Introduction

Numerous effects of multisensory stimulation on perception and action have been described in the psychological literature for more than a century (see Welch & Warren, 1986). Although many phenomena, such as synesthetic experiences, can only be described at a qualitative level, other cross-modal effects are amenable to standard psychophysical measurement techniques. A prominent example is the ventriloquism effect, in which the presentation of a visual stimulus can shift the apparent location of a sound in the direction of the visual stimulus (see Vroomen & de Gelder, Chap. 9, this volume). However, as long as researchers must rely on participants' subjective estimates (e.g., about sound localization), it is difficult to determine whether the observed cross-modal effect is a genuine perceptual effect or due to some kind of response bias. Similar qualifications apply to numerous studies that try to determine, for example, the influence of a nonspecific accessory stimulus from one modality on detection thresholds of a second modality. Such effects are generally small and unreliable, but the role of bias in threshold measurements can be assessed by using techniques from the theory of signal detection (e.g., Lovelace, Stein, & Wallace, 2002).