But it’s not just this flexible behavioral repertoire that’s so amazing. It’s not the fact that somehow, this dumb little spider with its crude compound optics has visual acuity to rival a cat’s (even though a cat’s got orders of magnitude more neurons in one retina than our spider has in her whole head). It’s not even the fact that this little beast can figure out a maze which entails recognizing prey, then figuring out an approach path along which that prey is not visible (i.e., the spider can’t just keep her eyes on the ball: she has to develop and remember a search image), then follow her best-laid plans by memory including recognizing when she’s made a wrong turn and retracing her steps, all the while out of sight of her target. No, the really amazing thing is how she does all this with a measly 600K neurons— how she pulls off cognitive feats that would challenge a mammal with 70M or more.

She does it like a Turing Machine, one laborious step at a time.

She’ll sit there for 2 hours, just watching. It takes that long to process the image: whereas a cat or a mouse would assimilate the whole hi-res vista in an instant, Portia’s poor underpowered graphics driver can only hold a fraction of the scene at any given time. So she scans, back and forth, back and forth, like some kind of hairy multilimbed Cylon centurion, scanning each little segment of the game board in turn. Then, when she synthesizes the relevant aspects of each, she figures out a plan, and puts it into motion: climbing down the branch, falling out of sight of the target, ignoring other branches that would only seem to provide a more direct route to payoff, homing in on that one critical fork in the road that leads back up to satiation. Portia won’t be deterred by the fact that she only has a few % of a real brain: she emulates the brain she needs, a few % at a time.

2022-06-24: Speaking of efficiency, the brain has a power saving mode.

When mice were deprived of sufficient food for weeks at a time — long enough for them to lose 15%-20% of their typical healthy weight — neurons in the visual cortex reduced the amount of ATP used at their synapses by 29%. Because the neurons in low-power mode processed visual signals less precisely, the food-restricted mice performed worse on a challenging visual task. The fact that these impairments in perception occurred long before the animal entered real starvation was unexpected.

A significant implication of the new findings is that much of what we know about how brains and neurons work may have been learned from brains that researchers unwittingly put into low-power mode. It is extremely common to restrict the amount of food available to mice and other experimental animals for weeks before and during neuroscience studies to motivate them to perform tasks in return for a food reward.