On the Eﬀect of Vibration on Slip Perception
During Bare Finger Contact
Hikaru Nagano1,2 , Yon Visell2 , and Shogo Okamoto1
1

Department of Mechanical Science and Engineering, Nagoya University, Japan⋆
2Electrical and Computer Engineering Department, Drexel University, USA

Abstract. This study investigated the inﬂuence of the presence andtiming of cutaneous vibration cues supplied to the ﬁnger pad on theperception of slip of a contact surface slid beneath it. We designed anapparatus that made it possible to supply precisely controlled shear force,
sliding displacement and vibration cues to the ﬁnger pad via a moving
surface. We conducted an experiment to assess the eﬀect, if any, of thepresence and timing of vibrotactile feedback presentation relative to slip
onset on the perceived duration of slipping between the ﬁnger and thesliding surface. We found that vibrotactile stimuli that are presented atslip onset or during the slip phase both increased the perceived durationof slipping. In contrast, if the same cues are presented during the stick
phase, they tended to decrease perceived slip duration. These results
support a perceptual role for cutaneous vibrations felt in slip estimation,and indicate an opposite perceptual interpretation depending on theirtiming relative to slip onset.
Keywords: Slip sensation, Stick-to-slip transition, Slip onset, Vibrotactile stimuli, Skin vibration

1

Introduction

Humans can readily perceive whether an object in contact with the ﬁnger isslipping or not, and slip detection in this form is assumed to be instrumental toa variety of manipulation tasks. However, the mechanism through which slip isperceived is not fully understood. During shear interaction between the ﬁnger
and a contacting surface, slip generally proceeds through three contact phases [1,2]. First, in the stuck phase, the tangential (shear) force grows without macroscopic relative displacement of the contacting surfaces. Second, at slip onset, a
stick-to-slip transition occurs, in which relative displacements begin propagate
locally in the contact area. Finally, in the full slip phase, the stuck contacting
area disappears entirely, and the contacting surfaces undergo macroscopic relative sliding. In each of these phases, transient local deformations of the ﬁnger pad
are generated, exciting concurrent activity in multiple types of mechanoreceptiveaﬀerents [3, 4]. The activation of these receptors is understood to contribute to⋆This work was in part supported by MEXT KAKNEHI 13J02247.

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H. Nagano et al.
Voice coil
actuator

CradleTouch surface
Sliding plate

Linearencoder

Linear guide rail

Fig. 1. Apparatus used to apply force proﬁles

the unconscious regulation of grip forces in ways that arrest slipping [5, 6], but
the precise way in which vibrotactile stimuli aﬀect slip perception has yet to berevealed. To this end, we designed a novel apparatus to stimulate the skin withshear sliding and vibration cues and undertook an experiment to investigate
the inﬂuence of the presence and timing of cutaneous vibration stimuli on slip
perception.
We speciﬁcally sought to investigate the role of vibrotactile stimuli that arepresented in each of the three phases described above, i.e.: sticking, slip onset,
and full slipping. As noted, we refer to the stuck phase as the period of time
during which a shear strain is applied to the ﬁngerpad without slipping. We
hypothesized that the combination of tangential (shear) force and imposed cutaneous vibration cues would lead to a perceptual interpretation that slip had
occurred. Although we refer to the second phase in our experiment as slip onset,
the precise timing of slip is diﬃcult to detect, since it accompanies a very rapid
collapse of the stuck area of contact [2]. We could identify the occurrence of
this transient phenomenon only immediately afterward, when the sliding speed
reached a threshold value (see methods, below). Nonetheless, we regard thisphase as the transient initiation of slipping. We hypothesized that an imposed
vibrotactile stimulus presented in this phase would enhance the detection of slip
onset. Finally, we hypothesized that during the full slipping phase, which temporally follows slip onset, imposed vibration feedback could enhance participants’sense that the contact surface was slipping beneath the ﬁnger.
In summary, if the sense that a surface is slipping under the ﬁnger is associated with the activity of mechanoreceptors in the ﬁngerpad, then imposed skinvibrations that activate these receptors may inﬂuence the perceived slip duration. However, the extent and nature of inﬂuence could depend on the relative
timing of presentation.
Motivated by the foregoing considerations, we designed an apparatus thatmakes it possible to supply precisely controlled force, sliding displacement andvibration cues to the ﬁnger pad. We conducted an experiment to assess theinﬂuence of the presence and timing of vibrotactile feedback on the perceived
duration of slipping between the ﬁnger and a sliding surface.

On the Eﬀect of Vibration on Slip Perception During Bare Finger Contact2

3

Methods

The experiment investigated the discrimination of slip duration. It was based on
the method of paired comparison.
2.1

Apparatus

The device we designed for this investigation is shown in Fig. 1. The moving partconsists of a plate with a contacting surface mounted on a low noise linear guide.
To ensure consistent friction conditions, during the experiment, a rectangle ofcardstock paper was aﬃxed to the plate, and the ﬁngerpad of the participant
was allowed to rest on this plate. A voice coil actuator (GVCM-019-032-02,
Moticont, CA, USA) drove the linear guide as commanded by a microcontroller (mbed NXP LPC1768, NXP semiconductors, Netherlands) anda voltage-controlled current ampliﬁer (4-Q-DC LSC 30/2, Maxon Motor,
Switzerland). The displacement of the sliding plate was captured by a linear
encoder (OEM-030U-01, Moticont, CA, USA). The sampling rate of thecontrol and measurement were set to 2.5 kHz and due to the controlled mechanical design, the usable frequency bandwidth of the device extended to morethan 1 kHz, ensuring that shear force, sliding displacement, and vibration cues
could be precisely delivered to the ﬁngerpad. A cradle was used to restrict kinematic movement of the ﬁnger; consequently, the displacement of the linear guidevirtually corresponded to the skin deformation until slip onset.
2.2

Procedure and task

Six participants, all of them are students in the authors’ department, volunteered for the study. Participants were naive with respect to the purpose of theinvestigation. They sat on a chair and placed the index ﬁnger of their dominant
hand on the sliding plate while maintaining an approximately constant normal
force of 1.5 N in order to avoid variations in friction forces. They were trained
to produce the speciﬁed normal force through feedback from a digital scale, andwere re-trained at maintaining this force level at breaks and wore sound insulating headphones playing a pink noise suﬃcient to mask any sounds produced by
the apparatus during experiments.
In each trial, participants felt the plate strain the ﬁngerpad, then slide beneath it while their ﬁnger remained passive. This occurred for two diﬀerent conﬁgurations of the slip stimulus. Participants then responded indicating whether
the duration of the ﬁrst or second slip was greater. They were instructed that“duration of slipping” referred to the length of the time period during which theyfelt slipping to occur. A slippage is diﬃcult to be subjectively scored, therefore
we employed the perceived slip duration as the evaluation index.
2.3

Stimuli and procedure

The experiment employed four diﬀerent conﬁgurations of vibratory stimuli applied to the ﬁnger pad, that consisted of commanded shear force trajectories

H. Nagano et al.

20

2

15

0

10

Increasing rate
is 100 mm/s

-2

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-4
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Time [s]

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20
15

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Time [s]0.8d) Stimulus D

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Time [s]0.8

Displacement ofsliding plate [mm]Force command [N]

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c) Stimulus C

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Displacement ofsliding plate [mm]

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Displacement ofsliding plate [mm]4Force command [N]b) Stimulus B

Force command
Displacement

Displacement ofsliding plate [mm]Force command [N]a) Stimulus A

Force command [N]

4

0

-4
0

0.2

0.4
0.6
Time [s]0.8

Fig. 2. Examples of force command and displacement of sliding plate in four types ofstimuliwith or without imposed vibrations. They were presented via the apparatus described above. The four types of stimuli used in the experiment (A, B, C, andD; see Fig. 2) diﬀered according to the presence and timing of vibrotactile cues.
The timing of the latter cues was speciﬁed relative to one of two events: the onset time t0 of shear stimulation of the ﬁnger or the subsequent time ts of onsetof slip between the plate and the ﬁnger. Slip onset was determined to be detected reliably as ﬁrst the time ts > t0 at which the rate of displacement reached
100 mm/s. This time varied outside the experimenters’ control from trial to trial
due to variations in frictional parameters. The stimuli are described in detail in
the following paragraphs.
Stimulus A: No vibrotactile stimulus For stimulus A, as shown in Fig. 2a), theforce command was increased at a rate of 1.78 N/s after a random time delay of
0–1 s. After slip onset, the force stopped increasing and was held constant for afurther 100 ms before it was reduced to 0 N.
Stimulus B: Vibrotactile stimulation during stuck phase An example of the force
command and the slider displacement for stimulus B are shown in Fig. 2b). The
diﬀerence from stimulus A is in the presence of vibrotactile stimulus. We adopted
a white noise as the vibrotactile stimulus for stimulating multiple mechanoreceptors in the ﬁngerpad skin, because these receptors react to the dynamic deformation of ﬁngerpad [3, 4]. This vibrotactile noise cue was initiated at a time
250 ms after t0 and lasted for 50 ms.

Probability of answers

On the Eﬀect of Vibration on Slip Perception During Bare Finger Contact

5

1
0.75

P1
P2

P3 P4P5

P6

0.5
0.25
0

a)
Probabilities
for A > Bb)
Probabilities
for C > Ac)
Probabilities
for D > C

Fig. 3. The frequencies with which the estimated slip duration in each stimulus condition is perceived longer than those in its paired conditions

Stimulus C: Vibrotactile stimulation at slip onset Fig. 2c) shows an exampleof the force command and the displacement for stimulus C. The vibrotactile
stimulus was applied coincident with the slip onset time, ts , and lasted for 50 ms.
Stimulus D: Vibrotactile stimulus presented during slip phase An example of theforce command and plate displacement for stimulus D are shown in Fig. 2d).
The vibrotactile stimulus was supplied at 50 ms after the time of slip detection
and lasted for 50 ms.
Pairwise comparisons Participants were presented with ﬁve diﬀerent paired comparisons (A-A, A-B, A-C, B-C, and C-D) in a two-alternative forced choice task
in which they judged which stimulus had a longer slip duration. The paired
stimuli and ﬁve combinations were presented to each participant in randomizedorder. The purpose of comparisons (A-B, A-C, C-D) was to assess the inﬂuenceof vibrotactile stimulation and timing on the perceptual estimation of slipping
duration. The remaining comparisons (A-A, B-C) were included in order to assess the extent to which participants’ slip duration judgements may have beenbiased by the presence of vibration stimuli independent for reasons unrelated toslip duration perception. These were, in short, used as control trials.
For each participant, 250 comparisons were conducted, so that each stimulus
pair was presented 50 times per participant. Participants were required to pause
and rest after each 50 trials. The experiment lasted approximately two hours per
participant.

3

Results

In analyzing the results, we removed trials in which slip occurred later or earlier
than expected due to changes in contact or frictional conditions, or variations
in normal force because these diﬀerences may aﬀect the perceived slip duration.
In each of ﬁve comparisons, 10 trials in each condition with the largest value ofdiﬀerence of ts between a stimulus pair and 10 trials that exhibited the largest

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H. Nagano et al.

diﬀerence of maximum slip displacement were excluded. We analyzed the remaining data, which consisted of 30 trials for each participant in each experimental
condition.
Figure 3 shows the probabilities at which the slip duration in each condition
was perceived longer than those other conditions with which it was paired. We
used hypothesis testing to assess whether the participants’ answer ratios weresigniﬁcantly greater than chance level (0.5).
Comparison between stimuli A and B As shown in Fig. 3a), the perceived slip
duration for stimulus A was signiﬁcantly longer than that for stimulus B (twotailed t-test, t(5) = 5.27, p < 0.01). This result indicates that the vibrotactile
stimulus presented at the sticking phase reduced the perceived slipping duration.
Comparison between stimuli A and C As shown in Fig. 3b), the perceived slip
duration for stimulus C was signiﬁcantly longer than that for stimulus A (twotailed t-test, t(5) = 5.93, p < 0.01). The participants tended to perceive the slip
duration longer when the vibrotactile stimulus was presented at slip onset. We
should note that the inﬂuence of stimulus B was opposite to that of stimulus A.
Comparison between stimuli C and D Fig. 3c) shows that there is no signiﬁcantdiﬀerence between the perceived slip durations between stimuli C and D (twotailed t-test, t(5) = 1.78, p > 0.05). This result implied that the vibrotactile
stimulus presented during the slipping phase has a similar eﬀect on the perceived
duration of slip to one presented at slip onset (stimulus C).
Comparison between stimuli B and C The perceived slip duration for stimulus C
was signiﬁcantly longer than that for stimulus B (two-tailed t-test, t(5) = 5.56,
p < 0.01). This result is consistent with the above comparisons in which A > B
and A < C, which indicates that the participants’ answers were not merely
aﬀected or biased by the presence of vibration.

4

Discussions

From the results of comparisons (A-B), it can be inferred that imposing vibration
at a time preceding slip onset tends to reduce perceived slip duration. This might
be explainable in terms of vibrotactile masking, which could reduce sensitivity
to slip onset cues. For example, when humans experience high and low-frequency
vibrations in succession, they perceive the latter vibration to be weaker than theformer one [7]. Thus, the pre-slip vibration stimuli used in our experiment could
have contributed to obscuring the detection of cutaneous vibrations caused by
the transition from sticking to slipping phases.
From the comparisons (A-C, C-D), it could be inferred that vibration cues
presented at slip onset or during the slip phase can inﬂuence the perception ofslip duration. Stimulus C, which presented vibration at slip onset, might havebeen perceptually fused with transient skin vibrations occurring at the transition

On the Eﬀect of Vibration on Slip Perception During Bare Finger Contact

7from stick to slip. If so, this could explain why stimulus C tended to increase
estimates of slip duration. Alternatively, stimulus D, in which vibration occurred
during mid-slip, could have been perceptually fused with skin vibrations elicitedby sliding. The latter occur due to transient contacts between epidermal ridges
and asperities constituting ﬁne surface texture. If this were the case, stimulus
D may have elicited a sensation similar to that experienced during continuousslipping.
In summary, we can conclude from these results that humans make perceptual
use of vibration information felt through the skin in estimating slip duration,and that the timing at which a vibration cue is felt also has an eﬀect on theperception of slip duration, to the extent that vibration presented during anappropriate phase can either increase or decrease the perception of slip duration.
These discussions are based on the results from six participants, therefore moreparticipants are preferable for precise discussions.

5

Conclusion

We investigated the perceptual inﬂuence of imposed vibrotactile cues during
shear interactions between a surface and the ﬁngerpad, by designing a novel
apparatus for presenting precise vibration and shear displacement cues, and by
conducting an experiment in which shear stimuli were presented with and without vibrotactile cues, and with variable timing of vibration presentation. The
results of these experiments indicate that slip duration can be aﬀected by vibrotactile stimuli in ways that depend on the timing of vibration. These support
the idea that similar vibrations may play a role in the perception and regulationof slip during bare ﬁnger contact with an object, and may have further implications for understanding the mechanisms underlying perceptual exploration andmanipulation of palpated objects.

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