What is smallest touch (pressure) sensation that a normal, healthy person (not hypersensitive nor insensitive) can feel on the palm of the hand?
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$\begingroup$ @fileunderwater - of course it's all about population averages. However I do agree with daniel; there are still many variables left; static pressure thresholds differ from hose obtained by vibrotactile stimuli. Furthermore, sensitivity goes down from fingertips > palm > back of the hand $\endgroup$– AliceD ♦Commented Mar 9, 2017 at 10:09
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$\begingroup$ What happened to all the previous comments? They are hardly irrelevant to the current question. @AliceD, do you have any idea? $\endgroup$– fileunderwaterCommented Mar 9, 2017 at 13:36
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$\begingroup$ @AliceD My point was not that it is about population averages, but that the smallest sensation/stimuli that a person can feel is likely to differ alot between persons, so the question is ill-posed. And removing hypersensitive persons feels like an arbitrary cut-off, when sensitivity is likely to differ dramatically between "healthy" persons. $\endgroup$– fileunderwaterCommented Mar 9, 2017 at 13:39
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$\begingroup$ @fileunderwater - the other comments were dealt with and I'm afraid I might have flagged your commentfor deletion - too quick perhaps. But let me put it this way - in psychophysics there is always a huge variation between people. Detection thresholds are subjective - people draw their own threshold. I, for example, generate relatively a lot of false positives, because I tend to feel stuff that is simply not phsysically present (it may be present in my mind for sure). Other folks pick a more stringent threshold and only say 'Yes' when they are absolutely sure they are feeling it $\endgroup$– AliceD ♦Commented Mar 9, 2017 at 13:43
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1 Answer
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Short answer
The detection threshold of static indentation stimuli on the palm of the hand is approximately 10 to 40 micrometers, dependent on the exact location under investigation.
Background
The sensitivity of a sensory system can be expressed as the detection threshold. This threshold is in psychophysics generally defined as that stimulus level where, on average, 50% of the stimuli are felt.
There are several types of touch receptors that can be investigated, each with specific types of stimuli that are associated with them (Fig. 1)
Fig. 1. Skin receptors and their response properties (Delmas et al. (2011)
The question specifically targets detection thresholds of indentation stimuli. These stimuli are typically expressed as distance units (meters) or pressure units (Newtons). The skin indentation under constant force is dependent on the area of contact, skin type and other parameters of lesser importance. Hence, indentation units using distance metrics may be the most appropriate way of expressing static detection thresholds. Johansson & Valbo (1970) investigated static detection thresholds on the palm of the hand (Fig. 1) and found average thresholds of 10 to 40 micrometers indentation. This variation was due to geographical differences; towards the fingers sensitivity is higher and thresholds are hence lower (Fig. 2).
Fig. 2. Detection thresholds of static stimuli on the palm of the hand. source: Johansson & Valbo (1970)
References
- Delmas et al. Nature Rev Neurosci (2011); 12: 139-53
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2$\begingroup$ Definitely seems like research that would've been done in the 70s :p. Anyways, nice answer! And interesting too! $\endgroup$ Commented Mar 9, 2017 at 16:14
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3$\begingroup$ I wonder if the number of false positives (or possibly the sensitivity) would change if the subject were first exposed to a video of fleas or other light-touch parasites. Or better, if they were exposed to live fleas jumping on them for a while... I know that I definitely feel hypersensitive to microtouches if I feel I might be crawled on, and I understand that this is not uncommon. $\endgroup$ Commented Mar 9, 2017 at 18:59
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1$\begingroup$ @DewiMorgan personally, I check for false positives by including catch trials where subjects are provided with the regular visual cue for "stimulation is up", but no tactile stimulus is presented. A "Yes" answer is per definition a false positive. If the false positive rate surpasses the 25%, the subject is instructed to raise their internal subjective threshold and the run is restarted. $\endgroup$– AliceD ♦Commented Mar 9, 2017 at 22:41
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$\begingroup$ @AliceD: Oh, I know it can be compensated for. I was just interested in whether the sensitivity or error rate changed with the same subject, according to mental stimuli like that. $\endgroup$ Commented Mar 10, 2017 at 1:40