Talk:Touch in aging
This is an interesting and well-written article on an important topic, and I enjoyed reading it. In several sections, it seems that additional relevant information should be incorporated into your article. Please see my suggestions below:
I have a technical comment here that concerns the fact that Semmes-Weinstein filaments are labeled according to a logarithmic scale.
You write that: “In this study, the mean detection threshold at the fingertip was 2.74 in the group of seniors aged over 60 years, whereas it was 2.18 in the participants aged less than 30 years, which corresponds to a 25% increase with age.”
The conclusion that this is a “25% increase” should be reconsidered or explained further, because the filament scale is logarithmic. The percent increase, therefore, with respect to grams force, will be greater than 25%. If filament value = log (10 x mg bending force), then we have:
2.74 filament = (10^2.74 / 10) mg = 55 mg = 0.055 g
2.18 filament = (10^2.18 / 10) mg = 15 mg = 0.015 g
Therefore, the increase (in force) is over 3-fold, a change much greater than 25%.
Non-Pacinian channels may be relatively less affected by age, but they are indeed significantly affected, and this should be pointed out. See, for example, the following papers:
Gescheider et al. (1994) The effects of aging on information-processing channels in the sense of touch: I. Absolute sensitivity. Somatosensory and Motor Research 11:345–357. This study showed significant loss of vibrotactile sensitivity with age - on the thenar eminence - in the P channel, the NP I channel, and the NP II channel. Although the loss was most pronounced in the P channel, it was significant in all channels.
Bhattacherjee et al. (2010) Vibrotactile masking experiments reveal accelerated somatosensory processing in congenitally blind Braille readers. Journal of Neuroscience 30(43): 14288-14298. This study investigated vibrotactile threshold detection (as well as amplitude discrimination and masking) on the fingertip, in blind and sighted participants of different ages. Among both groups, age significantly affected vibrotactile detection of a brief 20Hz pulse, suggesting a significant loss of sensitivity in the Meissner (i.e., RA1 or NP I) channel.
You write that: “Although still widely used clinically, the two-point test and its variants remain problematic for their interpretation is blurred by the presence of intensive cues affecting the perception of single vs. double point stimuli (Johnson and Persinger 1994). In fact, careful investigations by Johnson and colleagues have shown that observers can actually discriminate between single and double-point stimuli even when there is no physical separation between the two probes; thus invalidating the two-point limen as a test for spatial acuity”.
I agree fully. In fact, a recent article from my lab studied just this issue: Tong J, Mao O, Goldreich D (2013) Two-point orientation discrimination versus the traditional two-point test for tactile spatial acuity assessment. Frontiers in Human Neuroscience 7: 579. doi: 10.3389/fnhum.2013.00579.
In this article, we confirmed that participants can distinguish two points at zero separation from one point, supporting the presence of an intensive cue. We showed further that an alternative test, two-point orientation discrimination (2POD), does not suffer from this shortcoming. 2POD provides an uncontaminated measure of spatial acuity. We propose that 2POD replace the classic test in clinical settings.
In addition to the excellent grating orientation studies referenced by the authors, research from our laboratory has also found significant age-related decline in grating orientation acuity. Using a precision-controlled stimulus system (Goldreich D, Wong M, Peters RM, Kanics IM (2009) A tactile automated passive-finger stimulator (TAPS). J Vis Exp 28: e1374. doi: 10.3791/1374), we found significant age-related decline in grating orientation acuity - see, for instance:
Wong M, Gnanakumaran V, Goldreich D (2011) Tactile spatial acuity enhancement in blindness: evidence for experience-dependent mechanisms. Journal of Neuroscience 31(19): 7028-7037.
Goldreich D, Kanics IM (2003) Tactile acuity is enhanced in blindness. Journal of Neuroscience 23(8): 3439-3445.
These studies tested large groups of sighted as well as blind participants. The studies showed significant age-related decline in grating orientation acuity among both groups (see Table 2 of Wong et al., 2011). Interestingly, while at any given age blind participants had on average better performance, spatial acuity in the two groups declined at an equal rate with age (see Fig. 4 of Goldreich and Kanics, 2003). These findings lend support to the hypothesis that the age-related decline is tactile spatial acuity is due to loss of peripheral receptors (e.g. SA1 afferents or Merkel cells), which would be expected to affect both groups equally. At the same time, it seems clear that blind people somehow process tactile input more effectively (presumably, due to central nervous system differences), because they perform better on the task than do their sighted peers.
Tactile recognition of 2D patterns
It would be interesting in this section to reference the following study by Legge and colleagues, which showed that blind participants, unlike sighted, did not decline with age on a 2D haptic test. The good correlation, therefore, between tactile sensing and 2D haptic sensing may not be airtight; apparently extensive haptic experience or some other feature of blindness is able to overcome this correlation:
Legge, G.E. et al (2008) Retention of high tactile acuity throughout the life span in blindness. Perception and Psychophysics 70(8): 1471-1488.
Additional minor comments
In the Pressure sensitivity section, please remove the space before the comma: “after 60 years , a substantial proportion”
In the Spatial acuity section, the semicolon here should be a comma: “In fact, careful investigations by Johnson and colleagues have shown that observers can actually discriminate between single and double-point stimuli even when there is no physical separation between the two probes; thus invalidating the two-point limen as a test for spatial acuity”
In the Conclusion, you refer to “this chapter.” It seems that “this article” would be a more appropriate term for an encyclopedia article.