Electrical Stimulation of Receptive Fields on the Fingertips

One of SMoRG's focuses is electrotactile simulation on the skin. One of their more recent studies has been on mechanoreceptors. Before I get into this area of study, let me tell you more specifically what mechanoreceptors are.

Mechanoreceptors are used to provide fine resolution tactile representations of our environment; they allow us to discern such elements as size, weight, and texture. These receptors are highly concentrated in the skin of human fingertips. In the case I am about to describe, experimentation was conducted on the tip of the index finger.

The point of this specific experiment was to explore our ability to discriminate electrotactile stimulation on our fingertips through an array of conditions. The conditions were electrode size, spacing, and frequency of stimulation, inclusive of combinations of these factors. SMoRG wanted to determine which elements were most influential on determining the effect of electrotactile stimulation. This was how the experiment was set up:

Four locations on the tip of the index finger were electrically stimulated. These electrodes were mounted on the central (flattest) portion of the fingertip. 






During this experiment, five electrode arrays were used. The electrodes were one of two sizes (1.75 mm or 3.5 mm) in diameter, and in two by-two arrays with inter-electrode spacings of 3, 6, or 9 mm.









The lab tested these various factors by delivering electrical stimulation to the various arrays of electrodes on the index fingertip. After each trial, the subjects were asked to verbalize  which of four locations of the fingertip were stimulated.

Results:
Electrode size and stimulation frequency did not significantly influence a subject's ability to discriminate the location of electrotactile stimulation. Increased inter-electrode spacing did. In addition, it was discovered that electrotactile stimulus localization on the fingertip has a dependency on inter-electrode spacing and direction (it was more effective in the medial -lateral direction); these two factors reduced the discrimination error significantly.

One of the goals of this type of research in electrical stimulation of tactile afferents (mechanoreceptors; afferent means sending information toward the Central Nervous System) is to replace or augment sensation associated with mechanical stimuli. Unfortunately, is it difficult to match electrical stimulation with the signals carried by afferent fibers serving mechanoreceptors. If we found a way to do this, not only would we be able to more easily detect feedback while performing telerobotic tasks and interact with virtual environments, a breakthrough in this area would allow doctors to better evaluate neural deficits associated with sensory recovery (carpal tunnel surgery, skin grafts, etc.). However, SMoRG, and other institutions have not yet achieved the data needed to transfer various types of information through electrotactile stimulation.

Bunnies!

No... I did not get to play with bunnies today... I actually greatly dislike rabbits. They're mean! But I did learn about an interesting phenomenon: the Cutaneous Rabbit Effect or Cutaneous Saltation.

The Cutaneous Rabbit Effect was discover by Frank Geldard and Carl Sherrick in the early 1970s. The Cutaneous Rabbit Effect, or Illusion, is a tactile trick created by tapping two separate regions of the skin. Usually performed on the forearm, a rapid series of taps are released first near the wrist and then near the elbow. This creates the perception of sequential taps “hopping” up from the wrist to the elbow (even though no taps were applied between the wrist and the elbow). What is interesting about this experiment is that the brain areas that are activated by real taps to the skin are activated for these illusory taps, a phenomenon yet to be explained.

                                       

Dr. Tillery and his associates are trying to incorporate this phenomenon to induce brain activation across fingers which are not physically stimulated. Instead of using physical stimulation like most tests on the CRE, SMoRG used electrotactile stimuli across four stimulation sites (one before and two stimuli after the illusory stimulus). Additionally, the experiment had the subjects focus on the site that they thought was most likely going to be illusory.

What is most interesting about this experiment is that the Cutaneous Rabbit Effect can be generated across fingertips. This test displays the first evidence that the CRE can still come into effect when the stimuli occur over a path in which the skin is not directly linked. Pretty cool, isn't it?