Abstract
The ventriloquism aftereffect (VAE) refers to a shift in auditory spatial perception following exposure to a spatial disparity between auditory and visual stimuli. The VAE has been previously measured on two distinct time scales. Hundreds or thousands of exposures to a an audio-visual spatial disparity produces enduring VAE that persists after exposure ceases. Exposure to a single audio-visual spatial disparity produces immediate VAE that decays over seconds. To determine if these phenomena are two extremes of a continuum or represent distinct processes, we conducted an experiment with normal hearing listeners that measured VAE in response to a repeated, constant audio-visual disparity sequence, both immediately after exposure to each audio-visual disparity and after the end of the sequence. In each experimental session, subjects were exposed to sequences of auditory and visual targets that were constantly offset by +8° or −8° in azimuth from one another, then localized auditory targets presented in isolation following each sequence. Eye position was controlled throughout the experiment, to avoid the effects of gaze on auditory localization. In contrast to other studies that did not control eye position, we found both a large shift in auditory perception that decayed rapidly after each AV disparity exposure, along with a gradual shift in auditory perception that grew over time and persisted after exposure to the AV disparity ceased. We modeled the temporal and spatial properties of the measured auditory shifts using grey box nonlinear system identification, and found that two models could explain the data equally well. In the power model, the temporal decay of the ventriloquism aftereffect was modeled with a power law relationship. This causes an initial rapid drop in auditory shift, followed by a long tail which accumulates with repeated exposure to audio-visual disparity. In the double exponential model, two separate processes were required to explain the data, one which accumulated and decayed exponentially and the other which slowly integrated over time. Both models fit the data best when the spatial spread of the ventriloquism aftereffect was limited to a window around the location of the audio-visual disparity. We directly compare the predictions made by each model, and suggest additional measurements that could help distinguish which model best describes the mechanisms underlying the VAE.
Highlights
The ability to associate auditory and visual spatial information requires the nervous system to maintain spatial congruence between two distinct spatial encoding mechanisms
We predict that a longer experiment, in which the mean encoded difference in auditory and visual target location reached zero but patterns of error were still present across space, would reveal changes in spatial gain as well. These results demonstrate how the ventriloquism aftereffect (VAE) acts over multiple time scales, but cannot determine the underlying mechanisms without a direct comparison of the predictions those mechanisms make about the data
The experimental data presented here demonstrates how the ventriloquism aftereffect occurs over different time scales, and how repeated exposure to brief AV disparities alters auditory perception by producing both a large, transient initial shift, as well as a smaller, more enduring shift
Summary
The ability to associate auditory and visual spatial information requires the nervous system to maintain spatial congruence between two distinct spatial encoding mechanisms. More recent studies have shown that repeated exposure is not necessary, but rather that shifts in auditory spatial perception can be observed after as little as a single exposure to an AV spatial disparity [14]. These studies indicate that the ventriloquism aftereffect can be observed on multiple time scales, but do not demonstrate how these observations are related to one another. The goal of the current study was to measure the time course of auditory shift in response to extensive exposure to AV spatial disparity, while ensuring that shifts could not be attributed to eye position rather than the ventriloquism aftereffect
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