Abstract
When stimuli from different sensory modalities are received, they may be combined by the brain to form a multisensory percept. One key mechanism for multisensory binding is the unity assumption under which multisensory stimuli that share certain physical properties like temporal and/or spatial correspondence are grouped together as deriving from one object. In humans, evidence for a role of the unity assumption has been found in spatial tasks and also in temporal tasks using stimuli that share physical properties (speech-related stimuli, musical and synesthetically congruent stimuli). In our study, we investigate the role of the unity assumption in an animal model in a temporal order judgment task. When subjects are asked to indicate which of two spatially separated visual stimuli appeared first in time, performance improves when the visual stimuli are paired (in time) with spatially non-informative acoustic cues, a phenomenon known as the temporal ventriloquism effect. Here, we show that European starlings perform better when one singleton acoustic cue is paired with the first visual stimulus as compared to pairing with the second visual stimulus. This shows, in combination with our previous study, that a non-informative singleton acoustic cue, when temporally paired with the first visual stimulus, triggers alerting while, when temporally pairing with the second visual stimulus, it prevents a temporal ventriloquism effect because the unity assumption is violated. Thus, the unity assumption influences sensory perception not only in humans but also in an animal model. The importance of the unity assumption in this task supports the idea that the temporal ventriloquism effect, similar to the spatial ventriloquism effect, is based on multisensory binding and integration but not on alerting effects.
Highlights
In our brain, stimuli from all our senses work in concert to evoke percepts of our environment
We could further show that a singleton acoustic cue improved performance when preceding the first visual stimulus and serving as an alerting signal
We investigated the question whether the unity assumption applies to an animal model by comparing the performance of starlings in a vision-based temporal order judgment (TOJ) task: a singleton acoustic cue was paired with either the first or the second visual stimulus and, in addition, the acoustic cue was presented either synchronous to the visual stimulus or with a short time offset producing a leading sound or a trailing sound
Summary
Stimuli from all our senses work in concert to evoke percepts of our environment. If noninformative acoustic cues (for example, short noise bursts from a central position) are added to the visual stimuli, the performance may change: one acoustic cue (A1) temporally leading the first visual stimulus (V1) and a second acoustic cue (A2) temporally following the second visual stimulus (V2) will yield better performance than acoustic cues presented simultaneous to the visual stimuli as has been shown in humans (Morein-Zamir et al, 2003) and in European starlings (Sturnus vulgaris, Feenders et al, 2017) This effect is known as the temporal ventriloquism effect ( TVE, compare with the spatial ventriloquism effect, see Chen and Vroomen, 2013). It is important to note that the time offset between the acoustic and visual stimuli has to be chosen carefully to match the size of the temporal binding window which is task-specific (Meredith et al, 1987; Vatakis and Spence, 2010; Vroomen and Keetels, 2010): this window is larger for visually guided than for acoustically guided stimuli (due to extended processing times of the visual as compared to the auditory system), a time offset which matches the size of the temporal binding window of both modalities is necessary (in case of audio-visual experiments this is usually around 75–100 ms)
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