The Legacy Of Donald O. Hebb

The Legacy ofDonald O. HebbHebb majored in Englishwith the intention ofbecoming a novelist.He graduated with a B.A.in 1925 and taught at hisold school in Chester fora year. This was not asuccess and his novelwriting did not progress.D.O. Hebb1904-1985He went out westand worked atharvesting in Albertaand then spent timeas a labourer inQuebecDonald Hebb was born inChester, Nova Scotia, in1904.Both his parents werecountry physicians.The young Donald Hebbwas a precocious andvoracious reader.When he was 16, the familymoved to Dartmouth NS andthe following year, Hebbentered the Faculty of Arts atDalhousie University in Halifax. and he began toread Freud!Hebb approached theChair of theDepartment ofPsychology at McGill,W.D. Tait, about doinggraduate work.He was given a reading list, which includedworks by William James and the Elements ofPhysiological Psychology by G.T. Ladd and R.S.Woodworth, and was told to come back in a year.1

A year later, in 1928, Hebb was acceptedas a part-time graduate student.At the same time, heobtained anappointment asheadmaster of aworking class school inMontrealThere was a high rate ofabsenteeism and poorperformance in theschoolTogether with two professors from McGill, Kelloggand Clark, Hebb improved attendance by makingthe school work more interesting and using ‘timeouts’ for disruptive behaviour. It worked!In 1931, Hebb wasbedridden with atubercular infection of thehip.Sherrington, who was awarded the Nobel Prizefor Physiology in 1932, was widely regarded asBritain’s greatest physiologist.He establishedthe nature ofspinal reflexes –and how theyinteracted in theproduction ofmore complexbehaviour.According to Sherrington, the nervous system actsas the coordinator of various parts of the body andthat the reflexes are the simplest expressions of theinteractive action of the nervous system.His book “TheIntegrative Action ofthe Nervous System” isoften compared toNewton’s “Principia” interms of its impact onthe field.Hebb also began to readPavlov’s ‘ConditionedReflexes’ which had beentranslated into English in1927.Ivan Petrovich Pavlov1849-1936During this time he readSherrington’s “IntegrativeAction of the NervousSystem”.Sir Charles Sherrington1857-1952Pavlov’s work was the culmination of a longphysiological tradition in Russia in whichphysiologists attempted to explain complexbehaviour by beginning with simple reflexes.2

Pavlov was interested in gastrointestinal secretions-- how different parts of the gastrointestinal (GI)tract responded to food items.Pavlov soon realized that such secretions couldbe studied systematically and he spent manyyears studying them.In fact, he got the Nobel Prize in Physiology in1905 for his work on the gastrointestinal tractBut that’s not why he is famous of course.In carrying out his work, Pavlov would make holes orfistula in different parts of the GI tract and put food(meat powder) directly into the mouth or stomach.He would then measure the time it took forsecretions to be produced and how much wasproduced.“Psychic llCSCRMeatPowderUCSBut he was botheredby a persistentphenomenon thatwould often disrupthis measurements.SalivationUCRThe term ‘psychic secretions’ was eventuallyreplaced with the term, conditioning, andeventually became known as ClassicalConditioning or Pavlovian Conditioning.Sometimes the dog would start to salivate as soonas Pavlov walked into the room!He called these secretions that occurred evenbefore the meat powder was put into the dog’smouth, psychic secretions.A Clockwork OrangeStanley Kubrick’s 1971 filmfrom a novel by AnthonyBurgess3

Hebb was enormously influenced by Sherringtonand Pavlov – and embarked on what was tobecome an immensely successful career instudying the neural substrates of learning andperception.He finished his M.A. thesis in 1931. It was atheoretical rather than an empirical thesis – andwas called “Conditioned and UnconditionedReflexes and Inhibition”.In that thesis, one can see the seed of his laterideas on what was to become known as “CellAssembly Theory” or reverberating circuits.The thesis was passed cumlaude by two examiners – oneof whom was Boris Babkin, whohad worked with Pavlov in St.PetersburgBoris Petrovich Babkin1877-1950Babkin arranged for Hebb to conduct researchon Pavlovian conditioning with Leonid Andreyev,who had also come from Pavlov’s laboratory topursue his research at McGill.In 1934, Hebb wrote an unpublished manuscriptentitled “Scientific Method in Psychology: ATheory of Epistemology Based on ObjectivePsychology”.Many of the ideas that Hebb later incorporatedinto his famous 1949 book “The Organization ofBehaviour” can be seen in this unpublishedwork.But by 1934, Hebb had become disillusioned withMontreal and McGill. His wife had died on his29th birthday, after a car accident. Moreover, hefelt that the Pavlovian conditioning he wasstudying was sufficiently related to brain.In Hebb’s own words:“An excited neuron tends to decrease its dischargeto inactive neurons, and increase this discharge toany active neuron, and therefore to form a route toit,whether there are intervening neurons betweenthe two or not. With repetition this tendency isprepotent in the formation of neural routes.”One can see the direct link with Pavlov andSherrington -- and also with Ariëns Kappers whoin1928 published his theory of neurobiotaxis,proposing that axons, regardless of whether theyare being fired, grow towards active cells duringdevelopment.After deciding that he wanted to study the brain,Hebb wrote to Robert Yerkes at Yale, but in theend, on the advice of Babkin, he decided to go tothe University of Chicago to work on his Ph.D. withKarl Lashley.Lashley was a brilliantexperimentalist who workedon memory and vision. Hechallenged the notion oflocalization of function andput forward the ideas of massaction and equipotentiality.Karl Spencer Lashley1890-19584

The principle of mass action states that in manytypes of learning the cerebral cortex acts as awhole.Lashley argued that the brain is sufficiently plasticthat when one region is damaged another regioncan take over the functions of that region. This is theprinciple of Equipotentiality. In its strongest form:any part of the brain can take on the function ofanother part.Hebb’s Ph.D. thesis was entitled “The problem ofspatial orientation and place learning”.But before the research wascompleted, Lashley hadaccepted a position at Harvard.Hebb went with him and wasaccepted as a Ph.D. student atHarvard in 1935,Penfield, an Americanwho trained at Princeton,Oxford, and JohnsHopkins, was heavilyinfluenced by Sherrington,who had met while atOxford. In fact, afterPenfield completed hisM.D. at Johns Hopkins, hereturned to Oxford forgraduate studies inneurophysiology underSherrington’s supervision.Merton College, OxfordPenfield abandoned neurophysiology to becomea neurosurgeon – in part because he wanted tolearn more directly from the exposed brain(“neurology-in-action”, he called it). He dreamedof an institute where teams of neurologists,neurosurgeons, and psychologists could worktogether to study the brain.In 1934, with money from theRockefeller Foundation, hefounded the MontrealNeurological Institute (MNI).In the spring of 1936, Hebb submitted a thesis onthe vision of rats reared in darkness, and hereceived a Harvard Ph.D.Penfield developed the technique of directelectrical stimulation of the brain, which he usedduring the operation to help plan the surgery.In 1937, Hebb wasappointed to a fellowship atthe Montreal NeurologicalInstitute. He was recruited tohelp Wilder Penfield assessthe psychological effects ofbrain operations. Hereturned to Montreal with hisnew wife, Elizabeth.Wilder Graves Penfield1891-19765

Servos, P., Engel, S., Gati, J., Menon, R. (1999). fMRIevidence for an inverted face representation in humansomatosensory cortex. NeuroReport 14: 1393–1395ForeheadThe Sensory HomunculusChinBut back to Hebb.To assess the effects of removing differentparts of the brain, Hebb assembled a set ofdifferent psychological tests. This was thebirth of neuropsychology at the MNI.Hebb showed that the righttemporal lobe played a role invisual perception and that thefrontal lobes play their mostimportant role in earlylearning.An error in Penfield’s sensory map!Hebb became more andmore interested in theeffects of early experienceon later cognitiveperformance. Hecompared learning in ratswho were reared in normalcages with learning in ratswho were kept in his homeas pets.Chester Nova ScotiaSummer 19426

Hebb concluded “there is a lasting effect of infantexperience on the problem-solving ability of theadult rat.”He also showed that the effects of brain damageon intelligence depended on the age at which thedamage occurred.All of Hebb’s thoughts on the formation of newbrain circuits, perceptual learning, the influenceof early experience, and the effect of brainlesions on behaviour were combined in his1949 book “The Organization of Behavior”.These ideas formed the basis of one of the mostpowerful ideas in developmental psychology: thatearly experience plays a powerful organizing rolein later behaviour.These ideas also led to the introduction of earlyinstruction programs such as “Head Start”In 1942, Karl Lashley took over as Director of theYerkes Laboratories of Primate Biology at OrangePark, Florida. He invited Hebb to join him to studyemotional and cognitive behaviour in chimpanzees.In the five years he spentthere, before returning toMcGill, Hebb studied theexpression of fear andanger in chimps. Heshowed that they had anapparently ‘innate’ fear ofsnakes and dead bodies.The first ideas was that connections (the synapses)between two neurons could alter with time. If theneuron providing the signal (the pre-synapticneuron) and the neuron receiving the signal (thepost-synaptic neuron) were both active, then thestrength of the synapse would increase. This hasbecome known as Hebbian learning. The idea wasexpanded later to include the strength of thesynapse weakening if one neuron was active andthe other was not.7

The second idea was Hebbian learning would set uploops or cell assemblies, i.e. if neuron A fired andactivated neuron B, which fired and activated neuronC, which in turn fired and activated neuron A again,then the synapses linking these neurons would allstrengthen and increase the possibility of the wholething happening again in the future.In practice, cell assemblies would consist of manythousands of cells, but the connections woulddevelop in such a way that activatingany part of the loop would causeThe rest of it to activate and thewhole process would beself-sustaining.“The Organization of Behavior” was a bookwell ahead of its time.It had a huge influence on the field ofbehavioural neuroscience, which in some waysHebb can be credited with founding.Object Constancy“Few people today would defend the nuts and boltsof Hebb’s neuropsychological theory of perceptuallearning, but it provided a goal towards whichpsychological theory should move, and showedwhat could be done as anatomical andphysiological knowledge expanded. The ensuingprogress in that direction more than justifies theacclaim that Hebb’s speculations have enjoyed forthe last half-century.”Hebb used cellassemblies to explainobject constancyRichard Brown and Peter Milner. The legacy of Donald O.Hebb: morethan the Hebb Synapse. Nature Reviews Neuroscience 4: 1013-1019.Hebb argued that ininfants and children,learning involves settingup new cell assemblies.In adults, however,learning more ofteninvolves fine-tuningexisting cellassemblies.Sensory deprivation“The aim of the project was to obtain basic information onhow human beings would react in situations wherenothing was happening. The purpose was not to cutindividuals off from any sensory stimulation whatever, butto remove all patterned or perceptual stimulation, so faras we could arrange it."8