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(the vibration of the benders was inaudible because of its low sound pressure level and
the MR-CompatibIe sound attenuating headphones worn by the subjects). Auditory
stimuli were delivered to only the left channel (left ear) of the headphones to produce
rough spatial correspondence with the left hand tactile stimulation.
The same waveform was used for Vibrotactile stimulation (delivered via the
piezoelectric benders) and auditory stimulation (delivered via headphones). A driving
voltage was generated by a 24-bit PC sound card and amplified by a multichannel
amplifier (Sony USA, New York, NY). The waveform consisted of a 200 Hz sinusoidal
oscillation in a 500 ms envelope. To prevent onset and offset artifacts, the first and last
100 ms of the 500 ms envelope consisted of the first and second quarter-cycle of a 5 Hz
sine wave, allowing the oscillation amplitude to gradually increase and decrease.
During experimental trials, subjects discriminated between the three trial types
(tactile-only, auditory-only, or auditory-tactile) by pressing one of three buttons on a
fiber optic response stick (Current Designs, Philadelphia, PA). No feedback was provided.
Subjects were instructed to fixate central crosshairs, back-projected from an LCD
projector (Sony Electronics, San Diego, CA) onto a Lucite screen (Da-Lite lnc., Warsaw,
IN) and viewed through a mirror attached to the MR head coil. An MR-CompatibIe
eyetracking system (Applied Science Laboratories, Bedford, MA) was used to monitor
fixation and behavioral state.
Two intensities of stimulation were used: strong and weak. The intensities were
adjusted for each subject in the MR scanner just prior to fMRI data collection, using the
same driving waveform as used in the fMRI experiment. A strong tactile stimulus was