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Transcript
The central nervous system (CNS) consists of the
brain and the spinal cord, immersed in
cerebrospinal fluid (CSF).
Weighing about 3 pounds (1.4 kilograms), the
brain consists of three main structures: the
cerebrum, the cerebellum, and the brainstem.
See http://www.brainexplorer.org
Cerebrum: divided into two hemispheres (left and
right), each consists of four lobes (frontal [at the
front], parietal [in the middle], occipital [at the back],
and temporal [at the bottom]). The outer layer of the
brain is known as the cerebral cortex or the ‘grey
matter.’ It covers the nuclei deep within the cerebral
hemisphere known as the ‘white matter’.
Cerebellum: responsible for psychomotor
function, the cerebellum co-ordinates
sensory input from the inner ear and the
muscles to provide accurate control of
position and movement.
Brainstem: found at the base of the brain,
it forms the link between the cerebral
cortex, white matter and the spinal cord.
The brainstem contributes to the control of
breathing, sleep and circulation.
Neurons use their highly specialised
structure to both send and receive
signals. Individual neurons receive
information from thousands of other
neurons, and in turn send information to
thousands more. Information is passed
from one neuron to another via
neurotransmission. This is an indirect
process that takes place in the area
between the nerve ending (nerve
terminal) and the next cell body. This
area is called the synaptic cleft or
synapse.
CT (roentgen-ray computed tomography): A beam of x-rays is shot straight
through the brain. As it comes out the other side, the beam is blunted slightly
because it has hit dense living tissues on the way through. Blunting or
“attenuation” of the x-ray comes from the density of the tissue encountered
along the way. Very dense tissue like bone blocks lots of x-rays; grey matter
blocks some and fluid even less. X-ray detectors positioned around the
circumference of the scanner collect attenuation readings from multiple angles.
A computerized algorithm reconstructs an image of each slice.
See http://www.med.harvard.edu/AANLIB/hms1.html
MRI (magnetic resonance imaging): When protons (here brain protons) are placed
in a magnetic field, they become capable of receiving and then transmitting
electromagnetic energy. The strength of the transmitted energy is proportional to the
number of protons in the tissue. Signal strength is modified by properties of each
proton’s microenvironment, such as its mobility and the local homogeneity of the
magnetic field. MR signal can be “weighted” to accentuate some properties and not
others.
When an additional magnetic field is superimposed, one which is carefully varied in
strength at different points in space, each point in space has a unique radio
frequency at which the signal is received and transmitted. This makes constructing
an image possible. It represents the spatial encoding of frequency, just like a piano.
SPECT/PET (single photon/positron emission computed tomography):
When radio-labelled compounds are injected in tracer amounts, their photon
emissions can be detected much like x-rays in CT. The images made represent
the accumulation of the labelled compound. The compound may reflect, for
example, blood flow, oxygen or glucose metabolism, or dopamine transporter
concentration. Often these images are shown with a color scale.
Areas of Experimental Work
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Consciousness in vision
Attention
The conscious present
Visual images and inner speech
Thresholds of consciousness
Consciousness and memory
Consciousness as a state: waking, deep sleep, coma,
anaesthesia, dreaming
• Empirical theories of the NCC
• The self
• Voluntary control
The key idea: treating consciousness as an experimental
variable
E.g.:
• between conscious and unconscious streams of stimulation
• between conscious and unconscious elements of memory
• between forms of brain damage that selectively impair
conscious process and those that don’t
• between wakefulness and unconsciousness
• between new and habituated events
The difficulty has been to discover comparison conditions:
science advances by discovering that an apparent constant is
actually a variable (e.g. gravity, species).
This means, that to study consciousness, conscious brain
events must be sufficiently similar to unconscious ones.
“We can state bluntly the major question
that neuroscience must first answer. It is
probable that at any moment some active
neuronal processes in your head correlate
with consciousness, while others do not:
what is the difference between them?
(Crick and Koch 1998)
This is the problem of the NCC.
Science cannot observe consciousness directly; experiments
can only confirm reports of conscious experience. So, for
science, consciousness is a theoretical construct based on
shared, public observations. But this does not make
consciousness unusual in science….
Conscious processes are operationally defined as events that:
• can be reported and acted upon,
• with verifiable accuracy,
• under optimal reporting conditions,
• and which are reported as conscious. (Baars 2003, 4)
Unconscious processes are operationally defined as events such
that:
• knowledge of its presence can be verified, even if
• that knowledge is not claimed to be conscious;
• and it cannot be voluntarily reported, acted on, or avoided,
• even under optimal reporting conditions. (Baars 2003, 5)
Examples of experimental paradigms in
vision science:
• blindsight
• on-line (“how”) system vs. seeing (“what”)
system
• bistable percepts (gestalts; binocular rivalry)
• electrical brain stimulation (e.g. Penfield)
• selective lesions (e.g. in monkeys)
Two Vision Systems
Milner and Goodale “…go further than their predecessors
by proposing that the division of labour is determined by
the use to which visual information is to be put, once it has
reached the striate cortex. They suggest that a ventral
stream, terminating in the inferotemporal cortex, is involved
in maintaining an enduring, viewpoint-independent,
representation of objects and their behavioural significance
(the so-called ‘what’ pathway). In contrast, they suggest
that a dorsal stream, terminating in the posterior parietal
cortex, is involved in providing an egocentric representation
of objects toward which goal directed actions are planned
(the so-called ‘how’ pathway).”
Jason B. Mattingley, “Attention, Consciousness, and the Damaged Brain: Insights From Parietal Neglect and
Extinction,” Psyche 5 (1999)
State Consciousness
Brainstem mechanisms control the state of
consciousness; cortical activity provides the
contents of consciousness. The reticular activating
system connects lower brain stem neurons to the
thalamus (and hence on to the cortex); it is
responsible for cortical EEG readings (‘brain
waves’). It used (1960s) to be thought that this was
the seat of consciousness, but now this is
generally doubted, and consciousness is ‘located’
in the cortex itself.
Theories of Consciousness
A small list of suggestions that have been put forward might include (from Chalmers, online notes):
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40-hertz oscillations in the cerebral cortex (Crick and Koch 1990)
Intralaminar nucleus in the thalamus (Bogen 1995)
Re-entrant loops in thalamocortical systems (Edelman 1989)
40-hertz rhythmic activity in thalamocortical systems (Llinas et al 1994)
Nucleus reticularis (Taylor and Alavi 1995)
Extended reticular-thalamic activation system (Newman and Baars 1993)
Anterior cingulate system (Cotterill 1994)
Neural assemblies bound by NMDA (Flohr 1995)
Temporally-extended neural activity (Libet 1994)
Backprojections to lower cortical areas (Cauller and Kulics 1991)
Neurons in extrastriate visual cortex projecting to prefrontal areas (Crick and Koch 1995)
Neural activity in area V5/MT (Tootell et al 1995)
Certain neurons in the superior temporal sulcus (Logothetis and Schall 1989)
Neuronal gestalts in an epicenter (Greenfield 1995)
Outputs of a comparator system in the hippocampus (Gray 1995)
Quantum coherence in microtubules (Hameroff 1994)
Global workspace (Baars 1988)
Activated semantic memories (Hardcastle 1995)
High-quality representations (Farah 1994)
Selector inputs to action systems (Shallice 1988)
Three general types of NCC theory:
• Resonant neuronal assemblies (c.f. neural nets and
biological selection theories): e.g. Edelman
• Temporal coordination of large groups of neurons
(e.g. 40 Hz oscillation): e.g. Crick
• Global workspace theory (a small central domain of
working memory plus a vast distributed set of
specialized unconscious processors): e.g. Baars
Binocular Rivalry
Figure 2. Localizer Data: FFA and PPA
(a) Two adjacent near-axial slices
showing the localized FFA and PPA of
one subject (S1). The FFA was
localized as the region in the fusiform
gyrus that responded more to faces
than houses. The PPA was localized
as the region in the parahippocampal
gyrus that responded more to houses
than faces. (These images follow
radiological convention with the left
hemisphere shown on the right and
vice versa.)
(b) MR time course on localizer scans
showing FFA (blue solid line) and PPA
(red dotted line) activity (expressed in
percent signal change relative to
fixation baseline) averaged across all
four
subjects.
Subjects
viewed
sequentially presented faces (F),
houses (H), or a static fixation point
(1).
(Tong, F., Nakayama, K., Vaughan, J.T. and Kanwisher, N., 1998: Binocular rivalry and visual awareness in human extrastriate cortex, Neuron 21, 753-759)
(A) Sagital view showing the
orientation of the images and
the position of the 10 slices. In
all images the left hemisphere
is shown on the right-hand
side of the image.
(B) Slice taken at 20 mm
above the AC-PC line (third
slice from the top in A),
showing the pattern of
increased neuronal activity
under the aware condition.
Regions of significant
increased
activation are shown in black
and include the right
dorsolateral prefrontal area
(Brodmann area 46).
(C and D) The pattern of
activity in two slices taken at z
5 27 and z 5 22 (third and
fourth slice from the bottom in
A), respectively, showing
activity in midbrain centers,
including superior
colliculus in the unaware mode
Sahraie et al., Pattern of neuronal activity associated with conscious and unconscious processing of visual signals, Proc. Natl. Acad. Sci. 94 (1997)
Binocular Rivalry
Neurons in the superior temporal sulcus
(STS) and inferior temporal cortex (IT)
change with the percept, but most of the
neurons in the medial temporal cortex (MT)
and V1/V2 do not.
(The STS is the red line)
Rotating Snake: The strong rotation of the “wheels” occurs in relation to eye movements.
On steady fixation the effect vanishes.
At the top of the figure are some receptors. Below them are two
kinds of synapses (neural connections): Excitation synapses are
ones that increase neural activity and inhibitory synapses
decrease neural activity.
The concentric circles represent the neural activity recorded with
the electrode. when the receptors are stimulated with light. When
one or all of the center receptors are stimulated an excitatory
increase in neural activity is obtained at the electrode. When the
receptors labeled surround are stimulated an inhibitory decrease
in neural activity is obtained.
The receptive field that lies at the intersection of the white cross
has more light falling on its inhibitory surround than does the receptive
field that lies between the two black squares. Consequently, the
excitatory center of this receptive field between the squares yields
a stronger response than that which lies at the intersection of the
white cross.
Close your right eye and look directly at the number 3.
Can you see the yellow spot in your peripheral vision?
Now slowly move towards or away from the screen.
At some point, the yellow spot will disappear.
http://www.michaelbach.de/ot/