Evolution of the Brain

600m ya - Bilaterians and Steering

Valence

• Bilaterians are the only animals that have brains
• Nematodes (legless wormlike creatures about the size of a grain of rice) emerge in the Edicaran period from 635 to 539m ya.
• Brain had 302 neurons (against 85 billion today
• Initial steering is obtained through assessing the valence (goodness or badness) of a stimulus, and going towards the things that smelled good and away from the things that smelled bad.
• There were negative and positive valence sensing neurons and move forward neurons and turning neurons.
• The various sensory inputs acted as votes for going one way or another and the first brains evolved as a mega-integration place to take in all these votes and then decide who had won and thus where to steer.

Emotions

• Affect is the name for the unifying foundation of emotions
• In addition to valance (good or bad), there is arousal (high or low)
• A primitive good mood encourages feeding, digesting, and sexual activity
• A primitive bad mood inhibits these activities
• An aroused good mood leads to exploiting nearby food sources or sexual partners
• An aroused bad mood leads to escaping from bad feelings - hunger, fear
• The brain generates affective states using neuromodulators like dopanmine and serotonin.
• In the nematode, dopamine is released to create arousal and drive the search for food and serotonin is released to suppress arousal and drive the enjoyment of digesting it.
• Dopamine is less about liking things and more about wanting them.
• Other neuromodulators - norepinephrine, octopamine, and epinephrine drive escape behavior by suppressing the effectiveness of serotonin and stopping an animal from being able to rest and feel safe - acute stress response.
• Opioids initiate recovery processes and inhibit negative valence neurons to stop and recover from stress episodes.
• Chronic stress turns off arousal and motivation, activates serotonin and leads to numbness and depression (anhedonia). It can cause learned helplessness
• Affect answers two questions:
  • Do I want to expend energy by moving?
  • Do I want to stay here or leave?

Associating, Predicting, Learning

• The digestive organs are under the control of the nervous system
• Conditional reactions are involuntary associations - associative learning happens automatically without conscious involvement.
• At the same time as valence, the ability to use experience to change what is considered good and bad also emerges.
• Learning in biological brains has always been continual.
• Pavlov’s conditional reflexes are always strengthening (acquisition) or weakening (extinction) with each new experience. Extinction may be followed by spontaneous recovery (instantaneous) or reacquisition (faster than first time)
• Spontaneous recovery is a primitive form of long-term memory.
• The credit assignment problem - which cue really predicted something subsequently happening?:
  • Eligibility traces - Immediately follows cue
  • Overshadowing - Pick strongest cue
  • Latent inhibition - Pick the cue you haven’t seen before.
  • Blocking - Use existing cues and ignore others.
• Learning occurs when synapses change strength or when new ones are formed or old ones are removed.
• Association, prediction, and learning emerged to tweak the goodness and badness of things

500m ya - Vertebrates and Reinforcing

Reinforcement Learning

• Cambrian period (Cambrian Explosion) is 540-485m ya.
• The brains of all vertebrates, from fish to humans, develop in the same initial steps:
  • Brains differentiate into three bulbs - a forebrain, midbrain, and hindbrain
  • The forebrain unfolds into two subsystems:
    • The cortex and the basal ganglia
    • The thalamus and the hypothalamus
• Animals learn by first performing random exploratory actions and then adjusting future actions based on valence outcomes.
• Reinforcement learning is the ability to learn arbitrary sequences of actions through trial and error with reinforcing and punishing depending on the valence outcomes.