title: Contrast discrimination by the methods of adjustment and two-alternative forced choice.
abstract: Fifty years after the advent of signal-detection theory, some visual scientists still adhere to the notion of a high, fixed threshold. Indeed, experienced observers are able to deliver consistent settings of thresholds using the method of adjustment, but those settings are different from the thresholds obtained with two-alternative forced choice (2AFC) procedures. This article illustrates the difference by reference to four existing studies of contrast discrimination. All four sets of data show clear subthreshold facilitation, but the method of adjustment produces a rectilinear characteristic that corresponds to the use of different criteria over different ranges of contrast, whereas 2AFC procedures produce a curvilinear characteristic with generally lower thresholds. This article proposes a theoretical explanation for the difference.
authors.affiliation: Department of Psychology, University of Cambridge, Downing Street, Cambridge, CB2 3EB, England, UK, drjl@hermes.cam.ac.uk.
authors.forename: Donald
authors.initials: D
authors.lastname: Laming
mesh_terms: D000223:Adaptation, Psychological; D002755:Choice Behavior; D003118:Color Perception / Q000502:physiology*; D015350:Contrast Sensitivity / Q000502:physiology*; D004192:Discrimination, Psychological / Q000502:physiology*; D006801:Humans; D012684:Sensory Thresholds; D017603:Signal Detection, Psychological / Q000502:physiology*