Marcel, Anthony; Mackintosh, Bundy; Postma, Peggy; Cusack, Rhodri; Vuckovich, Joseph; Nimmo-Smith, Ian and Cox, Sylvia M. L.
Is susceptibility to perceptual migration and fusion modality-specific or multimodal?
A previous paper reported high susceptibility to spatial migration (allochiria) of tactile stimuli in about 25% of healthy individuals (High Error subjects). When synchronous stimuli touched the two hands, if the unattended stimulus was temporally modulated when the attended one was not (and was thus more salient than the latter), it "migrated" to and fused with or replaced the stimulus on the attended hand. When subjects rated similarity of the attended stimulus accompanied by a distractor to each stimulus alone, scaling distributions tested against a sampling model showed most High Error subjects experienced fused stimuli, others experienced replacement and Low Error subjects experienced neither. We argued that these migrations are equivalent to allochiria and that this underlies neglect and extinction. This study assessed whether the individual difference is modality-specific or not. In auditory and visual equivalents of the tactile rating experiment, the difference between High and Low Error subjects was replicated in audition, but no migration occurred in vision. However, when two words were briefly presented visually before a mask with cued report of one, letter migrations to equivalent locations did occur and the individual difference was reproduced. This constitutes the first report of individual differences in auditory fusion and visual letter migration. Migration occurred in egocentric coordinates but apparently preserved structural homology. Different migration rates between the modalities paralleled relative salience of the unattended to the attended stimulus. The multimodality of the individual difference suggests that its source is supramodal, in deficient binding of perceptual content to location. (c) 2005 Elsevier Ltd. All rights reserved.
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