingulate cortex also become active when
the subject sees someone else being pricked with a needle. Thus, for these kinds
of neurons, known as mirror
neurons, there is no boundary between the self and the other.
including humans, are highly social creatures. Knowing other individuals’
intentions has always been crucial for our survival. That is why we are past masters
of the art of internally
simulating other people’s minds, perhaps with the help of such mirror
neuroscientists, such as V.S. Ramachandran, even suggest that
this ability to decode other individuals’ states of mind may
even have evolved first, and subsequently been applied to the self, to become
what we call self-consciousness. And in Ramachandran’s view, not only the
mirror neurons, but
all parts of the brain that contribute to language, such as Wernicke’s
area in the temporal lobe, must inevitably play a role in this process.
important role is ascribed to language in several models of higher consciousness,
including that proposed by Michael Gazzaniga, who is known for
his work with “split-brain”patients. But while Gazzaniga's model identifies
the language hemisphere as the locus of this “interpreter” that makes
us conscious of ourselves, other authors, such as Edelman,
argue that consciousness
cannot be attributed to any specific structure in the brain.