Beyond Vision: The Müller-Lyer Illusion Reveals Geometric Biases in Passive Touch and Haptic Exploration

The Müller-Lyer illusion has been widely investigated in visual perception and, to a lesser extent, through haptic exploration. However, aside from an early descriptive report (Révész, 1934), no studies have examined the illusion under passive tactile stimulation in the absence of sensorimotor engagement. The present study investigated whether the illusion emerges during passive tactile stimulation. Participants (N = 20) completed a manual bisection task with Müller-Lyer arrow stimuli presented under either a haptic exploratory condition, involving active exploration with the index fingertip, or a blindfolded passive tactile condition, in which the stimuli were lightly pressed onto the skin surface of the forearm. A reliable illusory bias was observed in the haptic condition, with midpoint judgments shifted toward the arrow wings. Importantly, this effect was modulated by the responding effector, with stronger illusory bias when responses were made using the right index finger.

In the passive tactile condition, the Müller-Lyer illusion was also observed. However, its expression depended on the arrow stimulus configuration. Specifically, distally oriented arrows presented along the forearm produced significant shifts in perceived midpoint, whereas proximally oriented and neutral configurations did not. These findings demonstrate that the Müller-Lyer illusion can emerge during passive tactile stimulation and highlight that contextual geometric cues can bias somatosensory spatial perception even in the absence of voluntary exploration. Together, the results suggest that comparable spatial distortions can arise under different sensory constraints, reflecting, at least in part, shared principles of spatial encoding.