The role of onset in the perception of sequentially presented vibrotactile patterns

Transcription

1 Perceptin & Psychphysics 1983,34 (5), The rle f nset in the perceptin f sequentially presented vibrtactile patterns JAMES C. CRAIG IndianaUniversity, Blmingtn, Indiana Interactin between sequentially presented vibrtactile patterns was measured bth in a discriminatin task and in a tempral masking paradigm. The vibrtactue patterns were generated n an array f stimulatrs, 6 clumns X 24 rws, in cntact with the subjects' fingertips. As the time between pairs f patterns presented t ne fingertip was increased, perfrmance in discriminating whether the tw patterns were the same r different imprved. When ne member f the pair was presented t ne finger and the ther t a secnd finger, discriminatin perfrmance first declined slightly then imprved as tempral separatin increased. Perfrmance n pattern recgnitin in the presence f either a frward r backward masker imprved as the tempral separatin between the target and masker increased. In bth the discriminatin and masking measurements, the time between the nsets f the tw patterns (stimulus nset asynchrny) was fund t be the best measure f interactin between the sequentially presented patterns. The implicatins f these findings fr vibrtactue pattern prcessing are discussed. The study f tempral interactins between sequentially presented vibrtactile patterns has paralleled similar studies f visual patterns. This is nt _surprising since bth the skin and the eye are capable f perceiving spatially extended patterns, and there is cntinuing interest in the designing f cutaneus cmmunicatin systems fr replacing the visual system (Geldard, 196, 1966, 1974; Schiff & Fulke, 1982; Sherrick, 1975; White, Saunders, Scadden, Bach-y-Rita, & Cllins, 197). Sme studies using cutaneus stimuli have measured changes in perfrmance as a functin f the amunt f tempral separatin between patterns and have prduced results that are in sme instances bth qualitatively and quantitatively similar t thse prduced by studies using visual stimuli. These include studies f apparent mtin n the skin (Sherrick, 1968a, 1968b; Sherrick & Rgers, 1966), saltatry cnductin (Geldard, 1975; Geldard & Sherrick, 1972), tempral integratin (Craig, 1982a), metacntrast (Weisenberger & Craig, 1982), and tempral masking f patterns (Craig, 1978, 198, 1982b). An imprtant questin that has emerged in examining at least sme f these changes in sequential interactin is hw best t characterize these changes: Are they dependent upn the stimulus nset asynchrny (SOA), the interstimulus interval (lsi), amunt f tempral verlap, r perhaps sme ther measure f tempral interactin? The answer t this generalquestin has had majr cnsequences fr mdeling the prcessing f visual patterns (e.g., Breitmeyer This research was supprted by Grant NS-9783 frm the Natinal Institutes f Health. The authr wishes t thank Rger Rhdes fr assistance in cnducting these experiments. The authr's mailing address is: Department f Psychlgy, Indiana University, Blmingtn, Indiana 474S. & Ganz, 1976; Felsten & Wasserman, 198; Massar, 1975; Turvey, 1973). In a recent study fvisual patterns, Di Lll (198) fund that the amunt f tempral integratin between tw patterns was severely reduced if the duratin f the first pattern was greater than abut 1 msec. Additinal results shwed that the amunt f frward masking (when the masker precedes the target) declined cnsiderably as the duratin f the masker was increased. These results suggested that icnic persistence was an inadequate explanatin fr the results and, mre imprtantly fr the present study, that SOA rather than lsi was the critical factr in these measures ftempral integratin. Recent wrk with vibrtactile patterns has als suggested the imprtance f SOA rather than lsi in measures f tempral integratin. In ne set f measurements, letters were divided in half, and the time between the presentatin f the tw halves was varied. As the time was increased, subjects had greater difficulty recgnizing the letters. With several different duratins tested, it was fund that the decline in recgnitin appeared t be mre directly related t the time between the nsets f the tw patterns, SOA, than it was t lsi (Craig, 1982a). A secnd measure f tempral integratin tk advantage f an earlier bservatin that a pattern masker prduces mre interference in letter recgnitin than des an energy masker (Craig, 1982b). A pattern masker is created by turning n randmly selected elements f letters t prduce a letter-like pattern. An energy masker is created by turning n all the elements in the same area f the array as that ccupied by the target letter. Figure 1 shws a representatin f these patterns. We presented a target pattern immediately fllwed by an energy masker, 421 Cpyright 1983 Psychnmic Sciety, Inc.

2 422 CRAIG TARGET ENERGY nitin, there is the further suggestin that pattern nset is imprtant fr the generatin f spatial cnturs. This paper will deal with the imprtance f nset in determining tempral interactin between successively presented vibrtactile patterns in a discriminatin paradigm (Experiment 1) and in a masking paradigm (Experiment 2). This paper will als cncern itself with discriminatin perfrmance when patterns are delivered t mre than ne site (Experiment 3) and examine the rle masking plays in discriminatin. EXPERIMENT 1 PATTERN COMPLEMENT I~T'ME -, Figure 1. A representatin f a target letter, an energy masker, and a pattern masker and its cmplement. The time between the nset f the pattern masker and its cmplement culd be varied (Craig, 1982a). a backward masking paradigm, and then measured the increase in the amunt f masking as we changed the energy masker int tw pattern maskers. This was accmplished by gradually increasing the tempral interval between the pattern masker and its cmplement. As the tempral interval increased, the pattern masker "emerged," and the result was mre interference in letter recgnitin. With the tw duratins f maskers tested, 13and 26 msec, it was fund that a single functin culd be fitted t the data if the results were pltted as a functin f SOA (Craig, 1982a). In additin t these measurements f tempral integratin, studies f tactile apparent mtin have demnstrated the imprtance f the time between nsets (Sherrick, 1968b; Sherrick & Rgers, 1966). Using pairs f stimuli with different duratins, the critical variable in determining the emergence f gd apparent mtin was SOA. These measures f tactile perceptin suggest that, in terms f interactin between tw stimuli n the skin, the apprpriate tempral dimensin is SOA, and based n thse measures invlving pattern recg- Several studies have examined perceptin f tactile spatial patterns using a discriminatin task (Geldard & Sherrick, 1965, Nte 1; Gilsn, 1968). These studies have generally nt manipulated the time between stimuli and therefre d nt prvide evidence n the rle f tempral separatin. Althugh nt cncerned directly with the rle f SOA and lsi, ne f these studies, invlving patterns generated by stimulating different fingertips, fund n difference in discriminatin when perfrmance was measured with 3 msec SOAs as cmpared with I,OOO-msec SOAs (Geldard & Sherrick, Nte 1). In Experiment 1, subjects were required t discriminate pairs f patterns presented t their fingertips. T assess the rle f SOA, the tempral interval between the patterns was varied, as was the duratin f the patterns. Given current views f perceptual prcessing, we expected that subjects wuld stre a cpy f the first pattern presented, that the image f this pattern wuld begin t fade, either immediately after the ffset f the pattern r immediately after nset f the pattern, and that subjects wuld cmpare the secnd pattern t this mre r less imperfect cpy. Thus, ne might expect that discriminatin perfrmance wuld decline as the time between patterns increased. On the ther hand, previus measures had shwn that tw patterns presented in tempral prximity may be integrated and frm a single pattern (Craig, 1982a), making discriminatin difficult. Als, measures f tempral masking, which are als thught t be dependent in part upn integratin, shw that perfrmance in identifying target patterns imprves as the time between target and mask increases. These cnsideratins lead t the ppsite predictin, that discriminatin perfrmance wuld imprve with increasing tempral separatin. Methd Subjects. The subjects were cllege students wh were paid emplyees f the labratry. They had passed an initial screening test and received several days f testing withut feedback prir t frmal data cllectin. Three wmen and ne man served as subjects. Apparatus. The apparatus cnsisted f a tactile display similar t that used in the Optacn, a reading aid fr the blind, interfaced

3 ONSET OF VIBROTACTILE PATTERNS 423 with a POP-1l/34 cmputer. The display is cmpsed f 144 elements, which can be made t vibrate at 23 Hz. The elements are arranged in a rectangular matrix, 6 clumns x 24 rws, measuring 1.1 x 2.7 em, The array fits against the distal prtin f the subject's fingerpad. A mre cmplete descriptin f the apparatus may be fund in Craig (198). Stimuli. The stimuli were 1 patterns that previus measurements had determined were relatively easy t identify. Figure 2 shws representatins f the patterns. When presented singly, the subjects were able t identify the patterns with better than 957 accuracy. The reasn fr using these easily identified patterns was that sme preliminary wrk with mre difficult patterns (uppercase letters) had prduced relatively pr discriminatin perfrmance. Prcedure. The subject was seated with the left index finger resting n the tactile display. The subject wre earphnes thrugh which lw-pass-filtered nise was presented t reduce auditry cues prduced by the tactile array. T'Q begin a trial, the subject pressed a key and received a tactile cue stimulus (the tpmst left and right pins were turned n fr 26 msec, a cue that als served t keep the subject's finger in the crrect psitin n the display) fllwed 1 sec later by the first f the pair f stimuli separated by sme tempral interval. On half the trials, the tw patterns were the same, and n half, they were different; the subject was required t indicate whether the tw patterns felt the same r different. The subject respnded by pressing either "S" r "" n the keybard. Trial-by-trial feedback was prvided by means f a CRT display in frnt f the subject. When the subject was crrect, the wrd "crrect" appeared; when the subject was incrrect, an "S" r a "" appeared indicating whether the pair had been the same r different. Tw cnditins were examined. In ne cnditin, each member f the pair f patterns was presented fr 4 msec. In a secnd cnditin, each was presented fr 52 msec. The same series f SOAs was tested in bth cnditins: 9, 17, 26, 56, 1, and 3 msec. Frm these SOAs and pattern duratins, ne can calculate the crrespnding ISIs. With the 4-msec patterns, these SOAs prduced ISIs f 5, 13, 22, 52, 96, and 296 rnsec. With the 52-msec patterns, these SOAs led t sme negative ISIs, that is, the nset f the secnd pattern ccurring befre the ffset f the first pattern. The ISIs were -43, -35, -26, 4, 48, and 248 msec. Prir t the cllectin f the discriminatin data, the 52-msec patterns were adjusted t be equal, in intensity, t the 4-msec patterns. This required setting the tactile array at a higher vltage level when the 4-msec patternswere presented than when the 52-msec patterns were presented. ~ ~ g g a a a a a a a a a a gggggg gggggg. a a a gg::gg a a a gg::gg gg~~gg. gggggg g g gg g g g g a a a a a g g g g g g : : :.. : : : a a a a a a a a a FIaure 1. A represeutatiu f tbe 1 patterns used lu Experiments1 and 1. All SOAs were tested within a 4O-trial blck f trials, that is, the sessins were nt blcked accrding t SOA. A testing sessin cnsisted f nine blcks f trials during which each SOA was presented fr 6 trials. The subjects were tested fr 2 cnsecutive days in the 4-msec cnditin, alternating with 2 days f testing in the 52-msec cnditin. Results and Discussin Each f the fur subjects was tested fr 24 trials at each f the six SOAs. The subjects shwed a similar pattern f respnding as a functin f SOA s their data were cmbined. The standard errrs f the means n the btained "same" and the btained "different" respnses were apprximately 3OT, with ne cnditin reaching 5%. The subjects shwed a large respnse bias in that they respnded "same" mre ften than they did "different," a bias that was particularly evident at the briefer SOAs. T avid sme f the prblems in cmparing results acrss SOAs when respnse bias was changing, the data were analyzed in terms f signal detectin thery measures. Specifically, trials n which subjects respnded "different" when the patterns were different were cnsidered "hits," and trials n which subjects respnded "different" when the patterns were the same were cnsidered "false alarms." Using the hit and false-alarm rates thus generated, d' was cmputed fr each SOA (Swets, 1964). Frm each d', a P(C) max was cmputed, which is the prprtin f crrect respnses the subject wuld have achieved had the subject shwn n respnse bias (McFadden, 197). It is this value that is pltted in Figure 3 as a functin fsoa. The figure shws the best-fitting straight line with a slpe f 12.7 and a Pearsn r f.96. The main questin Experiment 1 was meant t answer was whether SOA r lsi r sme ther measure best characterizes any changes that might be seen in discriminatin perfrmance. The fact that a single functin can be fitted (Figure 3) t the tw different duratin cnditins and accunt fr 92% f the variance (r 1 =.96 1 =.92) suggests SOA is a gd chice. T see hw well lsi might accunt fr the changes, the data frm Figure 3 were repltted in Figure 4. There is clear separatin between the tw cnditins, and a single functin fitted t the data pltted in this way accunts fr 76% f the variance. Anther measure f tempral separatin, the amunt f tempral verlap, likewise des a prer jb than SOA in accunting fr the changes in perfrmance. The data shw that discriminatin perfrmance des, as predicted, change as a functin f the tempral separatin between the tw patterns. The fact that perfrmance imprves as the time between the tw patterns increases might be cnsidered surprising, particularly when the sequence f patterns the subject was receiving is cnsidered. Fr example, with the 52-msec patterns at the briefer SOAs, the subject was presented either a single pattern cntin-

4 424 CRAIG Ul., 9-4msec 8-52 msec Ul ~., a: t)., U +' c., Q; - 7 Figure 3. Percent crrect respnses, P(C) max, in a discriminatin task as a functin f SOA. Btb patterns presented t the Index finler. Patterns were presented fr eltber 4 r 52 msec, A nne f best fit bas been dnwn tbrulb tbe data pints. uing n fr sme perid f time (a "same" trial) r tw patterns cming n sequentially and cntinuing tgether fr sme perid f time befre then ging ff sequentially (a "different" trial). Why shuld discriminatin be s difficult at the briefer SOAs and imprve with lnger SOAs? The fact that recgnitin perfrmance is gd with these patterns and discriminatin is at first pr but imprves with tempral separatin suggests that masking may be respnsible fr the discriminatin functins. The time curse fr the imprvement f discriminatin is cnsistent with previus measurements f tempral masking functins with vibrtactile patterns (Craig, 1976, 1978, 198), and suggests that had the task in this experiment been changed slightly t make it a studyf masking, similar results wuld have been btained. Experiment 2 deals directly with the questin f SOA and tempral masking functins, and Experiment 3 deals with the pssible rle fmasking in pattern discriminatin. maskers wuld be 47 and 4 msec. In the backwardmasking cnditin, the maskers were presented fr the same duratin (26 msec) and the targets fr either 9 r 52 msec, again permitting a separatin f lsi frm SOA. The results frm Experiment 1 and the attempt t use masking t explain the discriminatin functins als imply that measures f tempral masking wuld shw SOA t predict perfrmance better than ther indices f tempral separatin. Methd Subjects. The subjects were selected as in Experiment I. Tw wmen and tw men were tested in the frward-masking cnditin, and three wmen and ne man were tested in the backwardmasking cnditin. Tw subjects served in bth cnditins. Nne f the subjects tested in Experiment 2 had been tested in Experiment 1. StimuU. The target patterns were the 26 uppercase letters f the alphabet. T avid pssible ceiling effects, letters were used as targets, rather than the 1 patterns frm Experiment 1. Previus measures had determined that letters were mre difficult t identify than the 1 patterns. The letters were generated n the tp 18 rws and, with the exceptin f letters I and J, all six clumns f the display. The masker was generated by turning n randmly selected segments f letters and generating what has been termed a "pattern masker." An example f ne f the pattern maskers is shwn in Figure I. These pattern maskers share features with letters and generally prduce mre interference in letter recgnitin than des an energy masker (Craig, 1982b). There were 26different pattern maskers. Prcedure. The apparatus used in Experiment I was used in Experiment 2. Masking was determined by measuring the percent crrect recgnitin f the target letters in the absence f and in the presence f a pattern masker. In the backward-masking cnditin, the masker was always presented fr 26 msec, whereas the target was presented fr either 9 r 52 msec. In this cnditin, the subjects matched the intensity f a 52-msec target t that f a 9-msec target. The targets were matched in rder t prduce equal levels f perfrmance in the absence f masking stimuli. Using the matching settings, preliminary measurements revealed an inequality in perfrmance in the absence f a masker. The intensity f the 52-msectarget was raised slightly, and the 9-msec target was lwered slightly, an adjustment that resulted in very sirnilar levels f perfrmance in the absence f a masker. Additinal measurements wuld have been required t see if the riginal intensity match was slightly in errr r if patterns unequal in intensity actually yield equal levels f perfrmance. Other results suggestthat the frmer is likelyt have been the case (Craig, 198). 9 4m#c EXPERIMENT 2 Experiment 2 measured interactin between tw sequentially presented patterns in a masking paradigm. Bth frward and backward masking were examined, and, fllwing the lgic f Experiment 1, several duratins f patterns were tested t separate the rles f SOA and lsi. In the frward-masking cnditin, the target patterns were presented fr the same duratin (26 msec), whereas the frward maskers were presented fr either 9 r 52 msec. Thus, fr an SOA f 56 msec, the ISIs with the 9- and 52-msec J e 7 lsi in MSEC Figure 4. Tbe data frm Filure 3 repltted as a functin f lsi.

5 ONSET OF VIBROTACTILE PATTERNS 425 With the 9-msec targets, five SOAs were tested, 13,26,56, 14, and 3 msec, The crrespnding ISIs were 4, 17, 47, 95, and 291 msec. With the 52-msec targets, the five SOAs tested were 56,69, 14, 147, and 347 msec, and the crrespnding ISIs were 4, 17, 52, 95, and 295 msec. The trials were presented in 3-trial blcks with a different SOA tested in each blck. Perfrmance was als measured in the absence f any masking stimulus fr bth the 9- and 52-msec targets. During the first half f the sessin, ne f the tw masking stimulus duratins wuld be used, with the secnd half f the sessin devted t the ther duratin. The rder f testing f the masker duratins was alternated daily. In the frward-masking cnditins, the subjects were presented a masker and then, after sme tempral interval, the target letter. The target was always presented fr 26 msec. The maskers were presented fr either 9 r 52 msec. Prir t the cllectin f the masking data, the subjects matched the intensity f ne f the 52-msec masking patterns t the intensity f the same pattern presented fr 9 msec, and these intensity values were maintained thrughut the experiment. Five SOAs were tested with the 9-msec masker, 13, 29, 56, ISO, and 5 msec (crrespnding t ISIs f 4,2,47, 141, and 491 msec), and five SOAs were tested with the 52-msec masker, 56, 72, 13, 25, and 5 msec (crrespnding t ISIs f 4,2,78, 198, and 448 msec). The targets and maskers did nt verlap ne anther temprally. There was als a singleletter cnditin in which n masking stimulus was presented. Trials were presented in 3-trial blcks with a different SOA tested in each blck. The 11 blcks, five SOAs with each f the tw maskers plus a single-letter cnditin, were presented in randm rder during a single testing sessin. Trial-by-trial feedback was prvided in bth the frward- and backward-masking cnditins. Results and Discussin Recgnitin perfrmance fr the 9- and 52-msec patterns in the absence f masking stimuli was very similar, 641 and 661 crrect and, as expected, prer than that seen with the 1 patterns frm Experiment 1. The amunt f masking was calculated by subtracting the percent crrect target recgnitin in the presence f the masking stimulus frm the percent crrect target recgnitin with n masking stimulus present. The standard errrs f the means fr bth the frward and backward masking cnditins were apprximately 21. The amunt f masking in the backward-masking cnditin pltted as a functin f SOA is shwn in Figure 5. Each pint represents the mean f 1,2 trials, 3 frm each f fur subjects. A line f best fit has been drawn thrugh the data pints with a slpef and an r =-.98. Repltting the data in terms f lsi again shws that the functins separate when presented in this way. Cnsistent with the apparent difference between the tw functins shwn in Figure 6 is the fact that the r value fr a line f best fit fr these data is lwer, -.85, than it is fr the data pltted as a functin f SOA. A similar picture emerges frm the frward-masking cnditins, as shwn in Figures 7 and 8. Each pint represents 3 trials frm each f fur subjects. In the absence f any masking patterns, recgnitin perfrmance fr these 26-msec target patterns was 691 crrect. The functin drawn thrugh the frwardmasking data in Figure 7 has a slpe, -21.6, shallwer than that seen with a backward masker and an r value 6 5 ' ' -9msflc - 52 msec 2-9msec 52 msec Figure 5. Amunt f masking (decune in percentage crrect frm cnditin In wblcb n masking stimulus was presented) as a functin f tbe time between tbe uset f tbe target and tbe uset f a backward masker. Targets f tw different duntius, 9 and 52 msec, were presented. The Unef best fit bas been dnwn tbrugb tbe data pints. f Pltting the results as a functin f lsi again shws the separatin f the tw functins and, fr a line f best fit, a reduced r value f -.9. Cmparing the tempral curse f the backwardand frward-masking functins with the discriminatin functin (Figure 3) suggests that the imprvement in discriminatin reflects a recvery frm masking. Cnsistent with that assertin is the fact that changes in bth discriminatin and masking are directly related t SOA rather than t lsi. The extent t which masking can predict discriminatin perfrmance in ther situatins and the implicatins that discriminatin is dependent upn recgnitin will be cnsidered in Experiment 3. The rle that nset apparently plays in a masking paradigm may als reflect n a distinctin referred t earlier, the difference between energy and pattern lsi in MSEC 81 Figure 6. Data frm Figure 5 repltted as a functiu f lsi

6 426 CRAIG ~ >.S u il C 3 ~ ~ - ~ 9msec msec E.S 2 5 & E c.. ~ U Figure 7. Amunt f masking (decline in percentage crrect frm cnditin in whicb n masking stimulus was presented) as a functin f tbe time Mtween tbe nset f a frward masker and tbe nset f tbe target. Maskers f tw different duratins, 9 and 52 msec, were presented. Tbe line f best fit bas been drawn thrugb tbe data pints. 5-9msec O 52 msec s 1 Figure 8. Data frm Figure 7 repltted as a functin f lsi.,,!,,!!! lsi in MSEC 2 I 2 4, " 4 masking. It has been suggested that, when a masker and target patterns are presented in clse tempral prximity, perceptin f the target is interfered with bth because the signal-t-nise rati is reduced by the energy cmpnent f the masker and because the features f the target are distrted in sme way by the features in the masker. The present study used pattern maskers exclusively, maskers that, while they are thught t be effective primarily because they alter the features f the target, are thught t affect the signal-t-nise rati as well. It might be expected that the energy cmpnent f the masker wuld be mre sensitive t changes in lsi (r perhaps the time between the middle f the target and the middle f the masker) rather than t SOA. Future studies might include using energy maskers t see if this alters the rle f SOA in tempral masking functins. The present masking results als affect ne f the cnclusins f an earlier study n pattern and energy maskers (Craig, 1982b). In the earlier study, the intensity f briefmaskers was increased by lengthening their duratin. With backward maskers, changing duratin did nt affect SOA-bth SOA and lsi remained cnstant-but with frward maskers, as duratin increased, SOA increased. One f the cnclusins f the earlier study was that, unlike the results frm studies f visual masking, increasing masker intensity has a greater effect n backward masking than n frward masking. What the present results suggest is that increasing the SOA as the duratin f the frward masker was increased tended t reduce the amunt f masking. Over the range f masker duratins tested in the earlier study, 13 t 52 msec, and the crrespnding range f SOAs, 17 t 56 msec, ne culd expect at least a 17 decline in perfrmance (Figure 7), mre than enugh t ffset the relative change in frward and backward masking prduced by increasing the duratin f the maskers, abut 77, and thus prducing results mre in line with visual studies. When the targets and maskers are equated fr intensity, the amplitude f vibratin at the nset f the lnger patterns is less than the amplitude at the nset f the briefer patterns; yet, the amunt f masking is the same. With the apparent imprtance f SOA, it may seem t be a paradx that the amplitude f vibratin at the nset f a pattern is nt critical, as lng as the verall intensity f the tw patterns is equal. The result may nt be paradxical, in light f the fact that the duratin f these patterns, less than apprximately 5 msec, is within the limits f tempral integratin fr vibrtactile patterns (Craig, 1982a), and that the distributin f energy acrss tempral intervals as brief as these may nt be critical. Accrding t this view, the nset f the pattern is a sharp discntinuity t which the rest f the pattern is smehw "funneled," in the manner that vn Bekesy has hypthesized (vn Bekesy, 196). EXPERIMENT 3 The results frm Experiments 1 and 2 suggested that masking might accunt fr the btained functins. At brief tempral separatins, masking between tw patterns wuld be cnsiderable, declining as the separatin increases. If masking were respnsible fr the results, then testing under cnditins that minimize masking shuld imprve discriminatin perfrmance and change the frm f the functin relating discriminatin t tempral separatin, perhaps seeing a decline in perfrmance with SOA. Experiment 3 measured discriminatin perfrmance with ne fthe pair f patterns presented t the index finger and the ther t the middle finger, a separatin that, based n previus measures f detectin masking (Sherrick, 1964), wuld be expected t reduce the amunt f masking between the tw patterns. Methd Subjects. The same subjects tested in Experiment 1 were tested in Experiment 3.

7 ONSET OF VIBROTACTILE PATTERNS 427 Prcedure. Prir t the measurements f discriminatin, it was necessary t measure the amunt f masking at a single site and between tw sites t be certain that masking between tw sites f stimulatin was less than masking at a single site. The target and masking patterns were selected at randm frm amng the 1 patterns used in Experiment 1. The target patterns were presented fr 26 msec t the subject's left index fingertip. The masking pattern was presented fr 26 msec t either the index fingertip, using the same tactile array n which the targets were presented, r t the left, middle fingertip, using a secnd tactile array. In the separate-site cnditin, fur SOAs were tested. The nset f the masker preceded the nset f the target by S6 msec, was simultaneus with it, r fllwed it by either S6 r 14 msec. In the same site cnditin, three SOAs were tested. The nset f the masker either preceded the target's nset by S6 msec r fllwed it by S6 r 14msec. Trials were gruped in blcks f 4, with each blck testing a particular SOA, including ne blck per sessinin which n masker was used. A daily testing sessin cnsisted f eight 4O-trialblcks, with the cnditins presented in randm rder. Trial-by-trial feedback was prvided. Fr the discriminatin measurements, the nly difference between the prcedure used in Experiment 1 and the prcedure used in Experiment 3 was the presentatin f ne f the tw patterns t the middle fingertip. The first pattern was always presented t the middle finger, fllwed by the secnd pattern t the index fingertip. The same pattern duratins, 4 and S2 msee, and the same SOAs were tested. Results and Discussin The masking data are presented in Figure 9. Each pint represents the mean f 1,12 trials, 28 trials u Q) 6.~8 4 ~ ~c ::!~ ' If 3 c. ::::l Q) Cl «t'll s: U 5 E e Same Site - Separate Site Frward Masking -5 5 Backward Masking Figure 9. Amunt f masking (decline In percentage eerreet wben cmpared witb a cnditin In wblcb n mukinl stimulus was presented) as a fnnctln f tbe SOA between tbe taraet and the muker. In tbe same-site cnditin, btb tbe taraet and masker were presented t tbe Index finler. In tbe separate-slte cnditin, tbe muker was presented t tbe middle fidler, and the target t tbe Indexfinger. frm each f fur subjects. The standard errrs f the means fr thesedata ranged frm 1% t 3%. Perfrmance in the absence f a masking stimulus was 981 crrect, with the amunt f masking indicated by the decline in perfrmance frm this level. As a number f previus studies have shwn, there is mre backward than frward masking (Craig, 1976, 1978, 198, 1982b). Als, there des appear t be slight interference in recgnizing a pattern presented t the index finger in the presence f a secnd pattern presented t the middle finger. Sme preliminary measurements f the amunt f masking resulting frm presentatin f the target t the middle finger and the masker t the index finger shwedmre masking than the reverse situatin; hwever, separate-site masking was still cnsiderably less than same-site masking. Finding mre same-site than separate-site masking is nt surprising in light f previusdetectinmasking results (Sherrick, 1964), and leads t the predictin that if masking were respnsible fr the relatively pr perfrmance in Experiment 1 and fr the imprvement in discriminatin with SOA, then verall the tw-finger discriminatin perfrmance shuld be cnsiderably better and the shape f the functin relating SOA t perfrmance might be altered frm that f same site perfrmance. The discriminatin data are shwn in Figures 1 and 11. The standard errrs f the means averaged less than 31, with the highest value being 41. The btained data were transfrmed as in Experiment 1, calculating hit and false-alarm rates and d', and frm d' cmputing P(C) max, which is pltted as a functin fsoa. The results are cmplicated in that perfrmance is nt a mntnic functin f SOA; hwever, when the dataare plttedas a functin f SOA, the 4- and 52-msec functins are similar. Bth sets f data shw a slight decline in perfrmance, abut 61 t 7/, frm the briefest SOAs ut t the middle SOA values. This slight decline in perfrmance may be mre significant when the results are cmpared with thse in Figure 3, where, ver the same range f SOAs, perfrmance imprves by 7% r 8%. Thus, the ttal change in perfrmance at the briefer SOAs prduced by mving t a separate site is mre than may at first be apparent. The 52-msec data d shw a definite minimum at an SOA f 56 msec, whereas the 4-msec data shw a brader trugh in perfrmance, extending frm 26 t 1msec. Presenting the data as in Figure 11 shws hw the tw functins separate when pltted as a functin f lsi. Thus, ne cnclusin frm Experiment 3 is that interactin between successively presented patterns appears t be related t the SOA, even when the patterns are presentedt separate sites n the skin. Experiment 3 was als undertaken t clarify why perfrmance in discriminating between these patterns is pr and what rle masking plays. A cmparisn f Figures 1 and 3 shws that fr mst SOAs, per-.

8 428 CRAIG msec - 52 msec 2 4 Figure 1. Perceut crrect respses, P(C) max, in a discriminatin task, as a functin f SOA. One member f the pair f patterns was presented t the middle finger, and the ther t the index finger. Patterns were presented fr either 4 r 52 msec. frmance was prer in the separate-site cnditin than in the same-site cnditin. It is nly at the briefer SOAs that perfrmance is better in the separate-site cnditin. It is tempting t attribute the small advantage in the separate-site cnditin at these briefsoas t the fact that the tw patterns mask each ther very little. Yet, even if masking were cntributing t the differences between the tw cnditins at briefer SOAs, there are prblems with interpreting all the results, particularly the separate-site results, in terms f masking. The large difference in the amunt f masking between the same- and separate-site cnditins wuld lead ne t predict a large advantage in discriminatin fr the separate-site cnditin, ne that is simply nt seen. Perhaps it is nt surprising that discriminatin perfrmance is pr in the separate-site cnditin, given that subjects may have difficulties in attending t tw sites at the same, r very nearly the same, time. If the skin is subject t sme attentinal limitatin when presented cmplex patterns, then the dividing r the switching f attentin required in the separatesite cnditin wuld be expected t reduce perfrmance. At lnger SOAs, the subject culd presumably switch attentin frm the first site t the secnd site and perfrmance shuld imprve, as indeed it des beynd 1msec. Several studies directly cncerned with attentin and vibrtactile stimuli have measured the detectin fweak vibrtactile stimuli presented t ne site, tw sites, r three sites and have prduced cnflicting results. One study (Franzen, Markwitz, & Swets, 197) fund n evidence that subjects culd attend t mre than ne channel f stimulatin at a time, whereas anther study (Craig, 1968) fund that subjects culd detect tw stimuli presented simultaneusly t tw different fingers mre readily than they culd detect a single stimulus. A third study (Shiffrin, Craig, & Chen, 1973) fund evidence supprting this latter result and prpsed an unlimited-capacity, nnattentin mdel fr detectin f weak vibrtactile stimuli. These results and mdels were cncerned with detectin f vibratry stimuli but did nt deal directly with hw patterned stimuli might be cmbined frm several lci. In several studies, the subjects' task was t discriminate pairs f patterns that stimulated a number f sitesn the skin simultaneusly (Geldard & Sherrick, 1965; Gilsn, 1968). The fact that subjects were able t discriminate between such patterns shws that they used the infrmatin resulting frm stimulatin f spatially distinct lci. Hwever, using tw Optacn displays, Hill (1974) fund n imprvement in reading when the material was scanned acrss tw fingers rather than a single finger, even thugh the tw-finger display prvided a wider field f view. Althugh such a result might suggest that subjects have difficulty in cmbining infrmatin frm spatial patterns presented t tw fingers, Hill als fund n imprvement when the display was widened t permit tw letters t be presented t a single finger. It might be expected, with regard t this latter result, that subjects culd cmbine patterns presented t a single finger. Their failure t d s suggests that. there may have been cnsiderable interference due t masking r that reading, because it invlves cnsiderably mre than simply recgnizing patterns, may nt be a gd measure f the skin's ability t cmbine spatial patterns. A simpler task, such as discriminatin, may prvide a mre accurate indicatin f this ability. Befre the pssible rle f attentin and prcesses related t masking can be cnsidered in greater detail, tw additinal items need t be discussed. One is the similarity f the separate-site functin t earlier studies, and the secnd is a clser analysis f the subjects' respnses in Experiments 1 and 3. The shape f the functin relating perfrmance t SOA is reminiscent f the functins btained in an earlier study in which recgnitin perfrmance in a masking paradigm was measured as a functin f SOA, when the target and masking stimuli were presented t different areas n the same fingertip I 8Il j 7 ~ 8Il 5. "mhc 52muc 25 1!i lsi in MSEC Figure 11. The data frm Figure 1 repltted as a functin f lsi. 25 3

9 ONSET OF VIBROTACTILE PATTERNS 429 (Weisenberger & Craig, 1982). In this metacntrastlike paradigm, the masking stimuli flanked the target stimuli and prduced maximum interference, nt when the target and masker were presented simultaneusly, but when the nset f the masker fllwed the nset f the target by 25 t 5 msec. The change, as a functin f SOA, in the amunt f separate-site masking shwn in Figure 9 is t small t assert that a metacntrast functin is respnsible fr the separatesite discriminatin functin. It is pssible that the masking functin generated with a target n the middle finger and a masker n the index finger might shw a greater change in masking as a functin f SOA; hwever, as already nted, the verall amunt f masking in the separate-site cnditin is t small t make a direct appeal t masking per se cnvincing. Still, the similarity f metacntrast and discriminatin functins suggests that they might be related t mechanisms such as integratin and interruptin, which have been used t explain these functins, but that the ways in which these mechanisms perate may differ in the separate-site and the same-site cnditins. Relevant t the questin f integratin f the tw patterns and t the cnclusin that masking might help explain the same-site results but nt the separatesite results is a clser analysis f the pattern f subjects' respnses in bth Experiment 1 and Experiment 3. As nted befre, there is a cnsiderable respnse bias in Experiment 1, and the bias changed as a functin f SOA. Fr this change t be seen, the subjects' respnses frm the 4- and 52-msec cnditins in Experiment 1 were cmbined and are presented in Figure 12, in which the percentage f crrect "same" respnses and crrect "different" respnses as well as the percentage f the ttal respnses that were "same" and that were "different" are pltted as a functin f SOA. The large bias twards respnding "same" at briefer SOAs shwn in Figure 12 may reflect tempral integratin f the tw patterns. That is, the tw patterns separated by a brief tempral interval are integrated int a single pattern that the subject reprts as "same." Previus results have shwn that tempral integratin may be fund ver intervals f up t apprximately 5 msec (Craig, 1982a), and, as nted befre, tempral integratin is dependent upn the SOA f the patterns being integrated. As the tempral separatin increases, the subject can differentiate the tw patterns mre readily, and at lnger SOAs, crrect respnses n bth "same" and "different" trials may rise as the subject can perceive bth members fthe pair. The same srt f bias twards respnding "same" exists with the separate-site data, as shwn in Figure 13. Figure 13 displays the data fr the separatesite functins in the same way as Figure 12 displays the data fr the same-site functins. The bias at the briefer duratins is as large as that seen with the same _ Percent COffeet ~SI~Rs [}--(J Difl lis Percent f Tts! SIm. R$.-"'Difl lis _A FJllIre 12. Results frm Experiment 1 sbwing the pereent erreet achieved n "same" trials and n "different" trlais u wed u the ttal pereentage f tbe respnses that were "same" and tbat were "different" u a functin f SOA. The data frm tbe 4- and 52-_cnditins bave been cmbined. site data, a result that again pses a prblem fr a masking interpretatin f the separate-site cnditin. It has been suggested that tempral masking functins fr vibrtactile patterns result frm tw prcesses, tempral integratin and interruptin (Craig, 198, 1982a, 1982b). If tempral integratin is a majr cmpnent f tempral-masking functins, then ne might expect t see little r n integratin under cnditins in which there was little masking (Kirman, 1973). Hwever, the separate-site data suggest the pssibility that integratin and masking need nt be as clsely related as riginally hypthesized. At brief SOAs, there is a large bias twards respnding "same," which, fllwing the abve reasning fr the same-site data, suggests cnsiderable tempral integratin, but under cnditins in which little masking is seen (Figure 9). In attempting t frmulate a general framewrk t accunt fr the same- and separate-site discriminatin perfrmance, it is necessary t accmmdate the bservatins n integratin, the masking results (Figure 9) and metacntrast effects, and pssible attentinal effects. It may be that the prcesses f integratin and interruptin perate in smewhat different ways, depending upn whether tw patterns are delivered t the same (r nearly the same) site r t different sites. In the same-site masking paradigm, integratin and interruptin may be unavidable because the spatial-tempral prximity f the tw patterns results in the tw affecting ne anther. In the dual-site masking paradigm, the subject may be able t attend t ne finger and have the prcessing f the pattern at that finger nly slightly perturbed by

10 43 CRAIG ~c: c. '" Q) a: 9 BO 7 3Ql..L..J..-'--_--I.._-'--...J-.J...l...J...L..l...J... -'----I..---J._ 1 Percent Crrect O----OSameRs Diff Rs 2 4 Percent f...-~ Same Rs.-... DiffRs Figure 13. Results frm the discriminatin measurements in Experiment 3 pltted as in Figure 12. a pattern presented t the adjacent finger. When required t, as they are in the discriminatin task, subjects can attend t bth fingers with sme lss f infrmatin abut the pattern presented t each finger. At briefer times, the subjects may be able t frm a single impressin, that is, they can integrate the patterns frm the tw fmgers. At slightlylnger SOAs, thse that exceed the tactile tempral integratin time (Craig, 1982a), a single impressin f the tw patterns can n lnger be frmed and attentin must be switched frm ne finger t the ther. In that sense, the subjects' prcessing f the first pattern is interrupted when attentin is switched t the secnd pattern. As SOA increases, additinal time is prvided t switch attentin, and perfrmance imprves. Mre wrk will clearly be necessary t determine hw useful this distinctin is between "autmatic" prcessing f tw patterns at a single site and the perceptin f patterns presented t tw sites, which is related much mre t attentinal prcesses. There is an implicatin in this framewrk that discriminatin perfrmance is based n recgnitin; that is, if cnditins are such that subjects can recgnize the tw patterns, then they can discriminate between them. Can recgnitin perfrmance be used t predict discriminatin? The masking results shwn in Figure 9, which used the 1 patterns frm the discriminatin task, prvide an estimate f recgnitin perfrmance when tw patterns are presented in clse tempral prximity. Frm the same-site masking data, ne can calculate that pattern recgnitin perfrmance at -54 msec is crrect (98% crrect in the absence fa masker minus the amunt f masking shwn, 22.5%) and at +54 msec is 42% (98% minus 56%). Based n these recgnitin estimates, 1 Ttal 2 4 discriminatin perfrmance at an SOA f 54 msec shuld be 66% crrect by the fllwing lgic: Subjects can recgnize bth patterns n 32% f the trials (.755 x.42), and thus culd discriminate between the tw patterns. On the remaining trials, 1-.32, r.68, subjects shuld be crrect half the time by chance,.6812, r.34. Ttal discriminatin perfrmance shuld be.32 plus.34, r 66% crrect, assuming n deficit due t prcessing tw patterns in a brief perid f time, that is, having t give tw different respnses t the tw patterns. Measured discriminatin perfrmance was better than predicted by the recgnitin data, 76% and 78% crrect (Figure 3), suggesting that additinal factrs might be perating t raise discriminatin perfrmance. At briefer SOAs, such phenmena as apparent mtin and saltatry cnductin might prvide cues as t whether tw patterns ccupied the same lcatin r nt and thus whether the tw were the same r different. At lnger SOAs such phenmena break dwn and discriminatin perfrmance culd be clser t that predicted by recgnitin. Unfrtunately, there are nt cmparable masking data n 1 patterns used in the discriminatin task at lng SOAs. Hwever, if ne uses the masking data btained with letters, Figures 5 and 7, the estimate f the masking at an SOA f 3 msec wuld be apprximately 1%, averaging acrss the frward- and backward-masking cnditins. Using this estimate f masking, discriminatin perfrmance at an SOA f 3 msec wuld be abut 89% crrect, ( )( ) + [1 ( )( »)/2. This predicted value is clse t the btained values,.85 and.87 frm Figure 3, and suggests that fr lnger SOAs subjects are nt picking up additinal cues in the discriminatin task t imprve their perfrmance. Finally, with the separate-site cnditin, the recgnitin data are likely t verestimate discriminatin perfrmance. With the small amunt f masking between separate sites, Figure 9, discriminatin shuld be cnsiderably better than we fund, suggesting, as nted befre, that limitatins prduced by such factrs as attentin are cntributing t the lwered perfrmance. GENERAL DISCUSSION One f the majr questins this study investigated was: What is the apprpriate tempral dimensin fr determining the interactin between sequential patterns? The results have been discussed primarily in terms f distinguishing between SOA and lsi; hwever, the results can als be used t examine ther measures 'f tempral separatin. Specifically, culd ther measures, such as amunt f tempral verlap, the time between ffsets f tw patterns, r the time between the middle f the first pattern t the middle f the secnd, d as well as SOA in accunting fr

11 ONSET OF VIBROTACTILE PATTERNS 431 the results? The answer appears t be n. The discriminatin results shw that pattern verlap fails t predict the changes in perfrmance. In Experiment I, fr example, the shrtest three SOAs fr the. 52-msec patterns prduced tempral verlap, whereas the same SOAs fr the 4-msec patterns did nt, yet the tw sets f data shwed the same changes in perfrmance. T separate SOA frm such measures as time between ffsets r the time frm the middle fthe first t the middle f the secnd pattern requires that the tw patterns whse interactin is being studied be f unequal duratin. The cnditins f Experiment 2 satisfy this requirement. Fr example, in the backwardmasking cnditin with an SOA f 56 msec, which prduced abut the same amunt f masking fr bth the 9- and 52-msec targets, the tempral separatin measured frm ffset f target t ffset f masker was 73 msec fr the 9-msec target and 3 msec fr the 52-msec target. Ifffset-t-ffset were the critical interval, unequal amunts f masking wuld be expected under thse cnditins. A similar picture emerges if ne thught that the cutaneus system was averaging acrss the presentatin time fr a masker and fr a target and thus the tempral center f the pattern shuld be used t measure interactins between patterns. Using the backward-masking cnditins and a 56-msec SOA, the center-t-center times were 64 and 91 msec, increasing the separatin between the 9- and 52-msec functins shwn in Figure 5. Calculating ther tempral separatins fr bth the backward- and frward-masking cnditins leads t the same cnclusin; that is, SOA is the best way t characterize the changes in perfrmance. The fact that SOA is apparently the critical variable in tempral masking functins is relevant bth t theretical issues relating t pattern recgnitin and discriminatin and t applied prblems such as reading thrugh the skin. When previus studies demnstrated that reasnably gd letter recgnitin was pssible with the Optacn at expsure times as brief as 4 msec, it was speculated that reading perfrmance might be imprved by presenting letters fr very brief times and increasing the amunt f "ff" time between letters, that is, letting SOA remain cnstant while lsi is increased, and thereby increase reading cmprehensin by reducing masking. The present results indicate that even if ISIs were increased, but SOA remained cnstant, there wuld be n imprvement in reading. Supprt fr this cnclusin cmes frm sme measurements f wrd recgnitin. Subjects were trained t recgnize three-letter wrds that were generated in 3 msec, In ne cnditin, the expsure time fr the letters was fairly brief with relatively lng ISIs;'in a secnd cnditin, the expsure time fr each letter was relatively lng with relatively brief ISIs. The resulting SOAs were fairly similar, and wrd recgnitin perfrmance was identical in the tw cnditins (Craig, 1983). The same cnclusin, that is, that the time between nsets is the critical variable, nt the "ff" time between letters, was reached in a study f visual wrd reading by Haber and Nathansn (1969). In that study, letters were presented rapidly at the same retinal lcus, very much as is the case with the tactile display. The result was that reading was a direct functin f SOA; increasing the "ff" time had n effect n perfrmance. The results f these experiments als suggest that the masking prduced by presenting patterns in clse tempral cntiguity can be vercme if the patterns are delivered t separate fingers. Thus, in palpating bjects, ne shuld expect that if attentin is directed t a single finger, little interference will be prduced by stimulatin at adjacent fingers; hwever, the discriminatin perfrmance suggests that there may be difficulties in cmbining the infrmatin frm separate fingers, at least at brief SOAs. By mving the hand ver an bject t be identified, a persn may be bth prducing the nsets necessary fr clear pattern recgnitin and keeping thse nsets temprally discrete, and thus permitting attentin t be shifted frm ne site t anther and the infrmatin frm the separate fmgers t be cmbined. That this is prbably nt the whle stry is suggested by peple's abilities t use Tadma, a system f speech receptin used by sme deaf-blind individuals. In using Tadma, the receiver places his r her hand n the face f the talker, typically with the thumb placed n the lips and the fingers spread fan-wise alng the cheek, jaw, and thrat. Deaf-blind users f this methd can understand speech, and, it is thught, must be cmbining infrmatin frm different fingers at fairly high rates (Reed, Durlach, & Braida, 1982; Reed, Durlach, Braida, & Schultz, 1982). This is a situatin in which users have little cntrl ver the rate f pattern presentatin, and the time between nsets f successive patterns is brief. Additinal wrk n multiple sites might well cncentrate bth n giving subjects much mre extensive training n multiple finger tasks and n develping patterns that might be cmbined acrss severalfmgersand that are similar t thse encuntered naturally in the cutaneus envirnment. REFERENCE NOTE 1. Geldard, F. A. &; Sherrick. C. E. Princetn cutaneus research prject (Reprt N.8). Princetn. N.J: Princetn University REFERENCES BREITMEYER, B. G., &; GANZ, L. Implicatins f sustained and transient channels fr theries f visual pattern masking. saccadic suppressin, and infrmatin prcessing. Psychlgical Review. 1976,

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