What is the frequency range of electrotactile stimulation?

Question

I have been trying to find out what the (human) electrotactile frequency range is. Since this is a much more experimental method (and more unpleasant) than the traditional vibrotactile method, the research is more limited.

E.g., for vibrotatctile I could find in Sonar and Paik (2016)

The mechanoreceptors responsible for vibrotactile sensation are the rapidly adapting (RA) and Pacinian corpuscle (PC) receptors with perceptible frequencies ranging from 3 to 100 and 100 to 400 Hz, respectively (Choi and Kuchenbecker, 2013).

Actually that seems to contain a typo or misquote of the paper they cite, which puts the PC at 10-500 Hz:

Anyhow, for electrotactile I could only find what some researchers have used in some of their devices, e.g. Okpara et al., 2007:

Based on our prior unpublished work, the four levels of frequency were set at 100, 35, 15 and 10 Hz.

That suggests a lower range perhaps of only 10-100 Hz. But is that the full range range that can be felt using electrotactile stimulation?

It's been suggested to me that it may be exactly the same as for vibrotactile, but a set of slides comparing various methods says about electrotactile:

Stimulate receptors and nerve endings with electric charge passing through the skin

Since there's some suggestion it may bypass receptor (in part), perhaps the results can be different.

What is the frequency range of electrotactile stimulation?

Answer 1

Short answer
Stimulus frequency does not matter too much for electrical neural stimulation.

Background
The frequency of electrical stimulation does not need to be in the same ball park as the physiological stimulus impinging normally on a tactile skin receptor. This, because the normal route of stimulation is bypassed and the axons can be activated directly (Kajimoto et al., 1999). In contrast, physiological tactile stimulation typically involves the deformation of a specialized dendritic region of a mechanoreceptors (Fig. 1). The effectiveness of mechanoreception depends highly on the physical properties of the dendritic sensory region in terms of responsiveness to various stimuli, including frequency selectivity, as you nicely illustrate in your answer. For electrical stimuli, all the dendritic mechaoreceptive selectivities are simply bypassed and pretty much any electric stimulus will do.

To give an example: a single jolt of direct-current electricity from a 9 Volt battery evokes a clear tactile sensation on the tongue. The BrainPort device, which is a sensory substitution device that aims to provide visual function to blind patienbts, relies on electrical stimulation of the tongue to provide the user with tactile images of the field of view as recorded by a head-mounted camera. The BrainPort uses a pulse rate of 200 Hz, which clearly electrically stimulates the tongue (Danilov et al., 2007). Hence, whether you provide a single jolt, or a continuous pulse train of 200 Hz doesn't really matter for perception, at least as far as I know.

<sub>References
- Danilov et al, J Vestib Res (2007); 17(2-3): 119-30
- Kajimoto et al., ICAT 1999</sub>


^{Fig. 1. Skin receptors. source: Lumen.}