Mushroom body
Subtraction is the rule.
A Kenyon cell active shortly before dopamine has that KC→MBON synapse depressed. Learning in a fly carves away responses that did not pay. Twenty rewarded encounters with one view: reward-side −6.0%, punishment-side −0.9% (flycoinrh measurement).
Reward-side · MBON01 02 03 · PAM
0.0%
mean gain 1.000
Punishment-side · MBON04 10 11 · PPL1
0.0%
mean gain 1.000
Kenyon cells · eligibility for this view
The reward signal is not real. Novelty standing in for sugar is a modelling choice — the fly has no say in it.