Differences in pleasant texture stimuli between
ﬁngertips and the palm
Sun Qingyu1 , Shogo Okamoto1,2 , Yasuhiro Akiyama1 and Yoji Yamada1
1. Nagoya University, Aichi, Japan
2. Tokyo Metropolitan University, Tokyo, Japan
Abstract—Because of the differences in the mechanical andanatomical properties of body parts, including the density ofmechanical receptors, humans’ perception of pleasantness varies
depending on which body part is affected. We tested thishypothesis on ﬁngertips and palm centers using 18 types of3D-printed dots scales. The participants pressed their ﬁnger orpalm to these dots scales with no sliding motions, and reported
the degree of pleasantness. For the ﬁngertip, ﬁner surfaces felt
more comfortable than rougher ones; this ﬁnding agrees withthe results of earlier studies. However, for the palm, pleasantness
reached its peak when a moderately rough scale was touched.
These ﬁndings will help design comfortable surfaces for different
body parts.
Index Terms—Dots scale, roughness, comfort

I. I NTRODUCTION
Thus far, several researchers have studied the perceptionof pleasantness when touching surface textures [1], [2], [3].
Existing studies agree that smoother surfaces, which present
less mechanical stimuli to the skin, feel more comfortable.
It is generally believed that this trend does not depend on
which body part is in contact with the surface. However, thesensitivity to surface textures largely varies among body parts.
The sensitivities are high at lips and ﬁngers, whereas they are
low at heels, back, and thighs [4]. These differences are in partcaused by the mechanical and anatomical differences between
body parts, including the varying densities of mechanicalreceptive units. Therefore, we reasonably hypothesize thatthe degree of pleasantness felt by touching surfaces also
varies among different body parts. However, the relationshipbetween pleasantness and surface roughness has yet to be thoroughly compared among different body parts. For example,researchers [1], [3] compared the pleasantness associated withactive versus passive rubbing motions for different body parts
using natural materials including cotton, denim, and sandpaper.
However, the perception of roughness has not been considered.
Therefore, we used surface specimens with different levels ofcoarseness in our experiment.
We investigated the pleasantness perceived when pressing
roughness scales with the ﬁngertip or central part of the palm,
instead of active or passive rubbing motions [1], [2]. The twobody parts included in our study have different mechanicalreceptors. Eighteen types of dots roughness scales were used,
This study was in part supported by MEXT Kakenhi 21H05819 and20H04263.

978-1-6654-1903-1/22/$31.00 2022 IEEE(a) Dot scales(b) 3D feature of specimens
Dot scales

Circular interval（1.5 times diameter)

Diameter(d mm)

30 mm
30 mm

Fig. 1. Specimens used in the experiment. (a) Surface parameters of the dotscales. (b) Overall size of the specimens

and the perception of pleasantness was then compared between
the two body parts.
II. M ETHODS
A. Stimuli: Dots scale
We used 18 types of 3D-printed dots scales. The printeremployed was Form 3 (Formlabs Inc., MA), with a nominal
resolution of 25 µm. The surface area of the specimen was
30 × 30 mm, and the height was 10 mm. The diameter d ofthe dot scales varied from 0.75 to 16.72 mm by a geometricfactor of 1.2. Accordingly, the interval between the centers of
the consecutive dots varied from 1.125 to 24.96 mm, always
1.5 times the diameter, as shown in Fig. 1 Because the ﬁngertip
would fall into the circular intervals of rougher specimens, weonly used the ﬁrst 14 of the ﬁnest specimen.
B. Task: Magnitude estimation of pleasantness perception
Magnitude estimation tasks were conducted to measure thepleasantness experienced from the specimens. We tested twoparts of the hand: ﬁngertip and palm. For the ﬁngertips,
the experiment was conducted with the ﬁrst 14 of the ﬁnest
roughness specimens. The smallest dot scale was used as themodulus. The participants were required to place their index
ﬁnger pad on randomly presented specimens and press with aforce of 200 gf, referring to an electric scale. Each specimen
was tested 3 times, and 96 trials (42 trials for the ﬁngertip and
54 trials for the palm) were conducted for each participant.
For our subsequent analysis, we used the median values of
the three trials for each roughness scale. The roughness values
were then normalized to scale their geometric mean to be 1 foreach individual. To ensure that they could not visually judge

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IV. D ISCUSSION(a) Variation of fingertip’s pleasantness perception

Pleasantness
perception (-)

1.5

1

0.5

00.75
(1.1251.08(1.62)

1.56(2.33)

2.25(3.36)

3.24(4.84)

4.66(6.96)

6.72
9.67(10.03) (14.44)

Diameter of dot scales (circular interval) (mm)(b) Variation of central palm’s pleasantness perception

Pleasantness
perception (-)

1.5
1
0.5

0
0.75(1.125)

1.08(1.62)

1.56(2.33)

2.25(3.36)

3.24(4.84)

4.66(6.96)

6.72(10.03)

9.67(14.44)

13.93(20.8)

16.72(24.96)

Diameter of dot scales (circular interval) (mm)Fig. 2. Variation of mean pleasantness experienced at the (a) ﬁngertip and(b) palm with 95% conﬁdence intervals.

the specimens’ surfaces, all participants wore glasses whose
lenses were partially blurred (upper part) using translucenttape.
C. Participants
Seven university students in their 20s participated in thetask after providing written informed consent. The objectivesof this study were not explained to the participants before theexperiments.
III. R ESULTS
The mean values of the pleasantness experienced at theﬁngertip and central palm are shown in Fig. 2. The dottedlines indicate 95% conﬁdence intervals. For the ﬁngertips, thepleasantness exhibited the highest value for the ﬁnest surface
texture. However, considering the conﬁdence interval, the true
peak may range from 0.75 to 3.89 mm. For the palm, thepleasantness reached a moderate peak at a diameter of 3.89mm.
It is generally believed that ﬁner surfaces are more comfortable to the touch than rougher ones [1], [2], [3]. Therefore,
we compared the pleasantness values associated with theﬁnest surface having a diameter of 0.75 mm with those thatexhibited the highest pleasantness values. For the ﬁngertip, theﬁnest surface exhibited the highest pleasantness value; hence,
we compared the ﬁnest surface with the 3.89 mm diametersurface. For the ﬁngertip, there was no signiﬁcant difference
between d = 0.75 mm and d = 3.89 mm (t = 0.36, p = 0.73).
However, for the palm, the specimen of d = 3.89 mm
exhibited a larger pleasantness value than the ﬁnest scale ofd = 0.75 mm (t = −3.47, p = 0.018).

Two suggestions were obtained from the present experiment
regarding the difference in pleasantness perception between
the ﬁngertip and central palm.
For the ﬁngertips, our results are largely consistent withprevious studies [1], [2], where ﬁner, smoother specimens
were shown to be more pleasant. However, our research
indicates a different result for the palm. This may be explainedby the fact that previous studies involved active or passive
rubbing motions, whereas our study involved pressing motions
without sliding. Therefore, the mechanical receptors mediating
the roughness perception and pleasantness feeling are also
different between the present and earlier studies. For thepressing motion, slowly adapting receptors are considered as
the main mediators, whereas, for the rubbing motion, fastadapting receptors are chieﬂy involved [5], [6].
As suggested by the conﬁdence intervals that are shownin Fig. 2, individual differences were greater for the ﬁngertip
than for the central palm. The individual differences in tactile
perception may be caused by the individual differences inmechanical and biological characteristics, including ﬁnger
size [7]. However, it is unclear why these differences are lesssigniﬁcant for the palm. Because our experiment focused on
pushing behavior, for which the neural activity is mainly mediated by slowly adaptive units [6], we suggest that individualdifferences in the units’ density are higher in the ﬁngertip thanin the palm.
V. C ONCLUSION
The mechanical and anatomical properties of body tissues
vary among body parts; hence, the pleasantness felt whentouching surfaces might also vary accordingly. However, earlier studies did not thoroughly investigate this point, and it
is generally believed that ﬁner surfaces feel more pleasantirrespective to body parts. We investigated the pleasantness
felt at the ﬁngertip and central palm using specimens with
14 or 18 levels of roughness. The relationships between
the pleasantness and surface parameters were substantiallydifferent between the ﬁngertip and palm. At the ﬁngertip,
similar to the results of previous studies, smoother specimens
were judged more pleasant, and there were no clear differences
between the results of ﬁne and moderately rough specimens.
In contrast, for the palm, a moderately rough specimen with adot diameter of 3.89 mm was determined to yield the highest
pleasantness value. These ﬁndings are expected to contributeto the design of pleasant surfaces.
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