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.