One signal, one number

The model now needs a usable version of that tasting record.

When Vincenza tastes a wine, activity spreads through many cells in her nose, mouth, and brain. Real sensory coding uses patterns across cells and across time. There is no single biological number called apricotty waiting to be read.

The model needs something much simpler. Its designer chooses one number to summarise how strongly each sensation came through. Call completely absent 0 and unmistakably strong 1. Everything else lands somewhere in between.

A trace of apricot might be 0.2. A wine drenched in it might be 0.9.

One place in the model that holds a number like that will be called a unit.

Earlier modules called the numbered parts of a network neurons. This chapter uses unit to keep the model separate from a biological cell.

The unit is not a photograph of one cell or one identifiable crowd of cells. It is a variable invented for this model. The designer labels it apricotty and supplies its value from the tasting record.

For now, a unit is simple. It does not decide anything or think about anything. It holds one number, saying how strongly one particular thing is present right now.

Four sensations, four units, four numbers between 0 and 1. That is the left-hand side of the model, and the picture on this slide shows it: each unit labelled with what it responds to, and holding how strongly that thing is present.

The four numbers there belong to one particular bottle, which arrives in a couple of slides. For now, only the shape matters.

Faint plum and overwhelming plum are genuinely different experiences. Turning that difference into one neat number is the convenience. It gives the model a graded input without pretending to reproduce the full sensory process.