The role of spatial disparity and hemifields in audio-visual temporal order judgments.

We explored whether sensitivity to audio-visual temporal order judgments (TOJs) was affected by the amount of spatial separation between a sound and light, and by whether the sound and light were presented in the same or in different hemifields. Participants made TOJs about noise bursts and light flashes, and judged whether the stimuli came from the same location or not. Flashes were presented either in the left or right hemifield (at +/-10 degrees from central fixation), and sounds either came from the same location as the lights, or at small or large disparities (20 or 40 degrees from the light, respectively), thereby crossing the hemifields or not. TOJs became more accurate (i.e., the just noticeable difference, JND, became smaller) when spatial disparity increased and when hemifields were crossed. Location discrimination of the sound and light was affected similarly. These results demonstrate that audio-visual TOJs are critically dependent on both the relative position from which stimuli are presented and on whether stimuli cross hemifields or not.