Abstract
Our somatosensory system deals with not only spatial but also temporal imprecision, resulting in characteristic spatiotemporal illusions. Repeated rapid stimulation at the wrist, then near the elbow, can create the illusion of touch at intervening locations along the arm (as if a rabbit is hopping along the arm). This is known as the “cutaneous rabbit effect” (CRE). Previous studies have suggested that the CRE involves not only an intrinsic somatotopic representation but also the representation of an extended body schema that includes causality or animacy perception upon the skin. On the other hand, unlike other multi-modal causality couplings, it is possible that the CRE is not affected by concurrent auditory temporal information. The present study examined the effect of a simple visual flash on the CRE, which has both temporal and spatial information. Here, stronger cross-modal causality or correspondence could be provided. We presented three successive tactile stimuli on the inside of a participant’s left arm. Stimuli were presented on the wrist, elbow, and midway between the two. Results from our five experimental manipulations suggest that a one-shot flash enhances or attenuates the CRE depending on its congruency with cutaneous rabbit saltation. Our results reflect that (1) our brain interprets successive stimuli on the skin as motion in terms of time and space (unimodal causality) and that (2) the concurrent signals from other modalities provide clues for creating unified representations of this external motion (multi-modal causality) as to the extent that “spatiotemporal” synchronicity among modalities is provided.
Highlights
Our daily lives are rich with information from the physical world
That ANOVA revealed only a significant main effect of tactile pattern, F (2, 24) = 5.97, p < 0.01; further, there were significant differences between L1–L2–L3 and L1–L1–L3 (620.9 ms) and between L1–L2–L3 and L1-L3-L3 (623.4 ms; ps < 0.05) in terms of reaction times (RTs), indicating that regardless of the presence vs. absence of the flash, the participants reacted most rapidly to the L1–L2–L3 stimuli. These results suggest that visual stimuli might not reduce the cutaneous rabbit effect” (CRE) though a flash on L1 at t 2 must be spatially incongruent with the L1–L2–L3 tactile sensation
The flash indicated the correct location of a second tactile stimulus, participants felt the CRE just as well as when no-flash was present
Summary
Our daily lives are rich with information from the physical world. While some limits are imposed by sensorineural imprecision (for a review, see Knill and Richards, 1996), the brain has developed strategies to deal with these limitations, including the utilization of prior knowledge and integration among multi-modal information. In the case of tactile perception, spatial imprecision due to low receptor density poses a particular challenge (Goldreich, 2007). The forearm has less acuity: it resolves detail on the order of 1 cm (Weinstein, 1968) This is the case even though the brain contains a representation of the body map in the primary somatosensory cortex (S1; Penfield and Boldrey, 1937) which reflects the locations of physical stimuli on the skin. The CRE is a subset of a larger class of tactile saltation illusions elicited when a mechanical stimulus is followed by similar stimuli rapidly applied at nearby locations (Geldard and Sherrick, 1972; Warren et al, 2010). The apparent location of each stimulus moves from the actual stimulation site toward the other stimulation sites in a predictable manner depending on factors such as stimulus location and frequency (e.g., Geldard and Sherrick, 1972; Kilgard and Merzenich, 1995; Cholewiak, 1999; Flach and Haggard, 2006)
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