A series of experiments measured the audiovisual stimulus onset asynchrony (SOAAV), yielding facilitative multisensory integration. We evaluated (1) the range of SOAAV over which facilitation occurred when unisensory stimuli were weak; (2) whether the range of SOAAV producing facilitation supported the hypothesis that physiological simultaneity of unisensory activity governs multisensory facilitation; and (3) whether AV multisensory facilitation depended on relative stimulus intensity. We compared response-time distributions to unisensory auditory (A) and visual (V) stimuli with those to AV stimuli over a wide range (300 and 20 ms increments) of SOAAV, across four conditions of varying stimulus intensity. In condition 1, the intensity of unisensory stimuli was adjusted such that d′ ≈ 2. In condition 2, V stimulus intensity was increased (d′ > 4), while A stimulus intensity was as in condition 1. In condition 3, A stimulus intensity was increased (d′ > 4) while V stimulus intensity was as in condition 1. In condition 4, both A and V stimulus intensities were increased to clearly suprathreshold levels (d′ > 4). Across all conditions of stimulus intensity, significant multisensory facilitation occurred exclusively for simultaneously presented A and V stimuli. In addition, facilitation increased as stimulus intensity increased, in disagreement with inverse effectiveness. These results indicate that the requirements for facilitative multisensory integration include both physical and physiological simultaneity.

The “temporal rule” of multisensory integration proposes that unisensory stimuli, and the neuronal responses they evoke, must fall within a window of integration. Ecological validity demands that multisensory integration should occur only for physically simultaneous events (which may give rise to non-simultaneous neural activations), and spurious neural response simultaneities unrelated to environmental multisensory occurrences must somehow be rejected. Two experiments investigated the requirements of simultaneity for facilitative multisensory integration. Experiment 1 employed an RT/race model paradigm to measure audiovisual multisensory integration as a function of audiovisual stimulus onset asynchrony under fully dark adapted conditions for visual stimuli that were either rod- or cone-isolating. Auditory stimulus intensity was constant. Despite a 155 ms delay in mean RT to the scotopic versus photopic stimulus, facilitative audiovisual multisensory integration in both conditions occurred exclusively at an audiovisual stimulus onset asynchrony of 0 ms. Thus, facilitative multisensory integration demands both physical and physiological simultaneity. Experiment 2 investigated the accuracy of simultaneity and temporal order judgments under the same stimulus conditions. Judgments of audiovisual stimulus simultaneity or temporal order were significantly influenced by stimulus intensity indicating different simultaneity requirements for these tasks. We consider the possibility that there are mechanisms by which the nervous system may take account of variations in response latency arising from changes in stimulus intensity in order to selectively integrate only those physiological simultaneities that arise from physical simultaneities. We propose that separate subsystems for audiovisual multisensory integration exist which pertain to action and perception.

Acute alcohol challenge has been associated with a selective impairment of right hemisphere function. A hallmark of visuospatial neglect syndrome is that patients with right hemisphere lesions misbisect horizontal lines far rightward of veridical center. Neurologically intact subjects misbisect lines with a systematic leftward bias (pseudoneglect). Neuroimaging studies in neurologically intact subjects reveal predominant right hemisphere activation during performance of line bisection tasks. The current study assessed whether acute alcohol challenge alters global visuospatial attention. Subjects (N=18; 10 male; strongly right-handed; mean age=23 years) engaged in a forced-choice tachistoscopic line bisection task in both ethanol challenge (mean BAC=.077) and no ethanol control conditions. Mean leftward bisection error in the control challenge condition was −0.238 degrees visual angle (1.05% line length), and leftward bisection error significantly increased (p=.001) under ethanol challenge (−0.333 degrees visual angle, 1.47% line length). Mean bisection precision in the control condition was 0.358 degrees visual angle (1.58% line length); bisection precision significantly deteriorated (p=.008) under ethanol challenge (0.489 degrees, 2.17% line length). Decreased bisection precision indicates that ethanol disrupts the fidelity of visuospatial performance. The exaggerated leftward bisection error implies that ethanol may exert a differential effect on left versus right hemispheric function with respect to the control of global visuospatial attention.

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