Engram (neuropsychology)
Encyclopedia
Engrams are a hypothetical means by which memory
traces are stored as biophysical
or biochemical
changes in the brain (and other neural tissue) in response to external stimuli.
They are also sometimes thought of as a neural network
or fragment of memory, sometimes using a hologram analogy to describe its action in light of results showing that memory appears not to be localized in the brain. The existence of engrams is posited by some scientific theories to explain the persistence of memory and how memories are stored in the brain. The existence of neurologically defined engrams is not significantly disputed, though their exact mechanism and location has been a focus of persistent research for many decades.
.
Karl S. Lashley's search for the engram found that it could not exist in any specific part of the rat's brain, but that memory was widely distributed throughout the cortex
. One possible explanation for Lashley's failure to locate the engram is that many types of memory (e.g. visual-spatial, smell, etc.) are used in the processing of complex tasks, such as rats running mazes. The consensus view in neuroscience is that the sorts of memory involved in complex tasks are likely to be distributed among a variety of neural systems, yet certain types of knowledge may be processed and contained in specific regions of the brain. Overall, the mechanisms of memory are poorly understood. Such brain parts as the cerebellum
, striatum
, cerebral cortex
, hippocampus
, and amygdala
are thought to play an important role in memory. For example, the hippocampus is believed to be involved in spatial and declarative learning, as well as consolidating short-term into long-term memory.
In Lashley's experiments (1929, 1950), rats were trained to run a maze. Tissue was removed from their cerebral cortices
before re-introducing them to the maze, to see how their memory was affected. Increasingly, the amount of tissue removed degraded memory, but more remarkably, where the tissue was removed from made no difference.
Later, Richard F. Thompson sought the engram in the cerebellum, rather than the cerebral cortex. He used classical conditioning
of the eyelid response in rabbits in search of the engram. He puffed air upon the cornea of the eye and paired it with a tone. (This puff normally causes an automatic blinking response. After a number of experiences associating it with a tone, the rabbits became conditioned to blink when they heard the tone even without a puff.) The experiment monitored several brain regions, trying to locate the engram.
One region that Thompson's group studied was the lateral interpositus nucleus (LIP). When it was deactivated chemically, the rabbits lost the conditioning; when re-activated, they responded again, demonstrating that the LIP is a key element of the engram for this response.
This approach, targeting the cerebellum, though successful, examines only basic, automatic responses, which almost all animals possess, especially as defense mechanisms.
Studies have shown that declarative memories move between the limbic system, deep within the brain, and the outer, cortical regions. These are distinct from the mechanisms of the more primitive cerebellum, which dominates in the blinking response and receives the input of auditory information directly. It does not need to "reach out" to other brain structures for assistance in forming some memories of simple association.
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
traces are stored as biophysical
Biophysics
Biophysics is an interdisciplinary science that uses the methods of physical science to study biological systems. Studies included under the branches of biophysics span all levels of biological organization, from the molecular scale to whole organisms and ecosystems...
or biochemical
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
changes in the brain (and other neural tissue) in response to external stimuli.
They are also sometimes thought of as a neural network
Neural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...
or fragment of memory, sometimes using a hologram analogy to describe its action in light of results showing that memory appears not to be localized in the brain. The existence of engrams is posited by some scientific theories to explain the persistence of memory and how memories are stored in the brain. The existence of neurologically defined engrams is not significantly disputed, though their exact mechanism and location has been a focus of persistent research for many decades.
Overview
The term engram was coined by the little-known but influential memory researcher Richard SemonRichard Semon
Richard Wolfgang Semon was a German zoologist and evolutionary biologist, who believed in the inheritance of acquired characters and applied this to social evolution....
.
Karl S. Lashley's search for the engram found that it could not exist in any specific part of the rat's brain, but that memory was widely distributed throughout the cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
. One possible explanation for Lashley's failure to locate the engram is that many types of memory (e.g. visual-spatial, smell, etc.) are used in the processing of complex tasks, such as rats running mazes. The consensus view in neuroscience is that the sorts of memory involved in complex tasks are likely to be distributed among a variety of neural systems, yet certain types of knowledge may be processed and contained in specific regions of the brain. Overall, the mechanisms of memory are poorly understood. Such brain parts as the cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
, striatum
Striatum
The striatum, also known as the neostriatum or striate nucleus, is a subcortical part of the forebrain. It is the major input station of the basal ganglia system. The striatum, in turn, gets input from the cerebral cortex...
, cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
, hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
, and amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...
are thought to play an important role in memory. For example, the hippocampus is believed to be involved in spatial and declarative learning, as well as consolidating short-term into long-term memory.
In Lashley's experiments (1929, 1950), rats were trained to run a maze. Tissue was removed from their cerebral cortices
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
before re-introducing them to the maze, to see how their memory was affected. Increasingly, the amount of tissue removed degraded memory, but more remarkably, where the tissue was removed from made no difference.
Later, Richard F. Thompson sought the engram in the cerebellum, rather than the cerebral cortex. He used classical conditioning
Classical conditioning
Classical conditioning is a form of conditioning that was first demonstrated by Ivan Pavlov...
of the eyelid response in rabbits in search of the engram. He puffed air upon the cornea of the eye and paired it with a tone. (This puff normally causes an automatic blinking response. After a number of experiences associating it with a tone, the rabbits became conditioned to blink when they heard the tone even without a puff.) The experiment monitored several brain regions, trying to locate the engram.
One region that Thompson's group studied was the lateral interpositus nucleus (LIP). When it was deactivated chemically, the rabbits lost the conditioning; when re-activated, they responded again, demonstrating that the LIP is a key element of the engram for this response.
This approach, targeting the cerebellum, though successful, examines only basic, automatic responses, which almost all animals possess, especially as defense mechanisms.
Studies have shown that declarative memories move between the limbic system, deep within the brain, and the outer, cortical regions. These are distinct from the mechanisms of the more primitive cerebellum, which dominates in the blinking response and receives the input of auditory information directly. It does not need to "reach out" to other brain structures for assistance in forming some memories of simple association.
Further reading
- Forgotten Ideas, Neglected Pioneers: Richard Semon and the Story of Memory, Daniel Schacter, 2001 ISBN 184169052X
- Holographic properties found in interference networks
- Dudai, Y. (2004). The neurobiology of consolidations, or, how stable is the engram? Annual review of psychology, 55, 51-86.