The relative timing of active and passive touch.

Tactile stimulation usually occurs as a combination of an active movement (reaching out to touch a surface) and a sensation (actually feeling the surface against the skin). The brain has information about the active component (the motor command) before it occurs because of efference copy, while the passive component must be transduced before it can be processed. Since the active and passive tactile components are available to the brain at different times, determining the time of touch requires calculation worked backwards from the passive sensation, and/or worked forward from the active motor command. In order to determine which touch process is perceived more quickly, we varied the relative delay between an active and a passive touch signal and determined the relative time perceived as simultaneous. A passive touch needed to be presented before an active key was pressed in order for the two touches to be perceived as simultaneous, but this timing difference was not significant. In order to test the plasticity of the active and passive touch systems, we exploited the fact that the point of subjective simultaneity between two stimuli can sometimes be altered by repeated exposure to asynchronous presentation. We exposed subjects to an active key press/ passive touch pair delayed by 250 ms. This exposure increased the range of relative delays between active and passive touches at which the pairs were judged as simultaneous. This is consistent with an adaptive change in the processing of active touch.