Box 4.3

• Limbic system: a group of structures that governs the emotional reaction to stimuli that lies under the cerebral cortex and encircles the upper brain stem
• Two very important systems for learning:

Amygdala

• Plays a key role in linking new affective responses to previously neutral stimuli
• Regulation of fear, why there was early interest in this structure
• Rare, but people who have experienced damage to this structure exhibit of lack of fear to stimuli that should signal danger
• Processing fear-related stimuli activates the amygdala
• Electrically stimulating the amygdala produces feelings of fear
• Mediates the conditioning of fear to signals of danger
• Motivates organism to avoid dangerous situations
• Fear can grow out of proportion to the true level of danger, which leads to a phobic response, interfering with everyday life

Different parts of the amygdala have different functions

• Lateral (side) nuclei
  • In fear conditioning, neural signals elicited by the CS and US converge within the lateral amygdala
• Basal (lower) nuclei
  • Neurons in this region along with the lateral region (together known as the basolateral amygdala) provide biological link that endows a CS with the capacity to elicit fear
• Central nuclei
  • Electrical stimulation → range of behavioral and physiological responses indicative of fear
    • Freezing
    • Enhanced startle to a loud stimulus
    • Change in heart rate
• Role in learning has been studied using stimulation, inactivation, lesioning, and recording
  • Lesioning
    • Leads to fearless creature that does not avoid dangerous situations
    • Ex: rats & cats
      • Rats are normally fearful of cats (predators)
      • If the amygdala of a rat has been lesioned, the rat will approach the cat with no fear at all
    • Leads to disruption of cues (CSs) that have been paired with an aversive event (a shock US)
      • Shocks are normally used to produce fear, like freezing or startling
• Multiple pathways transmit information about the CS
  • Direct path from sensory thalamus provides a coarse input that sacrifices stimulus detail for speed
  • Other CS inputs arrive from the cortex and provide a slower, but more precise, representation of the features of the CS

<aside> 💡 NMDA receptor: gated channel in the nervous system that many forms of learning depend on

• Engaging NMDA receptor initiates cellular process that enhances synaptic connectivity allowing a previously neutral cue to elicit a new response

• Requires strong input to unlock (shock US)

• Cue-elicited input to the basolateral amygdala may gain capacity to drive the neural machinery that generates fear

• Chemically inactivating region blocks fear conditioning </aside>

• Output of fear circuit → organizes the expression of fear