Spatio-temporal prediction modulates the perception of self-produced stimuli

We investigated why self-produced tactile stimulation is perceived as less intense than the same stimulus produced externally. A tactile stimulus on t he palm of the right hand was either externally produced, by a robot or sel f-produced by the subject. In the conditions in which the tactile stimulus was self-produced, subjects moved the arm of a robot with their left hand t o produce the tactile stimulus on their right hand via a second rebut. Subj ects were asked to rate intensity of the tactile sensation and consistently rated self-produced tactile stimuli as less tickly, intense, and pleasant than externally produced tactile stimuli. Using this robotic setup we were able to manipulate the correspondence between the action of the subjects' l eft hand and the tactile stimulus on their right hand. First, we parametric ally varied the delay between the movement of the left hand and the resulta nt movement of the tactile stimulus on the right hand. Second, we implement ed varying degrees of trajectory perturbation and varied the direction of t he tactile stimulus movement as a function of the direction of left-hand mo vement. The tickliness rating increased significantly with increasing delay and trajectory perturbation. This suggests that self-produced movements at tenuate the resultant tactile sensation and that a necessary requirement of this attenuation is that the tactile stimulus and its causal motor command correspond in time and space. We propose that the extent to which self-pro duced tactile sensation is attenuated (i.e., its tickliness) is proportiona l to the error between the sensory feedback predicted by an internal forwar d model of the motor system and the actual sensory feedback produced by the movement.