Most illusions are not only fun to experience but are also interesting to study in depth for what they can reveal about perceptual purposes. One of the most interesting illusions discovered in recent times in the rubber hand illusion. It was first reported in a paper in Nature in 1998, titled "Rubber hands 'feel' touch that eyes see" by Matthew Botvinick and Jonathan Cohen. So how does it work? A subject is seated with the left harm resting on a table while a standing screen is positioned besides the arm to hide it from the subject's view. A life-sized rubber model of a left hand and arm is placed on the table, directly in front of the patient. The experimenter uses two paintbrushes to stroke the rubber hand and the real hidden hand, synchronizing the timing of strokes as closely as possible.
In the original study by Botvinick and Cohen, subjects were asked to complete a two part questionnaire that asked for an open description of their experience and also to affirm or deny the occurrence of nine specific perceptual effects (see Fig. 1).
Fig. 1: Questionnaire includes nine statements presented in random order. Subjects indicated their response on a seven-step visual analogue scale ranging from 'agree strongly (+++)' to 'disagree strongly (---)'. Points indicate mean response and bars indicate response range. Underlined questions show a significant tendency to evoke an affirmative response (Adapted from Botvinick and Cohen (1998))
Most subjects indicated that they seemed to feel the touch not of the hidden brush, but that of the viewed brush, as though the rubber hand had sensed the touch. It was hypothesized that the illusion may arise due to a spurious reconciliation of visual and tactile inputs while distorting position sense (proprioception). In a second experiment, subjects were exposed to the illusion for a prolonged period and were then probed for distortion in proprioceptive information. Before and after the viewing period, subjects completed a series of three intermanual reaches. With eyes closed, the right index finger was drawn along a straight edge, until it was judged to be aligned with the index finger of the hidden left hand. The authors found that the subjects' reaches after experiencing the illusion were displaced towards the rubber hand, the magnitude of displacement varying in proportion to the reported duration of the illusion (see Fig. 2)
Fig. 2: Results of reaching experiment. x-axis indicates the percentage of 30-min viewing period during which the illusion was experienced. The y-axis indicates displacement of the three reaches made after the viewing period from the three made before. Data is fitted with a least-squares regression line (adapted from Botvinick and Cohen (1998))
In more systematic explorations by Tsakiris and Haggard (2005), it was shown that the drift, while indicating the position of the real left hand, in the direction of the fake rubber hand not only depends upon synchrony of strokes, but also on the position of the rubber hand (spatial congruency) as well as visual characteristics of the hand (body top-down effects) (see Fig. 3 for experimental setup)
Fig. 3: Participants saw in different conditions (a) a rubber hand in a congruent position, (b) a rubber hand in incongruent position, or (c) a wooden stick. The participant's left hand was out of view for the whole duration of the experiment (Adapted from Tsakiris and Haggard (2005))
The congruent posture elicits the maximum proprioceptive drift while having the rubber hand in an incongruent position or replacing the rubber hand by a wooden stick does not (see Fig. 4).
Fig. 4: Mean proprioceptive drift towards the rubber hand. Error bars indicate standard error. Asterix indicates significant difference between synchronous and asynchronous stimulation (Adapted from Tsakiris and Haggard (2005))
The body is distinguished from other objects as belonging to the self by participating in inter-modal perceptual correlations. In the experiments of Botvinick and Cohen, subjects who referred the tactile sensation to the rubber hand also reported experiencing the rubber hand as belonging to themselves. Indeed eight out of ten subjects employed terms of ownership in the free descriptions (Botvinick and Cohen (1998))
The rubber hand illusion presents a very intriguing case. It shows that certain forms of inter-modal correlations may be sufficient for self attribution even in the face of contradicting signals from other sensory modalities.
Botvinick M, & Cohen J (1998). Rubber hands 'feel' touch that eyes see. Nature, 391 (6669) PMID: 9486643
Tsakiris M, & Haggard P (2005). The rubber hand illusion revisited: visuotactile integration and self-attribution. Journal of experimental psychology. Human perception and performance, 31 (1), 80-91 PMID: 15709864