An paper has surfaced on Sandygautam Cognitive Daily (thanks to @mocost tweet) called ‘Why can’t you tickle yourself?’ by Blakemore, Wolpert and Frith, published 11 years ago. (cited below). Here is the abstract:
It is well known that you cannot tickle yourself. Here, we discuss the proposal that such attenuation of self-produced tactile stimulation is due to the sensory predictions made by an internal forward model of the motor system. A forward model predicts the sensory consequences of a movement based on the motor command. When a movement is self-produced, its sensory consequences can be accurately predicted, and this prediction can be used to attenuate the sensory effects of the movement. Studies are reviewed that demonstrate that as the discrepancy between predicted and actual sensory feedback increases during self-produced tactile stimulation there is a concomitant decrease in the level of sensory attenuation and an increase in tickliness. Functional neuroimaging studies have demonstrated that this sensory attenuation might be mediated by somatosensory cortex and anterior cingulate cortex: these areas are activated less by a self-produced tactile stimulus than by the same stimulus when it is externally produced. Furthermore, evidence suggests that the cerebellum might be involved in generating the prediction of the sensory consequences of movement. Finally, recent evidence suggests that this predictive mechanism is abnormal in patients with auditory hallucinations and/or passivity experiences.
We distinguish sensations that we produced from those that happen in the environment, and this would be important to appropriate responses. We also need to monitor our actions, to be sure they produce the outcomes we planned. For these reasons, our senses need to know what motor commands are being executed and to predict what sensory information to expect.
In order to generate sensory predictions, it is postulated that the central nervous system contains a central monitor or internal `forward model’. Forward models mimic aspects of the external world and the motor system in order to capture the forward or causal relationship between actions and their outcomes.
The experiments used an imposed robotic arm between the agent producing a tickle action and the part of the body being tickled. This allowed the subject to administer stimuli to themselves that could be delayed or rotated.
The results showed that subjects rated the self-produced tactile sensation as being significantly less tickly, than an identical stimulus produced by the robot (under the experimenter’s control). Furthermore, subjects reported a progressive increase in the tickly rating as the delay was increased between 0 ms and 200 ms and as the trajectory rotation was increased between 0 and 90 degrees. These results support the hypothesis that the perceptual attenuation of self-produced tactile stimulation is due to precise sensory predictions, rather than a movement- induced non-specific attenuation of all sensory signals. When there is no delay or trajectory rotation the model correctly predicts the sensory consequences of the movement, so no sensory discrepancy ensues between the predicted and actual sensory information, and the motor command to the left hand can be used to attenuate the sensation on the right palm. As the sensory feedback deviates from the prediction of the model (by increasing the delay or trajectory rotation) the sensory discrepancy between the predicted and actual sensory feedback increases, which leads to a decrease in the amount of sensory attenuation.
Next they measured brain activity associated with the prediction and attenuation of tickling.
There were four conditions: self-generated tactile stimulation; self-generated movement without tactile stimulation; externally generated tactile stimulation; and rest. Using this design we were able to assess the difference in brain activity during self-generated relative to externally generated tactile stimulation while factoring out activity associated with self-generated movement and tactile stimulation. Analysis of the imaging data resulted in the creation of statistical parametric maps refl̄ecting the two main effects, movement and tactile stimulation, and the interaction between these two factors.
They found the secondary somatosensory cortex and the anterior cingulate gyrus had attenuated activity by the movement. In contrast, the right anterior cerebellar cortex was deactivated only by movement that resulted in stimulus and not movement alone. It was also activated by externally produced stimulation.
We suggest that the cerebellum is involved in predicting the specific sensory consequences of movements and in providing the signal that is used to attenuate the somatosensory response to self-produced tactile stimulation.
Tickling self-produced and externally produced stimulation was compared for schizophrenic patients with auditory hallucinations and/or passivity, patients without these symptoms. with match controls.
The results demonstrated that normal control subjects and patients with neither auditory hallucinations nor passivity experienced self-produced stimuli as less intense, tickly and pleasant than identical, externally produced tactile stimuli. In contrast, patients with these symptoms did not show a decrease in their perceptual ratings for tactile stimuli produced by themselves as compared to those produced by the experimenter. These results support the proposal that auditory hallucinations and passivity experiences are associated with an abnormality in the forward model mechanism that normally allows us to distinguish self-produced from externally produced sensations. It is possible that the neural system associated with this mechanism, or part of it, operates abnormally in people with such symptoms.
There it is – another bit of evidence for near-future prediction in our experience of the world.
Blakemore SJ, Wolpert D, & Frith C (2000). Why can’t you tickle yourself? Neuroreport, 11 (11) PMID: 10943682