Association
Learning what is bad
New: the first learning within a life. After a bite that hurts, the look of what was bitten comes to mean bad.
Result: in a world where every food is new, the animals learn which ones are poison and live about twice as long as animals that cannot learn. Without the learning they lose more than half their fitness.
Until now everything the animal knows was put there by evolution. That works for poison that looks the same in every generation, but not for food that is new. In this stage all food is new: every kind of berry gets a new colour in every life, and which kinds are poison is drawn anew.
The circuit
One new cell, and one change to existing synapses:
us_pain, the teacher, is driven by pain alone. When it fires, it sends a teaching signal. (US stands for unconditioned stimulus: something that matters without any learning, as pain does.)- The synapses from the look onto the bad cells (
valence_av) can now change during a life. A synapse grows when its input was just active and the teacher fires. These are the purple arrows in the diagram.
Nothing else has to change. The bad cells already turn the animal away and block the bite, so once the look of a berry excites them, the animal treats that berry like the poison it was born knowing.
What gets the blame
A bite hurts, and something has to take the blame. Getting this right was most of the work of this stage:
- The lesson is about what sets the food apart. If pain strengthened the whole look, one bad red berry would make the animal wary of every berry. Here the adapting eyes of the previous stage pay a second time: what all bushes share cancels out before it reaches a synapse, and what is left to take the blame is the colour.
- The pain is brief. When pain lasted several ticks, the teacher was still firing after the animal had turned to the next bush, and it learned to distrust the neighbour as much as the poison.
- Every learnable synapse exists. A synapse that is not there cannot learn, so all of them are present, the new ones starting at zero.
Watching it work
One animal from the last generation, at two moments of its life. First, its first bite of a kind that is poison in this life. Before the bite the bad cells are silent and grasp fires. One tick later the pain arrives, the teacher fires, and the bad cells and no_feed switch on by reflex, as before this stage. What cannot be seen is the change the teacher leaves in the synapses.
Later in the same life the animal comes up to a bush of the same kind. Nothing hurts now, and yet the bad cells fire and no_feed holds the bite back. The animal moves on. It learned this; no ancestor could have told it.
Biology
A sea slug that is shocked after a touch learns to pull back from the touch. A rat that gets sick after a new food avoids that food afterwards. This is associative learning: something neutral (the touch, the taste) comes to predict something that matters (the shock, the sickness), and the animal responds to the first as it would to the second.
At the level of synapses, worked out in the sea slug Aplysia and the fruit fly, it has three parts. A sensory neuron that was just active leaves a short trace at its synapses. A teaching signal, released when something bad happens, reaches many synapses at once. Only the synapses with a fresh trace are strengthened. The circuit above is this picture in its simplest form.
The world
Four kinds of berry bush, all new in every life: each life draws a new colour for every kind, and which two of the four are poison. The bushes still look like berry bushes; a new kind differs from the others in colour, the way a new berry is still recognisably a berry.
Half of all bushes are poison, so an animal that cannot learn pays on every second bush it tries, all its life. Leaving everything unknown alone is not an option either: there is nothing else to eat.
Did it work?
Learning carries this stage. Silence the teacher, or freeze every synapse at its inherited strength, and the animals keep less than half their fitness. The old brain feels pain and turns away from it, but cannot change, so it pays for every poison bush it tries, every time.
The learners still lead short lives: half of what they try is poison until they have learned it, and nothing yet tells an animal that it is getting hungry.
Full results
Learning switched off freezes every synapse at its inherited strength; us_pain silences the teacher.
An earlier design had a second cell that lowered the aversion whenever something distrusted turned out to taste fine. It worked when meals were fed to the brain in a test. In a living animal it never fired: aversion blocks the bite, so food that is distrusted is never tasted again, and the mistake is never found out. That is why the lesson has to land on the right food in the first place.
The first versions of this world kept a familiar, safe kind of berry as half of all bushes. An animal that learned nothing got by very well, and a perfect learner would have gained less than a tenth of its food. With so little to win, evolution switched learning off whenever a lesson had any side effect. Removing the familiar food turned this around.