Week 10
Brain Lateralization, Plasticity, Consciousness, Sleep and Attention
This week, we’ll explore how the two cerebral hemispheres are different in their
function, and what studies of people with two disconnected hemispheres tell us
about the complementary function of the two sides of the brain. The four different
lobes of the brain appear to serve different aspects of cognition and behaviour –
we’ll outline their different functions. Then, we’ll introduce the idea of plasticity –
that how the brain supports cognition and behaviour can change with experience.
Finally, we’ll explore the topic of consciousness and alterations in consciousness
including sleep and disorders of consciousness such as coma and persistent
vegetative state.
Lateralization of Function.
Generally, we now agree that functions are localized in the brain, especially given
the evidence from neuroimaging studies. Different brain areas appear to do
different things. Furthermore, the left and right sides of the cortex (the two
hemispheres) appear to be somewhat specialized for certain functions. This is
referred to as "lateralization of function". The two sides of the brain appear to be
relatively symmetrical; but, actually, they are somewhat different both in structure
and in function. Language seems to be more dependent on the left hemisphere than
on the right –
it is lateralized to the left.
We
know this, in part, from the
study of patients with unilateral
brain damage
(e.g., Pierre Paul
Broca's patient), as well as from
imaging studies. In
addition,
motor and sensory function are
crossed; meaning that the right
motor
cortex controls the left
side of the body and vice versa;
similarly, the sense of
touch
from the right side of the body is
transmitted to the left
somatosensory
cortex.
Information from the visual
fields of the two eyes is also
crossed.
Wherever you look,
there is a fixation point – a point
your eyes are focusing on
(the
point at which the four coloured areas in the diagram above intersect). Visual
information in the LEFT half of the visual field relative to the fixation point (i.e., red
and purple) goes to right visual cortex. Visual information in the RIGHT half of the
visual field relative to that fixation point (i.e., yellow and green) goes to the LEFT
visual cortex.
Brain Structure and Experience: Plasticity
Experience (learning) can change brain structures – this is a major area of
psychological enquiry, and falls under the general name of ‘brain plasticity’. Classic
experiments conducted in the middle of the 20th century demonstrated that the
cortex of rats that were reared in enriched environments were thicker and had
more developed synaptic structures and function than the brains of rats reared in
impoverished environments. It was thought that adult brains were relatively fixed
in their functional organization, but that view is now changing – a growing body of
evidence suggests that even adult brains are quite plastic. A
neuropsychologist/neurologist named V.S. Ramachandran has worked extensively
with patients with phantom limbs – after amputation, people often feel that the
removed limb is still there, and these can often be painful (see page 160 of your
text). Ramachandran has devised a clever way of treating the phantom pain, and
recounts interesting stories of the experiences of people with phantom limbs that
illustrate neural reorganization and plasticity. Psychologists have also been working to discover the neural mechanisms that
govern learning and memory. In the 1940s, a major figure in neuroscience, Donald
Hebb, from McGill University, developed a theory about how learning is
accomplished at the cellular level in the brain. He proposed that during learning, a
selective number of existing synapses between cells in the relevant part of the brain
are fired each time the animal responds to the task. The repeated pattern of firing
strengthens these synapses, forming a neural circuit that did not previously exist
(Hebb called this circuit a cell assembly; the basic idea is captured in the phrase
“neurons that fire together, wire together”). Hebb’s postulated such such changes in
synaptic connections in 1949, but it wasn’t until the 1970s that the physiological
mechanism of the Hebbian synapse – called long term potentiation (LTP) -- was
found by Bliss and others. This work is discussed in the green box on (p. 215-216)
Consciousness
Have you ever stopped and wondered whether animals, maybe a pet dog, have the
sort of vivid, inner life that you have? That they don’t just behave, but experience?
Perhaps you have wondered what sorts of experiences they may have, or might not
have. Maybe you have even gone so far as to wonder whether the other people in
your life have such personal experiences, or are simply automata – moving and
speaking, but without the feelings or thoughts that you have? If you’ve
contemplated these sorts of things before, then you’ve wrestled with one of the
problems of consciousness. And you’re certainly not alone – questions about
consciousness have plagued the minds of philosophers for thousands of years. One
might think that this means we shouldn’t hold our breath for an understanding of
consciousness, but as we will see, psychological research has produced a number of
interesting findings and tools for chipping away at the mysteries of consciousness.
What is Consciousness?
Let’s start with the biggest question of them all: what is consciousness? There are an
abundance of beliefs on the matter(p. 267), in fact too many to list! First, let’s get a
better idea of what we mean when we talk about “consciousness”. Often the word is
used as a replacement for “awareness”. We often talk about being aware of facts,
and “raising consciousness” about issues, like affordable housing, fair-trade
chocolate, and the national debt. This is not what we mean when we refer to
consciousness. Instead, we use the word consciousness to refer to subjective
experience, or the felt aspect of some mental state. We can talk about someone
being conscious, that is, having the capacity to experience, or we can talk about the
contents of consciousness – the specific character of an experience, often referred
to as qualia (more on this later). For now though, we will discuss beliefs on what
consciousness is – what allows it to exist.
Philosophical Positions
One possibility is that consciousness exists because it is a substance, and that this
substance is separate from, and has different properties than, physical matter. We
refer to this view as Dualism (p. 11 of your textbook), which was famously
advocated by the philosopher René Descartes. Descartes meditated on the problem
of consciousness, and concluded that it must depend on a substance distinct from
physical matter, since it has properties that we cannot ascribe to matter (it doesn’t
take up space, weigh anything, and so forth). We see similarities to this idea in
religions that speak of a soul, or an immaterial, immortal aspect of a person. A completely different view is that there is only one kind of substance in the
universe, matter, and that consciousness is a physical phenomenon. This view is
often called Materialism (p. 12 of your textbook), but you may encounter it also as
Physicalism, or even Monism (the view that only one kind of substance exists).
There are differing opinions about exactly what the physical nature of
consciousness is (whether it is specific brain states or the type of computation
being performed by a system). Only the materialist interpretation lends itself
to
the scientific study of consciousness. One can only ask testable questions about
what consciousness is if one assumes that its basis is ultimately material, otherwise
one can never take measurements of anything that could support or disconfirm a
hypothesis. So for any scientific findings we will discuss, they will be interpreted
under the assumption that the ultimate basis of consciousness is physical.
Psychological Investigations of Consciousness
Operational definitions
In order to ask scientific questions about consciousness, it is important to have an
operational definition of consciousness. As you might recall from an earlier lesson,
an operational definition is a restricted definition of a broad idea that allows us to
generate testable hypotheses about it – to measure it in some way. If our definition
of consciousness is “having subjective experiences” then it is very difficult to
propose testable research questions. We are only able to measure observable,
physical events, and so it is important to have a definition of consciousness that
allows us to specify what objectively observable events we should see if someone or
something is conscious. A common operational definition of consciousness is that a
person is conscious when they are able to report their own mental state. Using this
operational definition, we can then establish measures or indicators of
consciousness. Common measures of consciousness include simple verbal report
(e.g., reporting that one was aware of a stimulus), accuracy (e.g., being able to
accurately detect a stimulus, which we can measure with signal detection theory (p.
132-134 in your text)), and confidence measures (e.g., assigning a number, from 1-7,
indicating how confident one is in a decision, such as a detection response).
Working memory and consciousness
Several prominent theories of consciousness attribute a central role to working
memory. The global workspace theory of consciousness, proposed by Bernard
Baars, proposes that working memory is a global workspace, which serves to
integrate, access and coordinate the functioning of large numbers of specialized
brain circuits that otherwise operate independently. The information that is present
in the global workspace is the information that we consciously experience.
Perceptual representations and memories compete for representation in this global
workspace, and the information that wins the competition is then accessible by
processes including attention, memory, high- level decision making, and verbal
report. According to this view, working memory can be thought of as the site of
consciousness, the contents of which at any given moment correspond to the
conscious experience that you are having. You will learn more about working
memory in a subsequent lesson.
Neural correlates of consciousness
In addition to developing psychological and neurological accounts of consciousness,
many researchers are interested in uncovering the so-called neural correlates of consciousness. This research studies how areas of the brain are involved in
producing certain kinds of conscious experience.
Researchers attempt to discover which neuronal circuits in the brain are involved in
producing conscious experiences of different types. For example, we may ask which
areas of the brain must be active in order to have a conscious visual experience of an
object. Your textbook discusses blindsight (p. 268), in which individuals with
damage to the visual cortex or visual pathways report no visual experiences, despite
being able to still accurately perform some tasks involving vision.
The phenomenon of blindsight implies that the damaged visual cortex is somehow
necessary for conscious visual perception. Although studies of people with
blindsight do not directly ask questions about the conditions of conscious
experience, they do provide insight into how consciousness works in the brain, and
therefore are informative for a general theory of consciousness.
What Are We Conscious Of?
If we can establish what allows consciousness to exist, that is one step, but what of
the contents of one’s conscious experience? We can know what it is that someone is
conscious of (say, a colorful sunset), but how can we know what the nature of its
conscious experience is? In “What is it like to be a bat?” Thomas Nagel argues that
we can never know what another’s conscious experience actually is like.
Psychologists take a more optimistic stance when tackling this problem, but we
should be aware of the difficulties in this research. Philosophers and scientists use
the word qualia (singular: quale) to refer to the quality of a given perceptual
experience – the colour magenta; the feeling of cold water on your hand; the taste of
chocolate. These are all intensely personal, subjective experiences that you can
discuss with others, but whether you both mean exactly the same perceptual quality
when you use the term ‘magenta’ is impossible to know.
The classic example of the problems associated with qualia comes from the Mary’s
Room thought experiment:
o Imagine that a woman named Mary has been raised her whole life in a room
where everything is black and white – there is no colour. Mary has spent her
whole life learning psychology, physiology, optics, and all other topics
relevant to understanding colour vision. We can even imagine that this takes
place in a time where we have a complete understanding of how colour
vision occurs. The question is this: when Mary finally leaves the room and
sees a red object for the first time, will she be able to know that it is red? Can
her understanding - of the physical basis of conscious experience - allow her
to predict what having a new conscious experience will be like despite her
never having had it? What this question really gets at is whether we can, in
principle, have a complete science of consciousness – one that explains and
predicts the nature of our experiences. The answer to this question is
anything but clear, and I encourage you to give it some thought yourself. In
the meantime though, let’s discuss what psychological research has been
able to reveal about the nature of our conscious experience.
Who Is Conscious?
Thinking about what produces or allows conscious experiences to occur is
important if we want to be able to know who is conscious, or when someone may be
conscious. This certainly has ethical implications in so far as the treatment of non-
human animals is concerned: do animals truly experience pain, or merely act like
they do? It also has ethical implications for humans. Take for example, a coma patient – how can we know whether they are conscious, having mental experiences,
or whether they are without consciousness altogether? We will discuss this example
more later, but it does exemplify some of the important applied reasons for which a
study of consciousness is important.
Split Brain Studies
As you know, your brain has two hemispheres, the left and right hemispheres.
Information from the cerebral cortex of one hemisphere reaches the other
hemisphere via the corpus callosum (highlighted in
the picture to the left), a bundle
of more than 200
million axons that connect cortical regions in the two
hemispheres. In some special
cases, such as severe
epilepsy, this connection is severed, leaving the
two
hemispheres unable to communicate. The operation
ensures that the epilepsy
cannot spread across to
the opposite hemisphere, and thereby helps to
protect a
large amount of the brain from the damage
that can result from epileptic seizures.
The result of
this operation is an individual who remains largely
normal and
functional, but who exhibits some
incredibly interesting characteristics that
have
implications for who has consciousness, and how.
Roger Sperry won the
Nobel Prize in 1981 (together with Hubel and Wiesel) for his studies of people in
whom the corpus callosum had been severed. In his words (1974),
o “each hemisphere [is] indeed a conscious system in its own right, perceiving,
thinking, remembering, reasoning, willing, and emoting, all at a
characteristically human level, and . . . both the left and the right hemisphere
may be conscious simultaneously in different, even in mutually conflicting,
mental experiences that run along in parallel”.
When studying patients in whom the corpus callosum is severed, as Sperry notes, it
is difficult to escape the conclusion that inside the head of the patient there are two
conscious individuals, with their own experiences and intentions. This is certainly
one of the eeriest findings in the history of psychology, and has become a hot topic
in philosophy of mind as well. Although the philosophical implications of such
research are outside the score of this course, I will introduce you to two of the
functional consequences of severing the corpus callosum and suggest a couple of
possible interpretations of these data.
Perception and Performance in Split Brain Patients
To study how stimuli are perceived, or not perceived, by the two hemispheres of a
split-brain patient, one must control the way that stimuli are presented. As you
know, visual information from the left side of space, the left visual hemifield, is
relayed to the right visual cortex, and the converse is true for the right side of space.
Researchers exploit this in split-brain patients by having patients fixate on a point in
space, and then briefly present a stimulus to either the right brain (stimulus in the
left visual field) or to the left brain (stimulus in the right visual field). Using this
methodology, one discovers remarkable things about the split brain patient’s
consciousness.
o For example, if an image, let’s say of a triangle, is presented in the right
visual field of a split-brain patient, the patient will be able to identify what
he or she has seen, by stating that a triangle was seen. Nothing odd yet. If the
same stimulus is presented in the left visual field, the patient will verbally
report having seen nothing. This is not, on its own, terribly surprising, since
the left visual field is represented only in the right visual cortex, and language is largely controlled by the left hemisphere (you will learn more
about language in a subsequent lesson). However, if you give the patient’s
left hand a pen and again pose the question, although the patient will
verbally report having seen nothing, their left hand will draw a triangle. It is
important that the left hand be used in this case because, as you have
learned, motor commands to the left side of your body are initiated from the
motor cortex in the right hemisphere – the hemisphere to which the stimulus
was presented. So here we see one person, one body, two hemispheres,
producing utterly conflicting responses.
Even more impressive is the case in which a split-brain patient is asked to arrange
blocks into a particular spatial configuration. In this example, a patient is provided
with a coloured blocks, and must arrange them into a spatial configuration that
matches a picture that they are shown – similar to completing a jigsaw puzzle.
Again, researchers control whether the left or right hand (i.e., the right or left
hemisphere) is used to complete the task. When the left hand is used to complete
the puzzle, patients have little difficulty in solving it, corroborating research that
shows that the right hemisphere is more adept in processing spatial patterns. When
the right hand is used, and the left hemisphere is now responsible for producing the
responses, performance suffers – patients now have considerable difficulty
arranging the blocks correctly. This finding is interesting, and very informative for
theories of hemispheric specialization, but it may not tell us very much about
consciousness. However, a startling finding emerges when both hands are allowed
to solve the puzzle. In this case, the hands actually compete, that is, while the right
hand is placing the blocks, the left hand will interfere! It appears that the two
hemispheres have different ideas about how to solve the puzzle, and we can actually
see these competing attempts manifested in the patient’s actions!
Although not all types of behaviours exhibit this special duality in split-brain
patients, the results of these studies must be considered when developing a theory
of consciousness. Your text mentions one interpretation of these findings: that
language lateralization plays a crucial role in consciousness – if the corpus callosum
is severed, the stimuli presented on the left side of space can never be consciously
perceived. This is how Michael Gazzaniga (who worked with Roger Sperry)
interprets the split-brain findings. Another possible interpretation is that there are
two consciousnesses in one individual - one that is mute, and one that is not. Which
view you favour may depend on your particular beliefs about the bases of
consciousness, and I leave it up to you to draw your own conclusions about what
split-brain research means for our understanding of who is conscious.
When Are We Conscious?
One last question related to consciousness is concerned with the conditions of
consciousness. This is directly related to the how of consciousness, in that we ask
what is necessary for conscious experience to occur. Often people wonder whether
they will be conscious after they have died, or whether they were conscious before
they were born. We will not discuss such issues in this course, but we can discuss a
closely related question, the question of how our states of consciousness may
change while we are alive. Carlson and Heth discuss two types of altered
consciousness in detail: hypnosis and sleep. Here I provide some more information
about sleep and also talk about the effects of coma on consciousness.
Sleep Although it is something we are very familiar with, sleep is a strange behaviour.
Every day, we shut our eyes and become largely immobile for six to eight hours, or
more. This behaviour is not unique to humans either. Mammals, birds, reptiles, and
even some fish and insects exhibit the same behaviour. Despite its ubiquity, the
purpose of sleep is not well understood.
Why do we (and other animals) sleep?
A number of theories exist to explain sleep. The first, discussed by Carlson & Heth on
p. 287-288, is the restoration theory of sleep. As the name implies, the theory
claims that the purpose of sleep is to allow the body and mind time to recover from
the stresses they endure during waking hours. The evidence for this theory is not as
compelling as one would hope, as athletic performance does not suffer greatly from
sleep deprivation. There is some evidence for a relation between sleep and physical
exertion. Shapiro et. al. demonstrated that in the nights following running a
marathon, participants’ overall sleep duration increased, as did the total amount of
time spent in deep sleep (a stage we will discuss more shortly). Even if sleep is not
crucial for keeping our bodies functioning, it may be important for keeping our
minds sharp. Indeed, performance of mentally demanding tasks tends to suffer
when we are deprived of sleep. When sleep deprived, we are much poorer at
maintaining vigilance, have poorer working memory function, and are slower at
processing information.
An alternative hypothesis for why sleep occurs is the preservation and protection
theory of sleep, which highlights the more indirect benefits that sleep might
provide. What all animals have in common is the need for energy, or food, and the
need to avoid harm. Sleep can be seen as evolutionarily adaptive in so far as it
allows an organism to be more conservative with the energy it collects, and reduces
the exposure of the organism to harm. In this sense, sleep may contribute to the
survival of an organism by reducing the amount of energy used – the number of
calories burned – during times when there is no need to use that energy. In theory,
an organism should spend only as much energy as is necessary to acquire more
energy and perform other necessary actions (such as reproduction). It should save
energy otherwise.
Supporting this idea, the patterns of sleep in different animals vary with animals’
feeding habits. Animals that consume large amounts of low-calorie food (such as
grass) tend to sleep less, as more time is required for them to consume a sufficient
amount of energy. Cows and horses, for example, only sleep between two and four
hours each day. On the other end of the spectrum are predatory animals such as
lions and tigers. Because it takes a great deal of energy to hunt and the prey
provides a relatively high number of calories, these predators sleep between one-
half and two-thirds of the day.
This concept may also explain why some animals sleep during the night and others
during the day. Since humans rely largely on vision for hunting and foraging, it is
best to do so when the sun is out and to conserve calories when the sun is down. For
mammals that are able to hunt or forage at night (such as bats, which rely primarily
rely on audition for perception), it is best to rest during the day and to save their
energy for finding food when it is more advantageous.
This brings us to protection, the second aspect of the theory. Sleeping at certain
times may be doubly advantageous for an organism in so far as it allows them to be
less exposed to possible predators. For example, it is safer for bats to hunt at night because many potential predators hunt primarily during the day. In this way, sleep
during particular times of day would confer the dual benefit of minimizing wasted
energy and maximizing safety.
Finally, many lines of converging evidence – drawn from many different lines of
research involving everything from molecular biology to cognitive psychology -
suggest that, during sleep, there is reprocessing of memories, and this is very
important for how memories are formed, stored and retrieved. Evidence suggests
that sleep is important for the consolidation of transient, short-term memories into
more stable long-term ones. Consolidation involves a variety of processes, and
seems to involve the ‘reactivation’ of the original firing patterns during sleep and
rest. Activity in the hippocampus during sleep seems to play an important role in
controlling the reactivation of memory traces during consolidation. Recordings from
the hippocampi of rats show that cells that were involved in learning a maze during
training are repeatedly reactivated during slow-wave sleep, supporting the idea that
sleep helps to consolidate memories. In humans, fMRI studies show that the
hippocampus is similarly reactivated during slow-wave sleep after having learned to
navigate through a virtual town. The extent of hippocampal activation also
correlates with recall in post-testing; the more active a subject’s hippocampus was
during sleep, the better they were able to perform in later memory tests. The
hippocampus also shows elevated activity during REM sleep when participants had
spent time learning to respond quickly to dots that appeared in a complex sequence.
These theories are not mutually exclusive. Sleep is a behaviour that has evolved
across a wide range of species in the animal kingdom, and it may have evolved for
multiple reasons. That is, it may be
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