Is it possible to perceive the shape of an object without exploring it?

Transcription

1 Is it possibl to prciv th shap of an objct without xploring it? Alssandro Formaglio Dpartmnt of Information Enginring, Univrsity of Sina, Italy Gabril Baud-Bovy Faculty of Psychology, UHSR Univrsity, Milan, Italy Domnico Prattichizzo Dpartmnt of Information Enginring, Univrsity of Sina, Italy ABSTRACT Th dformation of th fingrpads along th contact surfacs in a ral grasp provids local information about th shap of th objct, which is crucial for th stability of th grasp. Svral xprimnts wr prformd to invstigat whthr it was possibl to transmit information about th shap of a virtual objct graspd via a singlcontact haptic intrfac, in spit of th fact that thimbls prvnt th dformation of th fingrpads. To that nd, w modifid th classic god-point algorithm by using a psudo-llipsoidal forc fild that provids information about th objct s shap by incrasing th complianc along th normal or tangntial dirction. Th position of th god-point was hld fixd during th whol contact. Th main finding was that participants could idntify th orintation of th contact surfac whn th complianc was maximum in th tangntial plan by using small xploratory movmnts allowd by th contact modl. Kywords: haptic intrfac, shap prcption, contact modl, godpoint algorithm shaps prcivd via an haptic dvic. In ths xprimnts, w trid to provid information about th objcts shap by incrasing th complianc along th normal (Exprimnt II) or tangntial dirction (Exprimnts III and IV). To that nd, w modifid th classic god-point algorithm by using a psudo-lliptical forc fild instad of cntral forc fild to comput th contact forcs. 2 REAL PINCH GRASP: EXPERIMENT I Th dirction of th contact forcs cannot diffr too much from th contact surfac normal without causing a slip of th fingr. Prvious studis hav shown that humans hav intrnalizd this constraint and, as a rsult, tnd to apply forcs in dirctions that provid a rasonabl safty margin against a fingr slip [6, 7]. Th objctiv of th first xprimnt is to show that this tndncy holds tru in th particular grasping contxt of this study, that is whn a fixd objct is squzd by th thumb and indx fingr. 2.1 Mthods 1 INTRODUCTION In absnc of vision, th shap of an objct is usually prcivd via th movmnts of th fingr on its surfac [1]. Whil such movmnts ar obviously ncssary to prciv th shap of th objct in its globality givn th limitd siz of tactil rcptiv fild, it is important to not that cutano- and mchano-rcptors distributd in th skin and undrlying tissus can yild local information about th objct shap in th absnc of any xploratory movmnts. In particular, prvious studis hav shown that ths rcptors can provid information about th orintation of th contact surfacs undr th fingrpads [4] or th dirction of th contact forc rlativ to th normal [5] in addition of information about th cofficint of friction of th contact surfac [2, 3]. Such information is crucial to control th dirction of th contact forcs during th grasp of ral objcts in ordr to avoid a slip of th objct. Th gnral objctiv of this study is to xamin ways of transmitting information about th orintation of th contact surfac undr th fingrtip to an obsrvr who is touching a virtual objct via an haptic intrfac without moving th fingr. Such a qustion is challnging bcaus th contact with th virtual objct is mdiatd by thimbls that do not allow th fingr pads to dform as thy would at th contact with ral objcts. Th only information that can b transmittd is th dirction of th applid forc. In this study, w rport prliminary rsults of four pilot xprimnts. Th first xprimnt shows that contact forcs tnd to b orintd prpndicularly to th contact surfacs. Th following xprimnts xamin th capacity of human oprators to discriminat -mail: formaglio@dii.unisi.it -mail: baud-bovy.gabril@hsr.it -mail: prattichizzo@dii.unisi.it Figur 1: Exprimntal st-up usd to masur fingr forcs in Exprimnt I (th scrn hiding th viw of th dvic to th subjct is not shown). Inst: Th thr shaps shown at th nd of th xprimnt. Thr mals and two fmals participatd in this xprimnt (man ag: 3 yars). Participants wr satd in front of th instrumntd dvic (Figur 1). Th forarm was supportd by th xprimntal stup a fw cntimtrs abov th objct. Th gasping axis corrspondd to th sagittal axis. A scrn (not shown) hid th viw of th objct. Contact surfacs (17 mm diamtr) wr covrd with faltr. Th orintation of th indx contact surfac was varid across xprimntal conditions. Th xprimnt comprisd thr blocks of trials that diffrd by th angl ( 27 o, o or 27 o ) btwn th two contact surfacs. Each orintation was prsntd 1 tims conscutivly insid a block. Th ordr of prsntation of th blocks was rvrsd btwn participants. At th bginning of ach trial, an initial bp instructd th participant to squz th objct with a comfortabl forc. Four sconds latr, a scond bp indicatd th tim to rlas th objct. Forcs and torqus wr masurd by two 6 DoFs snsors placd undr ach fingr (Nano-17, ATI Tchnology) with a sampling rat

2 of 2Hz and savd for off-lin analysis. At th nd of th xprimnt, participants wr askd to match ach block of trials with on of thr possibl shaps prsntd on a sht of papr (s inst in Figur 1). 2.2 Rsults Contact forcs pakd shortly aftr th initial contact bfor stabilizing at a platau lvl until rlas tim (s Figur 2A). Th dirction of th contact forcs changd littl during th whol trial. Th indx contact forc tndd to rmain prpndicular to th contact surfac as xpctd (s Figur 2B,C). This chang was accompanid by a smallr rotation of th thumb in th opposit dirction, crating a significant torqu around th vrtical axis, as if th subjct trid to scrw or unscrw th objct. Non of th participants had difficulty matching ach block of trials with th corrct shap at th nd of th xprimnt. A Forc (N) B f x C -1 i t Tim (sc) f x Indx Thumb t i xprimnt, howvr, th quilibrium condition did not nd to b satisfid bcaus th objct was fixd. 3 EXPERIMENTAL SETUP AND FORCE RENDERING MODEL 3.1 Introduction As mntiond in th Introduction, human oprators ar usually abl to prciv th shap of th graspd objct vn in absnc of any xploratory movmnts. This prcption is lacking in virtual grasping via haptic intrfacs, whr thimbls prvnt th dformation of th fingrpads on th contact surfacs, which is crucial for th transmission of information about th objct s shap. In th following sctions, w will prsnt th prliminary rsults of svral psychophysical xprimnts whr participants squzd a virtual objct btwn th thumb and indx fingr. Th objctiv of ths xprimnts was to find a way of transmitting information about th shap of virtual objcts via a singl contact point haptic dvic and without xploratory movmnts. In this sction, w prsnt th lmnts of th xprimntal procdur that wr common to all haptic xprimnts. 3.2 Exprimntal procdur Th xprimntal stup consistd of a fixd (passiv) thimbl and of th thr DOFF haptic dvic that was usd to simulat th contact forc btwn th indx fingr and th virtual objct. Th participants insrtd th thumb in th fixd thimbl and th othr indx in th thimbl mountd at th xtrmity of th haptic dvic via a cardanic joint. At th bginning of ach trial, th indx fingr did not touch th virtual objct. A bp instructd th participant to flx th indx fingr. Participants wr instructd to maintain th contact until a scond bp, 3sc latr, indicatd th momnt of rlasing th objct by xtnding th indx fingr. Th lvl of squzing forc was slctd frly by th subjct. At th nd of ach trial, th participant was askd to match th prcivd shap of th virtual objct with on of svral shaps dpictd in a figur. Th only diffrnc btwn th dpictd objcts shaps consistd in th orintation of th indx contact surfac. 3.3 Forc rndring modl Figur 2: A: Thumb (gray lins) and indx (black lins) forcs rcordd during a singl trial. Th solid and dottd lin corrspond to th normal and horizontal forc componnts rspctivly. Th indx contact surfac orintation was orintd at 27 o rlativ to th thumb contact surfac. Th contact forcs wr avragd ovr a 5ms tim-window at th nd of ach trial (gray ara). B: Dirctions of th contact forcs in th horizontal plan producd by on subjct. Th orintation of th indx contact surfac was varid across xprimntal conditions (lft: 27 o, middl: o, right: 27 o ). Each lin corrsponds to a diffrnt trial. C: Avrag dirction of th forc (data from all subjcts, N = 5). Th shortr gray lins dnot th standard dviation of th forc dirction. 2.3 Discussion Th rsults of this xprimnt confirmd that contact forcs tnd to b orintd prpndicularly to th contact surfacs. In particular, this xprimnt showd that this obsrvation holds tru whn a fixd objct is graspd with a pinch grasp. Whil xpctd, this rsult ndd to b confirmd xprimntally bcaus th dirction of th contact forcs in th pinch grasp ar normally constraind by th position of th contact points whn th objct is fr. In this Th haptic rndring of th contact forc was prformd via th classic god-objct algorithm [11]. Th position of th god point was hld constant during th whol contact, i.. th fingr could not slip along th virtual surfac. In ordr to provid information about th orintation of th indx contact surfac, w modifid th classic god point algorithm by using a psudo -llipsoidal forc fild instad of a sphrical forc fild to comput th contact forc. Lt p F =(x F,y F,z F ) b th position of th fingrtip xprssd in a local rfrnc fram with its origin at th god point p G, as shown in Figur 3. W dfin th llipsoidal stiffnss matrix E = diag{k 1,k 1,k 2 }, whr k 1 and k 2 ar th stiffnss along th tangntial and normal dirction, rspctivly. Hnc, th contact forc for an llipsoidal forc fild is computd as F = Ep F (1) It is worth noting that th dirction of th forc F computd via quation 1 is not always dirctd toward th god point, and this might yild a glu-lik or rpllnt-lik bhavior whn th main axs of th forc fild ar not alignd with th contact surfac. To ovrcom this undsird ffct, w dfind th psudo-

3 x p F Figur 3: Local fram of rfrnc associatd with th contact surfac. p G and p F dnot th position of th god point and th position of th fingrtip insid th objct. llipsoidal forc fild as z p G y F = F p F p F (2) From this dfinition, it is clar that, for vry position p F of th fingrtip, th magnitud of th forc F is th sam as F, but its dirction is always orintd toward th god point as in a sphrical forc fild. n n 3.4 Data Analysis For ach combination of shap, ccntricity and stiffnss valus, w computd th proportion of corrct rsponss. Th main ffct of ach on of ths factors as wll as th intraction btwn ccntricity and stiffnss wr analyzd with a rpatd-masur ANOVA, aftr using th arcsin( x) transformation on th proportion of corrct rsponss to stabiliz th varianc. Th dgrs of frdom wr adjustd with Grnhous-Gissr s psilon ε in ordr to account for a possibl dviation from sphricity. 4 HAPTIC PINCH GRASP: EXPERIMENT II 4.1 Introduction This xprimnt aimd at inducing th participant to squz th objct with contact forcs similar to th ons obsrvd in a ral grasp. To that nd, th dirction of maximum complianc was alignd with th dirction of th normal. Th ida was (1) that th participant s fingrtip would pntrat th virtual objct along th dirction of maximum complianc and would thrfor also produc a forc along this dirction, and (2) that participants would infr th orintation of th contact surfac from th dirction of th forc on th basis of past xprinc with grasping ral objcts, as in Exprimnt I. 4.2 Mthods 4 adults, agd btwn 22 and 28, participatd in th xprimnt. All participants wr right-handd with no known snsorimotor impairmnts. Thir prior xprinc with haptic dvics varid from naïv to xprt. (a) (b) Figur 4: Psudo-llipsoidal forc filds. Th arrow dnots th forc vctor. Th complianc is maximum along th normal (a) or tangntial dirction (b). Stxt. A convnint way to charactriz th llipsoidal forc fild is rprsntd by its ccntricity and total stiffnss k: = k 2 k = k 1 k k [ Nmm ] Th rciprocal quations to comput th valus of th stiffnss along th tangntial and normal dirction ar: k 1 = k 2 = k 2 3k For > 1 th dirction of maximum complianc is orintd along th normal to th surfac at contact point (i.., k 2 > k 1, s Figur 4-(a)). On th contrary, < 1 dtrmins maximum complianc in tangntial dirction (s Figur 4-(b). Whn = 1, th psudo-llipsoidal forc fild dgnrats to a common sphric fild with stiffnss k, k >. In th haptic xprimnts prsntd in th following sctions, w manipulatd th complianc of th contact by varying th ccntricity and th stiffnss k of th llipsoidal forc fild. In Exprimnt II (Sction 4), th dirction of maximum complianc was alignd with th normal, i.. > 1.. In th last two xprimnts (Sctions 5 and 6), th dirction of maximum complianc was tangntially alignd, i.. < 1.. In all xprimnts, th ccntricity and stiffnss of th forc fild wr systmatically varid in ordr to idntify th optimal valus of th paramtrs for th task. (3) (4) (a) (b) (c) Figur 5: Top viw of th shap of th virtual objcts displayd in scond xprimnt. Figurs (a), (b) and (c) corrspond to th 27 o, o,and27 o conditions. Th xprimnt comprisd thr virtual objcts with shaps faturing an angl of 27 o, o, and 27 o btwn th thumb and indx contact surfacs (s Figur 5). Th indx contact surfac was titld around th vrtical axis. Th stup was arrangd so that th indx and th thumb wr laying on th sam plan as th normal dirctions to th surfacs. Th task consistd in touching th virtual objct for th prst tim intrval, and in rporting th prcivd shap as dscribd in Sction 3.2. Each virtual objct was rndrd with diffrnt paramtrs of th psudo-lliptical forc fild. Each shap was combind with thr stiffnss valus (k =.7, 1. and 1.25N/mm) and thr ccntricity valus ( = 1, = 2 and = 3) to yild a total st of 27 diffrnt stimuli. Th xprimnt comprisd 1 blocks of 27 runs. Insid a block, all stimuli wr prsntd onc in a random ordr. 4.3 Rsults To analyz th ffct of th stiffnss and ccntricity valus, w prformd a rpatd-masur ANOVA on th prcntags of corrct rsponss (s Sction 3.4). Non of th factors was statistically significant (s Tabl 1). Figur 6 shows that th prcntag of corrct rsponss did not diffr markdly from th chanc lvl (33%). Participants oftn

4 Factor DF hyp DF rror F ε P valu Shap Eccntricity Stiffnss Ecc:Stiff Tabl 1: Effcts of th main xprimntal factors in xprimnt II (rpatd-masur ANOVA) Th paramtrs of th psudo-lliptical forc fild wr systmatically varid. Thr stiffnss valus (k =.75, 1. and 1.25N/mm) and thr ccntricity valus ( =.4,.7 and 1.) wr usd. Ths paramtrs wr combind with th two orintation angls ( 45 o and 45 o ) to yild a st of 18 diffrnt stimuli. Th xprimnt comprisd 1 blocks of trials. Each stimulus was prsntd in a random ordr onc insid ach block. Stimuli wr prsntd according th xprimntal procdur dscribd in Sction Rsults %.75 k[ N mm ] Figur 6: Prcntag of corrct rspons rats for ach ccntricity and stiffnss valu (all shaps and subjct poold togthr) % smd confusd during th xprimnt and xprssd frustration at not bing abl to xplor th surfac of th objct. In fact, visual obsrvation of th subjcts bhavior and vrbal rports rvald that participants wr activly moving th fingr along th dirction of maximum complianc as if trying to xplor th surfac of th objct. 5 HAPTIC PINCH GRASP: EXPERIMENT III 5.1 Introduction On th basis of th rsults of th prvious xprimnt, w dcidd to modify th dsign of th xprimnt: first, th complianc of psudo-llipsoidal forc was maximum in th tangntial plan. Scond, only two shaps wr includd in th xprimnt, and th amplitud of angl btwn th orintations of th thumb and indx contact surfac was incrasd (45 o instad of 27 o ). Finally, th postur of th hand with rspct to th objct was modifid, so that th indx and th thumb would b laying in a plan orthogonal to th on dfind by th normals of th contact surfacs (s Figur 7). 5.2 Mthods (a) Figur 7: Top viw of th shap of th virtual objcts displayd in third xprimnt. 5 adults, agd btwn 22 and 28 (3 of thm participatd also in th Exprimnt II), prformd th xprimnt. All participants wr right-handd with no known snsorimotor impairmnts. Thir prior xprinc with haptic dvics varid from naïv to xprt. (b) 2.75 k[ N mm ] Figur 8: Bhavior of corrct rspons rats ovr stiffnss and ccntricity for th third xprimnt. As in prvious xprimnt, w computd th prcntags of corrct rsponss with rspct to stiffnss and ccntricity of th llipsoid. Figur 8 shows that ths prcntags incrasd markdly whn ccntricity dcrasd, that is whn th complianc along th tangntial dirction incrasd rlativly to th complianc along th normal dirction. In contrast, ths prcntags rmaind approximatly constant across stiffnsss for a sam ccntricity valu. For ccntricity = 1. th psudo-lliptic forc fild dgnrats to th classic sphrical fild, i.. th complianc is constant along all dirctions. As xpctd, th prcntags ar clos to chanc lvl (5%), which indicats that participants wr unabl to prciv th objct s shap. Factor DF hyp DF rror F ε P valu Shap Eccntricity Stiffnss Ecc:Stiff Tabl 2: Effcts of th main xprimntal factors in Exprimnt III (Rpatd-masur ANOVA). Th rpatd-masur ANOVA confirmd that th prcntag of rsponss dpndd only on th ccntricity of th forc fild (P <.5, s Tabl 2). Th absnc of ffct for th shap factor indicats that th proportion of corrct rsponss was th sam for both shaps. Ths rsults show that it is possibl to convy information about surfac orintation via a singl contact point by stting small valus of ccntricity, i.. incrasing th complianc of contact along th tangntial dirction

5 6 HAPTIC PINCH GRASP: EXPERIMENT IV 6.1 Introduction This xprimnt aimd at xtnding th rsults of th prvious xprimnt by changing objct position and hand postur. In this xprimnt, th indx contact surfac was tiltd around an horizontal axis. Morovr, participants graspd th virtual objct from asid, i.. th hand was sidways with rspct to th virtual objct, as shown in Figur 9. Not that th positions of th hand rlativ to th objct did not chang: th plan dfind by indx and thumb was orthogonal to th plan dfind by th normal dirctions as in th prvious xprimnt (s Figur 9). (a) Figur 9: Sid viw of th shap of th virtual objcts displayd in th fourth xprimnt. 6.2 Mthods 4 adults, agd btwn 22 and 28 (all of thm also prformd Exprimnt III), participatd in th xprimnt. All wr right-handd with no known snsorimotor impairmnts. Thir prior xprinc with haptic dvics varid from naïv to xprt. Th stimuli wr similar to th on of th prvious xprimnt, xcpt for th fact that th indx contact surfac was titld around an horizontal axis instad of a vrtical on (s Figur 9). As prviously, th two shaps wr combind with thr stiffnss valus and thr ccntricity valus to yild a total of 18 stimuli. Th xprimntal procdur was th sam as in prvious xprimnt. Each subjct prformd 1 xprimntal blocks, ach of thm consisting of displaying all 18 stimuli in a random ordr. 6.3 Rsults Th rsults of this xprimnt wr vry similar to thos of th prvious on. Figur 1 shows th prcntag of corrct rspons rats for ach valu of th paramtrs of psudo-lliptic forc fild. % k[ N mm ] Figur 1: Bhavior of corrct rspons rats ovr stiffnss and ccntricity for th fourth xprimnt. (b).4 Factor DF hyp DF rror F ε P valu Shap Eccntricity Stiffnss Ecc:Stiff Tabl 3: Effcts of th main xprimntal factors in Exprimnt IV (Rpatd-masur ANOVA). Th statistical analysis of th rsponss confirmd that incrasing th complianc of contact in th tangntial plan hlps th subjct to prciv th shap (P <.5, s Tabl 3). In addition, this analysis rvald that th stiffnss factor was also statistically significant (P >.5), indicating that lowr stiffnss valus might slightly facilitat th task by allowing largr movmnts. Not that th small incras of corrct rsponss for th stiffnss valu corrsponding to a sphrical forc fild (i.., = 1.), which provids no information about th orintation of th shap, is mostly likly to b du to th limitd numbr of subjcts who prformd th xprimnt. With a largr sampl, on would xpct to find prcntags closr to chanc lvl as wll as a statistically significant intraction btwn th ccntricity and stiffnss factors. 7 GENERAL DISCUSSION AND CONCLUSIONS In a ral grasp, snsory fdback from tactil and mchano rcptors distributd in th skin and undrlying tissus play a doubl rol in absnc of vision: first, thy provid information about th shap of th objct to th prcptual systm and, scond, thy allow th motor systm to control fingr forcs, in particular thir dirction, in an optimal way. In contrast, as mntiond in th Introduction, such snsory fdback is absnt in a virtual grasp bcaus th fingrtips ar usually insrtd in thimbls mountd at th xtrmity of th dvics usd to simulat th contact forcs. This fact that thimbls prvnt th dformation of th fingrpads ld us to considr using th dirction of th contact forcs as a cu about th orintation of th contact surfac. As a mattr of fact, prvious studis hav shown that contact forc tnd to b orintd prpndicularly to th contact surfacs whn a ral objct is graspd [7]. Th first xprimnt (s Sction 2) dmonstratd that this obsrvation holds tru whn th graspd objct is fixd. Such a bhavior is obviously bnficial to prvnt a slip of th fingr. Our first ida for transmitting information about th orintation of th contact surfac was thrfor to incras th complianc of a psudo-llipsoidal forc fild along th dirction of th normal in ordr to induc th subjct to pntrat th objct along this dirction. According to th classical god-point algorithm, such a pntration of th objct would b accompanid by a contact forc in th sam dirction (Sction 3). On th basis of th rsults of this initial xprimnt, w hypothsizd that participants would b abl to prciv th shap of a virtual objct from th pattrn of contact forcs producd whn grasping it. It is important to not that this hypothsis rsts on two assumptions: first, that a psudo-llipsoidal forc fild will induc th participants to squz th objct along a dirction that corrsponds to th normal and, scond, that participants will associat this pattrn of contact forc with a spcific shap. Th scond assumption is supportd by a growing body of vidnc supporting th ida that motor procsss ar involvd in th idntification of maning of th snsory signals. Functional imaging tchniqus in humans and singl-cll lctrophysiology studis in monkys hav shown that motor aras ar activatd during prcptual tasks (rviws in [13, 12]). This thory xplains th obsrvation that prcption is oftn influncd by action. For xampl, psychophysical xprimnt hav shown that th vlocity profil of th movmnt can influnc th prcption of th trajctory s shap

6 [8] or that th application of tangntial forc on th xtrmity of th fingrtip during th xploration of a planar surfac can induc illusory prcptions, such as a tilt of th plan [9] or a bump [1]. Th first assumption took for grantd that th participants would squz th virtual objct as a ral objct and that, during contact, th fingrtip would stabiliz at som distanc of th virtual contact surfac. Th rsults of th scond xprimnt (Sction 4) suggst howvr that this was not th cas. Participants wr unabl to idntify th shap of th virtual objct whn th dirction of maximum complianc was alignd with th normal. Rsponss wr at chanc lvl for all ccntricity and stiffnss valus. Visual obsrvation and vrbal rports pointd out that participants, contrary to our xpctations, did not squz th virtual objct as thy would hav with a ral objct. In fact, som participants rportd making micro xploratory movmnts along th dirction of maximum complianc in ordr to idntify th objct s shap. Th rcours to such a stratgy might xplain th ngativ rsults of this xprimnt and th confusion of th participants. As a mattr of fact, th dirction of maximum complianc mad an angl of 27, o or 27 o with rspct to th normal in this xprimnt. If subjcts prcivd ths dirctions as paralll to th surfac as is th cas during th fr xploration of a ral objct s shap, thn th virtual objcts would hav bn prcivd likwis it was faturing two contact surfacs making an angl of 63 o,9 o and 63 o rspctivly, which dos not mak much sns indd and, in any cas, did not corrspond to any of th visually prsntd rfrnc shaps. In addition, it might also b possibl that th diffrnc btwn th xprimntal conditions was too small to b prcivd clarly nough. In th two following xprimnts (Sctions 5 and 6), th dirction of maximum complianc of th psudo-llipsoidal forc fild was align with th tangntial plan. In addition, th xprimntal paradigm was simplifid by including only two shaps. Finally, th contrast btwn th xprimntal conditions was starknd by incrasing th angular diffrnc ( 45 o and 45 o ) btwn th thumb and indx contact surfacs. Th rsult of ths xprimnts dmonstratd that participants wr abl to haptically prciv th shap of th objct. This rsults hld tru whthr th orintation was tiltd around a vrtical axis (Exprimnt III in Sction 5) or around an horizontal axis (Exprimnt IV in Sction 6). Hr again, visual obsrvation and vrbal rports suggst that participants movd th fingrtip latrally to th xtnt allowd by th contact modl. A dtaild look at th rsults showd that th main xprimntal factor prtaining to th prformanc lvl was th ccntricity, that is th ratio btwn th compliancs along th tangntial and normal dirction. In contrast, stiffnss had almost no ffct in th Exprimnt III and only a small ffct in Exprimnt IV. REFERENCES [1] S.J.Ldrman and R.L.Klatzky, Extracting objct proprtis by haptic xploration, Acta Psychologica, 84, 29-4, [2] V.G.Macfild and R.S.Johansson, (1996) Control of grip forc during rstraint of an objct hld btwn fingr and thumb: rsponss of muscl and joint affrnts from th digits, Exprimntal Brain Rsarch, 18: , [3] V.G.Macfild, C.Hgr-Ross and R.S.Johansson, Control of grip forc during rstraint of an objct hld btwn fingr and thumb: rsponss of cutanous affrnts from th digits Exprimntal Brain Rsarch, 18: , [4] P.Jnmalm and R.S.Johansson, Visual and somatosnsory information about objct sha control manipulativ fingrtip forc, Th Journal of Nuroscinc, 17(11):4486:4499, [5] I.BirzniksI, P.Jnmalm, A.W.Goodwin and R.S.Johansson, Encoding of dirction of fingrtip forcs by human tactil affrnts, Th Journal of Nuroscinc, 21(2): , 21. [6] R.S.Johansson and K.J.Col, Grasp stability during manipulativ actions, Canadian Journal of Physiology and Pharmacology. 72(5):511-24, [7] G.Baud-Bovy and J.F.Sochting, Two Virtual Fingrs in th Control of th Tripod Grasp, Journal of Nurophysiology, 86:64-615, 21. [8] P.Viviani, G.Baud-Bovy and M.Rodolphi, Prciving and Tracking Kinsthtic Stimuli: Furthr Evidnc of Motor-Prcptual Intraction, Journal of Exprimntal Psychology: Human Prcption and Prformanc, 23(4) , [9] W.L.Sachtr, M.R.Pndxtr, J.Biggs and M.A.Srinivisan, Haptically prcivd orintation of a planar surfac is altrd by tangntial forcs, in Procdings of th Fifth Phantom Usr s Group Workshop, Aspn, Colorado, 2. [1] G.Robls-d-la-Torrs and V.Hayward, Forc can ovrcom objct gomtry in th prcption of shap in activ touch, Natur, 412: , 21. [11] C. Zills and K. Salisbury, A constraint-basd god-objct mthod for haptic display, in ASME Haptic Intrfac for Virtual Environmnt and Tloprator Systms, Dynamic Systm and Control, Chicago, Illinois, USA, vol. 1, pp [12] S.J.Blakmor and J.Dcty, (21) From th prcption of action to th undrstanding of intntion, Natur Rviws Nuroscinc 2: , 21. [13] G.Rizzolatti and L.Craighro, Th mirror-nuron systm, Annu Rv Nurosci, 27:169-92, 24. To conclud, this work rportd som prliminary rsults dmonstrating that it is possibl to convy information about th orintation of a virtual surfac vn via singl-contact haptic dvic. This study also rvald that prcpts wr likly to b basd on micro-movmnts allowd by th contact modl. Currntly, w ar xamining th fingrtip trajctoris rcordd during th contact in ordr to find out if it is possibl to prdict th prformanc at th trial or subjct lvl on th basis of th charactristics (.g., numbr or xtnt of th to-and-from movmnts ralizd during th xploration of th objct s shap). In addition, this analysis will also b aimd at finding out whthr participants, aftr an initial xploration phas, stabilizd th position of thir fingrtip and whthr th contact forc at this position corrsponds to th on obsrvd during a ral grasp.