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Research of Conditions of stimulus for Inducing Grasping ForceControl Reflex
Tatsuma Sakurai
Faculty of Information and Science Engineering
Tohoku University
Shogo Okamoto
Faculty of Mechanical Science and Engineering
Nagoya University

Masashi Konyo
Faculty of Information and Science Engineering
Tohoku University
Satoshi Tadokoro
Faculty of Information and Science Engineering
Tohoku University

Abstract— Humans can change their grasping force before
the grasped object unconsciously slips off their fingers. This
reflex is called the grasping force control reflex. This paper
describes the methods used to induce reflective grasping force
control and presents the observed results. To clarify the stimulus
conditions needed to induce reflective grasping force control, wedeveloped an observation device with piezoelectric actuators
and a force sensor. Vibration stimuli and force senses were
presented to a human finger, and a change in the grasp force
was observed by the force sensor. In this study, we changed
the parameters of the vibration stimuli and force senses anddetected the difference in the grasp force.

control. This grasping force feedback control based on thetactile information obtained from the fingertip is executed
unconsciously and is called the grasping force control reflex.
Previously Nakamoto and Konyo (2007) conducted aseries of experiments to investigate the manner in which
the grasping force control reflex should be induced. They
presented a vibration stimulus to the finger as tactile information and simultaneously presented a force stimulus as thevirtual object’s weight. In Nakamoto and Konyo experience, it is suggested that presenting a vibration stimulus, whichmost do by firing the Meissner disk for 0.5 [s] in the partial
slipping area shown in Fig 1, is most likely to induce thegrasping force control reflex. Partial slipping is said to beperceived by tactile receptors such as the Meissner disk.
In this case, a vibration stimulus, which fires the tactile
receptors, represents partial slipping. However , the tactile
conditions for inducing the grasping force control reflex
are still unclear. Thus, it is suggested that a force stimulus
perceived by muscle tissue and tendons could also affect thegrasping force control reflex; however, it is not clarified.
In this study, our objective was to learn more about theconditions for inducing the grasping force control reflex bychanging the parameters of the vibration stimuli and force
senses. To determine the grasping force change, we developed an experimental device, which is capable of presenting
various vibration stimuli and force senses and observing thegrasping force. We conducted three experiments using thisdevice.
In the first experiment, the objective was to determine
which tactile disk was the most effective in inducing thegrasping force control reflex. Partial slipping is said tobe perceived by the Meissner disk; however, other tactile
receptors can also be effective in inducing the grasping
force control reflex. While the subjects were grasping thevirtual object on the device, tactile receptors were stimulated
selectively by vibration and changes in the grasping force
were observed.
The second experiment was designed to investigate theI. I NTRODUCTION
By obtaining optical and tactile information, humans
change the grasping force and operate the target object
naturally. Grasping force control occurs either by feedback
control or by feed forward control. When optical information is available for predicting the object’s weight, humans
use feed forward control to change their grasping force.
Otherwise, humans use tactile information obtained fromthe tactile disk in the finger surface and provide feedback

Stick area

Partial slip area
Fig. 1.

Partial slip area and Stick area

T.Sakurai, M.konyo, S. Tadokoro, Graduate School of Information Sciences, Tohoku University, 6-6-01 Aramaki Aza Aoba, Aoba-ku, Sendai,
980-8579, Japan sakurai@rm.is.tohoku.ac.jp , S.Okamoto, Graduate Schoolof Engineering, Nagoya University Yamada-Obinata Laboratory

978-1-4244-9314-2/10/$26.00 ©2010 IEEE

- 408 -SI International 2010

effect of the vibration spatial pattern presented to the finger
surface. The actual partial slipping area is shown in Fig
1, and in this experiment, the spatial pattern of slipping
area differs from the actual slipping area spatial pattern.
The objective of this experiment was to determine whether
humans could recognize and use the spatial pattern of partial
slipping area when inducing the grasping force control reflex.
In the third experiment, the effect of changing force
senses was studied. In the previous research conducted byNakamoto and Konyo , force senses were changed to represent a sudden change in the virtual object’s weight; however,
the effect of force sense was not investigated. Therefore,
in this study, we presented a change in force sense, which
represented a sudden weight change of the virtual object,
as well as presented various vibration stimuli to the finger
surface. Also comparing to the first experiment, it’ll beclarified whether force senses sudden change is necessary
in or not.
In the previous research, it was unclear as to whether
a change in the grasping force was induced by muscle
constriction. It was also unclear as to when the grasping force
control reflex occurred. In this paper, while the subjects are
grasping a virtual object with the device, the electromyograph observes muscle potential. With this muscle potential,
we can determine the degree of muscle constriction and thetiming of the grasping force control reflex. If the induced
timing is within 50 [ms], grasping force control reflex canbe clarified as myotatic reflex. Also if the induced timing iswithin 200 [ms], the reflex is brainstem reflex.
II. M ETHOD OF INDUCING GRASPING FORCE CONTROLREFLEX

Fig. 3.

Spatial pattern conditions

Pacini disk. These tactile receptors have different frequency
response characteristics to mechanical stimuli.
Fig 2 shows the vibration perception threshold of tactile
receptors when presenting a vibration stimulus to the finger
surface. According to this graph, the Merkel disk is mostsensitive to vibrations at approximately 5 [Hz]; the Meissner
disk is the most sensitive at about 30 [Hz]; and the Pacini
disk is most sensitive at approximately 300 [Hz]. Considering
this knowledge, tactile receptors can be selectively stimulated
by presenting a vibration stimulus with the amplitude andfrequency around the perception threshold of each tactile
receptor. For example, the Pacini disk can be selectively
induced by a vibration of 300 [Hz] and an amplitude ofapproximately 1 [μ m].
In this paper, this method is called selective stimulation oftactile receptors. The perception threshold amplitude of eachfrequency is estimated individually prior to the experiment.
Then, during the experiments, the subjects are presented thevibration amplitudes estimated in the preliminary tests. By
varying the vibration frequency and amplitude and triggering
each tactile receptor by the selective stimulation of tactile
receptors, we investigated the relationship between the grasping force control reflex and the tactile receptors.

A. Frequency and Amplitude

B. Spatial Patterns of Vibration

When grasping an object, humans perceive partial slippingwith tactile receptors and respond with a grasping force
control reflex. Tactile information is mainly obtained bytactile receptors such as the Merkel disk, Meissner disk, andIn grasping a real object, the partial slipping area and stickarea of a human finger surface is as shown in Fig 1. The outerarea is the partial slipping area, and the inner area is the stickarea. Presenting vibration stimuli of 5, 30, and 300 [Hz] tothe outer area represents an actual contact surface between
a human finger surface and an object.
In this study, vibrations different from the actual spatial
patterns were presented to the finger surface. These spatial
patterns are shown in Fig 3.
C. Vibration presenting time
In previous works, Nakamoto and Konyo studied thegrasping of a virtual object with different vibration presenting times to gain new knowledge about the effect ofvibration presenting time on grasping force control reflex.
The vibration presenting times were 0.1, 0.3, 0.5, 0.7, and
1.0 [s]. It was found that a 0.5 [s] vibration resulted in thebest-induced grasping force control reflex. According to thisfinding, we set our vibration presenting time at 0.5 [s].

Fig. 2.

Vibration perception threshold of tactile receptors- 409 -SI International 2010

D. Force sense
In Nakamoto and Konyo’s experiment, a force sense waspresented to subjects to represent a virtual object’s weight,
and then, it was changed from 1 [N] to 3 [N] suddenly. Thischange in force represents a sudden change in the object’s
weight.
III. G RASPING FORCE CONTROL REFLEX OBSERVATION
DEVICEAn overview of the grasping force control reflex observation device is shown in Fig 4. The device is composed ofthree parts.
The first part is the structure to present vibration stimuli
to the human’s finger surface. The second part presents aforce sense to the human’s finger. The third part enables us
to observe the grasping force control reflex.
The vibration stimulus structure is shown in Fig 6. The
piezoelectric actuators are placed as vibrators, as shown in
Fig 4. These vibrators are capable of firing every tactile
receptor because they can present an amplitude of morethan 80 μ m and the response is more than 400 [Hz]. An
attachment is placed on a piezoelectric actuator to present
a vibration to the finger surface’s inner area, as shown in
Fig 1. A second attachment is placed on the other actuatorto present a vibration to the outer area. The attachments
used in this experiment are shown in Fig 5. The structureof attachments and actuators is shown in Fig 6.
The force sense presenting structure is composed of a
Maxson DC Motor 118751, a wire (stainless diameter 0.5[mm]), and a jig for contact to the finger. The force sense
is presented by winding wire with the motor and hanging
the device along the linear slider. The force sense is directly
presented to the finger side with the jig, and the jig slides
with the grasping device along the linear slider. By presenting
a force sense to the finger side, the finger surface is free fromtangential force and deformation. By presenting the force
in this way, there is no displacement between the virtual
grasping object and the finger surface; in other words, there
is no real partial slipping. No subjects felt uncomfortable
when the force sense was presented to the finger side.
The grasping force observing structure is composed of
3 axis force sensors (NITTA EFS-18M20A25-M10) and 1
electromyograph (NIHON KODEN multi-channel telemetersystem) Fig 7. The force sensors measure the normal line
grasping force change, and the electromyograph observesmuscle constriction and the timing of the active muscle
constriction. Electromyograph is attached on the skin surface
shown in 8. The target muscles are used when bending
fingers. Vibration presenting attachment is surrounded byfinger support so that grasping force does not deforms piezoelectric actuator and attachments. Finger support also limits

the contact area between attachments and finger surface toprevent individuals differences.
IV. E XPERIMENT FOR INDUCING THE GRASPING FORCECONTROL REFLEX

A. Preliminary experimentFirst, to estimate the perception threshold for vibrationindividually, subjects were presented with vibrations of different amplitudes. The frequencies used in this experimentwere 5 [Hz], 30 [Hz], and 300[Hz]. A headphone withpink noise and an eye mask were placed on the subjects
so they did not perceive any environmental changes. While
the subjects were grasping the device, we changed thepiezoelectric actuators’ input voltage by 2 [V]. Each time
subjects were asked if they perceived a vibration stimulus ornot. The input was increased by 2 [V] from 0 [V], and thethreshold amplitude was recorded.
Again, the input was decreased by 2 [V] from a welldefined input, and the threshold amplitude when the subject
could not perceive a vibration stimulus was recorded. Then,
using the method of limits, the perception threshold forvibration was calculated. This process was repeated 4 times
in each frequency for a total of 12 times individually.
The results are shown in Table 1. The decline amplitudetendency was almost the same for every subject. This data
does not contradict the information shown in Fig 2.
B. Experiment 1:Influence of Tactile receptors
In this experiment, subjects were presented with vibrationsof various frequencies to investigate which tactile receptor affects the grasping force control reflex. The amplitude was set
to the perception threshold individually for each frequency,
as examined in the previous experiment. A headphone withpink noise and an eye mask were placed on the subjects sothey would not perceive any environmental changes.

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L in er slid er

Mo to rA ttach m en t

F o rcesen so r

J ig

A ctu al pattern D ifferen t patternFig. 5.

Patterns of attachment

P iez o electric actu ator
Fig. 4.

Device overview

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TABLE I
P ERCEPTION THRESHOLD AMPLITUDE

Subject1 Amplitude [μ m]Subject2 Amplitude [μ m]Subject3 Amplitude [μ m]Subject4 Amplitude [μ m]Subject5 Amplitude [μ m]Subject6 Amplitude [μ m]Fig. 6.

Fig. 7.

30 Hz
24.5
23.7
20.3
22.1
23.8
19.1

300 Hz
1.7
1.9
1.3
1.6
1.3
1.1

Vibration Structure

Figure of electromyography(NIHON KODEN)

1) Conditions: The subjects were 6 healthy males withages ranging from 20 to 25 years. While the subjects had
to keep grasping the device so it did not fall off, a force
sense of 1 [N] was presented to the finger. Then, a vibration
stimulus was presented suddenly at 7 [s] for 0.5 [s] sinceafter 7 [s], the grasping force was almost stable.
The experiment to induce the grasping force control reflex
with vibration was conducted under the 4 conditions below.
The vibration stimulus was presented to the outer area shownin Fig 1.
Condition 1: Stimulate Pacini disk with 300 [Hz] vibration.
Condition 2: Stimulate Meissner disk with 30 [Hz] vibration.
Condition 3: Stimulate Merkel disk with 5 [Hz] vibration.
Condition 4: No stimulation presented.
Each condition was carried out 10 times randomly for a
total of 40 times.
F lexor hallucis longus

F lexor digitorum superficialis F lexor hallucis brevis
Fig. 8.

5 Hz
57.2
54.2
55.6
48.5
56.7
51.7

Position of electromyography2) Results: Fig 9 shows an example of the force sensor
output. The output is normal to the direction of the grasping
force. The grasping force change index is calculated by
dividing the grasping force average of the previous 1.0 [s]vibration by the following 1.0 [s] vibration. A time of 1.0[s] is sufficient to increase the grasping force and become
stable.
An example of an increase rate is shown in Table 2. Values
greater than 1.0 indicate the grasping force increased and thegrasping force control reflex was induced.
Fig 11 shows the complete data for all the subjects. Underconditions 1 and 2, an increase in the grasping force wasobserved. As for the level of significance, condition 1’s pvalue was 0.0013, and condition 2’s p value was 0.0002.
On an average, the number of times the grasping force wasincreased by the Meissner disk was larger than the numberof times the grasping force was increased by the Pacini
disk. Therefore, the Meissner disk was more effective ininducing the grasping force control reflex. And what is new
in this experiment is we could induce grasping force control
reflex without presenting force senses change. In the previous
research, it is suggested that sudden force change is needed
to induce grasping force control. However in this experimentgrasping force increase was observed in spite of no force
senses change were presented. This result shows tactile
stimulus is used in grasping force control reflex especiallytactile stimulus for Meissner disk.
C. Experiment 2:The affection of the spatial patterns ofvibration
This experiment is conducted to investigate the affectionof vibration spatial pattern presenting to finger surface. In
this experiment, the spatial pattern of slipping area differsfrom the actual slipping area spatial pattern.
1) Conditions: While the subjects had to keep grasping
the device so it did not fall off, a force sense of 1 [N]was presented to the finger. Then, a vibration stimulus waspresented suddenly at 7 [s] for 0.5 [s] since after 7 [s], thegrasping force was almost stable. Attachments are placed on
the piezoelectric actuators and the vibration spatial patterns
were changed like the patterns shown in Fig 3. Vibration
used in this experiment is 30 [Hz] and amplitude is perceived- 411 -SI International 2010

threshold of preliminary experiment. 30 [Hz] vibration is thebest induced grasping force control reflex in experiment 1.
The subjects were 6 healthy males with ages ranging from
20 to 25 years. Each condition was carried out 10 times
randomly for a total of 40 times.
2) Result: The result of grasping force increase rate isshown in Fig 12. The figure shows grasping force wasincreased in condition 1, 2, 3. In other words grasping force
control reflex was induced when presenting vibration. But inthis experiment there was no change among condition 1 andcondition 2. This shows grasping force control dose not beaffected by the vibration spatial pattern and it can be said
only certain vibration is needed for inducing. Condition 3
also induced grasping force control reflex and the number oftimes grasping force increased was larger than condition 1
and 2. Maybe vibration area is also affected to grasping force
control reflex, since spatial pattern condition 3 is double
the area of condition 1 and 2. No significant changes ofelectromyography output were found in the experiment. Thisis because grasping force change is very little so the changeof muscle potential could not be observed.

Fig. 9.

Grasping force change
TABLE IIE XAMPLE OF GRASPING FORCE CHANGE RATE

D. Experiment 3:Influence of force sense change
In this experiment, we not only presented a vibration
stimulus to the subject’s finger but also changed the force
sense suddenly. In this case, force senses sudden change
represents sudden change of the virtual object’s weight. This
is to estimate the affection of changing force senses tograsping force control reflex.
1) Conditions: The subject had to keep grasping thedevice so it did not fall off. The vibration was presented
suddenly at 7 [s], as in Experiment 1, but the force sense
was also changed suddenly from 1 [N] to 3 [N]. Force senses
are presented to finger side by the jig. The conditions ofvibration were the same as in Experiment 1. The subjects
were 6 healthy males with ages ranging from 20 to 25 years.
Each condition was carried out 10 times randomly for a totalof 40 times.
2) Results: The result of increasing rate of grasping force
is shown in Fig 13. In the level of significance 5%, allconditions are observed significant increase of grasping force
after presenting vibration and force change. And no significant difference were found between the conditions. Thismeans change of the force senses is greatly affected to induce
grasping force control. Considering grasp force increase incondition 4 of no vibration, force stimuli change 1 [N] to
3 [N] is more effective than vibration stimulate in inducing
grasping force control reflex. But in the previous experiment,
no grasping force increase were found in vibration conditionsof 5 [Hz], 300 [Hz] and no vibration with force change. And
vibration of 30 [Hz] with force change could induce grasping
force control reflex. Maybe force change is affected but inthis experiment the stimulation was too big so vibration
effect could not be observed.
Fig 10 shows one example of the average of 10 trials electromyography output. After presenting vibration and forcechange at 7 [s], active muscle constriction was observed. This
result shows grasping force increase was induced by muscle
constriction. And the timing of grasping force control reflex
is between 50 [ms] and 200 [ms].
V. C ONCLUSION
In this study, to learn more about inducing the grasping
force reflex, we observed the grasping force change when

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Fig. 10.

Electromyography Output

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Fig. 11.

Affection of tactile receptors

Fig. 12.

Affection of spatial patterns

subjects were presented with different vibration parametersand force senses while grasping a virtual object.
First, we developed an experimental device that canpresent various vibrations and force senses simultaneously
to represent the grasping of a virtual object. This device canalso change the spatial pattern of vibrations presented to thefinger surface. Using the observation system of the device,
which includes an electromyograph, we can determine thegrasping force change, muscle constriction, and the timingof active muscle constriction. With this experimental device,three experiments were conducted.
Experiment 1: The objective of this experiment was todetermine which tactile disk was the most effective in

Fig. 13.

Affection of force sense change

inducing the grasping force control reflex. In this experiment,
while subjects were grasping the virtual object with thedevice, tactile receptors were stimulated by vibration, andchanges in the grasping force were observed. The results
show that the grasping force control reflex occurs only
when stimulated by the Meissner disk and the Pacini disk.
In addition, the Meissner disk has a controlling effect on
inducing the grasping force reflex.
Experiment 2: The objective of this experiment was toinvestigate the effect of the vibration spatial pattern presented
on the finger surface. In this experiment, the spatial patternof slipping area differs from the actual slipping area pattern.
The results show that the grasping force control reflex was
induced when vibrations were presented; however, there were
no changes between conditions 1 and 2. This shows that thegrasping force control dose is not affected by the vibration
spatial pattern and only certain vibrations are needed.
Experiment 3: The objective of this experiment was todetermine the effect of changing the force senses. In addition
to presenting a vibration stimulus to the finger surface, we
presented a change in force sense that represents a suddenchange in weight of the virtual object.Because of muscle
potential activity, muscle constriction was observed and thetiming was between 70 [ms] and 200 [ms] on average.
According to this knowledge, grasping force control reflex
supposed to be brain stem reflex.
The grasping force control reflex was induced under all
conditions. Therefore, force sense has a greater effect on
inducing the grasping force reflex than a vibration stimulus.
In experiment 1 and experiment 2, grasping force control
reflex were observed while no force sense change waspresented. This result shows tactile stimulus can itself induce
grasping force control reflex especially for tactile stimulus tofire Meissner disk.
VI. ACKNOWLEDGMENTS
Grant in Aid for Scientific Research(No21656065) Germinating Research
R EFERENCES- 413 -

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