  Hallucinations and delusions are often grouped together within the positive
symptoms of psychosis. However, recent evidence suggests they may be driven by
distinct computational and neural mechanisms. Examining the time course of
their emergence may provide insights into the relationship between these
underlying mechanisms. Participants from the second (N = 719) and third (N =
699) iterations of the North American Prodrome Longitudinal Study (NAPLS 2 and
3) were assessed for timing of CHR-P-level delusion and hallucination onset.
Pre-onset symptom patterns in first-episode psychosis patients (FEP) from the
Prevention and Early Intervention Program for Psychosis (PEPP-Montreal; N =
694) were also assessed. Symptom onset was determined at baseline assessment
and the evolution of symptom patterns examined over 24 months. In all three
samples, participants were more likely to report the onset of delusion-spectrum
symptoms prior to hallucination-spectrum symptoms (odds ratios (OR): NAPLS 2 =
4.09; NAPLS 3 = 4.14; PEPP, Z = 7.01, P < 0.001) and to present with only
delusions compared to only hallucinations (OR: NAPLS 2 = 5.6; NAPLS 3 = 11.11;
PEPP = 42.75). Re-emergence of delusions after remission was also more common
than re-emergence of hallucinations (Ps < 0.05), and hallucinations more often
resolved first (Ps < 0.001). In both CHR-P samples, ratings of delusional
ideation fell with the onset of hallucinations (P = 0.007). Delusions tend to
emerge before hallucinations and may play a role in their development. Further
work should examine the relationship between the mechanisms driving these
symptoms and its utility for diagnosis and treatment.
