Apparatus
Tactile stimulation was delivered using a tactor (Dancer Design), a miniature electromagnetic solenoid type stimulator. The tactor provided vibrotactile stimulation at 40 Hz. Four different levels of stimulation intensity were provided: 0%, 20%, 50%, and 100%. Stimulus intensity was treated as an ordinal variable (i.e., 20%: low intensity, 50%: medium intensity, 100%: high intensity) because no information on absolute intensities was available. The intensity of the vibrotactile stimuli is exponentially related to the perceived intensity of the vibrotactile stimuli (Stevens, 1959), so we chose for larger percentage differences between medium and high intensities than between low and medium intensities. Participants had to wear earplugs to ensure that they did not hear the sound produced by the tactor. As the tactors produced almost no sound, neither sound detection control task, nor additional headphones were considered necessary.