Technically true! But one, very few neurons have connections going both ways to each other- any two neurons will be lined up axon to synapse one way or the othe

Technically true! But one, very few neurons have connections going both ways to each other- any two neurons will be lined up axon to synapse one way or the other. And two, the specific structures that compose most of the cortex are remarkably unidirectional. The neocortex consists mostly of little chunks of neurons arranged in layers, themselves hooked up snout to tail. There are loops, eventually, but only on the macro scale. Outside the cortex, and even moreso outside the brain, there is a far greater diversity of structures. Two neurons plugged into each other form an excellent basis for a timer, damper, clock, or even primitive stimulus response. There are structures taking proprioceptive info from the joints and performing integrals on them. It's neat because the circuits are so small we can actually understand them completely! But within the brain things definitely are not laid out right for backprop.