-- KarlinBark - 30 July 2004
I re-organized the 5 N difference threshold test to include a wider range of comparison forces applied to stretch the skin on the forearm. So instead of ranging from 4.4 N to 5.6 N, the comparison forces ranged from 3.8 N to 6.2 N. This was quite a large increase, instead of varying each comparison stimuli by 0.2 N, each had a difference of 0.4 N (in other words, the forces tested were 3.8, 4.2, 4.6, 5.0, 5.4, 5.8, and 6.2 N). I wanted to keep the differences between the comparison forces equal, and the spring scales we were using to test measured in increments of 0.2 N, so trying to test differences of 0.3 N would be slightly more difficult to estimate. Judging from Li and my data, at 1 N, the difference threshold is somewhere near 0.15 N, and the difference threshold at 5 N is near 0.5 N, which is quite a difference.

So we re-tested Li and myself. What was interesting was that we both performed better than the previous experiment (obviously, since the range of forces was greater), but we both noted that it was more like a guessing game. Although we did fairly well in distinguishing the greater force, we both did not feel confident in our answers. I certainly felt as if I was playing a guessing game for most of the time. However, even though we thought we were guessing, there?s no way anyone could guess as well as we did, so there must be something in the brain or sensors that detect the forces correctly...just not enough to give the brain a strong confirmation. Perhaps it is an issue of learning/not being used to the skin stretch feeling, or perhaps there is some error inherent in the experiment, I don?t know yet.

Another extremely simple, ghetto type of testing we tried today was to compare vibration to skin stretch. We used a small, vibrating pager motor and taped it to my arm. I walked around, {our hypothesis was that you can get desensitized to the vibration easily when performing some sort of physical activity (this was from personal paper bike experience)} with the motor taped on my arm, and first noticed that as I swung my arm back and forth, there was a distinct difference in the force felt. I also noticed that as I walked at a moderate pace, with wind blowing gently, I did not feel the vibration on my arm within 30 seconds or so. However, after I took the motor off my arm, we noticed that the connections weren?t too great, so the vibrations I felt were probably weaker than normal. Once we fixed the connections, Li tried the experiment. With the stronger vibration, he could still feel the vibration as he walked along. When I asked him to lightly jog, he immediately noticed that the vibration couldn?t be felt anymore. But as soon as the jogging stopped, he said the vibration was apparent. To test the desensitization of skin stretch, we applied a piece of tape and pulled on the tape to stretch the skin, and applied another piece of tape on the arm to keep the skin stretch in place. Then Li jogged around again and noticed that he could always feel the skin stretch.

This was exciting, because we think it helps us to validate the necessity of skin stretch and gives us a clearer direction and idea of what the benefits might be. Obviously the vibration vs. skin stretch test needs to more detailed than our testing today, but it definitely brightened our day. It would give a reason to say that vibration should not be used in virtual environment training purposes, where there is a small amount of physical activity necessary. And who?s to say that even in a non-physical environment that skin stretch would not be better either? Anyway, it was a good day.