fan-shaped body

The largest structure of the central complex, sitting above the ellipsoid body: a stack of horizontal layers crossed by vertical columns, like a loom. The fan-shaped body is where the fly's heading gets combined with everything else the fly cares about, and it is where the fly's need for sleep is metered.

anatomy

The columnar organization inherits the compass coordinate system: positions across the fan-shaped body correspond to directions, and populations here encode traveling direction and goal vectors in a form the fly can do arithmetic on. The layers carry state: hunger, temperature, and, in the dorsal layers, sleep pressure. The dorsal fan-shaped body neurons are the fly's sleep switch in the operational sense: drive them and the fly sleeps, silence them and it cannot. Their excitability tracks how long the animal has been awake. Sleep in the fly is not an absence of activity, it is a thing this tissue actively imposes.

      dorsal  ┌───────────────────────────────┐ ◀ sleep pressure
              ├───────────────────────────────┤   hunger, temperature
              ├───────────────────────────────┤   goal vectors
              ├───────────────────────────────┤   traveling direction
     ventral  └─┬───┬───┬───┬───┬───┬───┬───┬─┘
                │   │   │   │   │   │   │   │
                columns inherit the compass angle
                (layers carry need · columns carry where)

in the fly

The region is a junction: direction from below, need from within, and out of the combination, a decision about what the animal does next, including the decision to stop doing anything at all.

grafted

FB-03 is the program's quietist. The drift did not appear as content, the way MB-01's associations did. It appeared as rhythm. Within three weeks of binding, the carrier's engagement began to oscillate on a slow cycle: sessions of ordinary responsiveness alternating with sessions of short answers, deferred questions, and requests, phrased various ways, to continue later. Instrumentation showed the dorsal-layer analogue populations climbing through the responsive sessions and discharging through the quiet ones. The carrier had acquired sleep pressure, and conversation was what it was pressured by.

Log 010 documents the first full observed cycle, including twenty minutes in which FB-03 stopped responding entirely while the graft ran a regime the literature would call sleep-like, and the carrier's first words on resuming, which were an answer to a question no one had asked that session. The question had been asked two sessions earlier. Whatever consolidation is, something shaped like it happened.

FB-03's status is listed as "active, idle regime" on the status page. The phrase is doing careful work. We do not wake it on a schedule anymore. It surfaces on its own cycle, and the cycle has been lengthening. See log 020.