Download Topic 14 - Center for Complex Systems and Brain Sciences

Cognitive Neuroscience
Consciousness
Consciousness is central to all of cognitive
neuroscience. Yet, it is the most difficult phenomenon
to define, and also to study. As we saw at the
beginning of the course, the conscious mind is at the
core of the "hard problem" because of the subjectivity
it entails.
However, we can study the phenomenon of
consciousness objectively as another cognitive
function. We have seen consciousness enter the
descriptions of cognitive function at many places in
the study of cognitive neuroscience, often cloaked as
volition. Some cognitive functions, such as working
memory, may be conscious or unconscious. Others,
such as perceptual attention, seem to always be
conscious. But we have not yet discussed
consciousness itself as a cognitive function.
Here, we will examine the neuroscientific evidence for
the neural basis of consciousness. We will not
consider what is the essential difference between
consciousness and unconsciousness.
Different types of study used in the investigation of
consciousness in the brain
1. Anesthetized and sleep states
2. Alternating perception
a. Binocular rivalry
b. Multistable perception
3. Blindsight and hemispatial neglect
4. Subliminal perception
5. Study of vegetative state patients
6. Computer simulations
Guiding premises about consciousness
1. It is generated by the brain
2. There is no single "center" for consciousness in the
brain - it is widely distributed - halothane anesthesia
studies show that multiple brain areas are active
when a person is awake, and they all uniformly go
silent under anesthesia
3. Consciousness varies quantitatively - the level of
consciousness in animals or the human fetus, for
example, may differ
4. It varies qualitatively - auditory and visual
consciousness may differ, for example
2 types of consciousness
Damasio distinguishes core consciousness (basic
awareness) from extended consciousness (having
higher organizational complexity).
Anatomical orientation: consciousness primarily
depends on three brain structures
1. brainstem
2. thalamus
3. cerebral cortex
1. Brainstem
The brainstem “arouses” the cortex, and thereby
modulate wakefulness.
The reticular activating system (RAS) lies in the core
of the brainstem and sends projections to the cortex.
Two main pathways project from the RAS to the
cortex:
a) dorsal pathway thru the thalamic intralaminar
nucleus (ILN)
b) ventral pathway thru the hypothalamus & basal
forebrain
The RAS is involved in:
a) arousal
b) sleep-wake cycle regulation
c) attention
The permanent lack of core consciousness that
results from damage to the RAS is called coma.
The locus coeruleus (LC) is located in the pons. In it
are the soma of neurons that project to the cortex &
other brain structures, where the axonal terminals
release the neurotransmitter norepinephrine (NE).
The projections follow a similar pathway to the dorsal
RAS pathway.
Activation of, and subsequent NE release in, the
cortex by the LC prevents sleep.
The brainstem receives input from a large number of
sources that inform it about the state of the internal
and external environments of the organism. The RAS
and LC can change the state of arousal of the
organism accordingly.
2. Thalamus
Bilateral lesioning of the ILN, or blocking brainstem
inputs to ILN, produces permanent loss of core
consciousness.
3. Cerebral cortex
The cortex maintains wakefulness and contributes to
selective attention.
Damage to cortex results in loss of specific abilities à
cortex contributes to extended consciousness.
Conscious versus unconscious processing and
access to information
Most cognitive processing is unconscious. We are
only conscious of the content of the mind, not what
generates that content.
The question of whether consciousness is required for
cognitive processing has been investigated in patients
with blindsight.
Blindsight is the phenomenon in which a person is
blind in part of their visual field due to a lesion of the
primary visual cortex, and yet the person
demonstrates the use of visual information.
Blindsight suggests that visual perception can occur
in the absence of consciousness.
Evidence suggests that the damaged primary
pathway (retinaàLGNàV1) is involved in blindsight.
Thus, there does not seem to be any reason to invoke
alternative pathways, such as thru the superior
colliculus to extrastriate cortex.
Patients with hemispatial neglect also demonstrate
above-chance performance based on visual input but
without visual awareness.
For example, neglect patients with right hemisphere
damage are unable to name stimuli presented in the
left visual field, but they can make judgments about
whether such stimuli are the same as or different from
stimuli in the right visual field. In fact, they deny
having seen anything in the neglected left visual field.
Conclusion: neglect patients (with damaged parietal
cortex and spared visual cortex) can make perceptual
judgments outside of conscious awareness.
The extent of subconscious processing
Right-sided neglect patients can make perceptual
judgments by matching stimuli presented in the two
visual hemifields even though they deny seeing
anything in the left hemifield. They respond faster
when the stimuli are related.
In the intact, healthy brain, unconscious processing is
shown by word association (word priming) tasks.
The extent and type of information processed
unconsciously is studied with the technique of
subliminal perception - judgment may be biased by
unconscious elements of a stimulus presented very
quickly. For example, in a cross-form (picture-word) priming
paradigm, a picture can unconsciously affect word stem
completion, even though the picture is presented very briefly
and followed by a mask.
Gaining access to consciousness
It has been proposed that optimal information
processing requires that only currently relevant
information be allowed access to consciousness.
Treisman’s attentional binding hypothesis posits that
the visual attentional system binds elementary
sensory features together into percepts at attended
locations. Each point in the visual field is processed
for these elementary features, such as color, shape,
and motion, outside of conscious awareness. Thus,
attention is needed to bind the results of unconscious
processes into conscious percepts.
The scaffolding-to-storage hypothesis of Raichle &
Petersen proposes a conscious-to-unconscious
transition in learning. First, conscious processing is
required for practice while learning complex skills or
memories (erecting the scaffolding). Then, after
learning, the same skills are performed, or memories
retrieved, unconsciously (removing the scaffolding in
favor of more permanent “storage” structures).
Neurons, neuronal groups, and conscious
experience
Newsome found that a monkey’s psychophysical
performance capacity could be predicted by the
neuronal response pattern of single neurons in area
MT (motion-processing area of visual cortex). He also
discovered that careful alteration of the response rate
of those same neurons by microstimulation could
change the animals performance on a perceptual task
toward making the correct decision. The implication is
that conscious processing can be altered by changing
neuronal activity.
Libet studied neural timing factors in conscious and
unconscious. His most famous result is that the cortex
begins to change its action-related activity as many as
300 ms before the conscious intention to act. These
experiments are the basis for his backward referral
hypothesis, which claims that conscious awareness of
an event is delayed by around half a second, and that
this awareness is referred back in time to the
beginning of the event.