L9 Non-associative learning

L9 Non-associative learning

1. Plasticity and learning

   • Learning that does not require linking to associating stimuli together i.e. only requires one stimulus. One of the simplest type of learning
     • 4. COMPLEXITY OF HABITUATION
       • a. Crab, Chasmagnathus granulatus
         • i. General
           • Chased by gulls in nature
           • Overhead visual stimulus induces an escape response
           • Two cycles of visual danger signals (VDS) induce escape movements
         • ii. Experiments
           • The reaction can be measured by an transducer by placing a crab in a dish and mimicking the activity of its predator
             • The dish senses crab movement
           • MASS TRAINING: Used 300 trials without intertribal intervals (It is)
             • RESULTED IN A RAPID DECREASE IN RESPONSE AND NEURAL ACTIVITY
               • Following day: response disappeared
                 • This is STRONG, SHORT-TERM HABITUATION (STH)
                   • Showing it has a different mechanism to LTH
           • SPACED TRAINING: Used 15 trials with an ITIS of 3mins
             • RESULTED IN A SLOWER REDUCTION IN RESPONSE AND NEURAL ACTIVITY
               • Following day: response didn't disappear
                 • This is WEAK, LONG-TERM HABITUATION (LTH)
         • iii. Environmental during LTH
           • Crabs trained in the same context showed long-term habituation, compared to those who changed environment
           • ASSOCIATIVE THEORY OF HABITUATION: Shows the importance of context and a phasic stimulus compared to STH
       • Eye stalk has an optic ganglion and motion sensitive giant cell within it - can add an electrode to this
         • Shining a light on the eye stalk whilst mimicking bird action causes no reaction to light

2. 1. BACKGROUND - LEARNING

   • a. Learning
     • The process of acquisition of information of consistent spatial and temporal relationship between evets in the internal and external environment
     • Two forms exist
       • Explicit
         • Related to the medial temporal lobe
         • Facts
         • Events
       • Implicit
         • Associative learning
         • Non-associative learning
         • Procedural learning
   • b. Memory
     • Storage of knowledge acquired by learning
     • The persistence of change in behavioural performance after learning has been completed

3. 2. HABITUATION

   • A decrease in behavior due to repeated exposure to innocuous stimuli. I.e. activity dependent presynaptic depression of synaptic activity
   • a. Parametric characteristics
     • i. Response decrement - a negative exponential function of the number of stimulus presentations
     • ii. Spontaneous restoration with rest
     • iii. Greater habituation with repeated periods of habituation and recovery
     • iv. Greater habituation with short rather than long inter-stimulus intervals (ISI)
     • v. Greater habituation with weak rather than strong stimuli
     • vi. Prolonged recovery following additional stimulation after the response has decremented to an asymptote
     • vii. Generalization of habituation to a stimulus in another part of the receptive field
     • viii. Dishabituation
     • ix. Habituation of the dishabituatory stimulus with repeated presentations
   • b. Aplysia - Gill withdrawal reflex
     • i. Background
       • Difficult to identify the underlying circuits in complex mammals
         • Invertebrates have basic forms of cellular activity
       • Aplysia has few nerve cells (~20,000) whereas human brain has 3 trillion
       • Simple aplysia behaviors may involve only 100 cells
       • Large neurons - 500µm, humans are 10-30µm
       • Cells are easy to dissect and record from using microelectrodes
       • Neurons are easy to identify
     • iv. Physiological response
       • Looking at the connection between motor and sensory neurons
       • We know that over time, motor neuron EPSPs stop despite continued sensory spikes
         • Can be observed in vivo using ganglions in a petri dish
           • With samples showing a decrease in response after one day that can last up to 30 days (compared to a control)
             • Showing habituation can occur in the long term
     • iv. Synaptic activity
       • Connection between sensory and motor neurons
       • Initially, excitation of sensory neurons causes multiple transmitters to be released that excite the motor neuron
         • After habituation, less transmitter is released leading to smaller contractions
           • This synaptic plasticity that only involves one synapse is called HOMOSYNAPTIC PLASTICITY
             • Not fully understood - all we know about decrease in activity
     • ii. Reflex
       • A stimulus applied to siphon = defensive gill withdrawal into mantle cavity
       • Transducer used to measure contraction
       • When the siphon is first touched, large contractions occur
         • 80th touch - contractions diminish
           • After waiting 2 hours, strong contractions occur again
             • Called a spontaneous recovery of habituation
             • To prevent habituation, placing a larger stimulus will cause a large contraction again.
               • Called dishabituation
               • THEREFORE: Two ways of testing habituation: spontaneous recovery (rest) or dishabituation (stronger stimulus)
       • Must use a mild stimulus, multiple strong stimuli doesn't change amplitude of gill withdrawal
     • iii. Individual cells
       • Motor neuron (L7) innervates the gill
       • Sensory neuron has a monosynaptic connection with the motor neuron
         • Evidence:
           • Single sensory neuron activation = EPSPS in motor neuron
           • Burst of sensory neurons = high firing frequency in motor neurons

4. 2. SENSITIZATION

   • An increase in behavior due to exposure to a noxious stimulus
   • a. Gill withdrawal reflex
     • i. Circuit
       • Applying a shock to the tail sensitizes the withdrawal reflex, causing a small stimulus applied afterwards to cause withdrawal
       • Siphon = 24 sensory neurons
       • That connect to 6 motor neurons
       • Stimulation of the tail via a tail shock acts on three classes of modulatory interneurons
       • Serotonergic (5HT) modulatory action is important
         • Tail shock activates serotonin containing facilitatory neuron, as a result it synapses onto the sensory neuron. Both synapses onto the neuron controlling gill withdrawal. CALLED HETEROSYNAPTIC FACILITATION
     • ii. Synaptic connections
     • iiii. Short and long-term sensitization
       • A single shock = enhanced response
         • Multiple shocks = higher response, longer duration
           • Shocks for four days = gill withdrawal up to 1 week
             • THEREFORE: one shock = short-term, multiple shocks = long-term