D.O. Hebb

1. D.O. Hebb Neurophysiology and Learning

2. Donald OldingHebb: 1904-1985 • Born in 1904 in Nova Scotia, Canada • Dalhousie University class of 1925- by the skin of his teeth! • Taught school for several years • McGill for grad school: still one of best in neuro and behavior • Trained as Pavlovian • Highly influenced by Gestaltists and Lashley • Finished at Univ of Chicago: worked with Lashley • Worked to show that “switchboard” model of brain functioning was incorrect • Brain was not stimulated in specific areas by specific events

3. Donald OldingHebb: 1904-1985 • Showed localization was not good model of brain • Did help show principle of mass action: correlation between loss of function and degree of cortical (and lower area) damage • Also: equipotentiality: ability of one brain function to be taken over by another area of brain • 1935/6: Both Lashley and Hebb moved to Harvard • 1937: moved to Montreal Neurological Institute to work with Penfield • to Yerkes primate labs in 1942 • Finally back to McGill ,where he retired • Most important work: The organization of Behavior • Even today considered a remarkable, critical book on brain function and physiology • Was far ahead of his time

4. Most important finding regarding intelligence: • Experience in childhood normally develops concepts, modes of thoughts, ways of perceiving that constitute intelligence • Injury to infant brain interferes with that process but the same injury at maturity does NOT reverse it • Three important observations • Brain not a simple switchboard: if it was, brain damage would have much more profound effect • Intelligence comes from experience, thus not genetically determined • Childhood experiences MORE important in determining intelligence than adult experiences

5. Restricted Environments • interest in interaction between • early experience and brain development/function • Thus focused on restricted vs. enriched environments • Many, many studies on restricted environments show differences • Von Senden (1932): congenital cataracts • Riesen (1974): chimps reared in darkness • Hubel & Weisel (1963): cats reared in visually restricted environments • Work in mental retardation institutions

6. Enriched Environments • Hebb was one of first to suggest that if less is bad, more may be better • Rats reared in enriched vs. impoverished environments • Reared in laboratory cages • Reared in home by Hebb’s daughters • Enriched group showed better maze learning • Most supporting research shows enriched = better learners

7. But not always the case: • B.A. HEIDENREICH1,2, K.M. LAKIN1, S.J. EYLER3, J.L. MOORE1, M.A. TIEMANN1, V. FARMER-DOUGAN1,2 • Rats reared in isolate, enriched or iso-play conditions • Examined growth patterns, thigmotaxis, learning rates, reward sensitivity, brain DA

8. Behavioral Results

9. Hmmm…….enriched group learned faster but showed lower performance………….

10. Hmmm…… • Hebb: more is better • Heidenreich, et al.: enriched group • Learned faster • were less sensitive to rewards • So is an enriched environment better or worse? • Let’s see…………………

11. Cell Assemblies • Hebb: each environmental object we experience stimulates a cell assembly • Cell assembly = • complex pattern of neurons • Would call it a neural circuit today • Neural circuits build into neural networks

12. Cell Assemblies • Neurophysiological postulate suggests: • There is a mechanism by which initially independent neurons become linked to stable cell assemblies and by which assemblies become linked to other assemblies: • When an axon of cell A is near enough to excite cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of cells firing B is increased • Was talking long term potentiation, but didn’t know it! • Suggest that cell assemblies can be either externally or internally elicited

13. Phase Sequences • Cell assemblies become neurologically interrelated to form phase sequences • Phase sequence: temporally integrated series of assembly activities • Suggested that when phase sequence fired, we experienced a stream of thought • Allowed series of ideas to be organized in logical order • Two kinds of learning • Association learning: • Slow buildup of cell assemblies early in life • formed via straight associations • Cognitive learning: • Rapid • Later in life • Involves rearrangements of phase sequences • Thus explains insight, etc.

14. But, need to first look at systems in the brain to understand these systems • Is brain organized into networks? • Do areas in the brain have • Specific areas for specific functions? • General areas which participate in many functions? • Appears the answer is BOTH • First let’s look at how a neuron works, then we will get to more detail…………………………..

15. Parts of a Neuron • Synaptic bulbs or terminal buttons: •   bulbous parts at end of end brush •   close contact with next neuron's dendrites •   contain neurotransmitters • Synapse or synaptic cleft •   space between neurons •   space between one neuron's synaptic bulb/other's dendrites • Myelin sheath: •   insulates neurons •   made of either glial cells or Schwann cells •   allows synaptic transmission to occur by jumping down axon •   exposed area between sheath = Nodes of Ranvier

18. Action Potential • Dendrites receive incoming neurotransmitter • Chemical fits in “lock” on dendrite • Alters the shape of the cell wall • Allows changes in cell wall that will change voltage inside the neuron • if incoming message sufficient in strength- causes an ACTION POTENTIAL

19. Again: Three Steps for firing • Resting potential: voltage is about -70mV • Dendrites receive incoming signals • If sufficient, cell goes into firing mode • Action potential • Voltage changes from -70mV to +40mV • Ions exchange places • Occurs rapidly down axon • Only in places where myelin sheath doesn’t cover: Nodes of Ranvier • Refractory Period: • below resting or lower than -70mV • Cell recovers from firing • Brief time period when difficulty for it to fire again. • Back to Resting potential.

23. Many different ways of manipulating neurotransmitters • Alter rate of synthesis: more or less NT • Alter storage rate: again, more or less NT • Leaky vesicles • Alter release: more or less release • Alter reuptake: more or less • SSRI’s • Alter deactivation by enzymes: MAO inhibitors • Block or mimic receptor site attachment • Block and prevent attachment to receptors • Mimic the NT at the receptor site

25. Now, back to Hebb • How could a group of neurons work to control behavior? • Let’s start with arousal: how do you control • What you pay attention to • What you remember • Examine the arousal system

26. Arousal Theory • Arousal theory: • relation between cognitive functioning and level of stimulation • Reticular formation or reticular activating formation (RF) • Brain stem organ • Regulates sleep, attention • to some degree emotional behavior (related to arousal)

27. Hypothesized: RF regulates arousal • The neural impulse generated by stimulation of sense receptors has 2 functions • Cue function of stimulus allows organism to gain info about environment • Must be optimal level of arousal for this to occur • This optimal level controlled by RF • Arousal function: elicitation of optimal level of arousal in RF • Different tasks require different levels of arousal • Habits: performed by lower brain areas, low level of arousal • Highly skilled task: require higher level of arousal, higher cortical areas

28. Arousal theory and Reinforcement • Arousal level can be reinforcing • If arousal level too high: work on environment to reduce it • If too low: work to increase it • This change is reinforcing • Reinforcement = drive reduction • Argued that seeking stimulation is critical behavior • Being stimulated (appropriately) is reinforcing • Seeking excitement = critical human behavior

29. Sensory Deprivation • Assumes arousal = reinforcing; deprivation or arousal = punishing, • Indeed, prolonged sensory deprivation has interesting effects: • Feel need to move • Cannot last longer than 2-3 days • Irritability • Infantile behavior • Hallucinations: means of brain to increase arousal, stimulation • Potentially: psychosis • Suggests that stimulation is critical for survival • Some real implications for those with disabilities • Obvious use in war: torture tactic • Fits in well with response deprivation theory, as well

30. Another network: Nature of fear • Believed fear may be innate, but not necessarily evident from birth • Some developmental time course • In chimps, at about 4 mos (which corresponds with movements away from mother) • Suggested that cell assemblies for certain fears were innate • Could become linked with other cell assemblies via association process • Thus build from the innate cell assemblies • Get phase sequences for fear • Thus: phase sequence that is triggered may not be supported by original stimuli that triggered it- but goes into this innate sequence • This will be supported by Bolles in a few years!

31. Long Term and Short Term Memory • One of first to distinguish between 2 types of memory • LTM: permanent changes in physical structure of neurons • STM: ongoing activity in cell assemblies and phase sequences • Tried to explain how we remember • Both in the short term, immediate sense • And how we build long term memories • Again, needs to use those neurons!

32. Reverberating neural activity: • Learning both “instantaneously established and permanent” • Reverberating neural circuit = STM • Consolidation theory: • reverberating neural circuit results in permanent changes in structure of the circuit • Long term memory forms a permanent, reverberating circuit in the brain.

33. Evidence: Memory Disruption studies • Support for his model comes from memory disruption studies • Used electroconvulsive shock (ECS) to disrupt memories • Reset the reverberating circuits (via ECS): prevented long term changes • Individuals receiving ECS showed memory loss • Retrograde amnesia: loss of memory for events prior to the traumatic event • Can also permanently damage brain, which disrupts LTM

34. But what is Consolidation • Limbic system • Amygdala: mediates memory and action • Hippocampus: critical for memory of emotion • Cingulategyrus: motor area • Involved in regulation of emotion • H.M. • Severe hippocampal damage • Difficulty consolidating memory • Some emotional violatility • No LTM

35. Consolidation • Damage to hippocampus results in loss of declarative memory • Higher order learning • Often language based • Damage to Basal Ganglia = motor memory loss • Procedural memory • Not impaired in Parkinsons (different problem) • Learning disabilities link • Unable to form “memory” for habits • Basal ganglia thus not process basic behavior • Remains on cortical level, causing interference with other cognitive tasks

36. Why remember?Reinforcement and Dopamine • But why only remember certain things? • Reinforcement • Must be “reinforcement center” in brain that signals that an environmental event is important • Olds and Milner: • Brain stimulation = lots of behavior • Animals would work until death to gain access to this brain stimulation • Thought had discovered pleasure center: • Nucleus accumbens • Mesolimbic pathway • Dopamine (DA) was neurotransmitter involved in these areas

37. Reinforcement and DA • EBS = releasing LOTS of dopamine (DA) • Results in lots of locomotion or exploratory behavior • Salamoneand Schultz’ modern work has shown this release • DA release modulates “appetitive behavior” • Occurs in modes or modules related to terminal event • E.g., food modes, sex modes • Which mode depends on context of environment • Search, capture, prepare, consume • DA not affect consummatory behavior

38. DA is regulated in two ways: • DA is released in pulses (phasic) and has an overall tone in synapse (tonic) • Result is feedback system • DA release in response to stimuli in environment • Motivated or energize appropriate behavior • Feedback system follows Rescorla Wagner model

39. DA regulation • DA also has a constant level or overall tone in the synapse: • More released when “surprised above what expected • Less when “surprised” below what expected • When get what expected- behavior is “well learned” and appears to become habit (not sure how this works yet) • Thus, fluctuations in DA as learn, and then serve as feedback regaring state of environment

40. So, was Hebb right? • Hebb was laughed at when originally wrote his book on neural networks • Work performed in absence of modern imaging/measurement techniques • Mostly theoretical • Wrong on the details, but got the jist of it correct!