Italy second stimulus was seen to the left of the first, as a function of actual physical displacement. Following 2 Auditory Neuroscience Laboratory Department of Physiology standard practice, the data were fitted with cumulative Gaussian functions free to vary in position and width: University of Sydney New South Wales 2006 the position of the median (50% leftward) is termed the point of subjective equality (PSE), and the width Australia 3 Department of Psychology represents the estimate of localization accuracy (pre-sumed to depend on internal noise). For all unimodal University of Florence 50125 Florence conditions, the PSE was near 0, but varied considerably. For visual stimuli, was smallest (approximately Italy 1) for the smallest Gaussian blobs and largest (approxi-mately 20) for the largest (in line with the results of [6]). Localization accuracy for the auditory click was around Summary 6, falling midway between the various visual stimuli. Note that this poor localization does not necessarily Ventriloquism is the ancient art of making one's voice reflect performance under natural or "free-field" condi-appear to come from elsewhere, an art exploited by tions where all auditory cues (including intensity differ-the Greek and Roman oracles, and possibly earlier ences and monaural cues) are available. Free-field local-[1]. We regularly experience the effect when watching ization of spectrally rich stimuli such as click trains television and movies, where the voices seem to ema-produces discrimination thresholds on the order of 1 nate from the actors' lips rather than from the actual [7, 8]. sound source. Originally, ventriloquism was explained With unimodal thresholds established, we then mea-by performers projecting sound to their puppets by sured localization for a blob and click presented simulta-special techniques [1], but more recently it is assumed neously. Observers were asked to envisage each pre-that ventriloquism results from vision "capturing" sentation as a single event, like a ball hitting the screen, sound [2-5]. In this study we investigate spatial local-and report which of the two presentations was more ization of audiovisual stimuli. When visual localization leftward. For one presentation (randomly first or sec-is good, vision does indeed dominate and capture ond), the visual and auditory stimuli were in conflict, sound. However, for severely blurred visual stimuli with the visual stimulus displaced degrees rightward (that are poorly localized), the reverse holds: sound and the auditory stimulus displaced degrees leftward captures vision. For less blurred stimuli, neither sense (S V S A 2, where S V and S A are the spatial positions dominates and perception follows the mean position. of the visual and auditory stimuli). The average position Precision of bimodal localization is usually better than of this stimulus was always zero, as in previous studies either the visual or the auditory unimodal presentation. [9]. On the other (non-conflict) presentation, the two All the results are well expla...

of the visual and auditory stimuli). The average position Precision of bimodal localization is usually better than of this stimulus was always zero, as in previous studies either the visual or the auditory unimodal presentation.[9]. On the other (non-conflict) presentation, the two All the results are well explained not by one sense modalities were covaried to the left or right of center by capturing the other, but by a simple model of optimal the amount shown in the abscissa (with positive meancombination of visual and auditory information. Figures 1B-1D show Results and Discussion results for ⌬ ϭ Ϯ5Њ, Ϯ2.5Њ, or 0Њ. The pattern of results depended critically on blob size, which increases Observers were required to localize in space brief light through (B)-(D). For small blob widths (4Њ; Figure 1B), "blobs" or sound "clicks," presented either separately the curves were displaced systematically in the direction (unimodally) or together (bimodally). In a given trial, two of ⌬ (to the side where the visual stimulus was dissets of stimuli were presented successively (separated placed), suggesting that vision dominated the perceived by a 500 ms pause) and observers were asked to indicate position of the incongruent stimuli (the ventriloquist efwhich appeared to be more to the left, guessing if unsure. fect). However, for large blobs (64Њ; Figure 1D), the reThe visual stimuli were low-contrast ( ing rightward). The different colored symbols in

To combine information from different sensory modalities, the brain must deal with considerable temporal uncertainty. In natural environments, an external event may produce simultaneous auditory and visual signals yet they will invariably activate the brain asynchronously due to different propagation speeds for light and sound, and different neural response latencies once the signals reach the receptors. One strategy the brain uses to deal with audiovisual timing variation is to adapt to a prevailing asynchrony to help realign the signals. Here, using psychophysical methods in human subjects, we investigate audiovisual recalibration and show that it takes place extremely rapidly without explicit periods of adaptation. Our results demonstrate that exposure to a single, brief asynchrony is sufficient to produce strong recalibration effects. Recalibration occurs regardless of whether the preceding trial was perceived as synchronous, and regardless of whether a response was required. We propose that this rapid recalibration is a fast-acting sensory effect, rather than a higher-level cognitiveprocess.Anaccountintermsofresponsebiasisunlikelyduetoastrongasymmetrywherebystimuliwithvisionleadingproducebigger recalibrations than audition leading. A fast-acting recalibration mechanism provides a means for overcoming inevitable audiovisual timing variation and serves to rapidly realign signals at onset to maximize the perceptual benefits of audiovisual integration.