Journal of Experimental Psychology: Human Perception and Performance 1994, Vol. 20, No. 1, 131-153 Copyright 1994 by the American Psychological Association, Inc. 0096-1523/94/$3.00 Visual-Auditory Interactions in Sensorimotor Processing: Saccades Versus Manual Responses Howard C. Hughes, Patricia A. Reuter-Lorenz, George Nozawa, and Robert Fendrich Reaction times (RTs) to bimodal (visual and auditory) stimuli were examined using 3 different response systems: saccades, directed manual responses, and simple manual responses. The ob- served levels of intersensory facilitation exceeded race model predictions and therefore support summation (coactivation) models of bimodal processing. However, response-dependent differences suggest that the processing of bimodal targets also depends on the relevant sensorimotor pathways and requirements of the task. Coactivation of response mechanisms might account for the effects found using simple RTs. The results for saccades are consistent with known patterns of auditory- visual convergence in the oculomotor system. Recently there has been renewed interest in understand- ing the manner in which redundant stimuli affect sensori- motor performance (e.g., Ashby & Townsend, 1986; Miller, 1982; Mordkoff & Yantis, 1991; Stein, Meredith, Honeycutt, & McDabe, 1989; Townsend & Ashby, 1983). In many cases, performance (usually reaction time, RT) to single-stimulus presentations is compared with perfor- mance under conditions of dual-stimulus presentations. The frequent finding is that RTs to dual-stimulus presenta- tions are faster than RTs to either stimulus presented alone (Miller, 1982, 1986; Mordkoff & Yantis, 1991; Raab, 1962; van der Heijden, La Heij, & Boer, 1983). This is re- ferred to as a redundant-signals effect. Central to an analysis of the redundant-signals effect is the question of whether the facilitation produced by redundant targets is sufficiently robust to rule out the possibility that responses to redundant targets are simply triggered by which- ever target is detected first (equivalent to the operation of a logical OR gate). Because the detection times associated with each modality are considered to be random variables, some reduction in RT is expected in a system that applied such an OR operation to the detection times in otherwise independent sensory channels, an effect known as probability summation (e.g., Miller, 1982; Raab, 1962; Townsend & Ashby, 1983). Probability summation assumes that a separate decision pro- cess accumulates information on each afferent channel and that the first channel to detect the target generates the re- sponse. For this reason, such models of redundant-signals processing are often called race models. Miller (1982, 1986) pointed out that the magnitude of the redundant-targets effect should be greater than that attributable to race models if the Howard C. Hughes and George Nozawa, Department of Psy- chology, Dartmouth College; Patricia A. Reuter-Lorenz, Depart- ment of Psychology, University of Michigan; Robert Fendrich, Center for Neurobiology, University of California, Davis. This research was supported by Air Force Office of Scientific Research Grant 89-0437. Correspondence concerning this article should be addressed to Howard C. Hughes, Department of Psychology, Dartmouth Col- lege, Hanover, New Hampshire 03768. activities of several parallel afferent channels were pooled prior to a single decision process. Combined activation would produce RTs that are faster than those predicted by race models (Nozawa, 1989). Thus, facilitation beyond prob- ability summation may be indicative of neural summation (coactivation) somewhere in the processing system. The pooling of information could occur at the level of a sensory decision (Blake, Martens, Garrett, & Westendorf, 1980; Fi- dell, 1970; Founder & Eriksen, 1990; Luce & Green, 1972; Rose, Blake, & Halpern, 1988; Wandell & Luce, 1978; We- stendorf & Blake, 1988) or at the level of response selection or execution (e.g., Diederich & Colonius, 1987; Eriksen & Schultz, 1977; Fournier & Eriksen, 1990; Miller, 1982, 1986). Although the sensory channels in many redundant-targets experiments may reasonably be regarded as being organized in parallel, the very existence of a redundant-targets effect means that information about the individual targets must con- verge at some point in sensorimotor processing. Recent elec- trophysiological studies have revealed a specific site of auditory-visual convergence within the oculomotor systems of cats and monkeys (Jay & Sparks, 1987, 1990; Meredith & Stein, 1987; Peck, 1987). These findings motivated the present analysis of the redundant-targets effect on the latency of saccadic eye movements in humans. Multimodal Convergence in the Saccadic Control System Recent electrophysiological studies have revealed a neural mechanism that appears to enable multimodal control of sac- cades: Individual neurons within the deeper layers of the superior colliculus receive convergent visual and acoustic inputs (e.g., Jay & Sparks, 1987; Meredith & Stein, 1987; Peck, 1987). The coordinates of the visual and auditory re- ceptive fields are usually in spatiotopic register, andspatially aligned bimodal inputs often elicit unit discharges that are greater than responses evoked from either modality alone (Meredith & Stein, 1987). The responses of single cells to bimodal stimuli are sometimes close to the sum of the uni- 131 This document is copyrighted by the American Psychological Association or one of its allied publishers. 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