In this study, we had non-amputated people using a prosthesis simulator that operates the same way as the most popular prosthetic limb used by people who have upper limb difference.
Figure 1 from the paper. (a) the task the people performed (b) how many times they performed it (c) and the prosthesis simulator that we used.
The participants moved disks from the right target area to the left target area. There were three disks, and they were different sizes so that the person would think about how wide the jaw of the prosthesis needed to be opened.
The interesting thing we did was to provide vibrotactile feedback to the participants’ forearm when they grasped the disk. We did this in half of the trials so that we could compare between vibrotactile feedback and not having vibrotactile feedback to see if it improved grasp efficiency and how brain activity changed.
When reaching, the amount that the jaws are opened above the size of the disc is a measure of efficiency in the grasp. We all do this when we are reaching for objects, and we become more efficient as we learn to grasp.
Figure 3 from the paper. (a) the profile of the aperture (distance the jaws are open) when the people were reaching for, moving, and releasing the disks. (b) the maximum aperture as the people reached for the disks.
The interesting finding here is that vibrotactile feedback did not change the grasp. You can see the grasp profile without vibrotactile feedback and with vibrotactile feedback do differ, especially at the peak.
We also wondered if vibrotactile feedback would result in people moving faster when they did the task. It’s kind of intuitive - if you have vibrotactile feedback that lets you know you are still securely grasping the disk, it would seem you could move faster.
Figure 4 from the paper. (a) the velocity while people reached for the disks (b) the velocity when people were moving the disk.
It turns out, people actually went slower when they have vibrotactile feedback, both when they were reaching for the disks, and when they were transporting the disks.
We also recorded brain activity while people were doing the task. Here, we see some very interesting differences.
Figure 5 from the paper. EEG power plots while the person is reaching for a disk (left), or moving the disk (right).
This is very interesting! When the vibrotactile feedback is actually on (while moving a disk), we see differences in sensory (left parietal) and motor (left motor) areas. That’s cool. Move information coming in due to the vibration, and there is more activity in the sensory area. Makes sense. The really interesting thing is that we see similar effects when the person in reaching for a disk. That’s before the vibrotactile feedback even comes on. These brain areas are anticipating, or predicting, that the vibrotactile feedback will come on when the person picks the disk up.
The differences in activity in the motor areas (lower activity when vibrotactile feedback is, or will be, available) is probably part of the brain changing its activity to help learn how the vibrotactile feedback works, and what it means to the person as they are doing the task.
Here’s a link to an open access version of the paper. Enjoy!