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MemoryIts History and our present understanding
(حافظه: تاريخچه و دريافت فعلي ما از آن)
سعيد عمادي
گروه علوم زيستي
دانشگاه تحصيالت تكميلي در علوم پايه زنجان
سمينارهاي هفته اي علوم زيستي
1390 آبان 14
BrainHistorical Milestones
Roman physician Galen (AD 129 – 199):
Argued for the importance of the brain, and
theorized
in some depth about how it might work.
Galen traced out the anatomical relationships
among
brain, nerves, and muscles, demonstrating that
all muscles in the body are connected to the brain
through a branching network of nerves.
2
BrainHistorical Milestones
Galen's ideas were widely known during the MiddleAges
Not much further progress came until theRenaissance, when detailed anatomical study resumedand combined with the theoretical speculations of Rene Descartes (1596 – 1650) and those who followed him.
Descartes believed that the highest cognitivefunctions are carried out by a non-physical res cogitans, but that the majority of behaviors of humans, and all behaviors of animals, could be explained mechanistically
3
BrainHistorical Milestones
The first real progress toward a modern understanding
of nervous function, though, came from the
investigations of Luigi Galvani (1737 – 1798), who
discovered that a shock of static electricity applied to
an exposed nerve of a dead frog could cause its leg to
contract.
Since that time, each major advance in understanding
has followed more or less directly from the
development of a new technique of investigation.4
BrainHistorical Milestones
Until the early years of the 20th century, the most
important advances were derived from new methods for
staining cells.
Particularly critical was the invention of a staining method
by Camillo Golgi (1843 – 1926), which (when correctly
used) stained only a small fraction of neurons, but not in
their entirety, including cell body, dendrites, and axon.
Without such a stain, brain tissue under a microscope
appears as an impenetrable tangle of protoplasmic fibers, in
which it is impossible to determine any structure.
5
BrainHistorical Milestones
In the hands of Camillo Golgi, and especially of the Spanish neuroanatomist Santiago Ramon y Cajal (1852 – 1934), the new stain revealed hundreds of distinct types of neurons, each with its own unique dendritic structure and pattern of connectivity.
Neuron Doctrine
6Drawing by Camillo Golgi of a hippocampus stained with the silver nitrate method
Drawing by Santiago Ramon y Cajal of two types of Golgi-stained neurons from the cerebellum of a pigeon
BrainHistorical Milestones
Charles Scott Sherrington (1857 – 1952) is a scientist
best known for his work on how the elements of the
nervous system join together functionally.
In 1897, Sherrington introduced the term synapse.
In the first half of the 20th century, advances in
electronics enabled investigation of the electrical
properties of nerve cells, culminating in work by Alan
Hodgkin (1914 – 1998), Andrew Huxley (1917-), and
others on the biophysics of the action potential, and
the work of Bernard Katz (1911 – 2003) and others on
the electrochemistry of the synapse.7
MemoryHistorical and Conceptual Perspective
Socrates (470-399 BC)
Human have preknowledge, certain knowledge about the
world is inborn.Middle of the 19th century:
Experimental science (chemistry and physics): Began to
attract students of behavior and the mind.
Philosophical exploration gradually replaced by
psychology.
First experiments by psychologists:
Sense perception, but gradually toward more complex
workings of the mind.8
MemoryHistorical and Conceptual Perspective
Pioneer in experimental psychology: German psychologist Hermann Ebbinghaus (1850 – 1909)
He discovered two key principles of memory storage:
1. Memories, some short-lived and retained for minutes, others long-lived and persist for days to months
2. Repetition make memories last longer
German psychologists Georg Elias Müller (1850 –
1934) & Alfons Pilzecker suggested that this
memory that lasts days and weeks becomes
consolidated with time.
9
MemoryHistorical and Conceptual Perspective
American philosopher William James (1842
– 1910): Short term and long term memory
Short term memory: Seconds to minutes,
essentially extension of the present moment
Long term memory: Weeks, months, even a
lifetime.
This distinction has proven fundamental to
understanding memory.
10
MemoryHistorical and Conceptual Perspective
Russian psychiatrist Sergei Korsakoff (1853-1900): First
description of a memory disorder (Korsakoff’s syndrome).
Memory impairment provide a great deal of useful
information: The study of amnesia showed that there are
multiple kinds of memory. Charles Darwin (1809-1882): Mental characteristics have continuity across species. Human mental capabilities have evolved from those of
simpler animals. Inspired by these ideas Russian physiologist Ivan Pavlov
(1849-1936) and American psychologist Edward Thorndike (1874-1949) developed animal models for studying learning.11
MemoryHistorical and Conceptual Perspective
12
Classical conditioning: Association of two events (sound of
a bell and the presentation of food):
Animal salivate whenever the bell
sounds, even in the absence of food.
The animal has learned that the
bell predicts the coming of food.
Instrumental conditioning: Association between a correct
response and a reward, or an
incorrect response and a
punishment that follows the
response, and in this way gradually
modifies its behavior.
MemoryHistorical and Conceptual Perspective
Laboratory-based learning psychology: behaviorism.
Behaviorists, led by the American John B. Watson (1878-1958)
Behavior could be studied with the same rigor as other
natural sciences
Studying only observable stimuli and responses:
Behaviorists lost sight of many other interesting and
important questions about mental processes such as those
within the brain that underlie: perception, attention,
motivation, action, planning, and thinking, as well as learning
and memory.
Behaviorism: Dominant in studying learning and memory
early in the 20th century, especially in the US.13
MemoryHistorical and Conceptual Perspective
Among researchers for whom mental processes had center stage: Feredric C. Bartlett (1886-1969) Founder of cognitive psychology
Bartlett studied memory by having people learn
material like stories and pictures:
Memory: Surprisingly fragile and susceptible to
distortion
Memory retrieval: Seldom highly exact
Retrieval: Not simply a replaying of passively stored
information
Retrieval: A creative and reconstructive process14
MemoryHistorical and Conceptual Perspective
By 1960’s, due to the work of Bartlett:
Perception and memory depend on information in the environment and on the mental structure of the perceiver
Cognitive psychology: Not only stimuli and responses but the processes that intervene between a stimulus and a behavior- precisely the domain ignored by behaviorists
Cognitive psychologists:
Flow of information from the eye to its internal representation in the brain for eventual use in memory and action
Internal representation: Take the form of a characteristic pattern of activity in particular groups of interconnectd cells in the brain 15
MemoryHistorical and Conceptual Perspective
Biological Revolutions in the 20th Century:
1. Our understanding of how cells function in molecular terms which
expanded considerably after discovering of the DNA structure by
James Watson (1928 - ) and Francis Crick (1916 – 2004).
2. Systems component of biological revolution: instruments such as
PET and functional magnetic resonance enabled scientists to study
brain processes during cognitive activities.
There is now two approaches in the studying the biology of memory:
1. At the nerve cells level: cellular and molecular mechanisms of
memory storage (neurobiology of memory)
2. At the level of brain structures, circuitry and behavior: neural
systems16
MemoryHistorical and Conceptual Perspective
Where memory is stored?
Actually the same question has historically been arisen
about other mental functions too.
Two viewpoints:
There are specialized areas in the brain for functions
such as language, vision, etc.
Different function are not localized, instead they are the
result of global and integrated activities of the brain.
Now the first view is predominantly accepted by
neuroscientists.
17
MemoryHistorical and Conceptual Perspective
Karl Lashley (1890-1958):
Pioneer in localizing memory.
His conclusions:
Actually no specific location in the cortex but memory impairment for the maze habit correlated with the size of cortex removed: formulated the law of mass action for memory impairment
More experimental works in the following years led scientists to arrive at different understanding of Lashley’s results:
Lashley’s maze-learning task was not suitable for the localization of memory because the task depended on different sensory and motor capabilities
He focused only on cerebral cortex and did not explored region below the cortex region (subcortical regions)
18
MemoryHistorical and Conceptual Perspective
Nevertheless Lashley showed that there is no single center in the brain
where all memories are permanently stored: many parts of the brain
must participate in representation of memory.
Early response to lashley’s challenge about the locus of memory came
from Donald O. Hebb (1904-1985), a psychologist at McGill University
He suggested that assemblies of cells, distributed over large areas of
cortex, work together to represent information.
This insight came to be seen as one of the key principles of
information storage in the brain.
The modern view:
Memory is widely distributed but different areas store different aspects
of the hole and there is little redundancy or reduplication of function
19
MemoryHistorical and Conceptual Perspective
Cerebral cortex divisions:
1. Frontal lobe: Planning and Voluntary
movement
2. Parietal lobe: Sensations of the body surface
and spatial perception
3. Occipital lobe: Vision
4. Temporal lobe: hearing, visual perception
and memory20
MemoryHistorical and Conceptual Perspective
First suggestions that memory might be stored in the temporal lobe:
Wilder Penfield (1891 – 1976), working on neurosurgical treatment
of focal epilepsy.
During surgery, weak electrical stimulations, and determined its
effects on the ability to speak and comprehend language.
Brain contains no pain receptors, patients remained fully conscious,
able to report their experiences.
Responses to electrical stimulations:
“ It sounded like a voice saying words, but it was so faint I couldn’t
get it”
“ I am seeing a picture of a dog and cat … the dog is chasing the cat”
These responses were elicited invariably only from the temporal
lobes of the brain.21
MemoryHistorical and Conceptual Perspective
Stimulated by Penfield’s experiments: William Scoville
(1926 – 2008) soon obtained direct evidence that the
temporal lobes are critically important for human memory.
In 1957, Scoville and Brenda Milner (1918 - ), a colleague
of Penfield reported the extraordinary story of a patient H.
M (Henry Gustav Molaison, 1928 - ).
At the age of about 9, H. M. was knocked down by a bicycle
and sustained a head injury that led eventually to epilepsy.
H. M.’s seizures worsened over the years.
H. M.’s epilepsy was thought to have its origin within the
brain’s temporal lobe.22
MemoryHistorical and Conceptual Perspective
Scoville decided, as a last resort, to remove the
inner surface of the temporal lobe on both sides of
the brain, including a structure called the
hippocampus, in an attempt to treat his epilepsy.
This experimental treatment did help his epilepsy,
but it left H. M. with a devastating memory loss
from which he has never recovered.
From the time of his operation in 1953 until present
day, H. M. has been unable to convert a new short-
term memory into a permanent long-term memory.23
MemoryHistorical and Conceptual Perspective
Larry R. Squire (1941 - )
His pioneering work in human memory helped
describe for the first time the role of the
hippocampus and surrounding cortex region of
the brain in human memory.
He has explored fundamental issues such as
whether memory is one thing or many, what brain
structures are important for memory, and what
happens to memory in disease. 24
MemoryHistorical and Conceptual Perspective
Eric Kandel (1929 - )
His groundbreaking research
revealed what happens to the
brain when memories are
formed.
Kandel explored how nerve
cells (neurons) change during
learning.
His research involving the sea
slug Aplysia and mice
uncovered the basis of short
and long-term memory.25
MemoryHistorical and Conceptual Perspective
Using chemical techniques to produce mutations in single genes
Seymour Benzer (1921 – 2007) began to examine the effects on behavior
of changing one gene at a time, with the use of Drosophila as his animal
model.
He first identified a number of fascinating mutants that affected
courtship, visual perception, and circadian rhythems.
Benzer then turned his genetic approach to the problem of learning and
memory storage.
From mutants with memory defects, Benzer was able to identify several
proteins that are important for nondeclarative forms of memory storage.
It was immediately evident that some of these proteins were the same as
those identified independently in molecular biological studies of
nondeclarative memory in Aplysia. 26
MemoryHistorical and Conceptual Perspective
Conclusions: First there was no separation between the mind and the
brain.
First philosophy: Until late in 19th century.
Then psychology.
And now biology.
Experimental studies first in psychology and more recently in biology.
New millenium: Questions posed by psychology and biology have begun to converge on common ground
From psychology perspective: How does memory works?
Are there different kinds of memory?
What is their logic?27
MemoryHistorical and Conceptual Perspective
Conclusions: From biology perspective: Where in the brain do we learn? Where do we store what is learned as memory? Can memory storage be resolved at the level of
individual nerve cells? If so, what is the nature of the molecules that underlie
the various processes of memory storage? Neither psychology nor biology alone can satisfactorily
address these questions. Common program of inquiry defined by psychologists
and biologists (Neuroscience): How are the various forms of memory organized in the
brain? How is memory storage accomplished?28
MemoryHistorical and Conceptual Perspective
Conclusions:
What we know now :
Many forms of memory
Different brain structures and specific jobs
Memory: Encoded in nerve cells; depends on changes in the strength of their interconnections, stabilized by the actions of genes in nerve cells
Findings about how the molecules inside nerve cells change the connection strength between nerve cells
29