title: P3a and P3b from typical auditory and visual stimuli.
abstract: OBJECTIVE
  Target/standard discrimination difficulty was manipulated systematically to assess how this variable affects target and nontarget P300 scalp distributions for both auditory and visual stimuli.
  
  
  METHODS
  A 3-stimulus paradigm (target, standard, nontarget) was employed in which subjects (n = 16) responded only to an infrequently occurring target stimulus. The perceptual discrimination difficulty between the target and more frequently occurring standard stimuli was varied as Easy or Difficult in different conditions, while holding the nontarget stimulus properties constant.
  
  
  RESULTS
  When target/standard discrimination was Easy, P300 amplitude was larger for the target than the nontarget across all electrode sites, and both demonstrated parietal maximums. In contrast, when target/standard discrimination was Difficult, target amplitude (P3b) was larger parietally and occurred later than nontarget components, whereas nontarget amplitude (P3a) was larger and earlier than the target P300 over the frontal electrode sites. Similar outcomes across task conditions were obtained for both auditory and visual stimuli.
  
  
  CONCLUSIONS
  The findings suggest that target/standard discrimination difficulty, rather than stimulus novelty, determines P3a generation for both auditory and visual stimulus modalities.
authors.affiliation: Department of Psychology, University of California, San Diego, La Jolla 92037, USA.
authors.forename: M D
authors.initials: MD
authors.lastname: Comerchero
authors.forename: J
authors.initials: J
authors.lastname: Polich
mesh_terms: D000161:Acoustic Stimulation; D000328:Adult; D001921:Brain / Q000502:physiology*; D004192:Discrimination, Psychological / Q000502:physiology; D004569:Electroencephalography; D005071:Evoked Potentials / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D010775:Photic Stimulation; D013647:Task Performance and Analysis