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Motion-Induced Position Shifts

The ability to correctly determine the position of objects in space is a fundamental task of the visual system. To perceive the world correctly, we must know not only what objects are present in a visual scene, but also where they are. Neurons throughout the visual system are activated by stimuli falling on specific regions of the retina, their receptive fields. It might seem reasonable that this explicit location information – the receptive field locations of cells that respond to an object – would specify an object’s perceived location. However, a wide range of illusions collectively known as motion-induced position shifts have demonstrated that motion can influence perceived location to a surprising extent. Indeed, a moving object may be perceived at a location quite distant from its current location in the visual field and the receptive fields that would be activated on the basis of retinal input alone. I have combined an especially strong version of this type of illusion, the flash grab effect, with a classic bistable motion stimulus for a project investigating the contribution of global and local motion on the shift size and direction. I found that while there is a clear effect of global motion, position shifts are also strongly influenced by local motion (Kohler et al., 2015). The demonstration video to the right presents the different experimental conditions used in that paper. I am very interested in determining how and where local and global motion contribute to the effect, and whether these contributions change as motion unfolds over time.

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