By Maryna Mullerman ’20

Rubber hand visual-tactile illusion (RHI) experiments involve multisensory exposure, allowing participants to experience ownership over realistic rubber hands. Precise stimulation technique and positioning induce people to believe that a rubber hand is an integral part of their body. Marilia Lira and researchers from the Mackenzie Presbyterian University in Brazil investigated skin color’s effects on the time to onset of RHI in white participants. The researchers assessed illusion intensity and its connection to implicit racial bias. They hypothesized that longer RHI onset times would be evident under black rubber hand (RH) conditions.

The study recruited 95 right-handed, white male and female university students who had not previously experienced RHI. The participants were divided into synchronous and asynchronous condition groups. The RHI apparatus was equipped with two left male prosthetic hands —  one white-skinned and one black-skinned — and two left female hands of the same colors. The prosthetic hand was put in the natural position of a participant’s left hand on the visible side of a wooden barrier. The prosthetic hand and the real hand were synchronously or asynchronously stimulated with a brush. A stopwatch recorded times at which the participants started to experience RHI. Before and after stimulation, participants were asked to blindly touch their left index fingers with their right hands in proprioceptive drift tests. Each subject completed a RHI Questionnaire, an Interpersonal Reactivity Index (IRI), and a Race Implicit Association Test (IAT) to target implicit racial bias.

Participants under both conditions exhibited strong implicit bias towards black individuals. During the synchronous condition, the onset times for white rubber hand illusions were smaller, while the proprioceptive drifts were greater. There was a significant correlation between illusion intensity, time of effect, and proprioceptive drift for the black RH.

White participants took more time to experience RHI with the black hand. The black hand caused smaller proprioceptive drift, indicating a lower physical association, shorter experience, and greater difficulty of fitting the black hand into a white body representation. The study was limited to a two-colored schema and only white participants; also, it utilized a stopwatch that could have produced imprecise onset times. Nevertheless, the experiment offered a new perspective on human perception, providing further research possibilities concerning the effects of racial bias.

References:

1. M. Lira, et. al., The influence or skin colour on the experience of ownership in the rubber hand illusion. Scientific Reports 7, (2017). doi: 10.1038/s41598-017-16137-3
2. Image retrieved from: https://www.pexels.com/photo/2-person-and-1-child-connected-hands-159827/