Fig. 1.

Illustration of the experimental setup and design. (A) Participants were seated in front of a desk with the force sensor, the object and the force feedback device placed in front of them. A mirror blocked the view of the hand and the object. Participants viewed the scene presented to them on the monitor through the mirror. (B) The objects with two different textured surfaces that would cause the participant to experience a low fundamental frequency (40 Hz) and a high fundamental frequency (240 Hz), respectively. The start position at which participants initially made contact with the object and the go position from which participants began the stroking movements along the texture are marked with arrows. These two positions were represented to participants visually by green spheres on the monitor and were not discernible by touching the object. (C) The tactile stimulation device (tactor) was placed on the ventral part of the proximal phalanx of the right index finger. (D) The combination of the low frequency (40 Hz) and high frequency (240 Hz) objects with the two probe stimulation frequencies that either matched or mismatched the fundamental frequencies experienced by moving along the textured objects resulted in four movement conditions: low frequency congruent, low frequency incongruent, high frequency congruent, low frequency incongruent. (E) Participants initiated each trial via a central, virtual button. A green circle (start position) appeared at the very left of the object in the virtual workspace. Once participants moved their finger to the start position, thus touching the real object, the circle vanished and reappeared at the go position, 7 cm further to the right (∼3 cm before the textured area). Participants moved their finger to this go position at their own pace. Once there, three auditory cues spaced 655 ms apart sounded, prompting participants to start a smooth continuous movement at the designated speed of 203 mm/s after the third cue (go cue). The probe stimulus was presented 100 ms before the anticipated movement onset. After reaching the end of the textured area, participants initiated the question about the tactile stimulus via a central, virtual button and responded with either “yes” or “no” by selecting the respective button.