Predictive attenuation of tactile sensation
It has been proposed that, in order to enhance sensitivity to novel information, the brain removes predictable components of sensory input. This thesis describes a series of psychophysical and behavioural studies investigating predictive filtering in the perception of touch. Using a novel force-matching paradigm, we demonstrate that self-generated tactile sensations are perceived as weaker than the same stimuli externally imposed. This attenuation is shown to be temporally tuned to the expected time of contact and modulated by the certainty with which a sensation can be attributed to self-action. We confirm experimentally that this attenuation results from a predictive, rather than postdictive, mechanism. Such a mechanism may predict the sensory consequences of action based on an internal model of the environment and an efference copy of the motor command. We investigate how prediction is acquired in a new environment and the coordinate systems in which the new environment is internally represented. Using a novel protocol of transcranial magnetic stimulation, we find evidence to suggest that the efference copy signal underlying the prediction arises upstream of primary motor cortex. Patients with schizophrenia are found to show less attenuation than healthy controls, consistent with models of the disease that propose an underlying deficit in sensory prediction. These experimental findings are discussed in relation to potential neural mechanisms of sensory filtering, and the many proposed roles for predictive mechanisms in human sensory and motor systems are reviewed.