Brains are of different sizes, so can be any size. What happens when the size is 0?

The data flow through any active thing is:
nature -> sensors -> brain -> motors -> nature

Remove Brain and you have:

nature -> sensors -> motors -> nature

The functions here have a functional complexity of 1. To get above one in complexity you must add a brain.
(functional complexity = sensors/motors.)

Notice that a brain has flow from motors to sensors.

Sensors remove units of measurment. Motors add units of measurement. So a brain reads numbers, adds units of measurment if needed, does a computation, then becomes a sensor and removes units of measurement again.

Replace brain with neuron or with node and all this above remains true.

Valentino Braitenberg Wrote a book "Vehicles", MIT Press 1994
"Experiments in Synthetic Psychology" is what he called its contents.
To bad he did not know how to make a brain.

I made some brain code for all the vehicles that had proper description. I sent this to a reviewer of the book. Got no response of any kind.
Perhaps it is good that he did not know how to build a brain.
It has not helped me.