ABSTRACT
Global psychophysics involves situations in which the stimulus intensities are clearly distinguishable from each other. The subject’s task is to scale stimuli and, in so doing, reveal the mapping between physical intensity and perceived magnitude. This chapter focuses on local psychophysics (Luce, 1993; Luce & Krumhansl, 1986). Here the stimuli are highly similar, and the subject’s task is to discriminate between them rather than to scale their magnitudes. The aspect of local psychophysics of special interest is the measurement of Just-Noticeable-Differences (JND) between stimuli and the functional relation between the JND and the stimulus intensity at which it is determined. This relation is typically linear over much of the dynamic range of a stimulus attribute (Weber’s Law), but deviates from this pattern near the ends of the stimulus range, especially in the vicinity of the lower absolute threshold. The size of the JND across changes in physical intensity is thought by many theorists to indicate the sensitivity of the sense modality. The relative size of JNDs in respect to the intensity at which it is measured is an index of potential neurophysiological significance.
