CHRONOPROTO: WHAT A SINGLE POSITIVE EPISODE CAN AND CANNOT PERSONALIZE

When a seizure forecaster is personalized after a patient’s first usable seizure, the only labeled target data are typically the preceding preictal EEG. This episode locates the patient’s preictal windows, while ranking later windows also requires a reference for the interictal background. We formalize a positive-only protocol that releases this first preictal episode and keeps target background out of fitting. Under this protocol, ChronoProto contrasts a first-episode prototype with a fixed reference in a frozen source representation. We split the pairwise ranking error into prototype error and reference mismatch, and further split prototype error into within-episode sampling, governed by effective support, and episode-to-future shift. A one-dimensional construction admits opposite oracle rankings under the same observed information even when the positive distribution is known, establishing the need for an explicit reference assumption. On 28 later CHB–MIT seizures from 13 patients, changing only the reference moves patient-macro AP from 0.2656 to 0.3191. A source-selected linear SVM reaches 0.3175 AP, with paired AP and AUROC difference intervals against CP-0 spanning zero. These results motivate stating and justifying the reference as part of positive-only personalization.