Modulation of implicitly perceived hand size by visuotactile recalibration

When concurrent visual and tactile stimuli are repeatedly presented with a spatial offset, even unisensory tactile stimuli are afterwards perceived with a spatial bias towards the previously presented visual stimuli. This so-called visuotactile ventriloquism aftereffect reflects visuotactile recalibration. It is unknown whether this recalibration occurs within a bodily map and interacts with perceived features like shape and size of body parts. Here, we applied tactile stimuli to participants hidden left hand and simultaneously presented visual stimuli with spatial offsets that - if integrated with the tactile stimuli - implied an enlarged hand size. We either used a fixed spatial mapping between tactile and visual positions ("congruent"), or a scrambled ("incongruent") mapping. We assessed implicitly perceived hand size via two independent behavioral assessments: pointing movements to unisensory tactile stimuli and tactile distance judgments. Moreover, we assessed explicitly perceived change in hand size with perceptual self-reports. Especially after congruent recalibration, participants localized unimodal tactile stimuli as if they were aiming at an enlarged hand. They also reported tactile distance as shorter after congruent than incongruent recalibration. These modulations resemble those obtained after using tools that prolong the arm and extend reaching space; they suggest that recalibration affected a common, implicit hand representation that underlies both tasks. In contrast, explicit perceptual self-reports did not differ significantly between congruent and incongruent recalibration. Thus, simple visuotactile stimuli are sufficient to modify implicitly perceived body size, indicating a tight link of low-level multisensory processes such as the visuotactile ventriloquism aftereffect and body representation.