Memory Its History and our present understanding (حافظه: تاريخچه و دريافت فعلي ما از آن)

MemoryIts History and our present understanding

(حافظه: تاريخچه و دريافت فعلي ما از آن)

سعيد عماديگروه علوم زيستي

دانشگاه تحصيالت تكميلي در علوم پايه زنجانسمينارهاي 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.

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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

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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


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.

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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


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.

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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.

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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.

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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.

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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 process

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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


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 systems

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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.

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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)

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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 function19


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 memory

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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


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


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.

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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.

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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.

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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


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


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


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

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Documents

Memory Its History and our present understanding (حافظه: تاريخچه و دريافت فعلي ما از آن)