Abstract
When a short flash occurs in spatial alignment with a moving object, the moving object is seen ahead the stationary one. Similar to this visual “flash-lag effect” (FLE) it has been recently observed for the haptic sense that participants judge a moving hand to be ahead a stationary hand when judged at the moment of a short vibration (“haptic flash”) that is applied when the two hands are spatially aligned. We further investigated the haptic FLE. First, we compared participants’ performance in two isosensory visual or haptic conditions, in which moving object and flash were presented only in a single modality (visual: sphere and short color change, haptic: hand and vibration), and two bisensory conditions, in which the moving object was presented in both modalities (hand aligned with visible sphere), but the flash was presented only visually or only haptically. The experiment aimed to disentangle contributions of the flash’s and the objects’ modalities to the FLEs in haptics versus vision. We observed a FLE when the flash was visually displayed, both when the moving object was visual and visuo-haptic. Because the position of a visual flash, but not of an analogue haptic flash, is misjudged relative to a same visuo-haptic moving object, the difference between visual and haptic conditions can be fully attributed to characteristics of the flash. The second experiment confirmed that a haptic FLE can be observed depending on flash characteristics: the FLE increases with decreasing intensity of the flash (slightly modulated by flash duration), which had been previously observed for vision. These findings underline the high relevance of flash characteristics in different senses, and thus fit well with the temporal-sampling framework, where the flash triggers a high-level, supra-modal process of position judgement, the time point of which further depends on the processing time of the flash.
Highlights
In the flash-lag effect (FLE), a moving stimulus is seen to be ahead a brief visual stimulus that appears in alignment with the moving stimulus [1,2,3]
We investigated the relevance of flash characteristics for the haptic flash-lag effect
A sensitivity analyses conducted with GÃPower 3 [38] revealed that a single-sided trend (α < .10) for a flash-lag effect would have been detected in this experiment with a power of more than 90% if the effect was mm or higher
Summary
In the flash-lag effect (FLE), a moving stimulus is seen to be ahead a brief visual stimulus that appears in alignment with the moving stimulus [1,2,3]. Some studies examined how additional haptic information and the participants’ control of the moving item or the flash appearance affect the visual FLE [5,6,7,8]. The moving hand was physically behind the non-moving hand, when the two hands were perceived to be spatially aligned, indicating that the moving hand is perceived to be ahead the non-moving hand under physical alignment (Fig 1A) Linked to this haptic flash-lag effect are mislocalizations of stimuli that are applied to the unseen hand during its movement [13,14,15]. In the visual FLE, both moving objects and non-moving objects are perceived to be shifted in direction of motion, but the mislocalization of the moving object is stronger [16,17]
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