Cutaneous and Kinaesthetic Perception of Traversed Distance

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1 Cutaneous and Knaesthetc Percepton of Traversed Dstance Wouter M. Bergmann Test L. Martjn A. van der Hoff Astrd M. L. Kappers Helmholtz Insttute, Utrecht Unversty, The Netherlands ABSTRACT Dscrmnaton thresholds for tactually perceved traversed dstance were measured n three condtons: cutaneous-only, knaesthetconly and combned nformaton. The results were mm (3 %) n the frst and 11 mm (14 %) n the latter two condtons. Although cutaneous length percepton was shown to be possble, percepton n the combned condton was found to be manly based on knaesthetc nformaton. The maxmum-lkelhood estmaton model of cue combnaton was not supported. Keywords: length percepton, cue combnaton, movement Index Terms: H.1. [Models and Prncples]: User/Machne Systems Human Informaton Processng; H.. [Informaton Interfaces and Presentaton]: User Interfaces Haptc I/O 1 INTRODUCTION In daly lfe, we often move our hands over objects or surfaces. We can obtan nformaton about the sze of these objects by determnng the dstance our fngers traversed over the surface. Ths haptc length percepton s about as accurate as vsual or bmodal length percepton [1, 1]. In haptc length percepton, we make use of two channels of nformaton: knaesthetc percepton tells us how much our hand has moved n space. At the same tme, cutaneous percepton tells us how much of the surface has sld over our fngers. These two sources of nformaton are combned n some way to produce a sngle percept of the traversed dstance. In ths experment, we wanted to nvestgate how these two cues are combned and what ther respectve roles are n percepton of traversed dstance. Two man types of haptc length percepton can be dstngushed: Frst, there s the fnger-span method, n whch the sze or length of an object s determned by percevng the dstance between thumb and ndex or mddle fnger that enclose the object. Wth ths method, lengths can be perceved that are related to the physcal length by a power functon wth an exponent of 1.3 []. The percepton s ndependent of pushng force or complance of the object [3], but there seems to be an adaptaton effect [14]. Usng dscrmnaton experments, the threshold for dstngushng dfferent lengths n ths way was determned to le n the range of. 1.3 mm [6, 13, 17]. Ths value ncreases wth ncreasng dstance between the fngers up to about 3 mm for the largest fnger span [7, 9, ]. Ths range of thresholds can be consdered as a base lne for length dscrmnaton between the fngers. Due to physologcal constrants, ths method s lmted to dstances of about 8 mm. For larger dstances, movement s requred. Wth ths second method, the length of the object s determned by percevng the traversed dstance between the endponts of the object. Ths method seems to be somewhat less accurate than the fngerspan method [1]. Interestngly, when a length perceved wth the movement method s reproduced wth the fnger-span method, t s greatly exaggerated up to a factor of.. An underestmaton occurs f the methods of percevng and reproducng are nter- emal: W.M.BergmannTest@uu.nl IEEE World Haptcs Conference June, Istanbul, Turkey /11/$6. 11 IEEE changed [1]. Also when a vsual match s to be made, lengths perceved through the movement method are judged to be larger than through the fnger-span method []. Length perceved usng the movement method also has a less steep relatonshp wth physcal length than wth the fnger-span method, wth a power functon exponent rangng from.89 [16] to unty (a lnear relatonshp) [1]. However, there are many nfluencng factors, such as orentaton effects [, 4, ] or adaptaton effects [3]. Also, the perceved length depends on the speed of movement, both wth passve movements (the subject s fnger s beng moved) [4] and actve movements (the subject moves hs/her own fnger) [11]. In general, the perceved traversed dstance s larger wth slower movement speeds. However, n the range of 4 1 cm/s, the perceved dstance does not seem to depend on movement speed [11]. A smlar plateau was found earler n experments regardng the perceved path length of a pont movng over the skn of the forearm [18, ]. Ths ndcates that ths range of speeds s sutable for mnmsng the effect of speed on length percepton. It s surprsng that for haptc length percepton wth the movement method, no dscrmnaton experments have been publshed. It s unknown how accurate length percepton n ths way s. Moreover, hardly anythng s known about purely cutaneous percepton of traversed dstance, apart from the two studes on the perceved path length of a pont movng over the skn [18, ]. Ths s a fundamentally dfferent stuaton (the pont moves over an extended area of skn, as opposed to a surface movng over a sngle locaton on the skn). Therefore, the purpose of the present study s to measure for the frst tme dscrmnaton thresholds for tactual length percepton usng the movement method. It should be noted that the experments descrbed n [11] and [4] only used knaesthetc nformaton: the subject s fnger rested on a plate or rder that could slde back and forth, ether drven by an electrc motor or the subject hm/herself. Ths s a hghly unnatural stuaton n the sense that n normal haptc length percepton, also cutaneous cues are avalable n the form of the skn of the fngers sldng over the surface of the object. It has been found that cutaneous nformaton also play a role n length percepton usng the movement method []. In that experment, magntude estmaton usng cutaneous, knaesthetc, and combned nformaton was performed. In all cases, lengths were underestmated, the effect beng the largest wth only cutaneous nformaton and the smallest wth combned nformaton. In the present study, we would lke to nvestgate the role of these cutaneous cues n length dscrmnaton. To ths end, a length dscrmnaton experment was performed wth three condtons: knaesthetc (the fnger moves wthout touchng a surface), cutaneous (the statonary fnger touches a movng surface) and a combnaton of knaesthetc and cutaneous (the movng fnger touches a statonary surface). A secondary goal of the experment s to determne the weght factors for cutaneous and knaesthetc cues n the combned condton. Ths enables us to check whether these cues are combned n a statstcally optmal fashon,.e. accordng to the maxmum-lkelhood estmaton (MLE) model of cue combnaton [8]. METHODS The experment conssted of measurng dscrmnaton thresholds for traversed dstance n three condtons. A two-alternatve forced- 93

2 C, C+K K Fgure 1: Photograph of the set-up n the cutaneous condton. The hand rests on a statonary surface whle the ndex fnger touches a movng surface. The other set of straps s used n the knaesthetc and combned condtons, where the hand moves and the surface s statonary. choce paradgm was used. Each tral, the subjects had to feel two lengths and say whch was the greater. Passve percepton was used n ths experment. In the case of cutaneous percepton, ths s justfed snce there was found lttle dfference n performance between actve and statonary passve length percepton [19]. A smlarly small dfference between actve and dynamc passve length percepton s assumed to exst, although ths was not tested. Passve percepton has the advantage that there s complete control over the movement speed, preventng any confoundng effects thereof..1 Subjects Twelve people (4 women) volunteered to partcpate. They were naïve wth regard to the purpose of the experment. They ranged n age from to 1 years. Eleven were rght-handed and one was ambdextrous. All used the ndex fnger of ther rght hand.. Apparatus The lengths were presented usng a computer-controlled lnear postonng system (Isel-automaton) nterfaced wth a CNC controller (Isel C14-1). The system provdes a postonal accuracy of.13 mm. Mounted on the system was a medum densty fbreboard (MDF) surface wth a smooth area for touchng and another area where the subject s hand could rest. On the edge of the board, there was a notch for the ndex fnger. The hand was held n place wth Velcro straps. A smlar MDF board was mounted on a lab stand and could be adjusted n heght to be just above or just below the movng board. A photograph of the setup s shown n fgure 1. The setup could be easly adapted to accomodate the three condtons..3 Condtons There were three condtons: cutaneous (C), knaesthetc (K) and combned (C+K). These are llustrated n fgure. In the cutaneous condton, the hand was attached to the statonary holder wth the ndex fnger n the notch touchng a smooth surface that was attached to the movement devce. In the knaesthetc condton, the hand was attached to the movng holder wth the ndex fnger n the notch, but not touchng anythng. The combned condton was smlar to the knaesthetc condton except that the fnger touched the statonary surface. In the C and C+K condtons, subjects were asked to let ther fnger touch the surface wth a natural force, but ths force was not controlled. Fgure : Illustraton of the three condtons (sde vew). The ndex fnger rests n an openng n the top board. Top: n the cutaneous condton (C), the fnger stays stll and the surface moves. In the combned condton (C+K), t s the other way around. Bottom: n the knaesthetc condton (K), the fnger moves but does not touch the surface. In all condtons, a reference length of 8 mm was used. Each tral, ths reference length was pared wth one of 8 test lengths, whch were 38,,..., 1 mm n the cutaneous condton, 4,,..., 11 mm n the knaesthetc condton, and, 6,..., 8 mm n the combned condton. These step szes (1, and 8 mm) were chosen to provde the optmum range for determnng dscrmnaton thresholds, based on plot experments. The three condtons were performed n a pseudo-random, counterbalanced order (.e. every possble orderng occured equally often)..4 Procedure After recevng nstructons, the subject was blndfolded and donned nose protecton earmuffs (Gamma, 3 db) to prevent the sound from the movement devce to be used as a cue. In addton, s/he wore n-ear earphones on whch whte nose was played at a volume just below the rrtaton threshold to mask any remanng sound cues. For each condton, 8 trals were performed n random order, every test/reference par occurng tmes. In a tral, the test and reference lengths were presented one after the other, n pseudo-random, counterbalanced order (.e. n half the trals, the reference was presented frst and n the other half the test was presented frst). The subjects were not told whch was whch. Presentaton of a length conssted of the fnger beng moved frst to the rght and then the same dstance to the left n the K and C+K condtons. In the C condton, the surface moved n the opposte drectons. The start pont was the same for all trals n all condtons. After a pause of 1. s, the second length was presented n the same way. The subject then had to say whch of the two was the greater length. Then, the next tral began. In half the trals (randomly dstrbuted), the two lengths were presented at the same speed (113 mm/s). In the other half, the two lengths were presented at the same duraton (.89 s), the speeds beng dctated by the chosen test lengths. In ths way, subjects could use nether speed nor duraton alone as a cue for length percepton, and had to rely on ther tactual senses. Each condton took about mnutes. Between condtons, there was a short break. The whole experment took 3 4 mnutes per subject.. Analyss For every subject and every condton, the number of tmes that the test length was chosen to be the greater one was plotted as a functon of the test length. An example s shown n fgure 3. To 94

3 tmes test chosen Σ 13mm threshold mm C K C K testlength mm mean subject Fgure 3: Example of a psychometrc curve from one of the subjects n the knaesthetc condton. The dots are the measured data and the sold lne s a ft. The dashed lne ndcates the length of the reference stmulus. The best-ft value of the fttng parameter σ s ndcated. Fgure 4: Dscrmnaton thresholds for traversed dstance n cutaneous (C), knaesthetc (K) and combned (C+K) condtons. The last set of bars shows the average over subjects. The error bars ndcate the standard error of the sample mean. these data, a psychometrc functon of the form ( ) x lref f(x) = +erf σ was ftted, where l ref = 8 mm s the reference length. The parameter σ corresponds to the 84 % dscrmnaton threshold. In ths way, three thresholds per subject were determned, correspondng to the three condtons. In addton, the data from the equal-speed and equal-duraton trals were also analysed separately to check for possble effects. When multple cues are combned to form a sngle percept, the nformaton from those cues s weghted wth a certan weght factor w. These weght factors should add up to 1. The varance σ that s present on these cues s also combned nto the fnal percept. When the varances can be assumed to be ndependent, the combned varance s gven by: (1) σtotal = w σ. () In the case of combnng cutaneous and knaesthetc cues, wth w c + w k = 1, we can wrte Solvng for w c yelds σ c+k = w cσ c +(1 w c ) σ k. (3) σk σ ± c+k σ k + σ ( c σ c+k σ ) k w c = σc + σk. (4) Usng ths equaton and the measured dscrmnaton thresholds n the three condtons, we can calculate the weght factors for each subject. From the MLE model of cue combnaton follows that the best result (lowest varance of the combned percept) s obtaned when the weght factor for a partcular cue s nversely proportonal to ts varance: w j,mle = σ j σ For combnng cutaneous and knaesthetc cues, ths yelds w c,mle = σ k σc + σk () (6) as the optmal weght factor for the cutaneous contrbuton. When we substtute ths n equaton (3), we obtan a value for the predcted dscrmnaton threshold n the combned condton, based on the MLE model and the measured dscrmnaton thresholds n the two separate condtons: σc σk σ c+k,mle = σc + σk (7) By comparng ths to the actual measured threshold n the combned condton, we can check whether the MLE model apples to percepton of traversed dstance and whether the cues are combned n a statstcally optmal fashon. 3 RESULTS The thresholds for all subjects n the three condtons are shown n fgure 4. On average, the thresholds are ± mm, 11 ± 1 mm and 11±1 mm for condtons C, K, and C+K, respectvely (value±se). A repeated-measures ANOVA shows an effect of condton: F, = 3, p = Bonferron-corrected parwse comparsons show that the thresholds n the knaesthetc and combned condtons are sgnfcantly lower than the threshold n the cutaneous condton (p = 1.6 4,. 4, respectvely). There s no sgnfcant dfference between the knaesthetc and combned condtons. As for the separate analyss of the equal-speed and equalduraton trals, t must be noted that those thresholds are less relable because n that stuaton, each pont of the psychometrc curve s only based on trals nstead of. That sad, t was found that n the K and C+K condtons, the thresholds were approxmately the same for the equal-speed and the equal-duraton trals. However, n the C condton, the thresholds were sgnfcantly hgher for the equal-duraton trals than for the equal-speed trals (t 11 = 3., p =.4). Usng equaton (7), we can calculate a predcted value of the thresholds n the combned condton from those n the two separate condtons, based on the MLE model. These predcted threshold values are plotted as a functon of the actual measured values n fgure. The correlaton between measured and predcted values s not sgnfcant (R =.44, p =.1). Ths means that on the level of the ndvdual subjects, the MLE model s not a good predctor of the thresholds n the combned condton. If the weght factors are not based on the MLE model, then what are they? Usng equaton (4), we can calculate the weghts that have 9

4 predctedcombnedthreshold mm mean measuredcombnedthreshold mm 4 8 Fgure : Predcted threshold values for the combned condton as calculated from the measured data n the other two condtons, plotted as a functon of the measured thresholds n the combned condton. The numbers refer to the subjects. The calculaton s also performed on the mean of the data. The straght lne ndcates a perfect match. been used n the cue combnaton. These are plotted n fgure 6. For subjects 6, 7, 9, and 1, equaton (4) dd not yeld a (real) outcome (although the magnary part for subject 6 s very small). Note that most weghts are negatve, meanng that for those subjects, the addton of cutaneous nformaton has a detrmental rather than a benefcal effect on haptc length percepton. The cutaneous weghts are overall low, meanng that length percepton n the combned condton must be prmarly based on knaesthetc cues. Therefore, one mght expect a strong correlaton between the thresholds n the knaesthetc condton and the combned condton. However, ths correlaton s not sgnfcant: R =.1, p =.9. 4 DISCUSSION AND CONCLUSIONS The man result of ths experment s that for a reference length of 8 mm, the dscrmnaton threshold for passve tactual percepton of traversed dstance s about 11 mm, or about 14 %. Ths s qute a bt hgher than the Just Notceable Dfferences (JND) of. mm [7] or 3.1 mm [] reported for the same dstance wth the fnger-span method. Although these JNDs were measured n a dfferent way from the thresholds n the current experment, they should be qute comparable wthn a factor of 1.. In the present experment, the cutaneous nformaton dd not contrbute sgnfcantly, so ths threshold must be manly based on knaesthetc cues. In the fngerspan method, cutaneous cues also do not play a role, and the nformaton s purely proproceptve. Apparently, the accuracy of the proproceptve percepton of the poston of the thumb and fngers s about 3 tmes better than the knaesthetc percepton of the movement of the hand. It s noted that n the present experment, the whole hand was moved. It could be that when the hand s kept stll and only the ndex fnger s moved, lower thresholds are found. Of course, ths s vable only up to dstances of about 6 mm, dependng on fnger length. A second result s that purely cutaneous percepton of the length of an object sldng over the fnger s possble, wth an accuracy of 1 cutaneous weght mean subject Fgure 6: Calculated weght factors for the cutaneous cue n the combned condton. For subjects 6, 7, 9, and 1, no value could be calculated. The last bar s not the average of the others, but the cutaneous weght factor calculated from the average over subjects. about mm or 3 %. Ths s a novel and nterestng result, that opens up possbltes for ungrounded haptc dsplays. Although length percepton wth the movement method s manly based on knaesthetc cues, no sgnfcant correlaton was found between the thresholds n the K and C+K condtons. Ths mght be due to a dsruptve effect of the cutaneous nformaton n the C+K condton. Although cutaneous length percepton was found to be possble and ths nformaton could potentally contrbute to length percepton n the combned condton, the cutaneous stmulaton mght have drawn attenton away from the knaesthetc percepton. The severty of ths effect could be dfferent for dfferent subjects, resultng n a loss of correlaton between the thresholds n the K and C+K condtons. For of the 1 subjects (nos. 1, 6, 7, 9, and 1), a more or less benefcal effect was found of havng cutaneous nformaton present n the combned condton. However, for only one of these (no. 1) could separate weghts be calculated: 3 % cutaneous and 97 % knaesthetc. For the other 4, the cutaneous-only threshold s so hgh compared to the other two that ts contrbuton cannot account for the dfference between the knaesthetc and combned thresholds. It s not clear why ths s. It must be that n the combned condton, more nformaton s avalable than just the cutaneous and knaesthetc nformaton, but t s unknown what ths nformaton s. There mght be some effect of the fact that the startng poston was the same for all trals, so that n the K and C+K condtons, the task could be performed by just percevng the end postons. If ths were the case, then the task n the C condton was somewhat dfferent than that n the K and C+K condtons. Future research wth randomsed startng postons should provde more clarty n ths respect. Other cues were present n the stmul, such as speed cues and duraton cues. Care was taken to elmnate the usefulness of these cues by randomly swtchng between equal-speed and equalduraton trals. But even f subjects stll managed to somehow use these cues, the effect of ths would be the same n the three condtons. The conclusons about the relatve roles of cutaneous and knaesthetc nformaton would not change. In the separate analyss, t was found that n the K and C+K condtons, the thresholds were approxmately the same for the equal-speed and the equalduraton trals. However, n the C condton, the thresholds were sgnfcantly hgher for the equal-duraton trals than for the equalspeed trals. Ths would suggest that n ths condton, n whch t s harder to determne the traversed dstance, subjects may have used the duraton of the movement as an addtonal cue, but not so much the speed of the movement. In the condtons wth knaesthetc n- 96

5 formaton present (K and C+K), they probably dd not need any addtonal cues. The MLE model does not seem to correctly predct the thresholds n the combned condton from the measured thresholds n the separate condtons, judgng by the low correlaton coeffcent. However, the model cannot be rejected, because a pared-samples t-test between predcted and measured thresholds dd not show a sgnfcant dfference (t 11 = 1., p =.17). It appears that the varance over subjects s too large to draw such a concluson. (Also wth the data of subject 1 removed, who mght have been an outler, was ths dfference not sgnfcant). Although the correlaton between knaesthetc and combned thresholds s also not sgnfcant, t mght be that ths correlaton became sgnfcant f t were based on more subjects. A model n whch the most accurate source of nformaton alone determnes the percept n the combned stuaton can be charactersed as a wnner-take-all model. The predctve power of the two models can be compared by lookng at the RMS dfference between the predcted and measured thresholds for the two models. The RMS devaton from the measured thresholds for the wnner-take-all model (4. mm) s slghtly smaller than for the MLE model (4. mm). Therefore, t seems that a wnner-take-all model of cue combnaton (.e. combned percepton equals knaesthetc percepton n ths case) s more applcable to ths stuaton. However, n a stuaton of magntude estmaton n whch conflctng nformaton was present, t was found that the combned percept was determned by the nformaton source that provded the greater length, not necessarly the greater accuracy []. Ths s another clue that the MLE model does not apply to haptc length percepton. In concluson, we have shown that cutaneous length percepton s possble, and n some cases contrbutes a lttle to percepton of traversed dstance, but on average, percepton of traversed dstance s based on knaesthetc nformaton, whereas cutaneous nformaton can even have a dsruptve effect. The MLE model does not seem to apply to ths type of cue combnaton, although an outrght rejecton of ths model s not possble. ACKNOWLEDGEMENTS Ths work was supported by a grant from the Netherlands Organsaton for Scentfc Research (NWO) and by the Collaboratve Project no. 4887, THE Hand Emboded, wthn the FP7-ICT program Cogntve Systems and Robotcs. [] A. Hohmuth, W. D. Phllps, and H. VanRomer. A dscrepancy between two modes of haptc length percepton. Journal of Psychology, 9(1):79 87, [11] M. Hollns and A. K. Goble. Percepton of the length of voluntary movements. Somatosensory Research, (4):33 348, [1] J. Jastrow. The percepton of space by dsparate senses. Mnd, 11(44):39 4, [13] R. P. Kelvn. Dscrmnaton of sze by sght and touch. Quarterly Journal of Expermental Psychology, 6:3 34, 194. [14] R. P. Kelvn and A. Mulk. Dscrmnaton of length by sght and touch. Quarterly Journal of Expermental Psychology, (4):187 19, 198. [1] A. Kumazak, K. Terada, and A. Ito. Role of vson on haptc length percepton. In Proc. nd Jont EuroHaptcs Conference and Symposum on Haptc Interfaces for Vrtual Envronment and Teleoperator Systems, pages , Tsukuba, Japan, 7. IEEE. [16] M. Lanca and D. J. Bryant. Effect of orentaton n haptc reproducton of lne length. Perceptual and Motor Sklls, 8: , 199. [17] H. S. Langfeld. The dfferental spatal lmen for fnger span. Journal of Expermental Psychology, (6):416 43, [18] N. Langford, R. J. Hall, and R. A. Monty. Cutaneous percepton of a track produced by a movng pont across the skn. Journal of Expermental Psychology, 97(1):9 63, [19] R. Schellngerhout, A. W. Smtsman, and G. P. van Galen. Texture nformaton n tactual space percepton. Acta Psychologca, 99:93 114, [] S. S. Stevens and G. Stone. Fnger span: rato scale, category scale and JND scale. Journal of Expermental Psychology, 7():91 9, 199. [1] M. Teghtsoonan and R. Teghtsoonan. Seen and felt length. Psychonomc Scence, 3:46 466, 196. [] K. Terada, A. Kumazak, D. Myata, and A. Ito. Haptc length dsplay based on cutaneous-proproceptve ntegraton. Journal of Robotcs and Mechatroncs, 18(4): , 6. [3] J. T. Walker. Smple and contgent aftereffects n the knesthetc percepton of length. Journal of Expermental Psychology: Human Percepton and Performance, 4():94 31, [4] S. Wapner, J. Wenberg, J. A. Glck, and G. Rand. Effect of speed of movement on tactutal-knesthetc percepton of extent. Amercan Journal of Psychology, 8:68 613, [] B. L. Whtsel, O. Franzen, D. A. Dreyer, M. Hollns, M. Young, G. K. Essck, and C. Wong. Dependence of subjectve traverse length on velocty of movng tactle stmul. Somatosensory Research, 3(3):18 196, REFERENCES [1] E. Abravanel. The synthess of length wthn and between perceptual systems. Percepton & Psychophyscs, 9(4):37 38, [] L. Armstrong and L. E. Marks. Haptc percepton of lnear extent. Percepton & Psychophyscs, 61(6):111 16, [3] L. J. Berryman, J. M. Yau, and S. S. Hsao. Representaton of object sze n the somatosensory system. Journal of Neurophysology, 96:7 39, 6. [4] M.-F. Cheng. Tactle-knesthetc percepton of length. Amercan Journal of Psychology, 81:74 8, [] R. S. Davdon and M.-F. H. Cheng. Apparent dstance n a horzontal plane wth tactle-knesthetc stmul. Quarterly Journal of Expermental Psychology, 16:77 81, [6] A. G. Detze. Knaesthetc dscrmnaton: the dfference lmen for fnger span. Journal of Psychology, 1(16 168), [7] N. I. Durlach, L. A. Delhorne, A. Wong, W. Y. Ko, W. M. Rabnowtz, and J. Hollerbach. Manual dscrmnaton and dentfcaton of length by the fnger-span method. Percepton & Psychophyscs, 46(1):9 38, [8] M. O. Ernst and H. H. Bülthoff. Mergng the senses nto a robust percept. Trends n Cogntve Scences, 8(4):16 169, 4. [9] H. F. Gaydos. Senstvty n the judgment of sze by fnger-span. Amercan Journal of Psychology, 71:7 6,