Thanks for the reply.
I want to create a device as was done in this study (
http://www.jneurosci.org/content/20/4/1597.full). Briefly, it found that variable frequency stimulation applied to finger tips in human subjects results in improvement in spatial discrimination ability.
The relevant section regarding the specifics of the device,
"The timing of the coactivation protocol was the same as that in our previous neurophysiological study. To prevent habituation during the long-lasting stimulation over several hours, we presented the applied stimuli at eight different interstimulus intervals (ISIs) between 100 and 3000 msec in pseudorandomized order, resulting in a mean stimulation frequency of 1 Hz (Godde et al., 1996). The duration of each pulse was 10 msec. This protocol was used for a group of 11 subjects. The remaining subjects were tested with a slightly modified protocol, in which ISIs were randomized between 8 and 1761 msec, resulting in a mean frequency of 1.7 Hz. Because the outcome of the experiments was unaffected by the slight differences in average frequency, we pooled the data for further analysis."Mean stimulation frequency was 1 Hz in standard protocol and 1.7 Hz in the modified protocol, with no change in outcome of the experiment. I wonder how they did it.