Redundancy Gains in Absolute Judgments of Loudness and Brightness

People's ability to discern the physical intensity level of visual and auditory events presented at the same time is investigated in a bimodal identification paradigm with stimulus redundancy. Two approaches to modeling redundancy gains in choice reaction time (RT) and response probability in this paradigm are advanced: first, a separate activation model where two sequential likelihood ratio tests (SLRTs) for exponentially distributed neural interarrival times operate on parallel channels, each capable to evoke a response, and second, a coactive model where the outputs of two SLRTs are superposed in a single processing channel to trigger a response. Although both models predict plausible error rates, the separate activation model accounts better for observed benchmarks in choice RT. However, a violation of the race model inequality at the smallest quantile for loud and soft bimodal stimuli hints that the separate activation model might nonetheless be untenable. These findings challenge both parallel and coactive processing accounts of performance in intensity identification tasks with redundant auditory-visual stimuli.