Exp Brain Res (2002) 142:342–348
DOI 10.1007/s00221-001-0939-y
R E S E A R C H A RT I C L E
Michel Paré · Heather Carnahan · Allan M. Smith
Magnitude estimation of tangential force applied to the fingerpad
Received: 12 February 2001 / Accepted: 11 October 2001 / Published online: 4 December 2001
© Springer-Verlag 2001
Abstract Prior research on large-fibre skin mechanoreceptors in humans and monkeys has demonstrated their
sensitivity to perpendicular skin indentation and to the
rate of force application. Although some studies have examined skin afferent responses to stretch, relatively few
investigations have examined the neural and perceptual
correlates of shear forces applied tangentially to the skin.
The present study assessed the ability of human subjects
to scale different levels of tangential force applied to the
distal pad of the index finger. Subjects were instructed to
choose their own magnitude estimation scale. Seven
force levels ranging from 0.15 to 0.70 N were delivered
randomly at rates of 0.10 N/s, 0.15 N/s or 0.30 N/s. Tangential forces were produced with a smooth metal spatula coated with an adhesive to insure a shear force on the
underlying skin without slip. The same procedures were
also used to generate skin indentation with normal forces. The results showed that most human subjects were
able to scale different magnitudes of both tangential and
normal forces applied to the tip of the index finger. The
rate of force change did not influence the perception of
the applied forces. These results highlight the potentially
important role of tangential forces in haptic perception.
Keywords Touch · Shear force · Psychophysics ·
Human
Introduction
Glabrous skin mechanoreceptors are especially accurate
in providing information about small changes in force
M. Paré · A.M. Smith (✉)
Centre de Recherche en Sciences Neurologiques,
Département de Physiologie, Université de Montréal, C.P. 6128,
Succ. Centre-ville, Montreal, QC, Canada H3C 3J7
e-mail: [email protected]
Tel.: +1-514-3436353, Fax: +1-514-3432111
H. Carnahan
Department of Kinesiology, University of Waterloo, Waterloo,
ON, Canada N2L 3G1
magnitude applied externally or self-generated. For instance, in object palpation and manipulation, the cutaneous sensory endings transduce the mechanical deformation of the skin at the contact interface into afferent activity carrying spatially and temporally relevant information to the central nervous system. Both palpation and
manipulation involve the generation of normal and tangential forces against an object surface but with a different balance between these two forces. For example, the
goal of a tactile search is to optimize the dynamic component between the explored surface and the skin to
maximally recruit the mechanoreceptive afferents
(Johnson 1983). In this case, the balance between normal
and tangential force will be adjusted to optimize slip in
order to maximize the extraction of tactile information.
A convincing example of this phenomenon is the demonstration by Morley et al. (1983), who showed that the
ability to discriminate differences in spatial gratings is
significantly better when subjects are allowed to move
the fingertip over the surfaces as compared to stationary
contact. In contrast, the goal of grasping is to eliminate
slip altogether to ensure grip stability. In order to achieve
the optimal balance between the normal and tangential
forces, an individual needs to perceive both forces independently, particularly when they are applied to the fingertip.
Considerable effort has been directed towards studying the influence of pressure applied normal to the skin
surface on the discharge of primary afferents and the associated magnitude estimations (Burgess et al. 1983;
Cohen and Vierck 1993; Greenspan 1984; Greenspan et
al. 1984; Hamalainen and Jarviletho 1981; Knibestöl and
Vallbo 1980; Mei et al. 1983; Pubols 1982a, 1982b;
Pubols and Pubols 1976). Taken together, these studies
show the effectiveness of the cutaneous afferent system
in resolving forces applied normal to the skin surface. In
contrast, the current knowledge of the neural and perceptual correlates of the forces applied tangential to the glabrous skin surface is very limited.
Srinivasan et al. (1990) have shown that subjects easily discriminate the direction of skin stretch regardless
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Fig. 1 A Schematic of the
hand-held probe consisting of a
spring-loaded metal rod producing a range of forces (up to
0.8 N) that was linearly proportional to the compression of the
spring. B Calibration curve of
the spring compression and resulting force, the dotted line
shows the regression line for
r=1. The displacement of the
rod was recorded with a linear
variable differential transducer
(LVDT). Applied perpendicularly to the skin surface, the
probe delivered a normal force;
the addition of a smooth metal
spatula (C) to the probe tip allowed the application of forces
tangential to the skin surface.
The spatula was equipped with
a strain gauge to monitor the
contact force normal to the skin
surface, while the LVDT measured the tangential force
of whether the surface stimuli slips or not over the skin.
However, slip is undetected for smooth surfaces where
no raised features are available. Neurophysiological recordings in monkeys have shown that rapidly adapting
type I and type II (RAI and RAII) afferents, as well as
slowly adapting type I (SAI) afferents, signal skin
stretch when a smooth glass plate is translated tangential to the skin surface with or without slip (Srinivasan
et al. 1990). However, only the SAI afferents respond
with a sustained discharge outlasting the initial stretching phase and exhibit a distinct directional bias. In contrast, both RAI and RAII afferents are silent even if
slips occur after the initial skin stretch. Srinivasan et al.
(1990) have also shown that the addition of a single elevated asperity, undetectable by static touch, to a smooth
flat surface is sufficient for the RAI afferents to detect
slip. Furthermore, Edin and colleagues have shown that
the application of both normal and tangential forces, using brushes with different bristle stiffness, evokes activity in all three types of peripheral afferents (SAI, SAII
and RAI) that increases with force but which also is
generally proportional to the stimulus velocity (Edin et
al. 1995; Essick and Edin 1995). Accordingly, the discharge pattern of all peripheral afferents is modulated
by skin stretch, but it is still unclear to what extent this
information can be used perceptually to estimate the
amount of skin stretch.
Recently, Smith and Scott (1996) have asked subjects
to stroke featureless smooth surfaces with the fingertip
and to scale them from “most slippery” to “most sticky”.
These authors found that all subjects scanned the surfaces by varying the tangential force while keeping the normal force relatively constant. They suggest that some
skin mechanoreceptors must be sensitive to a wide range
of tangential forces applying stretch to the skin during
tactile exploration. These latter results support the idea
that individuals are capable of perceiving normal and
tangential forces independently. To further assess this
hypothesis, subjects were asked to scale shear forces applied to the fingertip in a magnitude-estimation experiment. Preliminary results have been presented in abstract
form (Paré et al. 1999).
Materials and methods
Subjects
Three women and four men (all right-handed), ranging in age
from 24 to 47 years (mean 32 years), were recruited among students and employees at the Université de Montréal. The protocol
was approved by the ethics committee of the Faculty of Medicine
and all subjects gave informed consent. To insure that all subjects
received the same information regarding the experimental procedures, written instructions were read by each subject.
344
Fig. 2 A Experimental paradigm with the pre-determined
rates and force magnitudes.
B Examples of single trial force
curves in the tangential plus
normal force and normal force
only conditions
Apparatus
A probe (Fig. 1) consisting of a spring-loaded metal rod produced
a range of forces (up to 0.8 N) that was linearly proportional to the
compression of the spring. The displacement of the rod was recorded with a linear variable differential transducer (Trans-Tek,
Ellington, Conn.), the output of which was digitized with a 12-bit
analog-to-digital converter yielding a theoretical resolution of
3 µm. When applied perpendicularly to the skin surface, the probe
delivered a force normal to the skin surface over an area of
81 mm2 for the entire range of normal forces used in this experiment. The addition of a smooth metal spatula (Fig. 1C;
30×13 mm) to the probe tip allowed the application of forces tangential to the skin surface. The contact area with the spatula glued
to the skin was approximately 130 mm2, but the effective stimulation area might have been greater. The spatula was equipped with
a strain gauge (Micro-Measurements, Raleigh, N.C.) to monitor
the contact force normal to the skin surface while the spring-loaded displacement transducer measured the tangential force.
Stimuli
Normal and tangential forces were manually generated by the experimenter and consisted of 7 magnitudes ranging between 0.15
and 0.70 N (Fig. 2) and were delivered at three different rates
(0.10 N/s, 0.15 N/s and 0.30 N/s). To accomplish this, the experimenter used an oscilloscope display to monitor the normal and
tangential force magnitudes. The experimenter controlled the
force magnitudes and the rate of the force output by matching preestablished force/time curves that were indicated on the display
screen. In the tangential force condition, the spatula was glued to
the finger by applying an adhesive coating (rosin dissolved in a
solution of mineral spirits) to ensure that the spatula did not slip
across the skin when the shear forces were applied. To generate
the tangential force stimuli, a normal force of 0.50 N was produced and kept as constant as possible before a tangential force
was delivered (Fig. 2). This procedure was used to avoid the simultaneous application of normal and tangential forces.
Procedure
The subjects were seated with their dominant (right) hand supinated on the table and fixed in a foam restraint. First, the cutaneous
punctuate pressure sensitivity threshold for the index finger was
established for all subjects using von Frey monofilaments. All
seven subjects tested had skin sensitivity within normal limits
(Semmes et al. 1960) according to the measured thresholds (mean
96 mg, SD 48 mg). Secondly either the normal or normal plus tan-
345
Analysis
The subjective ratings by the participants were normalized by dividing each individual’s response by their own grand average, resulting in an overall mean of 1.0. The force signals produced by
the experimenter for the 7 predetermined force magnitudes were
digitized at 500 Hz and stored on a laboratory computer. The
mean applied force calculated over the plateau phase was used for
the regression analyses as well as for the ANOVAs. Since the
stimuli were manually controlled, the rate of force change was
somewhat variable compared with the intended rates of 0.10 N/s,
0.15 N/s and 0.30 N/s (Fig. 3). To compensate for the minor variations in the rate of force change, the mean rate of each trial was
calculated and the stimuli were grouped into three rate categories
(slow, intermediate and fast) and included as a factor in the
ANOVAs.
Results
Accuracy of force application
Fig. 3A, B Force application curves for a typical subject for both
tangential (A) and normal (B) force conditions
gential forces were applied to the pad of the subjects’ right index
finger using the probe. The tangential forces were applied across
the finger in the radial to ulnar direction. Subjects were allowed to
choose their own magnitude estimation scale (Marks 1974). To assist in scaling the forces, the subjects were presented with the minimum and maximum force stimuli during familiarization trials. No
additional criteria were provided at any time during the testing. A
French translation of the following statement, based on a sample
provided by Gescheider (1985) was read to the participants:
You will be presented with a series of forces applied either parallel or perpendicular to the surface of your index finger. These
forces will be applied at various rates in a random order. Your
task is to assign a number to the intensity of the forces applied
on your finger. Assign successive numbers in such a way that
they reflect your subjective impression. There is no limit to the
range of numbers. You may use whole numbers, decimals or
fractions.
The subjects were not informed about the number of different
stimuli they were about to experience and no indication about
their performance was given until all of the tests were completed.
Each stimulus was presented five times in a pseudo-random order
for a total of 105 trials of normal force application and 105 trials
of tangential force application. The subjects were instructed to
wait for the experimenter’s signal, that is, at the plateau phase, before rating the stimulus verbally. Testing took place in two 40-min
sessions on consecutive days to avoid fatigue, and the conditions
were randomized across subjects. That is, the normal forces were
presented first for three subjects whereas four subjects started with
tangential forces.
Despite the fact that the stimuli were manually produced,
the experimenter was able to generate relatively constant
forces at different predetermined force magnitudes. Figure 4 shows an example of the mean forces applied by
the experimenter for one subject in both the normal and
tangential conditions. According to the error bar indicating the minimum and maximum force applied for each
magnitude and their rate of force change, the force magnitudes were almost always distinct from one another.
There were a few exceptions, as depicted in Fig. 4B, for
which there was some overlap in the range of forces presented in the 2nd and 3rd force magnitudes. Multiple regression analyses were performed separately for both the
normal and tangential force conditions to test the relation
between the expected force magnitude, the generated
force and the rate of force change. Correlation values of
0.98 (n=735, P<0.001) for the normal force condition
and 0.97 (n=697, P<0.001) for the tangential force condition suggest that in both cases the generated force reliably matched the expected force magnitude. The addition of the rate of force change variable to the regression
model did not contribute significantly the prediction of
the expected force. Furthermore, a Tukey’s HSD multiple-comparison test was performed to make paired comparisons between the different force magnitudes. The
analysis showed that all force levels were significantly
different from each other (P<0.001) for both the normal
and tangential conditions, even though the stimuli were
manually generated.
Psychophysical results
The data set for each subject consisted of the magnitude
estimates derived from two sessions involving stimulation of the same skin locus with normal and tangential
stimuli. That is, a data set involved 210 magnitude estimates, five trials at seven stimulus intensities and three
rates of force application for the two experimental conditions. Out of the 1,470 estimates given by the seven sub-
346
Fig. 4A, B Histogram of mean applied force (5 trials) for the 7
force magnitudes and 3 rates of force application displayed with
the minimum and maximum values (error bars) for both tangential (A) and normal (B) force conditions of one subject
jects, 43 (3%) were excluded from the analysis because
of problems encountered during the recording of the
force data. The data from an eighth subject was dropped
from the analysis because he appeared to be unable to estimate the tangential force although his normal force rating was comparable with the other subjects. This subject
used a magnitude scale with a range of 1–5 in the tangential force condition and only differentiated between
the lowest and the highest force magnitudes applied to
his fingertip; the middle range forces were randomly attributed to either low or high force categories.
Generally, psychophysical relationships are best described by a power function (Marks 1974). However,
graphic examination of the data shows an obvious linearity, a tendency for homoscedasticity and a normal distribution of the magnitude estimates for a given stimulus
intensity. All these conditions favor the use of a simple
linear model to describe the psychophysical functions.
Figure 5 shows the relationship between the applied
force and the normalized subjective scaling for two subjects. Figure 5A, B shows the subject with the best scaling ability defined by the highest goodness of fit values,
whereas Fig. 5C, D shows the subject who had the most
difficulty in estimating normal and tangential force stimuli. The perceived intensity was a monotonic function of
the force magnitude and the correlation between the
Fig. 5A–F Normalized subjective estimates plotted against the
applied force displayed with the linear regression curve of the
most (A–B) and the least (C–D) accurate subject. The black rectangular boxes along the x-axes display the range of forces applied
at each of the 7 force magnitudes defined in the experimental paradigm. E–F The linear regression curves of seven subjects for the
tangential and normal force conditions, respectively
magnitude estimate and the applied force ranged between 0.48 and 0.96, and 0.70 and 0.97 for the tangential
and normal force conditions, respectively. Figure 5E, F
shows the linear regressions calculated between the
arithmetic mean of 15 normalized magnitude estimates
combining the rates of force change and the applied forces for each stimulus intensity of the seven subjects.
Qualitatively the slopes look steeper for the normal force
condition, suggesting that perhaps subjects were better at
estimating normal forces applied to their fingerpad compared with tangential force stimuli. However, a paired ttest revealed no significant difference between the regression slopes of the two force conditions.
ANOVAs on subjects’ magnitude estimates with the
force magnitude and the rate of force change as factors
(7×3) were calculated separately for the normal and tangential force conditions. The analysis showed a significant effect of force magnitude (F6, 2=273.09, P<0.001
and F6, 2=147.11, P<0.001) for both the normal and tan-
347
gential forces, respectively, but no significant effect of
the rate of force change. Furthermore, stepwise regression analysis showed that adding the rate of force change
variable did not increase significantly the variation already accounted for by the applied force both for the tangential force and the normal force conditions. Tukey’s
multiple comparison test revealed that each force magnitude was significantly different from the other (P<0.05)
for both force conditions, suggesting that subjects were
able to differentiate each of the 7 force magnitudes.
In the tangential force condition, a force normal to the
skin was first applied to the skin, followed by the application of the tangential force. The experimenter tried to
keep these normal forces as constant as possible, but
variations occurred between trials as well as between
subjects. The normal force variations ranged between 0.1
and 2 N. A multiple regression analysis was performed
to test whether the level of normal force applied simultaneously with the tangential force had an impact on the
subjects’ magnitude estimates. The mean tangential and
normal forces were calculated over the plateau phase of
the stimuli for all subjects. The stepwise regression analysis revealed that the normal force did not contribute to
the variance of the subjects’ estimation (r=0.042, n=692,
P>0.05).
Discussion
The present study demonstrates that subjects were able
to accurately estimate shear forces applied to the glabrous skin. These results support the recent findings by
Birznieks et al. (2001) and Bisley et al. (2000) which
have found that a large portion of SAI and RAI afferents
responded to tangential forces applied to the fingertip.
Furthermore, electrophysiological recordings from Smith
and colleagues also suggest that the discharge of neurons
in the primary somatosensory cortex is modulated by
tangential forces generated on the skin of the index and
thumb (Paré and Smith 1998; Salimi et al. 1999a,
1999b). The present study showed that the subjective intensity estimates were accurately scaled to the magnitude
of the shear force on the skin over a range of forces from
0.15 to 0.75 N. The magnitude estimates were unaffected
by either a threefold increase in the rate of force application or a range of normal forces applied concurrently
with the tangential force.
Nature of the stimulus
The use of a manually controlled stimulator in this study
raised some concern about the repeatability of the skin
deformation. First, it is known that the force-indentation
relation depends on the skin compliance at the site of
stimulation and the duration of stimulation (Petit and
Galifret 1978). We did all of our experiments on the glabrous skin of the distal phalanx of the index finger, but
we did not control for either skin compliance or the in-
dentation depth of the stimulated skin, although it is
clear that both stress and strain in the adjacent skin must
have been higher with greater force magnitude. In regard
to the duration of the stimulation and also the delay between successive stimulations, Petit and Galifret (1978)
have shown that both these factors affect the force-indentation relation. Both the trial and intertrial duration
varied somewhat throughout the testing sessions, but the
normal distribution of the estimates for each force magnitude suggest that these time factors do not create a systematic bias in the results.
Secondly, Hamalainen and Jarvilehto (1981) have
shown that, in order to produce sensations of equal magnitude with a single mechanical pulse, the displacement
amplitude decreases with increasing probe area. For the
normal force condition, the surface of the probe used to
stimulate the skin was an area of 81 mm2 for the entire
range of normal forces used in this experiment. However, we were unable to estimate the exact skin area stimulated with the spatula in the tangential force condition.
The area of skin stimulated was undoubtedly influenced
by the magnitude of the shear force as well as the normal
force applied concomitantly. The estimated 130 mm2
contact area in the tangential force condition is higher
than in the normal force condition, which may partly explain the steeper slope for the normal force condition
even if the difference is not significant between the two
force conditions. However, the normal distribution of the
magnitude estimates for a given stimulus intensity suggests that the variations of the effective stimulation area
has not significantly influenced the performance of the
subjects. Finally, it has been reported that increasing indentation rate will elicit increasing magnitude estimates
for the same indentation depth (Greenspan et al. 1984).
In contrast, Burgess et al. (1983) have found that indentation depth judgements were not greatly influenced by
changes in the indentation rates. In agreement with
Burgess et al. (1983), the data in the present study did
not indicate that the rate of force change had any major
effect on the magnitude estimates for either the normal
or tangential forces.
Tangential force encoding
It was not clear whether the subjects relied entirely on
the amount of skin stretch in scaling the tangential forces
or whether other stresses and strains might have been involved. Primary afferents sensitive to stretching of the
glabrous and hairy skin have been reported frequently
(Burke et al. 1988; Del Prete and Grigg 1998; Edin
1992; Edin and Abbs 1991; Greenspan 1992; Grigg
1996; Hulliger et al. 1979; Khalsa et al. 1997; Knibestöl
and Vallbo 1970). Edin and Abbs (1991) have shown
that slowly adapting afferents of the hairy skin of the
dorsal hand reliably signal small changes in skin tension,
whereas rapidly adapting afferents respond to movement
around one joint that presumably causes skin stretch over
the joint. Moreover, Edin (1992) has applied quantita-
348
tively controlled skin stretches to the back of the hand
and shown that both SAI and SAII afferents are sensitive
to static as well as dynamic skin stretch. Consistent with
these results, Srinivasan et al. (1990) have shown that
RAI and RAII afferents respond only during the skinstretching phase, whereas the SAI afferents respond continuously during stroking of a smooth glass plate over
the skin. This suggests that the peripheral cutaneous receptors can encode the mechanical shear force applied to
the skin through the resulting stretching of the skin. Furthermore, the results of the present study as well as those
of Smith and Scott (1996) suggest that the shear forces
applied to the skin can be evaluated independently of the
normal forces applied simultaneously with the tangential
force.
Acknowledgements The authors gratefully acknowledge P.
Drapeau for computer programming, L. Lessard for help in data
analysis, C. Gauthier for computer graphics, and J. Jodoin and G.
Richard for their technical assistance. We thank Dr. C.E. Chapman
for helpful comments on a previous version of the manuscript.
This work was supported by a Canadian Institutes of Health Research grant to A.M Smith and a Natural Sciences and Engineering Research Council of Canada grant to H. Carnahan. M. Paré
was supported by the Fonds de la Recherche en Santé du Québec
(FRSQ) and the Université de Montréal.
References
Birznieks I, Jenmalm P, Goodwin AW, Johansson RS (2001) Encoding of direction of fingertip forces by human tactile afferents. J Neurosci 21:8222–8237
Bisley JW, Goodwin AW, Wheat HE (2000) Slowly adapting type
I afferents from the side and end of the finger respond to stimuli on the center of the fingerpad. J Neurophysiol 84:57–64
Burgess PR, Mei J, Tuckett RP, Horch KW, Ballinger CM, Poulos
DA (1983) The neural signal for skin indentation depth. I.
Changing indentations. J Neurosci 3:1572–1585
Burke D, Gandevia SC, Macefield G (1988) Responses to passive
movements of receptors in joint, skin, and muscle of the human hand. J Physiol (Lond) 402:347–361
Cohen RH, Vierck CJ Jr (1993) Population estimates for responses
of cutaneous mechanoreceptors to a vertically indenting probe
on the glabrous skin of monkeys. Exp Brain Res 94:105–119
Del Prete Z, Grigg P (1998) Responses of rapidly adapting afferent neurons to dynamic stretch of rat hairy skin. J Neurophysiol 80:745–754
Edin BB (1992) Quantitative analysis of static strain sensitivity in
human mechanoreceptors from hairy skin. J Neurophysiol
67:1105–1113
Edin BB, Abbs JH (1991) Finger movement responses of cutaneous mechanoreceptors in the dorsal skin of the human hand. J
Neurophysiol 65:657–670
Edin BB, Essick GK, Trulsson M, Olsson KA (1995) Receptor encoding of moving tactile stimuli in humans. I. Temporal pattern of discharge of individual low-threshold mechanoreceptors. J Neurosci 15:830–847
Essick GK, Edin BB (1995) Receptor encoding of moving tactile
stimuli in humans. II. The mean response of individual lowthreshold mechanoreceptors to motion across the receptive
field. J Neurosci 15:848–864
Gescheider GA (1985) Psychophysics methods, theory and application. Lawrence Erlbaum, Hilsdale, N.J.)
Greenspan JD (1984) A comparison of force and depth of skin indentation upon psychophysical functions of tactile intensity.
Somatosens Res 2:33–48
Greenspan JD (1992) Influence of velocity and direction of surface-parallel cutaneous stimuli on responses of mechanoreceptors in feline hairy skin. J Neurophysiol 68:876–889
Greenspan JD, Kenshalo DR, Henderson R (1984) The influence
of rate of skin indentation on threshold and suprathreshold tactile sensations. Somatosens Res 1:379–393
Grigg P (1996) Stretch sensitivity of mechanoreceptor neurons in
rat hairy skin. J Neurophysiol 76:2886–2895
Hamalainen H, Jarvilehto T (1981) Peripheral neural basis of tactile sensations in man. I. Effect of frequency and probe area on
sensations elicited by single mechanical pulses on hairy and
glabrous skin of the hand. Brain Res 219:1–12
Hulliger M, Nordh E, Thelin A-E, Vallbo AB (1979) The responses of afferent fibers from the glabrous skin of the hand during
voluntary finger movements in man. J Physiol (Lond)
291:233–249
Johnson KO (1983) Neural mechanisms of tactual form and texture discrimination. Fed Proc 42:2542–2547
Khalsa PS, LaMotte RH, Grigg P (1997) Tensile and compressive
responses of nociceptors in rat hairy skin. J Neurophysiol
78:492–505
Knibestöl M, Vallbo AB (1970) Single unit analysis of mechanoreceptor activity from the human glabrous skin. Acta Physiol
Scand 80:178–195
Knibestöl M, Vallbo AB (1980) Intensity of sensation related to
activity of slowly adapting mechanoreceptive units in the
humand hand. J Physiol (Lond) 300:251–267
Marks LE (1974) Sensory processes: the new pyschophysics. Academic, New York
Mei J, Tuckett RP, Poulos DA, Horch KW, Wei JY, Burgess PR
(1983) The neural signal for skin indentation depth. II. Steady
indentations. J Neurosci 3:2652–2659
Morley JW, Goddwin AW, Darian-Smith I (1983) Tactile discrimination of gratings. Exp Brain Res 49:291–299
Paré M, Smith AM (1998) Somatosensory cortical responses to
stimulation of the hand glabrous skin with normal and tangential forces in the monkey. Soc Neurosci Abstr 24:1125
Paré M, Carnahan H, Smith AM (1999) Tactile discrimination of
tangential force: psychophysical performance in humans. Soc
Neurosci Abstr 25:2196
Petit J, Galifret Y (1978) Sensory coupling function and the mechanical properties of the skin. In: Gordon G (ed) Active
touch: the mechanism of recognition of objects by manipulation. Pergamon, Oxford, pp 19–27
Pubols BH Jr (1982a) Factors affecting cutaneous mechanoreceptor response. I. Constant-force versus constant-displacement
stimulation. J Neurophysiol 47:515–529
Pubols BH Jr (1982b) Factors affecting cutaneous mechanoreceptor response. II. Changes in mechanical properties of skin with
repeated stimulation. J Neurophysiol 47:530–542
Pubols BH, Pubols LM (1976) Coding of mechanical stimulus velocity and indentation depth by squirrel monkey and raccoon
glabrous skin mechanoreceptors. J Neurophysiol 39:773–787
Salimi I, Brochier T, Smith AM (1999a) Neuronal activity in somatosensory cortex of monkeys using precision grip. I. Receptive fields and discharge patterns. J Neurophysiol 81:825–834
Salimi I, Brochier T, Smith AM (1999b) Neuronal activity in somatosensory cortex of monkeys using precision grip. III. Responses to altered friction perturbations. J Neurophysiol
81:845–857
Semmes J, Weinstein S, Ghert L, Teuber H-L (1960) Somatosensory changes after penetrating brain wounds in man. Harvard
University Press, Cambridge, MA
Smith AM, Scott SH (1996) Subjective scaling of smooth surface
friction. J Neurophysiol 75:1957–1962
Srinivasan MA, Withehouse JM, LaMotte RH (1990) Tactile detection of slip: surface microgeometry and peripheral neural
codes. J Neurophysiol 63:1323–1332