INTRODUCTION TO COGNITIVE NEUROSCIENCE I
IC Bruce, 2005
Each one of us lives in a unique & personal universe that is created by his or
her brain. What is the evidence that this is so? How, exactly, does the brain
do this?
Evidence concerning brain functions has accumulated from several kinds of
investigation:
Warfare: describing the consequences after recovery from penetrating head
injuries (e.g. the case of Phineas Gage)
On
13-Sep-1848,
25-year-old
Phineas Gage suffered an accident
in which an iron rod, 3 feet 7 inches
long & 1¾ inches in diameter, was
blown through his head. “The
damage involved both left & right
prefrontal cortices in a pattern that,
as confirmed by Gage’s modern
counterparts, causes a defect in
rational decision making & the
processing of emotion” (Damasio et al, 1994). Twelve years later, after
several seizures, he died. After some time, his bones were dug up. Today, his
skull & the iron rod are in Harvard’s anatomical museum. His web site is at
http://www.hbs.deakin.edu.au/gagepage/pgage.htm. A detailed analysis of his
case is provided by Macmillan (2000).
Diseases: describing the consequences of “experiments of nature” (e.g. the
cases of “Tan” & “HM”)
“Six days after Broca saw
him, Leborgne died. His
brain, which proved to be
in poor condition due to
infarctions, was removed &
presented to the Société
d’Anthropologie the next
day… Broca issued a weak
statement about localization at this time, but at a decidedly more animated meeting of the Société
d’Anatomie later that year [1861], he boldly proclaimed that his case study of
Leborgne placed him in firm agreement with the localizationists. Broca was
especially careful to emphasize that his localization of a faculty for articulate
language in the frontal lobe differed from that proposed by Gall…” (Finger,
1994).
Anatomy: analysis of the detailed structure of the brain (e.g. Brodmann’s
“cytoarchitectonic” maps of the human cerebral cortex)
“In the early part of the 20th century,
Korbinian Brodmann divided the
human cerebral cortex into 52 discrete
areas on the basis of distinctive nerve
cell
structures
&
characteristic
arrangements
of
cell
layers.
Brodmann’s scheme of the cortex is
still widely used today & is continually
updated… Several areas defined by
Brodmann have been found to control
specific brain functions. For instance,
area 4, the motor cortex, is responsible
for voluntary movement. Areas 1, 2, & 3 comprise the primary somatosensory
cortex, which receives information on bodily sensation. Area 17 is the primary
visual cortex, which receives signals from the eyes… Areas 41 & 42 comprise
the primary auditory cortex…” (Kandel et al, 2000)
Animal experiments: recording the activity of single neurons during specific
behaviours (e.g. Eric Kandel’s work on the neural basis of learning & memory
in the sea snail, Aplysia)
“Responses of a single cell to the 14 facial stimuli… A neuronal response was
evoked by all the animals with horns (including the schematic drawings),
although small horns… were less effective.” (Kendrick & Baldwin, 1987)
Correlations between cognitively significant stimuli, such as faces, & neural
activity, reveal how such stimuli are processed & represented in the brain.
Surgery: observing the consequences of
direct brain stimulation during epilepsy
surgery (e.g. Wilder Penfield’s maps of
the functional organisation of the human
cerebral cortex)
“17 Patient was counting when stimulus
was applied. Stimulation arrested
speech completely. Patient added, “It
raised my right arm.”…
19 Patient was counting. When
stimulation was applied, she made an
exclamation & then was silent. Her head
turned to the right, & there was
movement of the right arm…
22 The patient said, “Oh.” There was a
movement of the body. The patient said afterward that her body seemed to
arise, but she did not seem to be doing it. She said she experienced this at
the beginning of her attacks.” (Penfield & Welch, 1954)
Imaging: direct observation of changes in the normal human brain during the
execution of specific tasks (e.g. PET, fMRI, MEG)
In this functional magnetic resonance imaging (fMRI) study, areas that
responded to real motion but not while motion was being imagined are shown
in red, and areas that were active during motion imagery but not to real
motion are shown in green. Areas that were active in both conditions appear
as orange & yellow (Thompson & Kosslyn, 2000).
So, how does the brain create our internal universe?
All brains, from the relatively simple ones that operate jellyfish, to complex
ones like those of primates, operate via networks of specialized cells:
neurons.
Any neuron is composed of
three parts: a cell body (soma)
that contains the genetic
material & metabolic machinery
to keep the neuron alive; a set
of
branching
processes
(dendrites) that constitute the
input surface of the neuron –
information arriving from other
neurons
arrives
on
the
dendrites; and a single process (the axon) that constitutes the output of the
neuron – conveying the results of the neuron’s calculation to other neurons.
Neurons represent information in
terms of brief (~1msec) electrical
events (action potentials). For
example, this temperature-sensitive
neuron in the skin represents
temperature as a frequency code.
Higher frequency indicates lower
temperature. The upper trace shows
the baseline skin temperature of
34°C, followed by a cooling step of 110°. Warming the skin at the end of
the step silences this cold-sensitive
neuron (Kandel et al, 2000).
Neurons are linked with one
another at synapses, where the
electrical events (action potentials)
are converted to chemical events, a
package
of
chemical
being
released by each action potential.
This chemical then diffuses across
the space (synapse) and affects the
dendrites of the neurons with which
the upstream neuron’s axon
terminals are connected. While
more than 100 of such chemicals
have been identified, there are only
two kinds of synapse.
Some synapses are excitatory: An
action potential in one (pink) neuron
releases a chemical that turns on
the second (blue) neuron. This
second neuron is therefore more
likely to transmit an action potential to other neurons. Some synapses are
inhibitory: An action potential in one (pink) neuron releases a chemical that
turns off the second (blue) neuron. So the second neuron is less likely to send
action potentials to other neurons.
Here, we have a dynamic network whose elements represent information as
electrical events & this information is manipulated by the processes of
excitation & inhibition. At this level of consideration, the human brain works
the same way as any other brain. However, the level of “complexity” in the
connectivity of the human brain is extraordinary.
Each cubic millimetre of your cerebral cortex
contains about 90,000 neurons, whose
dendritic processes added together would
extend about 400 meters, & whose axons
would sum to about 3.4 kilometres. In this cubic
millimetre, the number of connections from
axons to dendrites (synapses) is about
700,000,000. The thickness of the cerebral cortex varies between 2 & 4 mm,
& its surface area is about 250,000 mm2. What is the total number of
synapses in your cerebral cortex? Since each synapse can be “on” or “off” (to
excite or inhibit) from millisecond to millisecond, how many different states
can your brain be in over a period of 1 second?
I don’t know how to measure “connectional complexity”, but I think that
between 50,000 & 100,000 years ago, the human brain may have crossed
some kind of threshold of complexity that allowed new properties & therefore
new functions to emerge. If this is true, then a machine of sufficient
connectional complexity should be able to do what the human brain does.
REFERENCES
Damasio H, Grabowski T, Frank R, Galaburda AM, Damasio AR (1994) The
return of Phineas Gage: Clues about the brain from the skull of a famous
patient. Science 264, 1102-1105.
Finger S (1994) Origins of Neuroscience: A History of Explorations into Brain
Function. Oxford University Press.
Kandel ER, Schwartz JH, Jessell TM (2000) Principles of Neural Science, 4th
Edn, McGraw-Hill, New York.
Kendrick KM, Baldwin BA (1987) Cells in temporal cortex of conscious sheep
can respond preferentially to the sight of faces. Science 236, 448-450.
Macmillan M (2000) An Odd Kind Of Fame: Stories Of Phineas Gage. MIT
Press, Cambridge.
Penfield W, Welch K (1954) The supplementary motor area of the cerebral
cortex. A clinical & experimental study. Archives of Neurology & Psychiatry
66, 298-317.
Thompson WL, Kosslyn SM (2000) Neural systems activated during visual
mental imagery: A review & meta-analyses. In: Toga W, Mazziotta JC (Eds)
Brain Mapping: The Systems. Academic Press, San Diego, pp 535-560.
RECOMMENDED READING
Calvin WH, Ojemann GA (1994) Conversations with Neil’s Brain: The Neural
Nature of Thought & Language. Addison-Wesley Publ Co. You can read or
download it free at: http://faculty.washington.edu/wcalvin/bk7/bk7.htm.
Carter R (1999) Mapping The Mind. Seven Dials Press, London.
The Hidden Mind. Scientific American Special Edition, August 2002.
Downloadable from www.sciam.com (for a small fee).
Society for Neuroscience information at:
http://apu.sfn.org/content/Publications/BrainBriefings/index.html
Recently, the world’s top science magazines “Nature” & “Science” have each
published special editions on cognitive neuroscience. They can be found
among the “e-journals” in the HKU library system. The references are:
Nature, Volume 431, Number 7010, 14 Oct 2004 (“Story of a Neuron”).
Science, Volume 306, Number 5695, 15 Oct 2004 (“Cognition & Behaviour”).