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PSYC 2330 (214)

Tuesday, Nov 27/2012 - Lecture 22
Tuesday, Nov 27/2012 - Lecture 22

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University of Guelph
PSYC 2330
Francesco Leri

Tuesday, November 27/2012 PSYC 2330 Lecture 22 Questions &Answers November 29 • MACK 238 2:30-3:30 • MACK 234 3:30-4:30 • Not a mandatory class Synaptic Plasticity • Slightly different from the slides that were posted at the beginning of the year • New slide organization is available on courselink • Plasticity refers to biological changes in the function and structure of the brain ◦ These changes are the result of learning ◦ These changes occur in the adult brain and are induced by learning ◦ If you have a memory of something (any kind of memory) your brain is different from before you had that memory ◦ What are the biological mechanisms that underly these changes? Hebb Rule • Learning is a process by which experiences change our nervous system. These changes are referred to as memories. • Changes occur in the systems responsible for thinking, perceiving, performing and planning • Hebb proposed a theory for how stimuli are represented in the brain ◦ MA(1932) : Conditioned reflexes and unconditioned reflexes and inhibition ◦ Spinal reflexes are the result of pre-natal pavlovian conditioning ◦ Analysis of neural learning mechanisms underlying Pavlovian conditioning, inspired by the rule of contiguity • Wrote several books including the Organization of Behaviour • Included his theories about how the brain stores information about memories • Neuroscientific methods were not as advanced in his time ◦ He was a pioneer in his theories ◦ His predictions were validated years later with modern techniques of neuronal analysis ◦ It is important that you understand his idea: ▪ How can we represent concepts in the brain in a permanent way ▪ Implies the creation of an association between one event and another ▪ If you have a representation S1 and another representation S2 that are activated at the same time, they will come to be connected together ▪ Connections between neurons if they are firing at approximately the same time ▪ Could be individual neurons or groupings of neurons Neurological Description of the Neural Mechanism • “Let us assume, then, that the persistance or repetition of a reverberatory activity (or 'trace') tends to induce lasting cellular changes that add to its stability. The assumption can be precisely stated as follows: When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes a 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 the cells firing B, is increased” • Important Idea: a connection between two neurons takes place only if both neurons are firing at about the same time. • Hebb Rule ◦ Abiological system that appears to follow the rule of Hebb is called a Hebb synapse ◦ You have an axon and a presynaptic terminal ▪ If Neuron Aand B fire at the same time and something occurs at the synapse to create a connection between the two, the synapse is called a Hebb synapse ▪ Hebb didn't know what changes had occured or what ▪ The neurons must be firing at the same time ◦ Once the bond has been formed, the activation of the presynaptic neuron will cause stronger activation of the postsynaptic neuron Classical Conditioning & The Hebb Rule • If a synapse repeatedly becomes active at about the same time the post-synaptic neuron fires, changes will take place in the structure or chemistry of the synapse that will strengthen it. • For simplicity each is represented by one neurone (Even though in reality it is much more complicated than that) • If you activate puff of air neuron, it activates the blink neuron ◦ This connection is already very strong, and does not need to be learned • The connection between the tone and the eye blink neuron is not very strong initially ◦ As the tone is heard and the eye is blinking, both neurons are activated at the same time ◦ The connection between the tone neuron and the blink neuron therefor becomes stronger ◦ Eventually the strength of the connection between the tone and blink neurons will become strong enough for the tone neuron to activate the blink neuron when the tone neuron is activated ◦ The chemical and anatomical changes should occur at the synapse T • The synapse P will always be stronger than the synapse T because it is the innate response that does not require learning to acquire • This is the biological explanation for classical conditioning Activity-Dependent Changes in Synaptic Transmission • Two types of learning: ◦ 1. Non-associative ◦ 2. Associative • Non-associative ◦ Use it or lose it learning ◦ Does not follow the Hebb rule • Associative learning ◦ Follows the Hebb rule ◦ More related to this course ◦ Classical conditioning ◦ Operant conditioning ◦ Habituation is not associative plasticity ◦ Type of plasticity demonstrated in previous graph with neurons (Synapses that follow the Hebb rule) Non-Associative • Modification of topographical maps in somatosensory cortex ◦ Cortical maps change with the use of afferent (input) pathways ◦ Examples ▪ Details of topographical maps vary considerably from one individual to another ▪ Increased use of select fingers enlarges the cortical representation of those fingers ▪ Intense disuse of a hand produces enlargement of cortical areas neighboring the hand- area (ie. The face) • This enlargement can cause a remapping of referred sensation • Many animal and human studies that explore non-associative learning • Example is the modification of the map of the somatosensory cortex ◦ Located behind the primary motor area ◦ Map of the body ◦ If you stimulate different areas of the somatosensory cortex you will experience sensations in corresponding parts of the body ◦ Map is organized in a logical way such that areas that are adjacent on the body are adjacent in the brain ◦ Those areas that are more likely to come into contact with the external context are more widely represented in the map ◦ These maps are modified by experience • Lines of evidence for modification ◦ Every individual's somatosensory cortex is highly different from everyone else ◦ If you surgically attach two fingers together of a monkey and then over stimulate both fingers together, the fingers will eventually be represented as one finger in the cortex ◦ If you remove a finger or physical body part, that area of the brain will adapt to use that space ▪ With no input coming to the area, the brain remaps itself ◦ As a result of increased or decreased stimulation, the brain will rewire and change Repeated Stimulation of a Single Synapse • Pre-synaptic action potentials leave an after-effect that alter the release of neurotransmitter in response to subsequent action potentials • If the after-effect increases the amount of neurotransmitter released by successive action potentials, it is referred to as synaptic enhancement • Conversely, if the after-effect reduces the release by subsequent action potentials, it is referred to as synaptic depression • Stimulator that goes into neuron A • Is not a single stimulus, but a train of stimulation • Check to see how much response is observed from repeated stimulation • What you'll observe is increased activation of B with each stimulation ofA • This train causes the synapse to be, for a certain
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