The Neural Mechanisms of Learning

Its all physical! The Neural Mechanisms of Learning

Neural basis of learing • Basic structure of the NS is set before birth • Neurons are however flexible living cells that can grow new connections • The ability of the brain to reorganise the way it works is referred to as plasticity

Neural Basis of learning Synaptic knob Axon terminals synapse Myelin sheath Axon

Neural Basis of learning

Hebbian theory • Learning results in the creation of cell assembles or neural networks • ‘neurons that fire together wire together’ • When a neurotransmitter is repeatedly sent across the synapse this can effect the strength of these connections • Neurons that do not fire together weaken their connections

Long term potentiation

Long term Potentiation New Receptor Formation Late LTP Long Term Memory New Synapse Formation

Long term potentiation and the hippocampus – rats in a water maze • Pool of milky water with platform to stand on (just under the surface) • 3 groups of rats • Group 1 – frontal lobe damage • Group 2 – hippocampus damage • Group 3 – no damage

Long term potentiation and the hippocampus – rats in a water maze • Results display the importance of the hippocampus in allowing LTP • Group 3 – no damage – located platform more quickly each trial • Group 1 – frontal lobe damage – performed about as well as group 3 • Group 2 – hippocampus damage – never got better, showed no evidence of learning

Neurotransmitter and LTP • Drugs that enhance synaptic transmission tend to enhance learning • NDMA (N-methyl-D-aspartate) a neurotransmitter found on dendrites in the hippocampal region • NDMA specialised to work with the neurotransmitter glutamate • Important role in LTP

MDMA and LTP • Genetically engineered rats with more efficient MDMA receptors • Better memory • Faster learning • As compared to rats with normal NDMA receptors

Neural Plasticity • The brain is adaptive • It changes as a result of experience (learning) • Remember LTP? • New connections • New neural networks • Genes govern overall brain structure • Unclear whether or not all brain structures are as plastic as the sensory and motor cortices?

Developmental plasticity • Babies born with all 100 billion nerve cells • Each cell at birth synapses with around 2500 other neurons • By late childhood the number of connections increases to around 15,000 per neuron • By adulthood this number decreases to around 8,000 as unused connections are destroyed • Children’s brains show greater plasticity than adults, this might explain why children learn languages faster than adults

Rozenweig studies • Lab rats placed in 3 different environments after birth with different opportunities for learning • 1 – standard environment – simple communal cage with food and water • 2 – impoverished environment – simple small cage housed alone • 3 – enriched environment – large, social, with lots of stimulus objects

Rozenweig studies • All rats stayed in their cages for 80 days • When their brains were dissected the rats with enriched experience had thicker, heavier cerebral cortex

Rozenweig studies • Differences were largest in the occipital lobes and smallest in the somatosensory cortex • Also showed new synapse formation • Thicker bushier dendrites • More neurotransmitter acetylcholine • Later studies showed changes in adult rat brains also placed into different environments

Later studies • Brain weight increase as much as 10% • Neural connections increase as much as 20% • Being raised in enriched environment can increase problem solving ability • Humans raised in isolation from proper stimulation can become severely retarded • genie & victor – the wild children

Genie

Later studies • The brains of university graduates have approx 40% more neural connections than those who leave school early! • Intellectual stimulation can protect against dementia! • This is even true for twins who have identical genetic make up

Developmental plasticity • Changes as a result of experience and maturation • Synaptogenesis– new neural connections • Synaptic pruning – removal of synaptic connections that are no longer needed • Adults have less neural connections than a 3 year old!

Developmental plasticity • Sensitive period – time an organism more responsive to certain stimulation • Lack of stimulation can lead to long term deficit • E.g. closed eye from birth leads to later blindness even when eye eventually opened • Language acquisition has a sensitive period (0 – 12) remember genie! • Learning a new language in teen years can lead to the development of a second Broca’sarea!

Adaptive plasticity • The brain reorganises the way neurons in different religions operate in response to a deficit • Deficits can occur from birth or as a result of brain damage

Damage from birth - congenital • Congenital – E.g. People who are blind from birth may have occipital lobes that are used for senses other than vision • this may explain why people who are blind from birth have very good hearing or tactile sensitivity

Damage from injury • When a particular brain area is damaged e.g. stroke other brain areas can ‘take up the slack’ • This is what happens when people ‘recover’ from brain damage • Nerve cells do not regrow, rather other neurons take over the functions of the damaged cell

Damage from injury • Rerouting – neurons near damaged area seek new active connections with healthy neurons • Sprouting – new dendrites grow • May occur near damaged area of in other parts of brain • Allows shifting of function from damaged area to healthy area • ‘Relearning’ tasks like walking, eating etc. helps these new connections form

Adaptive plasticity and experience • Musicians motor and sensory areas • Taxi drivers parietal lobes • Dancers motor areas

Other bits -The Basil Ganglia • Well learned responses • Neural network ‘transfers’ to the basil ganglia

Other bits - The Dopamine Reward System • Relevant to operant conditioning • Behaviours that produce a positive consequence make us ‘feel’ good • Release of dopamine at a neural level

The Neural Mechanisms of Learning