The somatic rubber-hand illusion. The experimenter moved each participant's left index finger so that it touched the right rubber hand on the knuckle of the index finger, and at the same time, the experimenter touched the participant's right index finger on the knuckle, synchronizing the touches on the two hands as closely as possible. Tapping movements were applied to the two hands at 1 Hz, which, after a period of ∼10 s, elicited an illusion that the participants were touching their own hand (illusion). The illusion was not elicited in the control conditions when asynchronous touches were applied (asynchronous), or if the participants were touching a brush rather than the rubber hand (incongruent). The top panel illustrates the setup with a sitting participant, as used in our initial psychophysical experiment. The bottom panel shows the setup used in the brain-scanning experiment. In the brain scan experiments (bottom), a small brush was attached to the lateral side of the rubber hand to be used in the incongruent control condition.
Testing the illusion in a group of 32 unselected participants. A, Results from the illusion questionnaire. The participants, on average, agreed with the statement that they felt as if they were touching their own right hand with their left index finger (“touching own hand”). The participants, on average, denied the four control statements (“larger hand”; “more than one”; “moving hand”; and “not feel hand”) (see Materials and Methods for details). The differences in ratings between the illusion and the control statements were significant (p < 0.001). B, Pointing errors after experiencing the illusion (sync) and after a control condition (asynchronous); the difference between ratings was significant (p < 0.001). C, D, Pointing error against the reported vividness (C) and continuance (D) of the illusion of self-touch. The data have been fitted with least-squares regression lines (C, y= 0.71 ×-0.94, r2 = 0.35, p < 0.001; D, y = 0.74 ×-1.05, r2 = 0.39, p < 0.001). Error bars indicate SD. Sync, Synchronous (illusion); Async, asynchronous.
Testing the illusion in 15 participants selected for participation in the fMRI scanning. A, Results from the rubber-hand illusion questionnaire for the three stimulation conditions (see Materials and Methods). The participants, on average, reported that they felt the illusion only during the illusion condition (Sync; p < 0.001), and on average they denied the four control statements. B, In the illusion condition, the participants experienced a strong and persistent illusion, as revealed by the vividness and continuance ratings. C, Finally, after the scans, participants reported having experienced a stronger illusion during the illusion compared with the control conditions (p < 0.001), consistent with the prescan psychophysical testing sessions. Error bars indicate SD. Sync, Synchronous (illusion); Async, asynchronous; Incongr, incongruent.
Activity in the premotor cortex (top panels), intraparietal cortex (left lower), and the cerebellum (right lower) that reflected the illusion of touching one's own hand (self-touch). The yellow/red activations correspond to the statistical parametric map of the contrast [(illusion - asynchronous) + (illusion - incongruent)] (p < 0.001, uncorrected). The significant activations are indicated by orange circles (p < 0.05, corrected). This activation map is super-imposed on the mean high-resolution anatomical MRI of the 15 participants on which the major sulci are visible. The right hemisphere is shown to the right. The coordinates in standard space are indicated. PCG, Precentral gyrus; PCS, precentral sulcus; IPS, intraparietal sulcus; IFG, inferior frontal gyrus.
Plots showing the size of activations in the relevant brain regions. The plots show the contrast estimates for each of the three experimental conditions relative to the resting baseline. Error bars show SEM across participants. The peaks are taken from Table 1 (p < 0.001, uncorrected). a.u., Arbitrary unit; L., left; R., right; inf., inferior; lat., lateral; g., gyrus; s., sulcus; Async, asynchronous; Incon, incongruent.
Relationship between activity in premotor (precentral, top panels) and cerebellar (bottom panels) cortices, and the reported strength of the illusion (linear regression analysis). The y-axis shows the contrast estimates for the contrast (illusion - asynchronous). The x-axis shows the illusion index (the product of the vividness and continuance ratings). The data have been fitted with least-squares regression lines (left precentral sulcus, y = 0.018 ×-0.726, r2 = 0.32, p < 0.01; right precentral sulcus, y = 0.016 ×-0.877, r2 = 0.20, p < 0.05; left cerebellum, y = 0.025 ×-1.21, r2 = 0.62, p < 0.001; right cerebellum, y = 0.024 ×-0.99, r2 = 0.37, p < 0.01). The peaks were located close to those detected in the main analysis. a.u., Arbitrary unit; L., left; R., right; inf., inferior; s., sulcus.
Small volume corrections based on an a priori hypothesis.
Only the most significant peaks from each region are reported.
This peak was not significant after correction for multiple comparisons (p < 0.001 uncorrected; p > 0.05 corrected), but we report it to illustrate the bilateral pattern of activity in the intraparietal cortex.