Editing Hebbian theory (section)

==Engrams, cell assembly theory, and learning==
Hebbian theory provides an explanation for how neurons might connect to become [[Engram (neuropsychology)|engrams]], which may be stored in overlapping cell assemblies, or groups of neurons that encode specific information.<ref>{{Cite journal |last=Sejnowski |first=Terrence J. |date=1999-12-01 |title=The Book of Hebb |url=https://www.cell.com/neuron/fulltext/S0896-6273(00)81025-9 |journal=Neuron |language=English |volume=24 |issue=4 |pages=773–776 |doi=10.1016/S0896-6273(00)81025-9 |issn=0896-6273 |pmid=10624941}}</ref> Initially created as a way to explain recurrent activity in specific groups of cortical neurons, Hebb's theories on the form and function of cell assemblies can be understood from the following:<ref name="Hebb 1949"/>{{rp|70}}

<blockquote>The general idea is an old one, that any two cells or systems of cells that are repeatedly active at the same time will tend to become 'associated' so that activity in one facilitates activity in the other.</blockquote>

Hebb also wrote:<ref name="Hebb 1949"/>
<blockquote>When one cell repeatedly assists in firing another, the axon of the first cell develops synaptic knobs (or enlarges them if they already exist) in contact with the soma of the second cell.</blockquote>

D. Alan Allport posits additional ideas regarding cell assembly theory and its role in forming engrams using the concept of auto-association, or the brain's ability to retrieve information based on a partial cue, described as follows:

<blockquote>If the inputs to a system cause the same pattern of activity to occur repeatedly, the set of active elements constituting that pattern will become increasingly strongly inter-associated. That is, each element will tend to turn on every other element and (with negative weights) to turn off the elements that do not form part of the pattern. To put it another way, the pattern as a whole will become 'auto-associated'.  We may call a learned (auto-associated) pattern an engram.<ref>{{Cite book|last=Allport|first=D.A.|editor=Newman, S.K. |editor2=Epstein R.|chapter=Distributed memory, modular systems and dysphasia|title=Current Perspectives in Dysphasia|publisher=Churchill Livingstone|location=Edinburgh|year=1985|isbn=978-0-443-03039-0}}</ref></blockquote>

Research conducted in the laboratory of Nobel laureate [[Eric Kandel]] has provided evidence supporting the role of Hebbian learning mechanisms at synapses in the marine [[Gastropoda|gastropod]] ''[[Aplysia californica]]''.<ref>{{Cite journal |last1=Castellucci |first1=Vincent |last2=Pinsker |first2=Harold |last3=Kupfermann |first3=Irving |last4=Kandel |first4=Eric R. |date=1970-03-27 |title=Neuronal Mechanisms of Habituation and Dishabituation of the Gill-Withdrawal Reflex in Aplysia |url=https://www.science.org/doi/abs/10.1126/science.167.3926.1745 |journal=Science |volume=167 |issue=3926 |pages=1745–1748 |doi=10.1126/science.167.3926.1745|pmid=5416543 |bibcode=1970Sci...167.1745C |url-access=subscription }}</ref> Because synapses in the [[peripheral nervous system]] of marine invertebrates are much easier to control in experiments, Kandel's research found that Hebbian [[long-term potentiation]] along with activity-dependent presynaptic facilitation are both necessary for [[synaptic plasticity]] and [[classical conditioning]] in ''Aplysia californica''.<ref>{{Cite journal |last1=Antonov |first1=Igor |last2=Antonova |first2=Irina |last3=Kandel |first3=Eric R. |last4=Hawkins |first4=Robert D. |date=2003-01-09 |title=Activity-Dependent Presynaptic Facilitation and Hebbian LTP Are Both Required and Interact during Classical Conditioning in Aplysia |url=https://www.sciencedirect.com/science/article/pii/S0896627302011297 |journal=Neuron |volume=37 |issue=1 |pages=135–147 |doi=10.1016/S0896-6273(02)01129-7 |pmid=12526779 |issn=0896-6273}}</ref>

While research on invertebrates has established fundamental mechanisms of learning and memory, much of the work on long-lasting synaptic changes between vertebrate neurons involves the use of non-physiological experimental stimulation of brain cells. However, some of the physiologically relevant synapse modification mechanisms that have been studied in vertebrate brains do seem to be examples of Hebbian processes. One such review indicates that long-lasting changes in synaptic strengths can be induced by physiologically relevant synaptic activity using both Hebbian and non-Hebbian mechanisms.<ref>{{cite journal |last1=Paulsen |first1=O |last2=Sejnowski |first2=T |date=1 April 2000 |title=Natural patterns of activity and long-term synaptic plasticity |journal=Current Opinion in Neurobiology |volume=10 |issue=2 |pages=172–180 |doi=10.1016/s0959-4388(00)00076-3 |pmc=2900254 |pmid=10753798}}</ref>