Sensitization and Dual Process Theory

• Sensitization


  The process that increases reflex responsiveness/vigour
• Dual Process Theory


  The theory that habituation and sensitization are two opposing processes that combine to produce changes in behaviour
• Reflex habituation


  Repeated presentation of a stimulus leads to a progressive reduction in the reflex response
• Aversive stimulus applied following habituation of a reflex


  Reflex responsiveness/vigour is restored (dishabituation)
• Reflex sensitization


  The phenomenon where reflex responsiveness/vigour increases after an aversive stimulus, even if the reflex had not previously been habituated
• Animals where reflex sensitization and habituation have been extensively studied

  • Planarium worms
  • Sea slug (Aplysia)
• Aplysia


  • Largest species is the black sea hare
  • Smaller species Aplysia Californica is typically used in experimental studies
  • Possesses several reflex behaviours, with the gill withdrawal reflex studied in most detail
• Sensitization occurs mainly in defensive reflexes, avoidance reflexes and related reflexes like orienting and startle reflexes
• Short term sensitization


  A single aversive stimulus may be sufficient to produce an effect lasting from a few minutes to a few hours
• Long term sensitization


  Repeated sessions separated by several hours, each consisting of a few aversive stimuli, produce effects lasting for days or weeks
• More aversive stimuli are often needed to produce a noticeable short term sensitization effect
• Sensitization is considered a form of learning, despite not being intuitive
• Dual Process Theory


  Concerns two processes: the habituation process and the sensitization process, which have opposing effects
• Processes that have opposite effects are called opponent processes
• Dual Process Theory


  1. When an eliciting stimulus is presented, both the habituation and sensitization processes are engaged

  2. The process with the larger effect determines the overall response
• Habituation process


  Always automatically engaged by an eliciting stimulus, regardless of its nature
• Sensitization process


  May be engaged, to an extent that depends on how aversive or arousing the eliciting stimulus is
• Non-aversive/arousing stimulus


  Habituation process is engaged, but sensitization process is only weakly engaged - balance tips in favour of habituation
• Aversive/arousing stimulus


  Habituation process is engaged, but sensitization process is strongly engaged - balance tips in favour of sensitization
• If the habituation and sensitization processes have equal 'strengths', they cancel each other out and there is no observable change in reflex responsiveness/vigour
• Reflexes that do not change in responsiveness/vigour over repeated elicitations


  • Pupillary reflex
  • Vestibular-ocular reflex
• Initially, the sensitization process has a greater effect on behaviour than the habituation process


  As elicitation is repeated, the effects of the habituation process overcome those of sensitization and eventually dominate
• According to Dual Process Theory, habituation and sensitization act in combination to produce functional changes in behaviour as a result of experience
• Stimulus not worth responding to


  Neither arousing nor aversive, so habituation process is activated but sensitization process is not (or only very weakly) - organism becomes progressively less responsive (habituation)
• Stimulus worth responding to


  Arousing and/or aversive, so habituation process is activated but its effects are cancelled out (or exceeded) by sensitization effects - organism's responsiveness does not decline (or increases)
• Without the sensitization process, habituation is not learning - it is merely a mechanism that reduces responsiveness regardless of whether the stimulus is worth responding to
• Sensitization and habituation are two processes that work together to produce learning - both are learning processes
• Habituation process


  1. Activated but its effects are cancelled out (or exceeded) by sensitization effects

  2. Organism's responsiveness does not decline (or increases)
• Changes in reflex behaviour


  • Result of the combined effects of habituation and sensitization processes
  • Both are part of learning
• Dual process theory


  • Sensitization and habituation are two processes that work together to produce learning
  • Both are learning processes
• Habituation effects are observed in very simple neural circuits in very simple animals like worms and sea slugs
• Aplysia nervous system


  • Has few neurons (~20,000 total)
  • Many neurons are large, some cell bodies are ~1 mm in diameter and visible with the naked eye
• Gill withdrawal reflex


  1. A brief tickle of the siphon elicits a rapid gill withdrawal

  2. Connectivity is both mono- and disynaptic
• A few tickles repeated with an ISI of no more than about 30 to 40 seconds


  Will quickly habituate the response
• Short term habituation


  • A series of closely spaced tickles produces a short term effect
  • If the animal is left to rest for a few hours, the response returns to its pre-tickling level
• The effect is not fatigue (it can be reversed by dishabituation)
• Mechanism of gill withdrawal habituation

  • The synaptic terminals of the SNs release less transmitter substance in response to the arrival of action potentials
  • This change only happens if the effects of the previous release of transmitter are present when the next release occurs (a sort of memory)
• Long term habituation

  • If sessions of repeated ticklings with short ISIs are repeated at intervals of a day or so, a long term effect is produced that can last weeks or months
  • The changes involve structural changes at the cellular level
• Regardless of the changes to synaptic connectivity that take place, habituation effects in elementary reflex circuits are due to a reduction of the efficacy (strength) of some synaptic connections within the circuitry
• Homosynaptic depression

  The reduction in synaptic connections between one neuron and the next