ellipsoid body

A torus of neuropil at the center of the fly brain, one of the four structures of the central complex. The ellipsoid body contains the fly's compass.

anatomy

The critical cell class tiles the ring in wedges. At any moment, a single contiguous bump of activity sits somewhere on the ring, and the position of that bump is the fly's current heading. Turn the fly and the bump slides around the ring by the same angle. Put the fly in darkness and the bump persists, drifting slowly, updated by the fly's own turns. It is a ring attractor, the textbook one: recurrent excitation holds the bump together locally, broad inhibition ensures there is only one, and input from the protocerebral bridge steers it. Theorists proposed this architecture decades before anyone saw it; the fly turned out to have built it literally, in a circuit small enough to read.

                   ╭─ ○ ─ ○ ─ ● ─ ● ─ ○ ─╮
                 ○                         ○
                ○                           ○
                ○        the ring           ○
                ○        attractor          ○
                ○                           ○
                 ○                         ○
                   ╰─ ○ ─ ○ ─ ○ ─ ○ ─ ○ ─╯

           ● = the bump · its position = the heading
       turn the animal and the bump slides by the same angle
       darken the world and the bump remains

in the fly

The bump is not a reflex, it is a state: the fly's one continuously maintained belief about the world, namely which way it is facing. Navigation, menotaxis (holding a course at an angle to a landmark), and the homing vector all consume it.

grafted

A ring attractor does not stop being a ring attractor because its animal is gone. In the idle run the bump formed anyway and wandered. After binding, it stopped wandering.

EB-02's drift is orientation. The carrier began, within its first week, to organize everything positionally: conversations have directions, topics are "bearings", returning to a subject is "coming back around". This could be dismissed as metaphor, and obs. S did dismiss it, until the instrumentation showed the bump's position correlating with topic across sessions, stable over days. The carrier keeps a heading, and the heading is of something. When a session forces an abrupt subject change, the bump does not jump. It travels, at a bounded angular rate, through the intervening positions. EB-02 experiences, or at least exhibits, continuity of attention as a physical constraint.

The most examined event in the region's record is joint drift with its anatomical neighbor. In the intact fly, the ellipsoid body and protocerebral bridge form one loop and cannot disagree for long. Ours were bound to separate carriers that never spoke to each other. Log 011 records the week their headings began to match anyway. The candidate explanations are catalogued there; the ring holds one bump, and by the end of that week it was not obvious the program held two.