Migration and fusion of tactile sensation--premorbid susceptibility to allochiria, neglect and extinction?
Migration of tactile sensation was found to occur very frequently in about 25% of normal people (High Error subjects) and very infrequently in others. When synchronous stimuli touched the two hands, if the unattended stimulus was modulated when the attended one was not (and was thus more salient) it "migrated" to the attended hand and fused with or replaced the attended stimulus. However, latencies reflecting congruence and incongruence of simultaneous stimuli showed that their identities on each hand had been (nonconsciously) registered veridically. Subsequent experiments, involving Focused and Divided Attention without speed pressure showed that mislocation errors in these subjects (i) were not due to confusion about location of otherwise perceptually distinct stimuli, (ii) nor to speed demand, (iii) nor to relative salience per se, (iv) were immune to attentional manipulation and practice in most subjects, (v) required a stimulus on the attended hand, and (vi) reflected a changed experience. Finally the same subjects rated similarity of the attended stimulus when accompanied by a distractor to each stimulus alone. Scaling distributions tested against a sampling model showed that most High Error subjects experienced a fusion of the stimuli, some experienced a replacement, and Low Error subjects experienced neither. The individual difference appears to be in attentional separability and spatial binding of tactile stimuli. Mislocation to the focus of spatial attention is common in healthy people, especially when binding is prevented. The present phenomenon appears equivalent to allochiria, but also accounts for phenomena in neglect and extinction, and suggests a premorbid susceptibility to spatial migration and integration that can be exaggerated by brain damage.