PSY260 Lecture 3: Nonassociative learning

Habituation

• “Simplest form of learning“

• Decrease in the strength or occurrence of a behaviour due to repeated exposure to the stimulus that produces the behaviour

Acoustic Startle Behaviour

• Initially, rodents respond to a loud noise with a reflexive muscular response (jump/freeze)

• After many repeated trials where the noise is not followed by any danger, the response decreases

  • Rodents are made to think that the stimulus is unimportant because nothing follows it

Infant Viewing Patterns

• Initially, infants fixate on a new visual stimulus

• After many repeated trials of seeing the same stimulus, infants appear to lose interest

  • They stare longer at novel stimuli

  • Habituation → ↓ staring

Characteristics of Habituation

• Habituation is ubiquitous → found throughout the animal kingdom

• 10 key characteristics

Decrease in Response

• Repeated application of a stimulus results in a progressive decrease in some parameter of a response to an asymptotic level

• Decrease is often exponential, but may also be linear

• We sometimes see an initial period of sensitization before this decrease in response

  • Small response increase, then larger decrease

• Ex. Acoustic startle

  • Starts at baseline behaviour

  • Repeated stimulus presentation → ↓ response intensity

  • Reaches some asymptote and levels off

Spontaneous Recovery

• If the stimulus stops and some time passes, behaviour gradually returns to normal

  • Spontaneous because all it needs to recover is the passing of time

Potentiation of Habituation

• After multiple series of stimulus repetitions and spontaneous recoveries, the response decrease becomes more rapid and more pronounced

  • Many repetitions of habituation/spontaneous recovery → effects become more permanent

  • Baseline is reduces

Frequency of Stimulation

• Taking breaks between sessions of repeated stimuli makes habituation develop more slowly but last much longer

  • Ex. studying → spaced repetition vs. cramming

• More frequent stimulation → quicker and more pronounced habituation

Weaker Stimulus = More Habituation

• The less intense the stimulus, the more rapid and pronounced the habituation process

  • Stronger stimulus → less habituation

  • Weak/useless stimuli are ignored, but stronger/painful stimuli are more likely to be important and will get attention

  • Noxious stimuli → sensitization

• Makes sense because weaker stimuli are less likley to be important and easier to get used to

Effects can Accumulate Beyond Behavioural Asymptotic Level

• Asymptote for behavioural response can be at or above 0

• Additional habituation training beyond the asymptotic level can be observed as delayed onset of spontaneous recover

  • Habituation seems to continue even after we can measure

  • Habituation training continues beyond asymptotic level

  • “Overlearning“

Stimulus Generalization

• Within the same stimulus modality, the response decrease shows some stimulus specificity

• Generally, responses only decrease to the habituating stimulus (specific)

• Very similar stimuli → may have some generalization

Dishabituation

• Presentation of a different stimulus results in an increase of the decremented response to the original stimulus

  • New stimulus can temporarily recover repsonses to habituated stimulus

  • Fades quickly if the original stimulus is presented repeatedly again

• Recovery, but not spontaneous → because a new stimulus is presented

Habituation of Dishabituation

• Repeated application of the dishabituating stimulus → amount of dishabituation decreases

  • You “habituate“ to the “dishabituating stimulus“ so that your original response doesn’t recover as much

  • If you present the different noise multiple times, it will gradually lead to less recovery of the initial response

Long-term Habituation

• Some stimulus repetition protocols may result in properties of the response decrement that last long-term

  • Long-lasting memory for habituation training

  • Believed to be related to changes in protein synthesis, LTD

Short-term Habituation

• Repeated stimulation depletes transmission:

  • Synaptic depression

• Presynaptic neuron: reduction in neurotransmitter release (uses up glutamate in available vesicles)

Long-term Habituation

• Related to LTD

• Presynaptic neuron:

  • May produce less NT over time (not just using it up, but producing less)

  • May alter neurotransmitter reuptake (may stay in synapse or not last as long)

• Postsynaptic neuron:

  • Receptos can become less responsive and less sensitive to NTs

  • Can lead to ↓ # of receptors on the membrane

  • Some sensory-motor synapses actually get entirely pruned away (most extreme)

• Can also alter gene expression

• These are the most long-term changes

Sensitization

• Increase in the strength or occurrence of a behaviour due to exposure to an arousal or noxious stimulus

• Stimulus is environmentally informative → we want to keep the response to it

  • Avoid pain in future

Sensitization: More Acoustic Startle

• Initial habituation to settle animals to a stable baseline (for sake of study)

• Foot shock applied at dashed line to the experiimental group only

  • Shock = noxious stimulus

• Sensitization → increased responding to startle stimulus (tone) after shock

Characteristics of Sensitization

• Sensitization is also ubiquitous → found throughout animal kingdom

• Set of common characteristics:

  • Noxious (painful) stimuli work better than weak

  • More generalization, less stimulus specificity (compared to habituation)

  • Can develop with just a single trial → enough to identify noxious stimulus and get sensitized

  • Mechanism to increase responses to stimuli that are important (alert)

Sensitization vs. Habituation

• Habituation

  • Decreases behaviour

  • Innocuous stimulus

  • Repeated exposure

  • Stimulus specific

• Sensitization

  • Increases behaviour

  • Noxious stimulus → environmentally relevant

  • Single trial (more can help)

  • Generalizes across stimuli

Types of Sensitization

• Peripheral sensitization

• Central sensitization

Peripheral sensitization

• Occurs in peripheral nerves

  • PNS

• When tissues are damaged/inflamed, sensory nerves can become more responsive. to pain signals

  • Ex. burn your hand → hurts when something else non-painful touches it

Central sensitization

• Occurs within brain and spinal cord

  • CNS

• This is more relevant to us

Role of Sensitization in Pain

• Both peripheral and central sensitization can lead to allodynia and hyperalgesia

  • Allodynia: ↑ response to non-painful stimuli

  • Hyperalgesia: increased pain sensitivity (↑ response to painful stimuli)

Aplysia Californica

• Habituation

  • Initially, touching the siphon leads to retraction of the gill

  • Repeatedly touching the siphon with a non-painful, benign stimulus leads to reduction in gill retraction (↓ sensitivity)

• Sensitization

  • If the touch is paired with a painful shock to the tail, this can override the habituation and lead to increased gill retraction after future siphon touches (↑ sensitivity)

Aplysia Californica: Baseline circuitry

• Siphon: attached to sensory neuron

• Tail: attached to sensory neuron

• Gill: motor neuron

• Pathway:

  • Sensory neuron detects benign tough on siphon

  • Sensory neuron triggers glutamate release at synapses on interneuron and motor neuron of gill

  • Triggers gill retraction

Aplysia Californica: Habituation

• Repeated stimulation of siphon eventually leads to habituation → ↓ gill retraction

• Sensory neuron triggers glutamate release, triggering motor neuron to retract gill

  • Repetition: glutamate gets used up

  • ↓ NT release → ↓ motor neuron stimulation

  • ↓ retraction response (habituation)

Aplysia Californica: Sensitization

• After habituation to siphon touch, introducing a shock to the tail can immediately cause gill retraction behaviour to increase again in response to siphon touch

• Shock to tail triggers tail sensory neuron

  • Passes signal to modulatory interneuron

  • Modulatory interneuron releases serotonin to override reduced glutamate release → trigger motor neuron firing → ↑ gill retraction

Mechanisms of Sensitization

• New/stronger synaptic connections between neurons

  • Increase in NT release

    • Typically excitatory NTs (glutamate)

  • Modulatory interneurons can override habituated responses in the presence of pain

    • In Aplysia, this is done via release of serotonin

Familiarization

• Reduction in the sense of novelty (sense that something is new) as an individual becomes more acquainted with a stimulus/environment

  • Similar to the concept of habituation

  • Related to integrating stimuli into new or existing schemas

  • Ex. walking into class 3+ times → you recognize it and it feels familiar

• Improved ability to recognize/process a particular stimulus

  • Increase synchrony and efficiency of neural pathways (LTP and LTD both play a role)

Familiarization: Remember-Know Paradigms

• Remembering: conscious recollection of specific details about a past event

  • Active recall of detail

• Knowing: a feeling of familiarity without specific details

  • Sense that you know without knowing detail

  • Tip-of-the-tongue

Aplysia Video

• Repeated touching of gill reduces gill withdrawal behaviour → habituation

• Shock to tail → sensitization → gill withdrawal returns

  • Repeated pairing of tail and siphon stimuli can cause prolonged sensitization

• Abdominal ganglion: holds many of the cell bodies involved in this behaviour

  • Connects to gill and siphon nerves

• Siphon: sensory neurons with release glutamate onto interneurons and gill motor neurons → gill retraction

  • Habituation: glutamate release is depressed, gill withdrawal weakens

• Tail shock: sensory neurons synapse onto modulatory interneurons, which release serotonin onto interneurons and motor neurons → override reduced response and gill retraction returns

  • Short-term: lasts ~ 1 hour