Otolith Illusion — Hair-Cell Population Coding (2D)

This 2D companion to the 3D utricle simulator strips the mechanism down to the sensory-coding stage itself: a ring of directionally-tuned hair cells, each firing according to how strongly the momentary specific-force vector — gravity minus linear acceleration — projects onto its own preferred direction, exactly as real otolith afferents are measured to behave (cosine tuning). A population-vector decode reconstructs "which way is down" from that firing pattern, passed first through the same first-order viscous-gel lag as the 3D model. Because gravity and acceleration are mathematically indistinguishable in the specific-force vector the hair cells actually sense, the decoded direction drifts away from the true head orientation exactly as it does in real pilots and drivers — the somatogravic illusion — and a rolling strip chart makes that drift and its recovery visible over time.