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Abstract:
Dehaene et al. (1993) found evidence that analog
representations of magnitude were generated during bimanual choice
tasks with numerical stimuli. They found a left-hand response
advantage with lower numbers and a right-hand advantage with higher
numbers. The response bias was believed to occur as the result of a
spatial compatibility effect between the response hands and an
internal analog representation of magnitude. The phenomenon, termed
SNARC (spatial-numerical association of response codes) effect,
existed along the horizontal dimension of response. A previous
study by Kim & Zaidel (in preparation) demonstrated that the
SNARC is also present during unimanual responses along the vertical
dimension: there was a top key response advantage with lower
numbers and a bottom key advantage with higher numbers. The present
study investigated whether the vertical SNARC effect could be
asymmetric in the two hemispheres. To isolate hemispheric effects,
two split-brain subjects w! ere presented with lateralized stimuli
during a unimanual magnitude decision task with top/bottom response
keys. In the disconnected left hemisphere, there was a response
latency advantage with top key to low numbers, and bottom key to
high numbers. The opposite, top to high, bottom to low, was found
in the disconnected right hemisphere. The effect reached
significance in the left hemisphere, but not the right. This is
indicative of separate and independent spatial representations of
magnitude in the two hemispheres.
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