The ‘Emotional Brain’ & Threat Conditioning

• The 'Emotional Brain'


  Developments over time
• Topics, first half


  1. The emotional brain – developments over time

  2. Subcortical and cortical contributions to emotion

  3. Pavlovian threat conditioning and learning

  4. The connectivity of the subcortical pathway involved in threat conditioning

  
• Cannon's theory of emotions


  Neuroanatomical scheme (Cannon-Bard theory)
• Cannon proposed the thalamus was the structure primarily responsible for the production of emotional experiences
• According to Cannon there is no emotional experience without the thalamus – the thalamus is the source of emotional experience
• Papez circuit


  Allows the regions responsible for 'feelings' to influence physiological and skeletomotor responses via the hippocampus
• Limbic system


  • Includes the amygdalae
  • Clusters of small nuclei (13 at the last count) that form 'lumps' on the ends of the tails of the caudate nuclei
• MacLean's limbic system has been called the 'emotional brain'
• Emotion and cognition are not separable in the brain

  
• Two parallel systems


  One low-level (subcortical) and one higher level (cortical) governing emotional experience and expression respectively
• Pavlovian threat conditioning


  Trainee learns to respond (CRs) to a stimulus (CS) that predicts an aversive event (US)
• Threat conditioning is a key empirical tool for studying how the brain learns to organize responses to threat and danger
• Auditory threat conditioning circuitry


  1. Auditory receptors transmit signals to neurons in the brainstem auditory nuclei

  2. CS and US information converges in the amygdalae

  3. The lateral nucleus (LA) sends signals to the central nucleus (CE) which in turn sends signals to regions that generate responses
• Lateral nucleus (LA)


  • Major site of learning and memory in threat conditioning
  • Where CS and US information is brought together, a possible site for changes in synaptic strength (memory)
• Damaging or destroying the LA impairs or prevents CR acquisition
• The subcortical circuitry operates beneath the level of conscious awareness in people
• Acquisition of CRs can occur beneath the level of conscious awareness
• Topics, second half


  1. Role of cortical areas in threat conditioning: (1) detailed sensory analysis

  2. (2) Contextual learning and declarative memory

  3. (3) Feeling afraid

  4. (4) Extinction

  5. Remarks on brain symmetry and functional specialization
• Role of cortical areas in threat conditioning


  1. Detailed sensory analysis

  2. Contextual learning and declarative memory

  3. Feeling afraid

  4. Extinction
• Remarks on brain symmetry and functional specialization
• Auditory threat conditioning circuitry


  • Operates beneath the level of conscious awareness
  • A threat CS or US presented subliminally (person unaware of the stimulus), behavioural and physiological responses are elicited
  • Participants do not report any feelings of fear, neither spontaneously nor when instructed to introspect
  • Brain imaging has shown that the amygdalae are activated when a person is unaware of the threatening stimulus
• Acquisition of CRs


  Can occur beneath the level of conscious awareness
• In amnesic patients with damaged hippocampi, CRs are acquired in threat conditioning, but no declarative memory of being conditioned
• Conversely, when the hippocampus is intact but the amygdalae are damaged, CRs are not acquired, but there is a declarative memory of the conditioning procedure
• Threat conditioning circuitry


  • Not directly involved in the generation of fearful feelings
  • The amygdalae are not responsible for generating emotional feelings
  • Such feelings must be generated in the cerebral cortex

  
• Sensory cortical areas are important for fine discriminations between CSs


  1. If auditory cortex is then lesioned or deactivated, the animal responds to both stimuli – as if they both signalled shock

  2. Only coarse sensory analysis is possible at the subcortical level

  
• More complete threat conditioning circuit

  • Subcortical circuit is responsible for quick, coarse responses to threatening/dangerous stimuli
  • Sensory cortex is responsible for discrimination between stimuli of similar type, identification of distal stimuli (what is it that is threatening)
  • Amygdala (particularly LA) is the primary site of the learned changes that result from threat conditioning
• Hippocampus


  • Crucial for explicit learning about the CS-US contingency
  • Critical for contextual learning
  • If threat conditioning occurs in a particular context, then that context is associated with the aversive US and the trainee can learn about this association
  • Contextual cues alone can evoke CRs (though typically weak); strongest CRs when CS + contextual cues are also present
• Contextual effects are reduced or absent if the hippocampus is damaged or removed
• Feelings of fear


  • Elicited by CSs following training (if person is aware of the CS)
  • Require cortical regions beyond the hippocampus and sensory cortices (removal does not eliminate feelings)
  • Many different cortical areas are associated with emotional experiences and some of these are connected to the amygdalae and/or sensory cortical areas
• More complete threat conditioning circuit


  • Context can evoke feelings of fear indicating that hippocampus is connected to the areas responsible for feelings
  • Other regions of cortex (including cingulate, prefrontal, orbitofrontal, insula and parts of temporal lobes) are involved
• Extinction


  1. Recall that extinction does not erase prior learning

  2. An inhibitory pathway is activated/strengthened

  3. Both acquisition and retention require cortex (unlike simple fear conditioning)

  4. Inhibitory pathway descends from the cortex to the amygdalae
• Prefrontal cortex


  • Implicated in the cortical area involved in extinction
• Subcortical circuit


  • Acts quickly to put the body in a state of readiness to deal with the upcoming danger or to escape from it
  • Acts beneath the level of conscious awareness but has limited discriminative ability
• Cortical components


  • Slow with discriminative/identification ability – can determine whether stimulus is worth responding to
  • Hippocampus important for contextual learning and explicit learning about threat
  • Other cortical areas (incl. insula, cingulate and frontal) generate feelings and are important in extinction
• The brain is cut into a left half and a right half by slicing it through down the longitudinal fissure
• All the structures discussed today exist in pairs: one in the right side of the brain, on in the left
• Symmetry of structure and function


  • Seems largely true for brainstem structures and forebrain nuclei
  • Also seems to be largely the case for the cerebral cortex
• Regions/structures are duplicated in the two halves and serve the same basic function
• Functional specialization


  • Similar for other sensory systems and motor systems
  • As the function is the same on both sides, it is not lateralized
  • However, the sensory or motor representation is lateralized (e.g., left side processes information from or for the right and vice versa)