title: When it's time for a change: failures to track context in schizophrenia.
abstract: BACKGROUND
  Reduction of P300 event-related potential amplitude in schizophrenia is perhaps the most replicated biological reflection of the illness. P300 is typically elicited by infrequent deviant events that are imbedded in a series of identical frequent standard events. Deviants have features that explicitly distinguish them from standards, whereas standards can be distinguished from each other based on their local sequential probabilities within the stimulus series. The improbable occurrence of a standard should generate a P300, but only if the implicit local context generated by the recent stimulus history is processed.
  
  
  METHODS
  To assess the ability of schizophrenia patients to process this implicit contextual information, ERPs were elicited from 22 controls and 16 schizophrenia patients during an auditory oddball task containing infrequent target tones (15%) and novel distracter sounds (15%) imbedded pseudo-randomly in a series of standard tones (70%). Consecutively presented standards following deviant stimuli varied in sequential probability from p=1.0 for the 1st standard to p=0.16 for the 4th consecutive standard.
  
  
  RESULTS
  Patients compared to controls demonstrated smaller P300 (P3a) to the fourth consecutive standard. However, in controls but not patients a contingent negative variation (CNV) was observed prior to the fourth standard, and an N2b/mismatch negativity (MMN) was observed following it.
  
  
  CONCLUSIONS
  These outcomes suggest that patients are deficient in using the implicit context established by recent stimulus history to anticipate that an otherwise standard stimulus was unlikely and its occurrence unexpected.
authors.affiliation: Psychiatry Service, San Francisco VA Medical Center, USA. judith.ford@ucsf.edu
authors.forename: Judith M
authors.initials: JM
authors.lastname: Ford
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Roach
authors.forename: Ryan M
authors.initials: RM
authors.lastname: Miller
authors.forename: Connie C
authors.initials: CC
authors.lastname: Duncan
authors.forename: Ralph E
authors.initials: RE
authors.lastname: Hoffman
authors.forename: Daniel H
authors.initials: DH
authors.lastname: Mathalon
mesh_terms: D000161:Acoustic Stimulation / Q000379:methods*; D000328:Adult; D018913:Event-Related Potentials, P300 / Q000502:physiology; D005072:Evoked Potentials, Auditory / Q000502:physiology*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D011930:Reaction Time / Q000502:physiology*; D012559:Schizophrenia / Q000503:physiopathology*; D055815:Young Adult