The Neural Mechanisms of Learning

Its all physical! The Neural Mechanisms of Learning

Neural basis of learning • Human brain follows a predictable pattern of growth and development, with different structures and abilities progressing at different rates and maturating at different points in the lifespan. • The brain is NOT a fixed solid organ. • The basic brain structure (i.e. Lobes, hemispheres, cerebral cortex) is established well before birth but the brain continues to develop after birth.

Neural basis of learning • Neural pathways extending within and between different areas of the brain are NOT “hardwired” like a computer. • Neurons are soft, flexible living cells. • Neurons can change their sizes, shapes, functions, connections with other neurons and patterns of connections. • These changes can occur at any stage in the lifespan, including old age. They are influenced by the interaction of biological processes that are genetically determined and by experiences in everyday life.

Neural basis of learning • When neurons communicate with each other they do so by sending a neurotransmitter comprising electrochemical messages across the tiny space between the axon ending of one neuron (that sends the neurotransmitter) and the dendrite of another (which receives the neurotransmitter). • The tiny space is called the synaptic gap.

Neural basis of learning • The synaptic gap is one component of the synapse. • The other two components of the synapse are the axon ending of the “sending” or presynaptic neuron and the dendrite of the “receiving” or postsynaptic neuron.

The Synapse • The synapse is the site of communication between adjacent neurons. It includes the synaptic gap and a small area of the membrane of each of the connecting neurons. • The act of sending a neurotransmitter across the synaptic gap actually changes the synapse. Some dendrites that receive the neurotransmitter messages can grow longer and “sprout” new branches or tips when used, whereas others are “pruned” away if not used. • Every day we form or “grow” millions of new synapses and millions of others disappear through disuse. • At least some of these changes seem to depend on our unique experiences of that day.

Neural basis of learning • As we learn by the constant stream of new experiences in everyday life, our brain modifies its neural pathway (or circuits) and neural connections within and between pathways, thus literally changing its structure and function by “rewiring” itself. • Existing connections between neurons can reorganise and new pathways can form and strengthen during the learning process, thus making communication across a connection and along a pathway easier the next time.

Plasticity • The ability of the brain to reorganise the way it works is referred to as plasticity. • Plasticity is a property that makes learning and memory possible, caters for the brain to be continually responsive to environmental input and thus assists us to adapt to life’s ever-changing circumstances.

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

Neural Basis of learning

Effects of neurotransmitters • Neurotransmitters work by attaching (“binding”) itself to the receptor site on the receiving neuron. It will have either of 2 effects: • 1. An excitatory effect which stimulates or activates a neural impulse in another neuron. • 2. An inhibitory effect which blocks or prevents the receiving neuron from firing.

Neurotransmitters • Each type of neurotransmitter has a chemically distinct shape and when released by the presynaptic neuron the neurotransmitter searches for the correctly shaped receptor sites on the dendrites of the postsynaptic neurons. • Like a key in a lock or a piece of a jigsaw puzzle, a neurotransmitter’s shape must precisely match the shape of the receptor site on the postsynaptic neuron’s dendrites for the neurotransmitter to have an effect on that neuron. • Post synaptic neurons can have many different shaped receptor sites on its dendrites and may therefore be able to receive several different neurotransmitters.

Neurotransmitters • The number of neurotransmitters that a neuron can manufacture varies. • Some neurons manufacture only one type of neurotransmitter whereas others manufacture two or more. • Researchers have identified more than 100 different neurotransmitters. • Two major neurotransmitters involve in learning and memory are glutamate and dopamine.

Neurotransmitters • Communication between neurons is usually a chemical process involving neurotransmitters. • Sometimes though, communication between neurons can be electrical when axons transmit messages directly to other axons or directly to the cell body of other neurons or when dendrites communicate directly with other neurons’ dendrites.

Physical brain changes • When learning occurs there are physical changes in the brain at the neuronal or “cellular” level. • Learning can result in new synapses forming or the connections between neurons at the synapses within neural pathways be strengthened. • Psychologists who adopt the biological perspective thus describe learning as a process that involves synapse formation and the building of neural pathways in the brain.

Hebbian theory (1949) • Canadian psychologist Donald Hebb had an idea that learning involves the establishment and strengthening of neural connections at the synapse. E.g. Learning the piano establishes new neural connections and practising strengthens the connections. • Learning results in the creation of “cell assemblies” or neural networks (interconnected groups of neurons that form networks or pathways. • ‘neurons that fire together wire together’

Hebbian theory (1949) • Neurons in a network send messages to other neurons within the network • Messages from one network may also be sent to other networks • Small networks may also organise into bigger networks. • As a result of the above, the same neurons may be involved in different learning or in producing different patterns of behaviour, depending on which combination of neurons is active.

Hebbian theory (1949) • When a neurotransmitter is repeatedly sent across the synapse, the presynaptic neuron and postsynaptic neuron are repeatedly activated at the same time. This has the effect of actually changing the chemistry of the synapse, leading to a strengthening of the connections between the neurons at the synapse. • When the synaptic connection is strengthened this makes the neurons more likely to fire again and to transmit their signals more forcibly in the future. • Neurons that do not fire together weaken their connections, making them less likely to fire together at the same time in the future.

Hebbian Learning • “Neurons that fire together, wire together”. • Kandel’s research on memory formation was influenced by and provided evidence for Hebb’s theory. Kandel had to induce learning in the sea slug and was able to observe changes at the synapse.

Long term potentiation • Long term potentiation refers to the longlasting strengthening of the synaptic connections of neurons, resulting in the enhanced or more effective functioning of the neurons whenever they are activated. • The effect of LTP is to improve the ability of 2 neurons to communicate with one another at the synapse. • LTP is now recognised as a crucial neural mechanism that makes learning possible in humans (and animals with nervous systems)

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 • Earliest studies of LTP in learning comes from studies with animals. • Morris et al (1982) investigated the role of both LTP and the hippocampus in spatial learning using rats and a water maze. • Pool of milky water with platform to stand on (just under the surface) – compared performance of rats swimming towards the platform. • 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 spatial learning and LTP in learning. • 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 • More evidence for the role of LTP in learning comes from studies indicating that drugs which enhance synaptic transmission tend to enhance learning • NMDA (N-methyl-D-aspartate) a neurotransmitter receptor found on dendrites particularly in the hippocampal region • NMDA is specialised to receive the neurotransmitter glutamate and together they have an important role in LTP.

Neurotransmitters and LTP • Without NMDA at the site of a dendrite where glutamate is received, any message carried in glutamate from a neuron cannot be “accepted” by a postsynaptic neuron. • This led researchers to examine whether they could influence learning by manipulating the capability of NMDA receptors in postsynaptic neurons during learning tasks.

NMDA and LTP • US psych. Tsien (2000) used genetically engineered mice with more efficient NMDA receptors. • Maze learning and object recognition tasks. • Mice had better memory (even a day later) • Mice had faster learning • As compared to rats with normal NMDA receptors in the control group. • This raises the possibility of developing drugs that might enhance the learning process (and memory) by activating or mimicking NMDA – but much more research remains to be done. • Other biological processes and psychological factors are also important though in learning.

Neural Plasticity • The brain is adaptive • It changes as a result of experience (learning) – neurons change as well as other types of brain cells. • Remember LTP? • New connections • New neural networks • These activities change the brain’s physical structure and function. • The brain can reorganise and reassign its neural connections and pathways based on which parts of it are overused or underused.

Neural plasticity • Brain structure is constantly remodelled by experience. • This ability to change is known as neuroplasticity, neural plasticity or simply plasticity. • Plasticity is the ability of the brain’s neural structure or function to be changed by experience throughout the lifespan.

Plasticity • Brain plasticity – from embryonic development to, and including, old age. • E.g. Learning our native language, to text message as an adult etc. • Genes govern overall brain architecture but experience guides, sustains and maintains the details. • The neural activity underlying learning occurs in a systematic, not haphazard way. • Unclear whether or not all brain structures are as plastic as the sensory and motor cortices which have a higher level of plasticity than others.

Plasticity • A developing individual’s brain is more plastic than an adult’s. • At specific times in development it seems the brain is more responsive to certain types of experiences. • Generally, the more complex the experience in terms of the variety of sensory input, the more distinctive the structural change in neural tissue involved in the experience – for both children and adults.

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 25 days after birth with different opportunities for learning • 1 – standard environment – simple communal cage with food and water (no opportunity for complex stimulation and informal learning) • 2 – impoverished environment – simple small cage with a single rat housed alone • 3 – enriched environment – large cage, social with 10-12 rats, with lots of stimulus objects changed daily (lots of opportunity for complex stimulation and informal learning)

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 in cortical tissue were unevenly distributed throughout the cerebral cortex. • Differences were largest in the occipital lobes and smallest in the somatosensory cortex • Also showed new synapse formation • Existing synapses were bigger • Thicker bushier dendrites, larger neurons • More neurotransmitter acetylcholine (prominent in cerebral cortex of animals) • Later studies showed changes in adult rat brains also placed into different enriched environments.

Further studies • Changes in young rats in further studies were more pronounced than those in adults. • Even when rats weren’t placed in differed environments until well into adulthood, changes occurred.

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 and to effectively deal with a more cognitively demanding and complicated environment. • Humans raised in isolation from proper stimulation can become severely retarded

Later studies • The brains of university graduates have approx 40% more neural connections than those who leave school early! (found in autopsies) • Studies conducted comparing life experiences of older people suggest a stimulating environment may delay the onset of some of the adverse effects of ageing. • Intellectual stimulation can protect against dementia and cognitive decline. E.g. Travel, extensive reading, short courses at TAFE, being socially active in groups. • This is even true for twins who have identical genetic make up

Developmental plasticity • Changes in brain’s neural structure as a result of experience and according to maturational blueprint or plan • Predetermined plasticity thus influenced by genes inherited but also subject to influence by experience. • After birth the infant brain forms far more synaptic connections than it will ever use. This process of forming new synapses is called synaptogenesis.

Developmental plasticity • Synaptogenesis– new neural connections • Happens so rapidly in the first year of life the total number of synapses increase 10 fold. • It is believed to allow the brain to initially have the capability to respond to the constant stream of new environmental input (e.g. To deal with all the sensory information that bombards the sense organs). • Following this proliferation though, the brain undertakes a process of eliminating synaptic connections.

Developmental plasticity • Synaptic pruning – process of elimination of synaptic connections that are no longer needed. • Synaptic pruning occurs in different areas of the brain at different times. e.g. Visual cortex – complete by age 10 Frontal cortex – complete by age 14 • Adults have less neural connections than a 3 year old by about 40%

Developmental plasticity • Experience determines which connections will be retained and those that are not decay and disappear. • It is a “use it or lose it” process. • Certain periods in an individual’s development are particularly well suited to learning certain skills and gaining knowledge. These periods are called sensitive or critical periods.

Developmental plasticity • Sensitive period – a specific period of time in development an organism is more responsive to certain environmental stimulation or experiences. • Sensitive periods usually have specific onset (“start”) and offset (“end”) times. • Lack of stimulation can lead to long term deficit e.g. Eye kept closed or doesn’t function properly due to an abnormality of cat, human or monkey from birth leads to later blindness even when eye eventually opened The changes response for this loss of visual function occur in the visual cortex. In cats and monkeys, the sensitive period extends to several months of age.

Developmental plasticity • Closing in the first 2 months has a much greater effect than closing it for the 5th. or 6th. months. • In humans the sensitive period is up to 6 years. Keeping one eye closed for several weeks during the sensitive period produces a measurable visual impairment. • Sensitive periods = “windows of opportunity for learning” They are the best possible or optimal times for relevant learning to occur. • Language acquisition has a sensitive period (0 – 12 years) – window gradually closing from 7 years of age. • Learning a new language in teen years can lead to the development of a second Broca’s area!

Developmental plasticity • Sensitive periods indicate that brain development goes through certain periods during which some synaptic connections are most easily made and some neural pathways are most easily formed, assuming there is exposure to the appropriate environmental stimulus. • Synaptogenesis early in development may reflect a genetically directed preparation by the brain to respond to certain types of experiences. This has been described as an experience-expectant process (the brain priming itself for expected experiences).

Adaptive plasticity • Adaptive plasticity refers to changes occurring in the brain’s neural structure to enable adjustment to experience, to compensate for lost function and/or to maximise remaining functions in the event of brain damage. • Most apparent in recovery from trauma due to brain injury (inflicted or acquired) • The brain’s response to injury depends on factors: • Location of damage • Degree and extent f damage • Age at which damaged is experienced

Adaptive plasticity • However, no clear line between two types of plasticity – both are influenced by experience. • Maturing brain of a child has the capacity to adapt to and therefore recover from trauma more effectively than the mature brain of an adult. • Reorganisation of brain structure that occurs can be immediately or continue for years involving a number of different processes. (e.g. Neuronal level, larger areas of brain tissue, transfer of function to another hemisphere).

Adaptive plasticity • Neuronal level – 2 processes for recovery • Rerouting • Sprouting • Both involve forming new connections between undamaged neurons. • Rerouting – an undamaged neuron that has lost a connection with an active neuron may seek a new active neuron and connect with it instead. • Sprouting – growth of new bushier nerve fibres with more branches to make new connections. Thus sprouting involves nerve growth AND rerouting.

Adaptive plasticity • Sprouting may occur not only in the damaged area but also in brain areas far away from the damaged area. • Both rerouting and sprouting enable the growth and formation of entirely new neural connections at the synapse to compensate for loss of function due to brain damage. • The brain’s adaptive plasticity enables it to take over or shift functions from damaged to undamaged areas.

Adaptive plasticity • Plasticity can occur at all levels of the CNS (cerebral cortex to spinal cord). • For neurons to reconnect or form new connections they need to be stimulated through activity. • The younger the individual, the greater the likelihood of successful “relearning” and subsequent new learning.

The Neural Mechanisms of Learning