Applying Incongruent Visual-Tactile Stimuli during Object Transfer with Vibro-Tactile Feedback

The protocol tests the effect of vibro-tactile feedback on the response to incongruent visual-tactile stimuli. A motion capture system calculates the 3D coordinates of a block transferred across a partition. So the analysis of the movement time and path of the block in each repetition can be performed efficiently and precisely at the end of the trial.

The reaction of the subject to the incongruent visual-tactile stimuli in the presence of vibro-tactile feedback can be easily quantified. Quantification of the positive or negative contribution of vibro-tactile feedback during motor function that involves incongruent visual-tactile stimuli can assist in future design of operative prostheses, haptics, and surgical tools, smart sportswear, or any other garments that incorporate vibro-tactile feedback. Any motion capture system and vibro-tactile feedback system can be used to replicate our protocol.

Begin by obtaining a wooden box with a 15.2 centimeter high partition in the center. Place a soft sponge layer on both sides of the partition. Then, place six passive reflective markers on the aspect opposite to the screen at the four corners and on both ends of the partition.

Use a 3D printer to manufacture a cube with the dimensions of 2.5 centimeters by 2.5 centimeters by 2.5 centimeters attached to a base with the dimensions of 4.5 centimeters by 4.5 centimeters by one centimeter. Next, place a large screen, approximately 1.5 meters, in front of a table and place the box on the table, 10 centimeters from the edge opposite to the screen. Finally, use a six-camera motion capture system activated at 100 hertz to visualize the partition and the movement of the block in real time.

Calibrate the motion capture system with an instrumented wand so that the block and the partition of the box are recognized as rigid bodies. Then, place the box on the table 10 centimeters from the edge opposite to the screen. First, instruct the subject to remove any watches, bracelets, and rings.

Attach the VTF system controller to their forearm. Then, attach two thin and flexible force sensors to the palmar aspect of the thumb and index fingers over a thin, spongy layer. Place a cuff on the skin of the upper arm of the subject and ensure it sits comfortably.

Press the button to activate the battery attached to the controller. And then ask the subject to press their thumb and index fingers together lightly. Instruct the subject to train for 10 minutes in grasping the block as lightly as possible, using only the two instrumented fingers.

Have them lift the block, move it, and place it back on the table several times, attempting to apply a minimal amount of force on the block. Next, instruct the subject to stand close to the table where the box and partition are placed. Place a divider at the edge of the table near the subject and above the box so that the subject is unable to see the box, but can easily see the screen in front of him or her.

Then, place the block in the middle of the right compartment of the box and guide the hand of the subject to it. Start the software controlling the cameras of the motion capture system. In the control panel of the visual feedback software, select with/without VTF, type the code of the subject, click run, connect, open, and start.

Instruct the subject to transfer the block 16 times with the force sensors instrumented hand, while viewing the movement of the virtual block on the screen. After each transfer, move the block back across the partition to its starting location. After the subject completes all 16 repetitions, click stop in the software.

Finally, ask the subject to rate the difficulty level of performing the task of transferring the block 16 times twice, with and without the VTF, according to the following scale. Zero is not difficult at all, one is slightly difficult, two is moderately difficult, three is very difficult, and four is extremely difficult. Results indicated that the perceived difficulty level from zero to four of performing the task with the VTF significantly correlated with the normalized path length of the block with VTF.

The normalized path length in the presence of incongruent visual-tactile signals was longer for subjects who perceived the task as more difficult when using the VTF. Further, the normalized path length was significantly longer for subjects who perceived the task as more difficult when using the VTF. It is critical that the subject is not aware of the possibility of misleading feedback during the trial.

To ensure this, the bottom of the box is lined with a soft sponge layer and the subject wears circum-ear earphones to eliminate auditory feedback of the block falling and hitting the wooden box. Also, the two transfers with incongruent feedback should be chosen randomly out of the 16 transfers performed.