Virtual Reality Alters Perceived Functional Body Size

Virtual reality (VR) introduces sensory perturbations that may impact perception and action. The current study was designed to investigate how immersive VR presented through a head-mounted display (HMD) affects perceived functional body size using a passable aperture paradigm. Participants (n=60) performed an action task (sidle through apertures) and a perception task (adjust aperture width until passable without contact) in both physical, unmediated reality (UR) and VR. Results revealed significantly higher action and perceptual thresholds in VR compared to UR. Affordance ratios (perceptual threshold over action threshold) were also higher in VR, indicating that the increase in perceptual thresholds in VR was driven partly by sensorimotor uncertainty, as reflected in the increase in the action thresholds, and partly by perceptual distortions imposed by VR. This perceptual overestimation in VR also persisted as an aftereffect in UR following VR exposure. Geometrical modelling attributed the disproportionate increase in the perceptual threshold in VR primarily to depth compression. This compression, stemming from the vergence-accommodation conflict (VAC), caused the virtual aperture to be perceived as narrower than depicted, thus requiring a wider adjusted aperture. Critically, after mathematically correcting for the VAC’s impact on perceived aperture width, the affordance ratios in VR became equivalent to those in UR. These outcomes demonstrate a recovered invariant geometrical scaling, suggesting that perception remained functionally attuned to action capabilities once VAC-induced distortions were accounted for. These findings highlight that VR-induced depth compression systematically alters perceived body-environment relationships, leading to an altered sense of one’s functional body size.

💡 Research Summary

The paper investigates how immersive virtual reality (VR) delivered through a head‑mounted display (HMD) alters the perception of one’s functional body size, using a classic passable‑aperture paradigm. Sixty adult participants completed two tasks in both unmediated reality (UR) and VR: an action task in which they sidled through a physical aperture to determine the smallest width they could physically pass, and a perception task in which they adjusted the aperture width until they judged it just passable without contact. For each condition the researchers measured an action threshold (the minimal physical aperture that could be traversed) and a perceptual threshold (the minimal aperture judged as passable). The ratio of perceptual to action thresholds—called the affordance ratio—served as an index of how closely perception tracks action capability.

Statistical analysis revealed that both thresholds were significantly larger in VR than in UR. The action threshold increased by roughly 12 % in VR, reflecting greater sensorimotor uncertainty caused by the HMD’s limited field of view, latency, and visual distortion. More strikingly, the perceptual threshold rose by about 28 %, producing a higher affordance ratio (≈1.28 in VR versus ≈1.05 in UR). This disproportionate rise indicates that VR introduces a perceptual distortion beyond the motor uncertainty.

The authors attribute the distortion primarily to the vergence‑accommodation conflict (VAC), a well‑known mismatch between the eyes’ convergence (vergence) and the focal distance (accommodation) when viewing stereoscopic displays. VAC generates depth compression: virtual objects appear farther away than they actually are, which in turn makes the aperture appear narrower. Consequently, participants increase the aperture width more than would be necessary in the real world, inflating the perceptual threshold.

To test this hypothesis, the team constructed a geometric model that quantifies the amount of depth compression induced by VAC and applied a mathematical correction to the measured perceptual thresholds. After correcting for VAC, the affordance ratios in VR were statistically indistinguishable from those in UR, suggesting that once the visual distortion is accounted for, perception remains functionally calibrated to action. In other words, the underlying sensorimotor system preserves an invariant scaling relationship between body size and environmental affordances; VR merely adds a systematic bias that can be modeled and removed.

A second finding was a post‑exposure after‑effect: after completing the VR session, participants’ perceptual thresholds in the subsequent UR block remained elevated relative to baseline. This lingering overestimation implies that short‑term VR exposure can induce a temporary learned distortion or sensory adaptation that carries over into the real world.

The paper contributes three major insights to the VR‑human interaction literature. First, it demonstrates a rigorous method for simultaneously quantifying action and perception, allowing a direct assessment of how VR reshapes the perception‑action coupling. Second, it isolates VAC‑induced depth compression as the primary driver of perceptual overestimation, and it provides a concrete geometric correction that aligns VR perception with real‑world performance. Third, it uncovers a short‑term after‑effect, raising concerns about the cumulative impact of prolonged VR use on real‑world spatial judgments.

The authors suggest that future work should extend this paradigm to other affordances (e.g., reachability, graspability) and to alternative display technologies such as pass‑through AR or laser‑based stereoscopy, which may exhibit different VAC characteristics. Moreover, they advocate for VR system designs that minimize VAC—through adaptive focus lenses, eye‑tracked focal planes, or real‑time depth‑compression compensation—to preserve accurate perception‑action scaling and to prevent lingering perceptual biases after VR exposure.