Habituation
Getting used to what is always there
New: no new cells. The eyes get used to what they keep seeing: what stays in view fades.
Result: the animals stop biting at bushes that never give anything, and spend far less of their life standing still. Switching the adaptation off costs them more than half their fitness.
The animals of the previous stage respond to a look in the same way every time they see it. An emptied bush still looks enough like food, so an animal can stand in front of it for a long time, biting. An animal should stop responding to something that stays the same and keeps giving nothing.
This stage changes one thing, in the eyes: they get used to what they keep seeing. To make that worth having, the world gets a dud, a bush that looks like food and never carries a berry.
The circuit
No new cells. Each of the eyes’ look inputs keeps a slow average of what it has seen over roughly the last fifty ticks, and passes on only the difference between what it sees now and that average.
Every neuron downstream of the eyes now gets this adapted look, and two things follow:
- What stays in view fades. An animal standing in front of a bush sees its look sink into the average. The good cells, driven by that look, fall silent, and with them the urge to bite. The animal moves on.
- What is unusual stands out. All berry bushes look alike in most respects, and that shared part is nearly always in the average. What passes through is the part that differs: the colour of this bush. Nothing uses this yet; the next stage does.
Watching it work
One animal from the last generation, at a dud. The two vis+0 rows are two look inputs of the eye that points straight ahead (vis for vision, +0 for its direction).
The animal walks up and bites. The dud does not change, yet what the eye reports sinks a little with every tick. The good cells and grasp hold for a while, then drop out, and the biting stops. When the animal turns away, the look inputs go below zero: the eye is still used to the dud and reports less of that look than usual.
Biology
Step into a room with a strong smell and after a few minutes it is gone. The smell is still there; the receptors have stopped reporting it. Sensory neurons respond to change, and a steady input carries no news. This is sensory adaptation.
Habituation is the same thing seen in behaviour. A snail that is touched pulls in its horns; touch it again and again and it stops bothering. It is the simplest kind of learning, found in every animal that has been tested, including ones with a few hundred neurons.
The world
The world of the previous stage with one more kind of bush, the dud: it looks like a berry bush and never carries a berry. Its colour is drawn anew for every life, so nothing an ancestor knew can tell an animal to leave it alone. When the dud happens to have an attractive colour, an animal whose eyes do not adapt will stand at it and bite for as long as it lives.
Did it work?
There is no group of neurons to silence here, since the change is in how the inputs work. The matching test is to switch the adaptation off in the evolved animals and let them live again.
The animals rely on it heavily: without adaptation they keep well under half their fitness. Against the old brain the gain is smaller and uneven across seeds. The control’s fitness swings from one generation to the next, depending on whether it happens to like the colour of that generation’s dud.
The clearest difference is in how a life is spent. Animals whose eyes adapt stand and bite at nothing less than half as often:
Full results
The first generations of main are a step back. Every inherited synapse was tuned to eyes that reported the whole look, and the eyes now report only the difference, so the population has to re-tune before the change pays.
The head-to-head test is left out here: in it all animals share one set of eyes, and the eyes are the difference.
Without the dud, adaptation was used only a little: the long lives had already selected most of the standing still away, and evolution can learn the look of an empty bush of every kind it has always known. Giving the good berries a new colour in every life did not change that. What makes adaptation worth having is something that looks like food and gives nothing, whatever colour it has.