Beyond vision: The Müller-Lyer illusion reveals geometric biases in passive touch and haptic exploration

The Müller-Lyer Illusion has been extensively studied in the visual modality and, to a lesser extent, in haptic exploration. However, with the exception of an early descriptive study in the 1930s, no reports have investigated the illusion during passive tactile stimulation. Previous research uses the terms "tactile" and "haptic" interchangeably, despite exclusively examining active exploration. The present study investigated whether the illusion emerges during passive tactile stimulation, and whether its expression differs from that observed during haptic exploration with sensorimotor engagement. Twenty participants completed a manual bisection task of Müller-Lyer stimuli in a haptic exploratory or passive tactile form. Reliable illusory bias was observed in the haptic condition, with midpoint judgments shifted towards the arrow wings. Notably, this effect was mediated by the effector, with stronger bias when responses were made using the right index finger. In the tactile condition, a configuration-dependent effect emerged, in that distally pointing arrows on the forearm elicited significant illusory midpoint shifts, whereas proximal and neutral configurations did not. These findings show that the Müller-Lyer illusion arises under passive tactile stimulation, specifically, we show that contextual geometric cues bias somatosensory spatial perception in the absence of voluntary exploration.