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I'm having some difficulty understanding how the afferent signals are sensed in the finger pads. My understanding is that for mechanoreceptors, as the indenting force increases, their effective response also increases. Additionally, as contact area increases, so does the overall stimulus area and hence a larger response amplitude from the mechanoreceptor afferents should be found.

However, the results shown in this article in the Journal of Neuroscience that discusses this physiology shows contradicting results. In this article, the closer the dot spacing (1 mm or less) and the higher the contact area the lesser the receptors seem to fire, and stop firing altogether at about 1 mm dot spacing?

Why would this occur?

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  • $\begingroup$ I made some edits to your question... not trying to change your question but only clarify it for other users so you can get a good answer from someone. $\endgroup$ – Vance L Albaugh Jul 9 '16 at 14:45
  • $\begingroup$ Unfortunately, this is not a "please hold my hand and walk me through this paper/concept/idea" site. I strongly suggest you take the tour and carefully read through the help center to learn more about the site, including what is on-topic and what is not, and how to ask a good question. $\endgroup$ – MattDMo Jul 9 '16 at 16:36
  • $\begingroup$ This is a really tough question to answer. I skimmed through the linked paper (authorative authors there!) and the last part of the discussion goes into more detail on this question, including references to other papers. I may get back to this question, but it's going to take me a while. Good question! +1 $\endgroup$ – AliceD Jul 9 '16 at 19:59
  • $\begingroup$ Can you also pinpoint the figure numbers so that the actual issue is easier to spot? $\endgroup$ – WYSIWYG Jul 11 '16 at 6:43
  • $\begingroup$ This took me a while and my apologies - I have to add that it's an excellent article with a set of awesome authors and it was an enlightening read. Thanks so much - I learned a great deal :) I'm actually writing a paper on this stuff as we speak, so questions like this make this site so fruitful :-) $\endgroup$ – AliceD Nov 24 '16 at 10:21
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Short answer
Tactile grating stimuli with a higher line density (number of lines per surface area) and applied with an equal force result in a lower amount of pressure per unit of skin area under the grating, and hence smaller afferent responses.

Background
In the Discussion section of the linked article (Phillips et al, 1993) the authors explain on p. 838 (left column) that the reduction in the response of SA afferents (slowly adapting fibers) when grating density is increased is due to the higher proximity of neighboring areas of stimulation, i.e., the lines in the grating stimuli are more closely spaced. The authors reason that this leads to a reduction in the effective stimulus (i.e., compressive strain) at the receptor terminal due to the presence of adjacent stimulus elements that distributes, and hence reduces the skin loads per line.

Reference
- Phillips et al, J Neurosci; 12(3): 927-39

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