# Illusory deformation of the finger is more extensive in the distal than the lateral direction

## TL;DR

People perceive finger illusions more strongly when fingers appear longer rather than wider, suggesting a cognitive bias toward distal deformation.

## Contribution

The study introduces a novel method using the numbness illusion to compare distal and lateral illusory finger deformation.

## Key findings

- Perceived distal distortion was superior to lateral distortion in ownership ratings.
- Illusory deformation was more extensive in the distal direction than the lateral direction.

## Abstract

Previous studies have examined the rubber hand illusion with finger lengthening, but there is limited research on finger widening. This suggests a strong cognitive bias toward the illusory expansion of the finger in a distal direction rather than lateral. To test this, we compared the illusory deformability of the finger in the distal and lateral directions through the generation of illusory finger deformation using a double-touch operation, referred to as the numbness illusion. Our results showed that perceived distal distortion was wholly superior to perceived lateral distortion in terms of sense of ownership ratings. Moreover, the extent of the perceived deformation was greater in the distal than lateral direction, supporting our hypothesis that there is a distal bias. We suggest that this preference may be because the presence of multiple joints is required to create illusory deformation in the target direction.