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TWK: 7th Tübingen Perception Conference
30th Jan - 1st Feb 2004
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Representational Momentum: Perception or Cognition?
Dirk Kerzel (Justus-Liebig-Universität Gießen)
When observers are asked to memorize the final position of a moving object, judgments are often displaced in the direction of motion. So far, this distortion of visual short-term memory has been explained with reference to high-level, cognitive processes. The classical explanation has been that mental representations of moving objects reflect properties of the physical world. In analogy to the physical momentum of moving objects, the remembered target position possessed “representational momentum” and overshoots the true final target position. This idea has its roots in Shepard’s proposal that many cognitive activities are guided by internalizations of invariant real-world constraints. In contrast to these knowledge-based proposals, more recent low-level accounts hold that errors in visual short-term memory are accounted for by properties of motion perception, attention and eye movements. First evidence for this account is the distinction between smooth and implied motion. Whereas a shift of the remembered final target position is reliably observed with implied motion, no shift occurs with smooth motion when fixation is maintained. Paradoxically, weak or second-order motion signals produce more forward displacement than real motion. The second important distinction concerns the response mode. When motor judgments such as mouse or natural pointing movements are used, displacement in the direction of motion is larger than with relative judgments that involve a comparison with a probe stimulus. Contrary to current theorizing, this suggests that the distortion is larger in a motor map of environmental space than in a cognitive/perceptual map. It may be that the forward error in pointing movements compensates for neural delays. |
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