Abstract
BackgroundIn von Schiller’s Stroboscopic Alternative Motion (SAM) stimulus two visually presented diagonal dot pairs, located on the corners of an imaginary rectangle, alternate with each other and induce either horizontal, vertical or, rarely, rotational motion percepts. SAM motion perception can be described by a psychometric function of the dot aspect ratio (“AR”, i.e. the relation between vertical and horizontal dot distances). Further, with equal horizontal and vertical dot distances (AR = 1) perception is biased towards vertical motion. In a series of five experiments, we presented tactile SAM versions and studied the role of AR and of different reference frames for the perception of tactile apparent motion.MethodsWe presented tactile SAM stimuli and varied the ARs, while participants reported the perceived motion directions. Pairs of vibration stimulators were attached to the participants’ forearms and stimulator distances were varied within and between forearms. We compared straight and rotated forearm conditions with each other in order to disentangle the roles of exogenous and endogenous reference frames.ResultsIncreasing the tactile SAM’s AR biased perception towards vertical motion, but the effect was weak compared to the visual modality. We found no horizontal disambiguation, even for very small tactile ARs. A forearm rotation by 90° kept the vertical bias, even though it was now coupled with small ARs. A 45° rotation condition with crossed forearms, however, evoked a strong horizontal motion bias.DiscussionExisting approaches to explain the visual SAM bias fail to explain the current tactile results. Particularly puzzling is the strong horizontal bias in the crossed-forearm conditions. In the case of tactile apparent motion, there seem to be no fixed priority rule for perceptual disambiguation. Rather the weighting of available evidence seems to depend on the degree of stimulus ambiguity, the current situation and on the perceptual strategy of the individual observer.
Highlights
The capacity of our sensory organs is restricted and the availability of the information from the world around us
In a series of five experiments, we presented tactile Stroboscopic Alternative Motion (SAM) versions and studied the role of aspect ratio (AR) and of different reference frames for the perception of tactile apparent motion
Increasing the tactile SAM’s AR biased perception towards vertical motion, but the effect was weak compared to the visual modality
Summary
The tactile motion perception was ambiguous for a broad range of SAM ARs with only a weak vertical but no horizontal bias. The ignorance of the forearm-crossing together with the stimulation mode from Condition 2 would lead to motion ambiguity, i.e. both horizontal and vertical motion percepts are possible, but the two dots should be perceived as moving antiparallel as illustrated in (Fig 5B.b1 and 5B.b2). Participant C further reported 100% parallel horizontal-motion percepts in Condition 2, which is only possible if he integrated the crossed-forearms in his motion processing, as illustrated in (Fig 5B.a). Participant D reported 52% horizontal motion percepts with both parallel and antiparallel motion percepts in Condition 2 This is only possible if the participant alternated between integration of the forearm-crossing and ignorance of the forearm-crossing within one experimental condition, as illustrated in (Fig 5B.a and 5B.b). Participant F's reports from Condition 2 nicely fit to our explanation He perceived horizontal and vertical antiparallel motion and must have ignored the forearm-crossing as illustrated in (Fig 5B.b1 and 5B.b2). Participants should report with each presentation both the motion direction and whether they perceived parallel or antiparallel motion
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