Snipping The Gordian Knot

I recently painted myself into a corner with the design rule "everything must be a perceptron". The problem is that just like with human perception, events need to bubble up from their source until they reach a threshold and "fire". A perceptron is a perceptual event that has fired - but how do I model the preconditions for a perceptron to fire? I need something smaller than a perceptron.

Borrowing from psychology again:

Sensation is the first stage in the biochemical and neurologic events that begins with the impinging upon the receptor cells of a sensory organ, which then leads to a perception.

Sensational!

So instead of tying the design in knots trying to model perceptrons with perceptrons I've snipped the proverbial Gordian Knot and now lesser perceptual events are simply sensations that can chain sensors together to make a perceptron.