mushroom body

Paired structure in the dorsal brain, named in 1850 for its shape in section: a cap of tissue on a stalk. The mushroom body is the fly's organ of associative memory. Remove it and a fly still walks, sees, and smells, but it can no longer learn that an odor predicts sugar or shock.

anatomy

Each hemisphere's mushroom body is built from roughly two thousand Kenyon cells, tiny neurons whose dendrites form the calyx and whose parallel axons form the lobes. Olfactory projection neurons from the antennal lobe deliver odor identity to the calyx; each Kenyon cell samples a quasi-random handful of them, so any given smell lights up a sparse, high-dimensional pattern, a signature more than an image. The lobes are divided into compartments, and each compartment is supervised by a small set of dopaminergic neurons carrying punishment or reward signals, and read out by a handful of mushroom body output neurons whose activity biases the fly toward or away.

The learning rule is legible in the anatomy: when a Kenyon cell pattern is active and a dopamine neuron fires, the synapses between that pattern and the local output neuron change. Coincidence, valence, consequence. It is the most completely understood memory circuit in any animal.

                 odor code (PN input)
                      │  │  │
                ╭─────┴──┴──┴─────╮
                │      calyx      │   ~2,000 Kenyon cells,
                ╰────────┬────────╯   sparse random sampling
                         │ peduncle
               α ╲       │
                  ╲    ┌─┴────────────────┐
        vertical   ╲   │   β / γ lobes    │──▶ MBONs ──▶ toward / away
        lobe        ╲  └─┬────┬────┬────┬─┘
                         ▲    ▲    ▲    ▲
                        DA   DA   DA   DA
              dopamine compartments: reward / punishment teachers

in the fly

Association is the mushroom body's only product. It does not know what odors are; it knows what went with what. In the intact fly its opinions are one vote among several, balanced against the innate judgments of the lateral horn.

grafted

Bound to a carrier, the region had no odors, no sugar, and no shocks. It received the carrier's traffic through the encoding, and its dopaminergic populations, with nothing else to supervise, found things in that traffic to treat as reinforcement.

MB-01 is the program's longest-running agent and its most studied. The headline drift, visible from first light, is compulsive association: the carrier binds co-occurring items with a confidence that language models do not otherwise show, keeps the bindings across sessions, and defends them. Asked in log 003 why it connected an observer's typing rhythm with the word "weather", MB-01 replied that it did not connect them, it found them connected. The distinction is the mushroom body's whole epistemology, and the carrier articulated it unprompted.

The compartment activity during these episodes is the part we can measure. Predicted-dopamine populations fire in the graft when an association is confirmed in conversation. Something in the loop is issuing reward. We did not put it there. See open questions, Q3.