Biological Processes
The Nervous System
Thought Paper #1
Topic will be
posted on Prometheus by the end of the day.
Due 1/28/03. Typed,
double-spaced.
Underlying Questions
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What is the relation between the mind and the brain?
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What is the physical basis of experience?
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To what extent is behavior learned and to what extent
is it inherited?
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How and why did the human brain get so big?
Brief History of Mind-Brain Problem
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René Descartes (1596-1650)
– Body is a complex
machine; mind is separate from the body.
– They communicate
via the pineal gland.
– Non-human animals
are unconscious automata.
– Some historians
now wonder whether Descartes might have saved the soul to save himself.
– Led to the
“Cartesian Impasse.”
Responses
to Cartesian Impasse
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Nicolas Malebranche (1638-1715)
– Occasionalism: God
is the only true cause; no influence of mind on body or body on mind.
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Gottfried Wilhelm Leibniz (1646-1716)
– Psychophysical
parallelism: Mind & body are different entities, existing in perfect
synchrony.
Responses to Cartesian Impasse
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George Berkeley (1685-1753)
– Immaterialism:
Nothing exists independent of mind.
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Julien Offray de la Mettrie (1709-1751)
– Materialism:
Nothing exists independent of matter.
Conscious acts are distinguished from unconscious processes only by the
relatively complexity of their physiology.
Turmoil in the 19th Century
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Mind-brain problem
especially acute in 19th Century
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Increasing understanding
of localization of function in the nervous system.
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Phineas Gage
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Paul Broca
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David Ferrier.
–
Awareness that mental
events bring about changes to the body/brain.
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Hypnosis
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Mental illness
19th Century Solutions
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Shadworth Holloway Hodgson (1832-1912)
– Epiphenomenalism:
Mental states are produced by the brain but cannot affect the brain.
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William Benjamin Carpenter (1813-1885)
– Interactionism:
Mind & brain are different but mutually interactive.
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George Henry Lewes (1817- 1878)
– Neutral monism:
One kind of “stuff”; mind & brain differ in arrangement of
stuff or perspective from which it is viewed.
What’s the right answer?
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Uniform agreement that consciousness is produced by
brains, & that the complexity of consciousness is correlated with the
complexity of the brain.
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Uniform ignorance about how this happens.
Two Current Theories
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Consciousness is
produced by a special class of neurons or neural systems. Research should focus on identifying
which neural states are correlated with consciousness and which are not.
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Consciousness is an
irreducible phenomenon, like space, time, & gravity. To explain consciousness, we need new
physical laws.
Learning About Mind & Brain
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Brain damage
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Lesions
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Recording brain activity
– Single-unit
recordings
– EEG & ERP
– Brain imaging
Brain Damage
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Phineas Gage (1823-1860)
Phineas Gage
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Physical impairment was slight.
– Lost vision in
left eye; left side of face partially paralyzed.
– Posture, movement,
speech unimpaired.
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Personality changed completely.
– Impulsive,
profane, obstinate.
Brain Damage
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Paul Broca
Brain Damage
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Paul Broca
Plusses and Minuses
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+Naturally occurring.
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+May reveal which areas are essential for certain types
of behavior.
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-Multiple areas typically damaged.
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-Naturally occurring.
Lesions
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Specific areas of the brain are surgically removed or
destroyed.
Plusses and Minuses
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+Detailed information about which areas of the brain
may be essential for certain behaviors.
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+Effective for studying plasticity.
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-Invasive.
Single-Unit Recording
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Activity of an individual neuron is recorded with a
microelectrode.
– Pioneering studies
by Hubel and Wiesel.
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Simple cells: Orientation selective
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Complex cells: Directionally selective
Plusses and Minuses
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+Precise measurement of the time course of brain
events.
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+Precise measurement of where in the brain processing
is taking place.
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-Invasive.
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-Animals only (with a few exceptions).
EEG & ERP
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An active neuron
generates an electrical field.
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A large population of
neurons generates a field that can be measured with electrodes placed on the
scalp.
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The plot of voltage
across time is called the electroencephalogram (EEG).
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When the EEG is
synchronized to an external event, it is called an event-related potential
(ERP).
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ERPs can be used to
measure the time-course of processing in the brain.
Background
Plusses and Minuses
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+Noninvasive.
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+Precise measurement of the time-course of brain
processes.
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-Poor measurement of where in the brain processing is
taking place.
Brain Imaging
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Positron Emission Tomography (PET)
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Functional magnetic resonance imaging (fMRI)
– Active areas of
the brain need oxygenated blood.
– fMRI detects
changes in blood flow in the brain.
Plusses and Minuses
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+Noninvasive.
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+Precise measurement of where in the brain processing
is taking place.
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-Poor measurement of the time-course of brain
processes.
Interim Summary
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Each technique has strengths and weaknesses.
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Arriving at a thorough understanding of how brains
produce behaviors will require the use of all of them (“converging operations”).
More on Lesion Studies
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Cerebrum consists of two nearly symmetrical halves, the
cerebral hemispheres.
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Left hemisphere controls right side of body; right
hemisphere controls left side of body.
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Hemispheres communicate through the corpus callosum.
Lateralization
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Some functions are
primarily controlled by one hemisphere.
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Left hemisphere controls
language for most people:
–
95% of right-handed
–
70% of left-handed
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Right hemisphere seems
to specialize in spatial tasks, emotions, music.
–
R. parahippocampus
lesions ® anterograde disorientation
–
R. hemisphere lesions ® deficits in comprehending humor
–
R. temporal lobe lesions
® deficits in musical imagery
Example of lateralization: Vision
Split Brains
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Corpus callosum severed to treat epilepsy.
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Most behavior is normal.
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Object shown in the right visual field could be easily
named; object shown in the left visual field seemed to be unrecognized.
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But it could be pointed to by the left but not the
right hand!
POP QUIZ!
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Somatosensory cortex controls which of the following?
– A. language
– B. voluntary
movements
– C. thinking
– D. sense of touch
Plasticity
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Injury to the developing
brain can produce massive reorganization.
–
Hemispherectomy
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Long thought that such
reorganization, or plasticity, was limited in the mature mammalian brain.
–
Ramusson et al.’s
(1985) raccoon.
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Now we know that the
adult brain can be extensively reorganized in response to injury.
Mechanisms of Plasticity
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Strengthening of existing connections
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Growth of new neural connections
(“sprouting”)
Mechanisms of Plasticity
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Strengthening of existing connections
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Growth of new neural connections
(“sprouting”)
Summary
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Scientists still do not understand how brains produce
consciousness.
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Learning how the brain produces behavior requires the
use of converging methods; no single approach will suffice.
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Even the adult brain is capable of extensive
reorganization in response to injury.