From Cognitive to Neural Theories
of Consciousness
Efraïm Salari
ABSTRACT
There seems to be a tendency within the education of neuroscience and cognitive
psychology to give little attention to what is not well understood. One subject that
tends to get left out of education for this reason is the concept of consciousness. There
have been, however, a number of studies that tried to address the question of how
brain processes are related to consciousness.
The main question of this thesis therefore is: what are, according to contemporary
views, the cognitive and neuronal processes that are needed for, or correlated with
conscious experience?
Through high level cognitive theories a scientific understanding of consciousness was
arrived. Based on this, lower level neural theories where developed, focusing on how
neurons interact with each other, and which neurons/areas interact with each other
during conscious and unconscious processing. Three important theories will be
discussed to illustrate this.
First, the global workspace hypothesis will be discussed which is a cognitive theory.
It was developed to, for the first time after behaviourism, address questions on
consciousness scientifically. It states that different forms of information are usually
autonomously and unconsciously processed in different brain areas. Consciousness is
however derived when brain areas share their output to a global ‘workspace’ onto
which different areas are connected such as for instance memory, language and
association areas. Second, the 40 Hz theory is discussed which was based on the
same notions as the global workspace theory but which takes a neural point of view,
and focuses on how remote brain areas communicate to form a unified conscious
percept. It states that neurons in different brain areas that are involved in conscious
percept, fire in synchronized oscillations in order to bind their information processing.
Finally, the recurrent processing theory is considered which is also a neural theory
of consciousness that is to some extend based on the notions of the global workspace
hypotheses but focuses on which areas communicate during conscious percept and
more specifically on the course of this communication. This theory deviates from
previous theories of consciousness in its’ notion of how to measure consciousness.
Most theories use an intuitive concept of consciousness, i.e. that people can report on
what they are conscious of, and so reportability can be used as a measure of
consciousness. However, the recurrent processing theory states that reportability
might not always be sufficient for measuring consciousness (for instance in split brain
and locked in patients) and states that a fully neural description of consciousness is
needed. According to this theory, consciousness is arrived when recurrent processing
between higher and lower order areas takes place, regardless of peoples reports.
Keywords: Consciousness, global workspace, 40 Hz, recurrent processing
“Conscious experience is at once the most familiar thing in the world and the most
mysterious.”
David J. Chalmers (1995)- The Conscious Mind: In Search of a Fundamental Theory
INTRODUCTION
The fields of neuroscience and cognitive psychology are concerned with one of the
most fascinating questions in contemporary science: how does the brain work? The
knowledge that is gained so far about the functioning of the brain is incredible.
However, there seems to be a tendency within the education of these fields to give
little attention to what is not well understood. One subject that tends to get left out of
education is the concept of consciousness. In 1990 Francis Crick and Christof Koch
already wrote:
“IT IS REMARKABLE that most of the work in both cognitive science and the
neurosciences makes no reference to consciousness (or 'awareness'), especially as
many would regard consciousness as the major puzzle confronting the neural view of
the mind and indeed at the present time it appears deeply mysterious to many people.
This attitude is partly a legacy of behaviorism and partly because most workers in
these areas cannot see any useful way of approaching the problem.” (Crick & Koch,
1990, p263)
Although in the education of neuroscience the neuronal correlates of consciousness
(NCC) are heavily overlooked, there have been a number of studies that tried to
address the question of how brain processes are related to consciousness. Therefore
the focus of this thesis will be on the following question: What are, according to
contemporary views, the cognitive and neuronal processes that are needed for, or
correlated with, conscious experience?
One can debate however whether there is a relationship between the processes that are
going on in the brain and consciousness in the first place. René Descartes, for
example, thought that there is a duality between mind and body; that the mind
originates from something non-physical. However, this not a very popular view
anymore (but see Chalmers, 1995 for a contemporary dualistic approach) and in this
thesis I will assume that a correlation between the physical state of the brain and
conscious experiences exists. Indeed, a lot of evidence indicates that specific neuronal
processes are associated with whether or not people are consciously aware of for
instance a presented stimulus (Baars, 2005; Dehaene & Naccache, 2001; Crick &
Koch, 1990, Lamme, 2006)
What is the problem?
For a lot of brain modalities, such as for instance motor processing or vision, there is
a good understanding of both the cognitive (high level) and the underlying neural
(low level) mechanisms of these functions. It is known for example which brain
regions are responsible for these functions and what neural activity is involved. For
instance, when a particular motor action is executed, neural activity originates in the
primary motor areas, which then terminates in the muscles. In vision, light hits the
retina, which activates ganglion cells from which neural activity travels towards V1,
followed by higher order areas, integrating the information into a complete image of
the visual field.
However, which neuronal responses are accompanied by
consciousness still remains to be a subject of debate and a lot of questions remain to
be answered. For example, where along the visual stream does consciousness of a
scene arise? There is no clear end point that has to be reached for consciousness to
arise (such as a muscle in motion). Do specific cells, areas or networks need to be
active for conscious awareness? Are there specific forms of cell communication
involved? Why are we aware of some things and not of others?
The reason why a lot of these questions have not been answered is due mostly to a
lack of consensus on how to define consciousness and how to measure it. Because of
that, early psychological research focused only on behavioural observations, without
concerning about the processes in the mind that led to them. This movement, called
behaviourism, did not address any questions on consciousness. However, with new
imaging techniques, such as fMRI, the brain’s processes could be studied while
functioning. This leaded to new insights, and cognitive models of consciousness
where developed to address the problem of consciousness scientifically. These models
where concerned with: what brain functions (i.e. attention, memory, vision) are
involved in consciousness, and what brain areas are responsible for them.
Subsequently, based on these higher level cognitive theories, lower level neural
explanations where developed, focusing on what neural mechanisms are involved
during conscious percept. To illustrate this, three important theories will be discussed.
The first theory that is discussed is the global workspace hypothesis, which is a
cognitive theory, describing the functions that are involved in consciousness and the
brain areas related to that. It states that different forms of information are usually
autonomously and unconsciously processed in different brain areas. Consciousness is
however derived when brain areas share their output to a global ‘workspace’ onto
which multiple areas are connected such as for instance memory, language and
association areas. This theory is discussed here because it was one of the first theories
that tried to break with behaviourism and start a scientific search for understanding
consciousness. Also, it has been very influential for later theories, among which
theories that focused on the lower level, neural mechanisms involved in
consciousness. Two of these neural theories are described next: the 40 Hz theory and
the recurrent processing theory.
The second theory that is discussed is the 40 Hz theory, which focuses on how
neurons involved in consciousness fire. It states that neurons in different brain areas
that are involved in conscious percept, fire in synchronized oscillations in order to
bind their information processing. This theory is discussed for two reasons. The first
reason is that it was a reaction to the cognitive theories of consciousness, to come for
the first time with a neural theory of consciousness, describing what neuronal
mechanisms are involved during conscious precept. The second reason is that this
theory is influenced by the global workspace theory and therefore comparisons can be
made between these theories.
Finally, the recurrent processing theory is considered which is also a neural theory of
consciousness and focuses on which areas communicate during consciousness and
more specifically on the course of this communication. This theory deviates from
previous theories of consciousness. Most theories use an intuitive concept of
consciousness, i.e. that people can report on what they are conscious of, and so
reportability can be used as a measure of consciousness. However, the recurrent
processing theory states that reportability might not always be sufficient for
measuring consciousness (for instance in split brain and locked in patients) and states
that a fully neural description of consciousness is needed. According to this theory,
consciousness is arrived when recurrent processing between higher and lower order
areas takes place, regardless of peoples reports. This theory is discussed for two
reasons. The first reason is that it’s fully neural perspective on consciousness deviates
from other theories in an interesting and unexpected way, which might be influential
to our understanding of consciousness. The second reason is that it is a rather new
theory, which is also influenced by the global workspace theory.
All theories are built on the notion that for people to be able to report on their
conscious content, different brain areas need to communicating with each other on a
global scale, as suggested by the global workspace theory. The 40 Hz theory however,
focuses on how different areas communicate during conscious content processing
whereas the recurrent processing theory focuses on which areas communicate and in
what way.
What is consciousness?
So far the term consciousness has been used without an explanation of what this
actually means. Consciousness can be described in multiple ways. Baars (2005) for
instance describes it in the following way:
“It is broadly true that what we are conscious of, we can report with accuracy.
Conscious brain events are therefore assessed by way of reportability.” Baars
(2005, p.45).
Lamme (2006) on the contrary argues that to use the term consciousness, it should not
only be described in terms of other function. Reportability for instance relies on
memory and language. Paralyzed patients however that cannot speak and move, still
show forms of consciousness (as suggested by fMRI experiments; see for instance
Owen et al., 2006) but have no way of expressing themselves. Lamme argues that
consciousness should therefore also be described purely in terms of neural activity.
Meaning that if specific neural activity is present, this should be considered a sign of
consciousness, whether or not people can report awareness.
Others, such as Crick & Koch (1990) have restrained themselves from a precise
description of consciousness but use both reportability measures as well as neural
evidence (for instance when using animal models).
Because consciousness can be described in multiple ways, some have argued that
there are multiple neural correlates of consciousness. Ned Block (2005) for instance
argues that consciousness can be separated into access- and phenomenalconsciousness.
Access-consciousness is the kind of consciousness that is involved when one can
actively think about something (and so usually report on it). This arises when
information is not only locally processed (for instance in visual areas only) but is also
available to multiple brain areas such as those important for memory, evaluation and
language. Access-consciousness, according to Block (2005), can be explained by the
global-workspace hypothesis (Baars, 2005, Dehaene & Naccache, 2001).
Phenomenal-consciousness is that what separates experiences from each other but
without further evaluation of these experiences. In a visual scene for instance, there
are different colours that one experiences, however one does not necessarily have to
actively think about all the colours that are seen. According to Block (2005) this form
of consciousness can be explained by the recurrent-processing theory (Lamme, 2006).
Because different authors have used different definitions of consciousness, it is hard
to give one description of the term ‘consciousness’. There is simply no consensus as
to what consciousness conveys. However, one useful way of differentiating
consciousness from unconsciousness is that the former comes with an ‘experience’ of
some kind whereas the later does not (Chalmers, 1995). This ‘experience’ can be
assessed in multiple ways, i.e. by reportability (Baars, 2005) or neural activity (see
Lamme, 2006) and in this paper consciousness is assessed as whether or not the
presentation of external stimuli are accompanied by an experience. Possible other
forms of consciousness, such as thoughts that are not directly related to something in
the environments are not discussed in this thesis to restrict the scope of this paper.
An important and useful implication of describing consciousness as the presence of
experience, is that some animals by definition need to have consciousness (although
possible in a different form than humans). Monkeys for instance can be trained to
report whether or not they saw a stimulus. Therefore animals can be used to
investigate some forms of consciousness. Also, when using neural measures of
consciousness, other animals such as cats can be used to study the neural correlates of
consciousness (see for instance Crick & Koch, 1990).
Because consciousness can conveys many things, this thesis focuses on theories that
try to explain consciousness only in the form of experiencing and being aware of
external input (stimuli perception). The terms consciousness, awareness and
conscious percept are used interchangeably (but see Chalmers, 1995, for a different
notion).
The purpose of consciousness:
Related to the question of what consciousness is, is the question of what the purpose
of consciousness might be. Some researchers have addressed this question. Lamme
for instance (2006) has argued that consciousness is necessary for learning and
memory. This is because the neural activity that according to Lamme correlates with
consciousness (recurrent processing) involves an optimal situation for brain plasticity.
Dehaene et al. (2001) have also suggested that consciousness is necessary for
particular mental functions. Like Lamme, they argue that consciousness is necessary
for information maintenance. That is, studies have shown that only information that is
consciously attended to can last longer than iconic memory. Studies for instance show
that the judgement of a target word is influenced by a previously presented masked
prime word, which was unconsciously processed but not consciously perceived. This
was the case however only if they are separated less than a hundred milliseconds in
time (Greenwald, 1996). This suggests that unconscious information decays very
quickly. This result is supported by single cell recordings in monkeys (Rolls & Tovee,
1994). The neural activity of masked stimuli last only for as long as the stimulus is
presented (ca. 50 ms) whereas the activity of equally long presented non-masked
stimuli last for up to 300 ms.
Other functions of consciousness, according to Dehaene and Naccache (2001), are its
requirement for novel operations and also for intentional behaviour.
However, all these purposes of consciousness do not give an answer to the
philosophical question as to why some brain processing has to be accompanied by
experience at all. One could argue for instance that consciousness has no purpose, and
is mere an epiphenomenon (Chalmers, 1995). Why are people (and probably animals)
not mere like machines, processing some input and producing some output
(behaviour), without experience? That is, if you could have (in theory) a brain
operating exactly the same with or without consciousness, than what is the purpose of
consciousness?
The hard problem:
David Chalmers (1995) makes a distinction between the ‘easy’ problems of
consciousness and what he has termed ‘the hard problem’. The easy problems are
those that are going to be discussed in this thesis, involving which brain processes are
related to consciousness. Chalmers however claims that there is no reductionist
explanation for experience. This means that there is no explanation with physics as
we know it, to explain everything that needs to be explained about consciousness. We
can explain which neural processes are correlated with consciousness with physics,
and even why the processing of some input to the brain leads some behavioural
output. However, why is this processing accompanied by an experience? That is,
broadly speaking one could argue that there is a logical (not empirical) possibility of a
brain executing the exact same neuronal responses as a conscious brain but without
being conscious all together. Think of creatures that are in every particle identical to
ourselves but that live in a different world without consciousness. Chalmers has
termed these copies ‘zombies’. A zombie will behave exactly the same as the original
person since all the neuronal responses are exactly the same, and it is therefore really
different from zombies usually seen in movies. It would even claim to be conscious,
except that it is not. The fact that some neuronal processes involve consciousness
does not mean that these processes could not have been there without consciousness.
However, in our world these processes are, according to Chalmers, accompanied by
consciousness like a law of nature, and consciousness might be a fundamental
building block of physics, equal to for instance mass, time and space.
Chalmers claims that with physics as we know it, we will never be able to explain
everything that needs to be explained about consciousness. Therefore we need
radically different theories about consciousness than those that seem intuitive to us.
Although different from the way dualism was specified by Descartes, Chalmers
adopts a particular form of dualism for explaining consciousness. Others have looked
at other forms of physics such as quantum physics to explain consciousness (Squires,
1990). Still others claim that all that there is to explain are the neural mechanisms of
consciousness, and that there is no hard problem. Dehaene and Naccache (2001) for
instance fall into this last category.
“Many scientists and philosophers still adhere to an essentialist view of
consciousness, according to which conscious states are ineffable experiences
of a distinct nature that may never be amenable to a physical explanation. Such
a view, which amounts to a Cartesian dualism of substance, has led some to
search for the bases of consciousness in a different form of physics (Penrose,
1990). Others make the radical claim that two human brains can be identical,
atom for atom, and yet one can be conscious while the other is a mere `zombie'
without consciousness (Chalmers, 1995).” Dehaene & Naccahce (2001, p.4)
The problem with the last sentence of this statement is that it is actually incorrect.
Chalmers (1995) does not claim that in this world two brain could exist atom for atom
identical, one being conscious and the other not. However, he claims that one could
think of a brain without consciousness, performing all the processes that a conscious
brain would perform. Therefore the question of why these processed are accompanied
with consciousness needs to be explained.
Lamme (2006) does not claim to solve the hard problem but thinks that the recurrent
processing theory brings it closer than previous theories. Crick & Koch (1990) do not
make any specific claims about the hard problem in their 40 Hz theory.
THEORIES OF CONSCIOUSNESS
In this paper, the hard problem is left aside and focus is lead upon the ‘easy’ problems
because these questions are interesting on their own, and they can scientifically be
addressed. These involve questions such as; what brain activity is correlated with the
conscious perception of a stimulus and how is this different from unconscious
perception? Multiple theories on this matter have been developed and three of them
are discussed here. First the Global Workspace hypothesis will be discussed followed
by the 40 Hz theory and finally the recurrent processing theory. For each theory, first
a short description will be given, followed by scientific findings supporting the
theory.
Global Workspace
The global workspace hypothesis was developed in the nineteen eighty’s, mostly by
Bernard Baars, as a rejection of behaviourism (Blackmore, 2005). It is one of the first
contemporary theories to address the problem of consciousness scientifically.
Description
Baars (2005) describes the global workspace hypothesis using a theatre metaphor in
which all of the theatre is dark except for the part that is within a spotlight. Everything
that happens in the dark is not known to the rest of the system and therefore
unconscious. Only the part that is within the spotlight is apparent to the whole system
and therefore in consciousness. The spotlight is controlled by attention processes,
which determine what the content of consciousness will be.
The global workspace hypothesis is build upon six hypothesis and assumptions. These
will first be described briefly and later support for them will follow.
- (1) The brain is modular
According to the global workspace hypothesis, the brain can be seen as a set of
parallel processors each tuned to its own specific function or modality (e.g.
perception, memory, motor planning) as indicated by imaging studies. This means
that different functions/modalities are processed in different parts of the brain.
- (2) Information can be processed unconsciously
Each of these modalities process information more or less autonomously and in an
unconscious manner, separate from the rest of the brain.
- (3) Information needs to be globally available for consciousness
Consciousness is derived however, at the moment information is shared and
exchanged between modalities (Baars, 2005; Dehaene & Naccache, 2001). There is a
centred and shared memory capacity, the global workspace, onto which otherwise
separated modalities are connected and through which information can be shared to
the rest of the brain. This makes information available not only at the local level but
also at a global level.
- (4) Minimum neuronal activation and attention attenuation is needed
The modality that can send information to this global workspace is determined by topdown attention (Dehaene & Naccache, 2001). Because of this top-down regulation of
which information enters the global workspace, information is shared with the system
as a whole in a controlled and coordinated fashion (Baars, 2005). For a modular
process to enter the global workspace -and therefore consciousness- neural activity
needs to be amplified by top-down attention for a sufficient duration (see support
section; Dehaene & Naccache, 2001). Subsequently, the information is thought to be
sent to the rest of the modalities via corticocortical and corticothalamic fibers (Baars,
2005).
- (5) Where in the brain is the Global Workspace located?
Because the global workspace is thought to be a form of memory capacity, a logical
question to ask would be where this workspace might be in the brain. However, there
is no fixed set of regions that is connected to the global workspace, as different brain
regions can be ‘mobilized’ and ‘demobilized’ (Dehaene and Naccache, 2001).
According to these authors the brain contains ‘long-distance’ neurons, which connect
remote brain areas and contribute to the global workspace. However, only some of
these neurons are active at any particular time. Therefore they conclude that there is
no particular need to point to one specific location in the brain that is active for all
conscious content.
However, these kinds of long distance neurons seem to mainly originate from
prefrontal cortices (PFCs) and anterior cingulate cortex (ACC; Dehaene & Naccache,
2001; see support section). Furthermore, frontal and parietal association areas have
consistently been indicated in processing of sensory information during conscious, but
not during unconscious states (Baars, 2005). Baars (2005) suggests that there seems to
be a special role for frontoparietal association areas in the formation of consciousness,
even though these areas themselves are not responsible for processing for instance
visual or sensory content. He proposes that these areas are important in the formation
of a ‘self’ as these areas have been shown to be involved in interpreting sensory input
and comparing it to expectations (Gazzaniga 1998; Baars, 2005) which is important
for maintaining a sense of a ‘self’ that is consistent over time.
- (6) Which modalities contribute to the Global Workspace?
Because different brain areas can contribute to the global workspace but we do not
become aware of all processes in the brain (e.g. you don’t have to be conscious to
breath) the global workspace theory predicts that some modalities can contribute to
the global workspace whereas others cannot. This would explain why we are
conscious of only some content that is processed in the brain. According to Dehaene
and Naccache (2001) at least five main modalities have the possibility to engage with
the workspace.
“[P]erceptual circuits that inform about the present state of the environment;
motor circuits that allow the preparation and controlled execution of actions;
long- term memory circuits that can reinstate past workspace states;
evaluation circuits that attribute them a valence in relation to previous
experience; and attentional or top-down circuits that selectively gate the focus
of interest.” (Dehaene & Naccache, 2001, p 14).
They argue that the global interconnectedness between these modalities makes
conscious experience a unitary feeling. Furthermore, connections to language and
motor areas make the content of workspace available for reportability.
Support
The previous assumptions will now be discussed again and support for them are
provided.
- (1) The brain is modular
The global workspace hypothesis poses that the brain works in a modular fashion, i.e.
that different functions are processed in different parts of the brain. This has been
suggested by many imaging and lesion studies. For instance, different aspects of
visual information are processed by different visual areas (V4 for colour, MT/V5 for
motion etcetera; McKeefry & Zeki, 1997; Antal et al., 2005) and lesions to specific
areas only disrupt specific features of vision (Livingstone & Hubel, 1988). In
language for example, Broca’s area has been related to speech production (Alexander
et al., 1990) whereas Wernicke’s area has been related to speech comprehension
(Naeser et al., 1987). Again, disruption of these areas show specific deficits in speech
production and speech comprehension respectively (Kertesz, Harlock & Coates,
1979). This suggests that certain computational operations in the brain are processed
in different parts of the brain and are to some extend independent.
- (2) Information can be processed unconsciously
Besides that the global workspace hypothesis poses that different modalities are
processed in different areas, it also claims that a lot of these modular processes take
place unconsciously. Support for this statement comes from three different patient
studies. These patients have in common that they are unaware of a particular form of
visual input, however still show the ability to process this information unconsciously.
First, people suffering from hemianopsic scotoma -blindness in only a part of the
visual field- sometimes show (depending on the lesion) to be able to react above
chance level to stimuli in the part of the visual field that is not consciously perceived
(Weiskrantz, 1996). This has been called the ‘blindsight-phenomena’.
Second, people suffering from prosopagnosia -a condition in which patients are not
able to recognize faces- show different electrophysiological reactions to familiar faces
than to unfamiliar faces although they claim not to recognize any of these faces
(Renault et al., 1989).
Third, people with hemispatial neglect tend to disregard their left visual field. They,
however still show an enhanced performance on a lexical decision task if a related
visual prime is shown in the left and visually unconscious field (McGlinchey-Berroth
et al., 1993).
Hemianopsic scotoma-, prosopagnosia - and neglect-patients all have in common their
lack of conscious awareness of a particular form of information, while still being able
to show behavioural or neural responses to this information. Therefore, processing of
this information needs to have been taken place in an unconscious manner.
Other forms of evidence that information can be processed unconsciously come from
studies in healthy subjects. Some studies have shown for instance that if isolated
stimuli are presented for very brief periods of time (29-50 ms), people can report
seeing them and name them correctly (Deheane et al., 2001). However, when these
stimuli are preceded and followed by a mask, they will not be consciously perceived.
Still, these stimuli are unconsciously processed. If unmasked words are preceded by a
masked prime word, reaction times where significantly faster if the prime was the
same word as the unmasked target word than when it was a different word (Dehaene
et al., 2001). This indicates that semantic information of unconsciously perceived
words has been processed. A lot of processing is thus taking place unconsciously.
- (3) Information needs to be globally available for consciousness
As shown before, information can be processed in an unconscious manner, however,
the global workspace hypothesis poses that this information enters consciousness at
the moment it is globally available. Indeed, studies have shown that unconsciously
perceived stimuli affect the primary cortices of the corresponding modalities but
consciously perceived stimuli on the contrary also affect other areas such as the
frontal and parietal cortex (Baars, 2005). Sleep experiments for instance show that
during unconscious states only the primary auditory cortex is activated after auditory
stimulation (Portas et al., 2000) but not frontal or parietal areas. Furthermore, studies
with people in vegetative state also show that in these patients only the primary
sensory cortices are active after presentation of auditory and tactile stimuli (Laureys
et al., 2000, 2002) similar to healthy controls. However, the healthy controls also
show activation in association cortices whereas the patients did not. Unconscious
processed stimuli therefore solely activate the primary cortices. Indeed, word masking
studies by Dehaene et al. (2001) have indicated that words followed by a mask where
only processed by word recognition areas. However, when the same words were
consciously perceived, other areas such as frontal and parietal regions where also
active. This indicates that people do not become conscious of a stimulus when it is
only locally processed but that they do when this information is also globally
available.
- (4) Minimum neuronal activation and attention attenuation is needed
The global workspace hypothesis claims that information enters consciousness when
it is globally available but only some of the neuronal processing that takes place
enters consciousness. It has been suggested that for content to enter consciousness,
first, there needs to be at least a minimum duration of neuronal activity (predicted 100
ms; Dehaene & Naccache, 2001; Lamme, 2006) and, second, this activity needs to be
amplified by attention.
First, the need for a minimum duration of neural activity will be considered. Indeed,
single unit studies (Rolls and Tovee, 1994) show that masked, and therefore
unconsciously perceived stimuli that were presented for only 20-30 milliseconds,
initiate neuronal firing for as long as the presentation time. However, neurons remain
active far longer than the stimulus presentation time for non-masked and consciously
perceived stimuli, up to 200-300 milliseconds. This extended neural activity has been
attributed to the influence of memory (Rolls and Tovee, 1994). This suggests that a
minimum duration of neural activity duration is needed for awareness.
Second, the need of attentional attenuation of this activity is discussed. For a stimulus
to be consciously experienced it needs to be presented for a minimum duration and
with sufficient clarity (Dehaene & Naccache, 2001). However, this is not enough for
it to be perceived consciously. Directed attention is needed to amplify the neuronal
activity that is encoding the stimulus (Dehaene & Naccache, 2001; Baars, 2005).
Evidence for this can be found in hemispatial neglect patients as well as healthy
participants.
Neglect is thought to result mainly from attentional deficits (due to parietal cortex
lesions) that draw the focus of attention completely to the right side of the visual field.
If two stimuli are presented, one on the left and one on the right, patients will only
report seeing the right stimulus. When, however, only the left stimulus is presented
people do report seeing it. This means that if attention is not available for the left
stimulus (because of the presence of the stimulus on the right), people will not
consciously perceive this stimulus. However, when attention is available for the
stimulus on the left (when there is no stimulus on the right), people will perceive it.
This shows that attention is needed to become consciously aware of a stimulus.
Furthermore, studies in healthy participants show that attention is needed for
conscious processing. One interesting phenomenon that supports this is called
‘inattentional blindness’ (Mack, 2003). In studies investigating this phenomenon,
people are instructed to direct their attention to a particular set of stimuli and to
perform a demanding cognitive task with these stimuli. Most of the participants
however failed to perceive an extra (un-attended) stimulus at the point where they
were looking. This un-attended stimulus had sufficient contrast and duration to be
perceived if the other stimuli where not displayed or if no task directing attention
away from them was involved. Again, this shows that attention is needed to become
consciously aware of a stimulus.
- (5) Where in the brain is the Global Workspace located?
The brain modalities that are attenuated by attention differ according to the situation
in which someone finds itself. There is therefore not one set of areas that is always
involved in the global workspace but different areas may contribute to the workspace
at different points in time. Support for this comes from for instance imaging studies
indicating that different brain areas are active during different experiences. Therefore,
depending on the experience, relevant areas contribute to the in global workspace
(Dehaene & Naccache, 2001). For instance, during the experience of a face, the
fusiform face area (Kanwisher, McDermott & Chun, 1997) will be contributing to the
global workspace whereas during the perception of motion, area V5 (Antal et al.,
2005) will be mobilized.
Furthermore, the PFC and ACC have also been suggested to play a crucial role for
consciousness as indicated by brain imaging studies, even though they are not directly
involved in sensory processing (Dehaene & Naccache, 2001). One interesting study
for example comes from McIntosh, Rajah, and Lobaugh (1999) where they showed
people multiple stimuli that were related to each other, which was only detected by
some of the participants. Interestingly they found that people that became aware of
this relationship showed increased activation in the left PFC. Moreover, in accordance
with the workspace hypothesis, they found long-distance functional connections
appearing at the moment people became conscious of the relationship.
- (6) Which modalities contribute to the Global Workspace?
Although the areas that contribute to the global workspace can differ, only some
modalities have the ability to contribute to the global workspace whereas others have
not. Anatomical studies with monkeys show that the PFC contains long-distance
neurons connecting this area to temporal and parietal areas (important for perceptual
categorization and attention), motor and language related areas (for action upon the
conscious content) and the hippocampus (for memory; Goldman-Rakic, 1988 in;
Dehaene & Naccache, 2001). This indicates that these modalities can contribute to
the global workspace.
Summery
The global workspace theory is one of the first contemporary theories that tried to
address the problem of consciousness scientifically. It poses that information is
processed in a different parts of the brain in an autonomous way. Information that is
processed in a particular brain area can however be shared with other brain areas
through a shared memory space: the global workspace. People stay unconscious of the
information that is not shared, however information that is shared with other brain
areas (for instance areas important for memory and language etc.) via this workspace
will enter consciousness. Long-distance neurons connect different brain areas to this
workspace. Which information is to be globally available is influenced by attention
processes. Although the workspace is not specifically located in one place and
dependents on the set of neurons that contributes to it at a given moment in time, the
PFC and ACC might have an important role in the formation of conscious experience.
Possibly these areas are involved in the formation of a sense of a ‘self’ that is
continuous and consistent over time.
Personal opinion
There is a lot to say in favour of the Global Workspace model. It is a simple concept
and it conceptually has been influential for other theories of consciousness.
Furthermore, there is a lot of evidence that confirms the proposed ideas. I think the
global workspace hypotheses lays down a good cognitive understanding of
consciousness.
However, in my opinion the role of attention has not been fully explored in this
theory. Furthermore, these authors have stated that the global workspace theory
explains consciousness, denying the hard problem of consciousness. A notion on
which I disagree with the authors. Therefore, here follow two personal critiques.
First, a big role is reserved for attention in determining what information enters
consciousness. However, there is no clear specification on how attention works and
how it makes ‘decisions’ as to what enters consciousness. Although this might need a
new theory altogether I think it is an important issue to address since, without an
explanation of how attention works there is still some part of the mechanisms behind
consciousness that is not explained. Furthermore, because no real description of the
attention is given, it remains unclear how attention and consciousness are different
concepts.
Second, Dehaene & Naccache (2001) claim that the question of consciousness can be
solved with cognitive psychology and neuroscience. Furthermore, Baars (2005) states
that:
“the primary functional role of consciousness [is]: to allow a theater architecture to
operate in the brain, in order to integrate, provide access, and coordinate the
functioning of very large numbers of specialized networks that otherwise operate
autonomously.” (Baars, 2005, p47-48).
However, in my opinion ‘the hard problem’ is still a valid problem. That the brain
might work in a global workspace fashion, with information globally available, might
only be because it is efficient. Indeed, computer simulations suggest that effortful
processing of information is accompanied by global information availability (Dehaene,
Kerszberg & Changeux, 1998). This does not mean however that information
processing needs to be accompanied by ‘awareness’ or ‘experience’ and the global
workspace theory therefore does not provide an answer as to why this is the case.
40 Hz Oscillations
Whereas the global workspace hypothesis is a high level cognitive model, Franscis
Crick and Christof Koch (1999) have argued that a low level model on the neural
basis of consciousness is needed. They build upon similar ideas as the global
workspace hypothesis but they focus on how neurons involved in consciousness
communicate. Their focus is mainly on the visual domain as that is well-studied
domain.
Description
Many ideas that were postulated in the global workspace hypothesis are also seen in
the 40 Hz oscillations hypothesis. For instance, Franscis Crick and Christof Koch
(1999), believe that that the brain can be seen as a set of parallel processors and that
during an experience, in multiple areas, neurons relevant to that experience are active.
In other words, that the brain is modular with different brain areas responsible for the
processing of different forms of information. Furthermore, they pose that
consciousness can arise when neural activity is global and unified and that “some
form of rather short-term memory and also […] some form of serial attentional
mechanism” is needed (Crick & Koch, 1990, p263). This ‘form of short-term
memory’ is comparable to the ‘global workspace’ on to which different brain areas are
connected to share information. They further assume that attention mechanisms are
needed to select what content is to get into consciousness. These are all higher order
cognitive notions comparable to the global workspace hypothesis. The 40 Hz theory,
however, emphasizes on what lower level neural mechanism exactly is used to create
this global unity.
There are a couple of questions that need to be answered according to these authors,
with respect to neurons that give rise to conscious experiences and they give possible
explanations for some of them.
The first and most prominent question for which Crick and Koch give an explanation
is: how do neurons responsible for consciousness fire differently (if at all) than other
neurons? Second, what is the location of these neurons in the brain? Third, are they
of any specific neuronal type?
According to these authors, the brain does not have the capacity to store/represent
every experience in a single cell. Although there are cells that have a very specific
tuning to particular stimuli (like the grandmother cells; Gross, 2002), most
experiences arise from the interaction of multiple neurons. However neurons that are
responsible for different parts of one experience are located in distant areas. For
instance in vision, neurons that are responsible for colour processing and neurons that
are responsible for motion processing need to act in unity to result in a unified
experience, even though they are located in different areas. How the brain creates this
unity is what has been called the ‘binding problem’ (Treisman, 1996).
To be able to experience a new ‘scene’, some sort of binding is necessary between
neurons in different areas. This needs to happen fast and dynamically (however, if
learning is involved however, bindings needs to be permanent). Crick and Koch argue
that, with respect to their first question, neurons involved in visual consciousness fire
in synchronization to create unified processing (see support section).
Attention is thought to select relevant neurons throughout the brain that need to fire in
synchronization. This synchronization is established when neurons in different areas
fire in oscillations at the same frequency. The frequencies with which this may occur
may vary between 35-75 Hz but they named this whole range the ‘40 Hz oscillations’
(in this thesis it is sometimes referred to as the gamma band oscillations).
Furthermore, they argue that, with respect to their second question, neurons involved
in consciousness are mainly located in the neocortex and that, with respect to their
third question, these neurons are ‘complex cells’ (as opposed to simple cells; see
support section).
Support
- (1) Neurons that are responsible for consciousness fire in synchrony
To become conscious of for example a visual scene, different features (colour, motion,
etc.) of that scene need to be bound together. The proposed mechanism responsible
for binding different features of an experience is to synchronize the firing of neurons
responsible for the processing of those features. It is believed that this synchronization
mainly happens at oscillations in the gamma range (according to these authors and
others; Crick & Koch, 1990; Koch & Rees, 2002).
Animal research has shown for instance that recordings in different neurons of the
visual cortex responded in an oscillatory fashion when stimulated by moving light
bars in a particular direction (Gray et al., 1989). These oscillations were mainly seen
in the gamma range. Recordings of two different neurons with the same preferred
movement orientation show that when two bars were presented, one in each neurons’
receptive field but moving in opposite direction, these oscillations where not
synchronized. When both bars moved in the same direction, only a weak
synchronization was found. However, when one bar (spanning both receptive fields)
was presented, the synchronization was enhanced remarkably. This suggests that the
processing of an object in different parts of the brain is being unified through
synchronized oscillations.
Moreover, in humans, gamma oscillations have been shown to be related to conscious
percept. Rodriguez et al. (1999) presented people stimuli that either could be
perceived as meaningless shapes or as faces. Long distance gamma oscillation
synchronization became apparent at the moment people start to see a face out of the
shapes.
Another line of research indicates that synchronized oscillatory neuronal firing is
important for consciousness comes from attention studies. Attention is thought to be
responsible for selecting what is to emerge into consciousness (based on the global
workspace hypothesis, see that section for support) and it was found that selective
attention facilitates synchronized neuronal activity in visual and somatosensory areas
(Rees, Kreiman & Koch, 2002). A study with monkeys showed for example that
neurons responding to an attended stimulus exhibited increased gamma oscillations
compared to nearby neurons responding to distracter stimuli (Fries et al. 2001). The
amplification of gamma oscillations with attention suggests that gamma oscillations
are correlated with conscious percept.
Even though the way attention influences consciousness is not fully known, Crick and
Koch (1990) provide some ideas about how exogenous visual attention might work.
There could be a topographical salience map (possibly located in the thalamus)
indicating where important information is located in the visual field. This salience
map would then be a combination of individual feature maps (e.g. where does color
stand out, where is there motion). The salient locations are those locations in a visual
scene where these features are different from the background. A particular location to
attend to is selected in a winner-take-all fashion. Once a location is selected, feedback
pathways (form the salience map to the involved visual neurons) will synchronize the
activity in all visual neurons that correspond to that location.
There might be a couple of reasons why synchronized oscillations would be useful for
unifying the processing of stimuli in different parts of the brain. One of the reasons
might be that synchronized activity means that incoming signals arrive at a neuron at
the same time which will lead to larger effects than random spike activity of the same
amount (Crick and Koch, 1990). Furthermore, according to Fries (2005) two different
brain areas communicate to each other most efficiently if the neurons in those areas
are both in their excitable phase. The excitability of the receiving neuron is known
when the two areas oscillate coherently. Therefore, synchronized oscillations between
different areas that process different features of a stimulus are thought to be used for
communication between these areas.
- (2) Where in the brain are the neurons responsible for consciousness?
Crick and Koch (1990) argue that neurons that are responsible for consciousness are
mainly located in the neocortex and paleocortex because damage to cerebral cortex is
often accompanied by disturbances in consciousness.
- (3) Are these neurons of any particular neuronal type?
Research of the visual cortex showed that only a subset of neurons show to fire in
oscillation. Gray et al. (1990) found that these neurons where mainly complex
neurons as opposed to simple neurons. Simple neurons process information about a
particular small part of the visual scene (think of it as the information of a pixel on a
computer screen, which in itself doesn’t provide much information about the shape or
form that pixel is part of). However, complex neurons integrate information from
different simple cells to derive at concepts of visual forms (by combining the
information in multiple pixels, forms and shapes can be made). Complex cells are
therefore logical candidates to fire in an oscillatory fashion and to be involved in
consciousness.
Summary
The 40 Hz theory was a reaction to cognitive theories of consciousness and was one
of the first theories to explain consciousness on a neural level. It agrees with most of
the ideas of the global workspace but focuses on what neuronal mechanisms are
involved during conscious experience. First, it poses that there are simply not enough
neurons to represent every experience by a different neuron and so neuronal
interactions are needed to build up an experience. Second, different features of an
experience are processed in distant brain areas and this information needs to be bound
together to create a unified experience. Third, the mechanism that is responsible for
this binding is the synchronization of neuronal oscillations (in the gamma range) in
these distant areas. Attention is thought to be responsible for synchronizing these
oscillations. Cells responsible for consciousness are mainly complex neurons located
in the neocortex.
Personal opinion
The 40 Hz theory has tried to give a neuronal explanation for consciousness.
Although the theory indeed has more of a low level focus than the global workspace
theory, more proof for it is needed in my opinion.
Although some studies (as outlined above) suggests that synchronized oscillations do
involve consciousness (at least for visual awareness), there is still debate on this
matter. For instance, disruption of synchrony in oscillations does not seem to impair
perception in primates and therefore a causal role for synchronous oscillations in
consciousness is not yet found (Rees, Kreiman & Koch, 2002).
Moreover, Crick and Koch themselves argue that “Our tentative theory, most of the
elements of which have already been proposed by others, is a program for research
rather than a detailed model”. (Crick & Koch, 1990, p273). Therefore, more research
and evidence is needed.
Furthermore, the question of why gamma oscillations in particular are necessary for
consciousness (and not just oscillations in general) is not addressed for instance.
Besides, although the authors indicate that their ideas on how attention works is still
incomplete, one important questions is left out all together, namely; how does
attention select a winner out of multiple locations to attend to if those locations have
the same level of salience. Furthermore, it only explains stimulus driven (exogenous)
attention but not internally driven (endogenous) attention. Moreover, this idea of how
attention works remains rather high level (cognitive) and a better low level (neuronal)
understanding of it is not provided.
Recurrent processing
Just like Crick and Koch, others have also argued for a neuronal exploration of
consciousness. One of them is Victor Lamme, who goes as far as stating that our
intuitive notions of what consciousness is (i.e., people can report what they are
conscious of), is constraining our understanding of it. In his 2006 paper ‘Towards a
true neural stance on consciousness’, he argues that both behavioural observations
(what people report as to what their conscious content is) as well as neural arguments
should be considered as equally contributing to the scientific understanding of
consciousness.
Description
There are conditions in which consciousness might exist, yet people fail to report it.
To illustrate this, Lamme (2006) refers to the behaviour of split-brain patients (see
also Gazzaniga, 1998). These are patients in which the corpus callosum, the massive
collection of neuronal tracts that connect both hemispheres, is surgically been
disconnected. These patients cannot verbally report on anything that is presented
solely to the left visual field. However, they still show forms of recognition as
indicated by non-verbal reports with the left hand. The reason for this is that left
visual content is processed mostly by the right hemisphere but language is usually
located mostly in the left hemisphere. Since communication between hemispheres is
disrupted, no verbal reports can be made and therefore these patients claim not to be
conscious of the stimulus. However, they can draw with their left hand what they
‘saw’ because the left hand is controlled mostly by the right hemisphere.
Does this mean that people are not consciousness only because there is no language
that can report on it? In considering this question, Lamme argues that neuronal
arguments should account for as much as proof of consciousness as do behavioural
measures, and uses research on ‘recurrent processing’ to illustrate how this can be
done.
What is recurrent processing?
Just like the global workspace hypothesis and the 40 Hz oscillations theory, the
recurrent processing (RP) theory notices that the brain acts in a modular fashion with
different brain areas devoted to different functions. Furthermore, it also states that
information exchange between different brain areas is needed for consciousness,
which is comparable to the previous two discussed theories.
However, according to this theory, this communication should be mutual in order for
consciousness to arise. For instance, when an image hits the retina, this information
travels very rapidly (10 ms per processing level) along the brain hierarchy from lower
brain areas to the higher ones (Lamme, 2006). This information flow is called the
‘feed-forward sweep’. Within 100-150 ms this information has reached all involved
brain areas. This feed forward of information alone, however seems not to be enough
for conscious awareness. Communication back from higher order areas (e.g. v4,
v5/MT) to lower order areas (e.g. v1) seems to be an important factor in the
establishment of consciousness. This form of communication has been called
recurrent processing.
Recurrent processing can happen locally and globally. Local recurrent processing
refers to communication between areas of a particular domain, such as the different
visual areas. Global recurrent processing means that sensory information is also
shared with non-sensory areas such as frontoparietal association areas.
People are thought to be able to report on, for example a visual scene, especially when
not only local information exchange but also global information exchange is
established (in agreement with the global workspace theory).
When only local recurrent processing is present, people do not generally seem to
claim to be aware of a stimulus. However, this processing can still influence some
forms of behaviour as indicated by masked priming studies (see support section).
Lamme argues that we should take this as much as prove for consciousness as reports
of awareness, even though this might seem counterintuitive.
The reason for this is that it might help us understand what the function of
consciousness is and separate it from other cognitive functions.
To show that recurrent processing is involved conscious percept, four questions need
to be addressed. First, is recurrent processing indeed involved during sensory
processing? Second, is it involved during conscious percept? Third, is it also
sufficient for consciousness? Fourth, what are the advantages of a pure neuronal
understanding of consciousness? Most of the support studies have focused on
awareness of visual stimuli.
Support
- (1) Is recurrent processing involved in visual processing?
Recurrent processing means that not only information is traveling from lower to
higher order areas (by means of the feed-forward sweep) but higher order areas are
also communicating back to lower order areas. This form of processing has been
indicated during visual perception and is indicated by neuronal activity after the feedforward sweep is finished.
The feed-forward sweep reaches all higher order areas within 100 ms (Lamme &
Roelfsema, 2000) but neurons in both lower and higher visual areas can stay active
after the feed-forward sweep is finished. Studies have suggested that this might be
due to recurrent processing. Neurons show an initial peak response to stimuli
presented in their receptive field (RF), which reflects the feed-forward sweep (Lamme
& Roelfsema, 2000; Supèr et al. 2001). However, the activity that is seen after the
feed forward sweep is finished has been shown to be influenced by visual information
outside a neurons receptive field (Allman et al, 1985). This is information outside a
neurons receptive field that determines the salience of the stimulus (responsible for
figure-background separation). For instance, if a line is surrounded by similar oriented
lines, it is less salient than when it is surrounded by differently oriented lines. And
this surrounding information (indicating how salience a stimulus is) influences the
later neural activity. For instance, when the same stimulus is placed on two different
backgrounds (different surroundings) in which it is more salient in one compared to
the other, the fast neuronal response will cause equal activity in both conditions, since
the stimulus is the same. However, the slower neuronal response is enhanced if the
stimulus is more salient due to the background. The influence of a neurons activity by
its surrounding is called context modulation. Because this late neural activity is
modulated by information that is processed by other neurons, it must be due to
recurrent processing.
- (2) Is recurrent processing necessary for conscious experiences?
Studies have suggested that feed-forward sweep alone is not sufficient for reportable
consciousness experiences and that recurrent processing is necessary (Lamme, 2006).
Backward masking studies show for instance that the presentation of a mask after a
short delay (<100) to the stimulus can disrupt awareness of the stimulus. The feed
forward sweep still seems to reach all areas (Lamme & Roelfsema, 2000) and it has
therefore been suggested that masking suppresses the recurrent processing.
Furthermore, transcranial magnetic stimulation (TMS) experiments in which visual
areas where disrupted after the feed-forward sweep is finished show impaired
perception (Lamme & Roelfsema, 2000). Therefore the decreased stimulus
awareness has to be due to disrupting the recurrent processing.
Furthermore, studies with monkeys suggest that recurrent processing is necessary for
the monkeys to report the presence of a target stimulus (Supèr et al. 2001). Both
detected targets and un-detected targets produced fast neuronal responses in V1
caused by the feed-forward sweep, showing that the detection failure is not caused
because signals do not the reach visual cortex. However, a slower recurrent
processing response (due to context modulation outside a neurons receptive field)
seems to be enhanced when targets are detected. This suggests that when someone
reports seeing a stimulus, recurrent processing is present.
- (3) Is recurrent processing sufficient for consciousness?
Although recurrent processing is present when people and monkeys report seeing a
stimulus, RP can also be seen when people or monkeys fail to report any awareness of
an object.
Studies in which monkeys had to make a saccade to a target stimulus and ignore other
stimuli (catch trials), have shown for instance that raising the number of catch trialsand so the decision threshold for detecting a target- induces instances of RP although
the monkeys did not make a saccade (Supèr et al. 2001). In other words, the monkeys
did not report to be aware of the stimulus but still RP was seen. Furthermore, human
studies show that when an object is shown but no attention is paid to it (e.g. during
inattentional blindness) and so the object is not reported to be seen, recurrent
processing is present (Lamme, 2006). However, it is present only locally between
visual areas. In this case there is no global recurrent processing (between visual areas
and frontoparietal areas). On the contrary, when global recurrent processing is present,
people usually report to be aware of the object (Lamme, 2006).
This seems to falsify the statement that recurrent processing is sufficient for conscious
experience but Lamme argues that this is because we only take consciousness as that
what people/monkeys report to be aware of. This however relies on other modalities,
such as language, attention and working memory.
For instance, in the monkey study it could be that the monkey did in fact see the target
but because of the many catch trials decided not to report it (Lamme, 2006). In the
case of intentional blindness, people could be forgetting the stimuli that where not
attended and therefore not report them although they might have been ‘conscious’ of
them at the moment of perception (Lamme, 2006).
Lamme (2006), therefore suggests that recurrent processing should be taken as an
indication of consciousness.
- (4) What are the advantages of a neural definition of consciousness?
It might seem counter intuitive to state that people can be conscious a stimulus even if
they don’t report on it. However there might be some advantages for doing so.
Because of the reliance of behavioral report on other modalities, Lamme argues that
our understanding of consciousness cannot be complete. He argues that if we attribute
neural evidence for consciousness (such as the existence of RP) equal weight as
behavioral reports, we might gain some insights.
The first advantage is that we can describe consciousness without the use of other
cognitive functions such as attention, language and memory etcetera, which are
needed for reportability. This enables one to study consciousness as a concept on its
own (Lamme, 2006), which is very useful in cases where all forms of communication
are lost (such as locked-in syndrome) or not available in the first place (animals).
Then a neural concept of consciousness is the only way to reach consensus on
whether or not consciousness is present.
Although it seems problematic that the neural concept of consciousness does not
always accompany reportability, this can still be explained very well. In the cases
where there is no reportablity (inattentional blindness, split brain), cognitive functions
other than consciousness might be responsible for this (e.g., memory, language). For
instance, because someone forgot about a perception or because, in split brain patients,
language areas cannot be reached. This means that consciousness can be seen as
separate from other functions without losing an explanation for why sometimes
people cannot report being aware of something.
The second advantage is that a neural definition for consciousness might help us
understanding what the direct function of consciousness is. Because during recurrent
processing, pre-and postsynaptic neurons are active at the same time, the Hebb rule is
met. This rule states that the more two neurons fire simultaneously, the more they
form a solid connection, which is important for learning. Therefore recurrent
processing might be a basis for learning (Lamme, 2006). Stimuli that only produce a
feed-forward sweep do not produce long lasting changes to the brain. Consciousness
can thus be seen as fundamental for learning.
Summary
The recurrent processing theory focuses, like the 40 Hz theory, on the neural
mechanisms of consciousness. However it deviates from an intuitive understanding of
consciousness that is based on whether or not people can report to be conscious, and
tries to explain consciousness fully by the presence of particular neural activity. It
poses that consciousness can be measured by whether or not recurrent processing is
present. Furthermore, if this is the case only locally, people are likely not to report on
an experience whereas if recurrent processing is also seen globally, people are likely
to report the experience. The advantage of seeing consciousness fully in neural terms
is that it can be studied in cases where no reportabilty exists. Furthermore, it might
give some insights in the purpose of conscious.
Personal opinion
Taking a neural concept of consciousness without linking it to reportabilty feels
counter intuitive, however there are some clear advantages in doing so. It will give
some insights in how consciousness is neuraly different from other functions and
what the function of consciousness is, which makes the theory valuable in my
opinion.
There are however some personal critiques on how recurrent processing is measured
and how consciousness as a modality is defined.
First, there are studies that support the idea that recurrent processes are involved in
consciousness. However, these studies involve assumptions or indirect measures of
RP (e.g. context modulation). Studies measuring recurrent processing directly would
therefore be a good addition, for instance by simultaneous single cell recordings of
two cells.
Second, whether consciousness can be completely separated from other function such
as language, attention and memory can be debated. The two types of consciousness,
with and without reportability, are neuraly different and so using the same
terminology (as Lamme does) might not be the most efficient in my opinion. Ned
Block (2005) may give a somewhat clearer distinction. He claims, as mentioned
before, that there are different types of consciousness: phenomenal- and accessconsciousness (see introduction; “What is consciousness?”). Phenomenal
consciousness, he links to Lammes’ recurrent processing and access consciousness to
the global workspace. This distinction might do more justice to the different forms of
consciousness.
Third, Lamme seems to equate consciousness to some extent to learning but is not
clear whether he thinks these processes are the same or still different. He argues that
to use a term such as consciousness it should be separated from other functions.
However, when saying that learning relies on consciousness (in both it’s reportable
and unreportable form) it is not clear what makes the distinction between learning as a
function and consciousness as a function.
Conclusion/Summary
In this thesis I gave an overview of three influential theories on the cognitive and
neural correlates of consciousness. First, the Global Workspace theory was explained
which states that consciousness arises if information is not only locally but also
globally available. This was an early cognitive theory that tried to find a scientific
explanation for the problem of consciousness and was influential for neural theories
of consciousness, two of which have been discussed in this thesis. Second, the 40 Hz
hypothesis was considered which was developed to move from the cognitive theories
of consciousness to a more neural understanding. It builds upon the same ideas as the
global workspace hypothesis but focuses on the neural mechanisms involved during
conscious awareness. It states that different parts in the brain process different
features of, for instance a visual scene, and these features are bound together if one
becomes conscious of the scene. The neural mechanism for this binding is through to
be the synchronization of gamma oscillations. Finally, the Recurrent Processing
hypothesis was considered, which also gives a neural explanation of consciousness.
However, it goes as far to state that consciousness should not be considered as only
that what people report on but also as what kind of neural activity is present.
According to this theory, recurrent processing can be viewed as a sign of
consciousness even though people might not report to be conscious.
Although these theories focus on different aspects they all share some common
notions. All theories originate from the cognitive perspective that the brain works to
some extent in a modular fashion, meaning that different functions are processed in
different parts of the brain. Also, all theories state that when people report to become
conscious, brain activity is seen on a global scale with different areas communicating
with each other. The 40 Hz theory however, focuses on how different areas
communicate during conscious content processing (through oscillations) whereas the
recurrent processing theory focuses on which areas communicate (local recurrent
interactions or global recurrent interactions) and in what direction this communication
goes.
As has been shown by the theories discussed in this thesis, a scientific understanding
of consciousness was arrived through cognitive theories. Based on this, neural
theories where developed, which focused on how neurons interact with each other and
which neurons/areas interact with each other during conscious and unconscious
processing. Furthermore, our perception of consciousness, its role and purpose, might
even change because of these theories.
Because of the many debates that are still going on about consciousness, this thesis
was not meant to give an answer to the questions on consciousness. However, it
discussed some of the ideas and theories concerning consciousness because these
have had (in my opinion) to little attention in the education of neuroscience and
cognitive psychology.
There remains a lot of research and debate as a lot of questions are still not clearly
answered. Consensus on the definition and purpose of consciousness is not yet settled
and the differences between consciousness, attention and awareness etcetera are not
yet clear. This makes consciousness still a fascinating subject of research.
Acknowledgement
Henk Aarts for the first directional advice, Albert Postma for finding the right people
to contact, Lenny Ramsey and Ravi Chacko for there comments and advice, my
fellow students for the discussions on the topic, with in particular Joost Haarsma and
Victor Manuel Saenger, Jan Brascamp for reviewing and Serge Dumoulin for his
advice and examination.
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