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A world-fixed sound presented to a moving head produces changing sound-localization cues, from which the audiomotor system could infer sound movement relative to the head. When appropriately combined with self-motion signals, sound localization remains spatially accurate. Indeed, free-field orienting responses fully incorporate intervening eye-head movements under open-loop localization conditions. Here we investigate the default strategy of the audiomotor system when localizing sounds in the absence of efferent and proprioceptive head-movement signals. Head- and body-restrained listeners made saccades in total darkness toward brief (3, 10 or 100 ms) broadband noise bursts, while being rotated sinusoidally (f=1/9 Hz, V(peak) =112 deg/s) around the vertical body axis. As the loudspeakers were attached to the chair, the 100 ms sounds might be perceived as rotating along with the chair, and localized in head-centred coordinates. During 3 and 10 ms stimuli, however, the amount of chair rotation remained well below the minimum audible movement angle. These brief sounds would therefore be perceived as stationary in space and, as in open-loop gaze orienting, expected to be localized in world-centred coordinates. Analysis of the saccades shows, however, that all stimuli were accurately localized on the basis of imposed acoustic cues, but remained in head-centred coordinates. These results suggest that, in the absence of motor planning, the audio motor system keeps sounds in head-centred coordinates when unsure about sound motion relative to the head. To that end, it ignores vestibular canal signals of passive-induced head rotation, but incorporates intervening eye displacements from vestibular nystagmus during the saccade-reaction time.

Orienting responses to audiovisual events in the environment can benefit markedly by the integration of visual and auditory spatial information. However, logically, audiovisual integration would only be considered successful for stimuli that are spatially and temporally aligned, as these would be emitted by a single object in space-time. As humans do not have prior knowledge about whether novel auditory and visual events do indeed emanate from the same object, such information needs to be extracted from a variety of sources. For example, expectation about alignment or misalignment could modulate the strength of multisensory integration. If evidence from previous trials would repeatedly favour aligned audiovisual inputs, the internal state might also assume alignment for the next trial, and hence react to a new audiovisual event as if it were aligned. To test for such a strategy, subjects oriented a head-fixed pointer as fast as possible to a visual flash that was consistently paired, though not always spatially aligned, with a co-occurring broadband sound. We varied the probability of audiovisual alignment between experiments. Reaction times were consistently lower in blocks containing only aligned audiovisual stimuli than in blocks also containing pseudorandomly presented spatially disparate stimuli. Results demonstrate dynamic updating of the subject's prior expectation of audiovisual congruency. We discuss a model of prior probability estimation to explain the results.