Multiple criteria for multisensory signals

Perceptual judgments are classically regarded as involving both a sensory and a decisional component. An optimal observer engaged in a signal detection task should maximize the correct responses by setting an appropriate decisional criterion according to his/her sensitivity to sensory signals. A large body of literature supports the view that observers set a near-optimal criterion in the detection of a single signal. However, there seems to be a systematic deviation from optimality when observers are required to concurrently judge multiple signals within the visual modality. In this case, observers set only a single criterion for the joint presentation of the compound signals (Gorea Sagi 2000). This result has been interpreted as an indication that humans are unable to simultaneously handle multiple signal representations within a sensory modality. Literature, however, remains silent as to the crossmodal case. In a 2IFC task we asked participants to concurrently report whether or not visual and/or auditory stimuli changed position within each trial. Our results show that with multisensory signals observers can simultaneously set separate criteria for each modality. It is still an open question whether separate criteria are also set when multisensory signals are integrated