Psychophysically scaled cue differences, learning rate, and attentional strategies in a tactile discrimination task

.JOIJRNAL

OF

ESPERI.MENTAL

Psychophysically and

(‘MILD

i’sYC~tri,oGY

Scaled Attentional

283-302

13,

(1972)

Cue Differences, Learning Strategies in a Tactile

Discrimination

Rate,

Task’

AND RICHARD

Florida

C.

State

URBANO Uhwrsit~

A series of three experimrnta with nursery school children is reported. Texture cues, which wrrc psychophysically scalrd in the first study, were used in the following two expcrimrnts to invcstignte the relationship between cue differences and learning rate. It was found that, psychophysically discriminable differences between cues were not used effectively in the learning situation. Escrpt with large cue differrnces markedly above thresho!d. chance performance, ~vvas obtained. Thr appropriat,eness of psychophysically stsaling cues for learning studies was questioned. In addition to accuracy scores, threcl “attentional” exploration stratrgic>s wprc observed and thr conditional probability of a corrrct response, given rrcordcd. .%doption of an efficient an appropriate attentional rcxponsr, clsploration strategy lid to improvrd performance. Thus the use of the haptic modality providrd an oppottunity to observe the “covert” attentionnl link in thr lcnrning chain ant1 information concrrning the ability of young childrrn to atlol)t approprintc, scar?11 st r:ltt=gifJs in a tactile discrimiI1ation tnsk.

A variety of theoretical models of learning postulate that the speed of discrimination learning is directly related to the degree of physical difference between the cues of the relevant dimension (Fisher, 1970; Kendler, Basden, & Bruckner, 1970; Lovejoy, 1968; Spence, 1960; Zeaman, 1971). The models differ in their underlying theoretical mechanisms. ’ Thrsc studies werr supported by United States Public Health Grants: MH 07346, and M-1099. The prepxrat,ion of the paprr was supported by a United States Public Health Postdoctoral Fellowship, HD 46628-01, to t,he first. author, and a Career Development Award: K4-HD-46,370, to the second author. The authors wish t,o thank the staff and children of the Orchard Downs Nursery School. Champaign, IL, for their cooperation in conducting these studies. The authors thank Francine Patterson for her help in preparing stimuli and data collection. 283 @ 1972 hy i\mdemic Press. Inca.

+CI~C~C (1960) YBS~~W that th(b atttouut~ of stimulus generalization hetwcen positive and negative cue q is inversely related to the differences between cues along a physical continuum. Accordingly, the rate of discrimination learning should 1~0 directly related to physical differences between discriminanda. The theory has recently been expanded (Kendler et al., 1970) to explain visual dimensional dotninance by suggesting that rcspontling differentially to a dimension is determined by the relative psychophysical differences between the cues of each ditnension. In a recent study of both visual and tactile discrimination learning (Gliner, Pick, Pick, Qi Hales, 1969), it was reported that patterns of dttnensional dominance were influenced by physical cue differences, but discrimin:ibility and preference were not found to be equivalent. Except at stimulus values close to threshold, preference was not’ a simple function of (liscritninability. Basic data are needed concerning discritnination learning cficicncy in children using cues of known psychophysical distinctiveness. Attention theories of learning also assume that increasing the degree of 1)sychophysical difference between CUES along a dimension enhances the probability of attending to that dimension. A cue-feedback tnechattism is incorporated in recent theories (Fisher, 1970; Lovejoy, 1968; Zcaman, 1971, to account for such cue effects. The S is assumed to scan the available ditttcttaions, notice specific cue values and then make a choice on the basis of the dimension containing the greatest difference between its constituent cues. Support for this postulate has been inferred from studies which show that experitnentally acyuired differential cue habit strength (Campione & Wentworth, 1969)) individual cue preferences (Brown & Campione, in ljress) and physical difference between CUCY (Shepp k Zeaman, 1966) all affect the probability that a particular dimension will be selected. The present st’udies attempted to expand the growing body of information by delineating the relationship between learning efficiency and psychophysically scaled cue differences which were assessed individually for each S. AtMtion t.heories postulate a chain of two responses, an attentional *‘observing” response to one or more of the dimensions present on a trial, followed by an instrumental choice response of one of the stimuli (Zeaman C! House, 1963). In typical visual discritnination learning studies, the instrumental choice response is observable but t,he attentional resllonsc is covert and must be inferred from the pat&m of instrumental responses. However, if the problem is presented in the hal)tic modalit,y, tactile exploration can be assumed to be the equivaIcnt of attentional scanning. Further, it can be observed and recorded, together with the instrumental choice response. Categorization of the exploration strategies employed provides an indicator of the underlying

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dimensional observing responses and could therefore serve as a preliminary attempt to externalize the covert attentional link in the learning chain. It would also be possible to consider the conditional probability of a correct instrument,al response given an appropriate> or inappropriate attentional response. These data are of basic interest in extending our knowledge of attentional mechanisms in learning. The two major aims of the present studies were first, to provide basic data on the relation between learning when the specific cues involved have been psychophysically scaled individually for each S. The second goal was to externalize the covert attentional responses in the chain of discriminative learning and to trace the relation between an observed instrumental response and observed attentional response which preceded it. A series of three studies is reported. Experiment 1 was a psychophysical scaling study which replicated and extended the data reported by Gliner (1967) and provided the cue information used in the two subsequent tactile discrimination learning experiments. EXPERIMENT

1: PSYCHOPHYSICAL

SCALI?;G

Subjects. Twenty-six Ss were selected from the Orchard Downs Nursery School, Champaign, IL. They ranged in age from 3 years 8 months to 5 years 2 months, with a mean of 4 years 2 months. Four S; failed the pretraining session and one X refused to cooperate on the second day of testing. The remaining 21 Ss consisted of 12 girls and 9 boys. Apparalus. The portable apparatus consisted of the following main features. A front vertical panel (18 X 18 in.) shielded the stimulus display from the S’s view. The panel contained two 5 in. diameter hand holes, set 4 in. apart. These holes were lined by foam rubber squares (6 X 6 in.) which were cut into quarters. As the S inserted his hands into the holes, the flaps of the foam squares opened, but reverted to a closed square when not in use. To further obscure the S’s view of the stimuli, an 18 X 15 in. curtain covered the front screen and t’he hand holes. The S was required to place his hands under the curtain and into the holes to feel the stimuli. Behind the front screen was a sliding tray (12 X 15 in.) containing two reward apertures (2 in. diameter, 6 in. apart). The reward apertures were covered with 2$$ x 2+$ in. bases containing a small hole in the center. The stems of the stimuli were mounted in these small holes for display. The dimensions of the apparatus and t,he mounted stimuli were such that each of the paired stimuli was directly behind the center of one hand hole and approxi-

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mately the same height as the S’s hand (when it was resting comfortably in the hole). StirrLuli. The stimuli consisted of a series of copper ,squares (l$,$ X I$ in.) soldered to 2 in. screws which served as the stems. The stems were inserted into the small holes in the reward aperture bases (described above). The stems of the screws V-W covered by shrinkable electrical spaghetti. The heads of the screws were color-coded with paint so that the E could rapidly discriminate the stimuli by sight. Each copper square was covered on both sides with sandpaper of different coarseness. The tcsturcs were the same as those used by Gliner (1967). The testurc series cons&et1 of 15 grits (24-500) of 3 14 aluminum oxide sandpaper. The stamlard (STD) stimulus was covered by grit 120 paper. There was a smooth series of 7 grits (150, 180, 220, 240, 280, 320, and 500, labeled l’-7’) and a rough series of 7 grits (100, 80, 60, 50, 40, 36, and 24, labeled l-7). Therefore stimuli 1 and 1’ were the closest in t’exturc to the STD and 7 and 7’ were those most different from the STD. As the thickness of t’he paper varied with the coarscness (e.g., grit 24 was much thicker than grit 500)) the stimuli covered by the finer grits were lined with one or more layers of thin cardboarcl to minimize thickness differences. One caach of the 14 stimuli 1-7 and l’-7’ and 14 versions of the STD were available, so that’ each stimulus object was used eclually often and was subject to t’he satne degree of change in surface texture due to wear.

Pretraining. The purpose of the pretraining procedure was to accustom Ss to the apparatus and to ascertain whether they could make same/different judgments with easily diacriminable pairs. A familiarization session was arranged where the Xs were taken in pairs to the trailer and acquainted with the apparatus. They were encouraged to insert their hands in the holes, look under the curtain and to look at the E’s side of the apparatus (this was found to be necessary when it was discovered that Xs run individually initially refused to place their hands in the hand holesj . On the day following the familiarization sessions the SJ were taken individually to the trailer and presented with the pretraining problems. They were told that they were> to play the “feely game” and shown ho\% to feel the comparison stimuli with both hands. The stimulus pairs used in pretraining were the pair presenting the maximum available texture differences (7 anal 7“) and an identity pair presenting two identical STDs. On the first trial a different I:air was presented and the E guided t,he s’s hands demonstrating how to feel the stimuli by rubbing the fingertips against, the grain of the grit’. The E said “Can you feel these things?

PSYCH~PHI-SICALLT

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DIFFEREK~ES

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This one is softer (indicating the smooth object) and this one is more .scratchy (indicating the rough object). They are not the same. Rub them with your fingers like this so that you can tell they are not the dame.” (Pilot testing had revealed that Ss of this age spontaneously used soft and scratchy, but had difficulty with ruugh and smooth as verbal descriptions.) Thc~ Nz; were then prcseu,ed with the identity pair and the E guided the S’s exploration of the stimuli, instructing him to “rub the two things like this with your finger tips. Can you feel they are just the same? They feel the same when you rub thetn up and down.” The pretraining series of 20 different and 20 ident)ity pairs werca then presented in a random order or until the & made eight consecutive correct responses of “same” or “not the same.” On each trial the E said: “DO these feel the same, rub them up and down and see.” Each correct response was answered by “that’s right” and each incorrect response with “No, thes;e two things arr not the same/the same, feel them again.” Four Ss failed to meet criterion in this phase and were clropped frorn the study. Test series. Immediately following criterion on the pretraining problems, the scaling portion of the experiment was begun. A total of six test series (each modeled after Gliner, 1967) was presented, two on Day 1 and four on Day 2. Each test series consisted of 16 trials, including three blocks of four test trials, with two trials present’ing the training pairs (the identity pair, and 7 with 7’) inserted between each block of test trials. On each of the test trials, the STD and one of the twelve comparison stimuli were presented and all responses were rewarded. When the training pairs were presented, only correct responses were rewarded. The order of presentation of the comparison stimuli was determined according to a concentric series such that on each bIoc,k of four trials, the two most similar and two tnost different remaining pairs were presented (i.e., 6, l’, 1, 6’; then 2’, 5, 5’, 2; then 4, 3’, 3, 4’; each paired with the STD). Three of the six se& were presented in an ascending order (4, 3’, 3, 4’ upwards) and the remaining series were presented in descending order (6, I’, 1, 6’ down). The order of ascending or descending presentations wan counterbalanced across Ss with the restriction that the first series of 16 trials was presented in the descending order for all 8; (to replicate Gliner, 1967). There were six measures on each comparison for each child and 126 measures on each comparison for all Ss. Kesdts

cm-1 Iliscussion

The same/different judgments for all 8s on the first series of comparison1: are presented in Fig. 1, together with comparable data from Gliner’s (1967) kindergarten 8s. It can be seen that the kindergarten 5’s made

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FIG. 1. The proportion of “not same” rxponaw to each stimulus pair on Trial 1, compared wit.h the comparable data from Gliner (1967). Stimuli are placed on the abscissa according to the log of the grit number.

finer discriminations than the preschool Ss of the present experiment. This superiority could reflect a developing capacity to discriminate texture differences, a developmental progression which Gliner’s study indicates continues until at least third grade (Gliner, 1967). However, minor procedural differences between the two studies may account for the differences observed, so suggestion of a tlcvelopment in tactile discrimination between 4- and 5-year olds can only he tentative. Inspection of Figure 1 reveals that the shape of the psychophysical functions of the two studies are remarkably similar. Of particular note is the fact that both groups of Ss performed considerably better on the rough series than on the smooth series, a reliable finding which prompted the use of the rough series as cuts in the subsequent discrimination learning experiments. In Fig. 2 the same/different judgments on the first and the sixth presentations of the 14 critical comparisons are presented. It can clearly be seen that performance improved with practice. If the two extreme comparisons, where 10070 accuracy was recorded, are excluded then of the remaining comparisons showed an improvement by Trial % 6 (p < .005, sign test). This finding suggests that previous studies (Gliner, 1967; Gliner et al., 1969), which determined psychophysical

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distinctiveness of cues from a single measure per S, probably underestimated the S’s discriminative ability. On the basis of theye psychophysical data, the cue values for the subsequent discrimination learning experiments were individually selected for each child. EXPERIMEST

2

A miniature experiment design (House 41 Zeaman, 1963) was used to investigate tactile discrimination learning when the differences between the cue values of the relevant dimension were psychophysically scaled for each S. Texture was chosen as the relevant dimension throughout, as Crliner et al. (1969) have shown that this dimension is dominant over form in tactile discrimination studies with young children. Problems with eit.her a constant or variable form dimension were used to assess the ability of these young children to focus on the relevant texture dimension in the presence of variability on the irrelevant form dimension (Lehman, 1971). Both accuracy scores and exploratory hand movements were recorded, so that information would be available on both the instrumental rhoires and the attentional responses which preceded them.

Subjects. Ten of the S.G who had provided psychophysical data were randomly selected to take Ijart in Exp. 2. Two Ss failed to meet pretraining criterion so that the data from eight Ss, four boys and four girls, were included in the analyses. They ranged in age from 3 years I I months to 5 years, niih a mean of 4 years 5 months. Appclmh~. The apparatus n a.c: the samC 3; that used in the psychoIjhysical scaling study n-ith OIIP additional fcatur~~. The bases ( 2% X 21/! in.) which covered the reward aperture n were mounted on sliding tracks. As before, the stems of the stimuli were inserted into these bans. After the S had c~xploretl the two stimuli, hc was trained to push the base of the chosen object back along its (3 in.) track. This exposed the rcnar(l aperture which contained/tlid not contain a colored bead. Stiu21di. The 27 stimuli consisted of t,hrec forms in each of nine available sandpaper grits. The three forms were: a 11/1X 11/z in. square (area 2.25 sq. in.) ; an equilateral triangle with base and height of 2 in. (,area 2 sq. in.) ; a circle with a 1% in. diameter (area 2.4 sq. in.). The nine available textures were: the STD (grit 120) ; the smoothest grit (500) and the seven grits of the rough series (l-7, grits 24-100). Several identical versions of each stimulus combination were available so t,hat the E could arrange the individual stimulus sets in advance for each 8. This also ensured that the items were used equally often. As an extra precaution, the sandpaper coverings were changed on approximately the fourth day of t,he experimental sessionsso that any noticeable change in texture due to wear was avoided. The stimuli were colorc~ocledso t,hat the E could rapidly recognize them visually.

Pretraininy. In order to enter the experiment. the 5’s were required to reach criterion on two pretraining problems. The stimuli for the first ~~robleiii were the two texture s representing the greatest range of texturc difference available (7 and 7’). The S’s were randomly assigned to one of the three available forms as the constant form dimension and to tither 7 or 7’ as the correct stimulus. The technique of baiting the correct stimulus and the sliding stimulus base was demonstrated to the Ss who were told t,hat this time they were “to try and guess which iv t,he winner.” The Sj were instructed to feel both stimuli, but to choose only one. They were shown how to indicate their choice by sliding back the base of the appropriate stimulus. Training was continued for a maximum of 50 trials or until a IO/10 criterion was met. On t’he following day the Ss passing criterion on t,he

PSYCHOPHYSICALLY

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first probkm were presented with a second pretraining problem. The second problem I)rcsented the same texture cues (7 and 7’) but introtluced a variable irrelevant form dimension. Again a IO/10 passing criterion wa.; demanded within 50 trials. A correction procedure was used throughout and correct choice, q were rewarded with a colored bead. Experimentwl sessiom. The experimental sessions consisted of eight days of testing. There were eight experimental problems consisting of two Problem Types in four levels of Problem Difficulty. Texture was always the rcalcvant dimension. The Problem Type referred to the values of the irrelevant form dimension. In Constant Form problems, form was constant within and between tht. three trials of each problem, but varied across problems. In Variable Form problems, the form dimension was variable nithin and between the three trials of a problem as well as across the problems. The four levels of difficulty referred to the psychophysical values of the relevant texture dimension. The psychophysical differences between the texture cues were scaled so that the easiest problem presented the over-learned pretraining texture pair representing the greatest available difference between texture cues (7 and 7’). The second level of problem difficulty consisted of the greatest difference between the textures of the rough scale (7 and STD), a difference detected by all Ss in the psychophysical scaling series. The third and fourth levels of difficulty consisted of the STD and the smoothest rough texture values that each S had successfully discriminated from the STD 6/6 times (level 3) and 5/6 times (level 4). These last two values were determined individually for each S. Each l)roblem presented three trials (Train, Test, 1, Test 2). On the training trial, the two st’imuli were presented to the S and he was req”ired to guess which one was the winner. On the two test trials the S was told to “find the winner.” A correction procedure was employed throughout, with the correct strmulus indicated by a colored head reward and verbal reinforcement. The position of the correct stimulus was determined by :L Gellermann (1933) sc>riea. The reward value of each sOimulus was determined randomly. There were 16 three-trial problems each day consisting of 2 repetitions of each of the 8 problems. The order of presentation was counterbalanced across Problem Tyljes, Days. and Ss. Two measures per clay on 8 Problems X 8 Days X 8 1% re+ultcd in 128 data points for each trial of each problem. E recortlcd the ty])e of hand exploration enlployed on eac+h trial jjy checking one of the following categories: (a) feels only form (by t,racinq outline), Ch) feels only texture (by rubbing), (c) feels outline first, then texture, (d) feels texture first, then outline. At the end of eaclh

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The proportion of correct respon:es as a function of Problem Type and Problem Difficulty are presented in Fig. 3. It can be seen that the Variable Form Condition n as more difficult than the Constant Form Condition and that the proportion of correct responses varied with the level of problem difficulty. Preliminary analyses of the data revealed no differences across clays, so this factor was not included in the subsequent analyses. All analyses were conducted separately on both the raw proportion of correct responsej and the arcsine transformation of these data. Since the pattern of results obtained from the t,wo analyses was identical, only the raw data analyses are reported here. A 4 (Problem Difficulty) X 2 (Problem Type) X 2 (Test 1 or 2 j X 2 (Replications Within a Day 1 X 8 (Subjects) factorial analysis of variance was conducted on the prolhortion of correct responses on Tests 1 and 2 for each problem. The main effects of Problem Difficulty and Problem Type were significant (F(1,7) = 150.94, and F(3,21) = 87.04,

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respectively, p < .OOl). Post hoc t tests revealed that the difference between the Constant Form and Variable Form conditions was significant f3r difficulty levels 1, 2, and 3, but was not significant for dificulty level 4. This factor, which resulted in a significant interaction of Problem Tyl:c X Problem Difficulty (F(3,21) = 4.44, p < .025), was due to the fact that performance was approximately at chance for the most difficult texture problems (difficulty 4). A second analysis of variancc nar conducted, identical to the first, but excluding the problems of level 4 difficulty. The exclusion of these problems did not significantly alter the pattern of main effects reported in the original analysis of variance, but the interaction of Problem Type X Problem Difficulty disappeared. In Fig. 3 it can be seen that two points (difficulty 3 and 4 of the Variable Form condition) fell below chance level of 50% correct, with a value of 47% and 4670, respectively. A test of the standard error of a proportion revealed that these points were not significantly below chance. The three-way interaction of Problem Type X Problem Difficulty X Test 1 or 2 approached significance in the main analyses (F(3,21) = 2.44, p < .lO). (Although this interaction did not reach an acceptable level of significance, the trend is important and will be discussed in a subsequent section.) This inl-eraction is illustrated in Fig. 4, where it can be seen that there is no improvement over test trials 1 and 2 for the Variable Form condition or for difficulty levels 3 and 4 of the Con-

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btant Form condition. However, for difficulty levels 1 and 2 improved ljerformance is shown on t’rial 2 of the Constant Form condition. Post hoc f tests revealed that there two points were significantly different but, tli(l remaining six comparisons were not reliable. Eaploratior~ strategies. There were three main exploration strategies cmployerl: (a) felt only texture, (b) felt texture, t’hen form, (c) felt form, then texture. The proportion of each type of exploration as a i’l:nction of Problem Type are prclsented in Table 1. When all observaI ens, collapsed across problems, test trials, Ss, and days, were conside~tl, there were no cases recorded where only the form dimension was observed (by tracing the outline). By comparison, 31% of all observat’ions were classified as felt only the texture dimension, with no attempt to trace the form outline. More responses investigating only the texture dimension were recorded under the Constant Form than the Variable Form condition. Sixty-three percent of all observations were classed as exploration of both texture and form with approximately equal distributions of form or texture first responses (see Table 1). Six percent of all responses remained unclassified due to experimenter error or the absense of any recognizable exploration response (i.e., the S pushed the stimulus base without feeling the stimuli), The conditional probabilities of a correct response given each of the t,hree main exploration strategies are shown in Fig. 5. Only responses to the first three levels of problem difficulty were considered due to the chance level of performance on the most difficult level 4 problems. When the Constant Form problem< are considered, it can be seen that there

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strategy (feel texture) increases over the three trials. However, when a Variable Form dmiension is present, the probability of adopting a particular exploration strategy dots not vary greatly over trials. In Fig. 7 the 1:attern of exploration response.- combined for difficulty I and 2 and for difficulty 3 ant1 4 is 1)rcscnted. As can be seen, a greater increase in texture resl~onscs occurred on problems 1 and 2 than on 3 and 4. In the Variable Form condition the l)aLtern of exploration respoiises remained fairly stable across l~roblem types. Thus, the basic patterns obtained with both the exploration strategies and percent correct measures were simiiar. For level 1 and 2 problems in the Constant Form condition, there is an increase from Trial 1 to Trial 2 in the incidence of texture only responses and a corresponding increase in the percentage of correct choice responses. In the level 3 and 4 problems and for a11 levc>ls in the Variable Form condition there is little suggestion of either increased texture responding or inrreascd accuracy. This same ljattern was obtained in Exp. 3. Discu~ssiof2 One of the main findings of Exp. 2 was that the presence of a variable f~rni dimension led to a dccrcaasc in performance under all levels of 1)rohlcm d’fficulty. Tllis influt>ncc of the variable form dimension was inv&igated in Esp. 3. All other aspects of the results will be discussed in the general discussion following Exp. 3. BSPERIMENT

3

That variability on an irrelevant dimension increases problem difficulty is consist.ent with previous reports of visual discrimination learning (Zeaman & House, 1963). However, in Exp. 2 the irrelevant form dimension never provided cues for problem solution and the Ss were specifically trained to attend to texture, but the attentional responses to the forms did not extinguish. One feature of the last experiment may account for the persistent, attraction of the irrelevant and probably nondominant (Gliner et nl., 1969) form dimension. The distinctive differences between the randomly selected form cues (square, circle, triangle) should be more apparent thsn those between the degrees of roughness/ smoothness presented on the Fcsalcdtexture dimension. If attention were directed to the dimension carrying the greatest difference between its constituent cues (Fisher, 1970; Zeaman. 1971), then the irrcvelant form dimension present in Exp. 2 might claim attention, even in the absence of reinforcement. Discrimination of the fine texture differences would be impaired by the presence of the distinctive cues of the variable form dimension. Similarly, the absence of a variable form dimension in the

scaling study could have unhanccd the discriminability of the relevant texture difference. Experiment 3 was conducted to investigate whether the presence of the variable form dimension in Exp. 2 was responsible for the poor discrimination learning with fine texture differences. To achieve this the main features of Exp. 2 were repeated, but a constant form dimension was employed. Method Subjects. Ten Ss, not included iu Exp. 2, were selected from thos;ts 8.1 who had provided reliable psychophysical data. C)ne S failed pretraining and another X failed to complete the eight days of the cxperimental Fessions. The remaining eight Ss, five girls and three boys, had :t mean CA of 4 years, 3 months (range 3 years 8 months to 5 years 2 months). dpparat~s. The apparatus was identical to that used in Exp. 2. Stbmz~li. The nine tcxturcs and three forms of Exp. 2 were retained. The stimuli were recovcrcd before pretrnining and on Day 4 of the experimental sessions. Procedure P~tr~~ining. The pretraining consisted of two problems so that an equal amount of pretraining was provided in both Exps. 2 and 3. The first problem was identical to Exp. 2, with a randomly selected constant form in each of the two highly distinctive textures (7 and 7’). The second problem repeatccl thcsc kxt.ure cues but introduced a second constant form. The problems were presented for 50 t.rials per problem, 1 problem a day, with a success criterion set at 10 consecutive correct, responses, exactly as in Exp. 2. Experimental sessions. Following the procedure of Exp. 2, the experimental sessionsconsisted of 8 days of testing a series of 16 three-trial problems. There were only four problem types in Exp. 3, as all problems presented a constant form. The remaining form, not used for each S’s prctraining problems, was selected as the experimental form. This form remained constant throughout the experimental sessions.The four problems corresponded to the four levels of psychophysical difficulty of the texture cues. These four levels of difficulty were identical to Exp. 2, with the values of the two most difficult problems again selected individually on the basis of each X’s psychophysical data. There were 16 problems a day, composed of 4 replications of cacll Inoblem. The problems were presented in a predetermined sequeucc which counterbalanced the order of presentation across &‘s and gags. Tile l)osition of the correct stimulus of each [)air was determined by a

Gellermann (1933) series. With 8 Ss X 8 Days X 4 Replications of each problem per day, the design yielded 256 data points for earh trial of each problem. The classification of exploration strategies introduced in Exp. 2 was continued in Exp. 3. At the end of each day of testing, the Ss were rewarded with the prepached lO$ toys, identical to those provided for the Ss in Exp. 2. Results All analyses were conducted separately on both the raw proportion of correct responses and the arcsine transformation of these data. Since the pattern of results obtained from the two analyses was identical, only the raw data analyses are reported here. A 4 (Problem Difficulty) X 2 (Test 1 or 2) X 4 (Replications within a Day) X 8 (Subjecs) factorial analysis of variance was conduct,ed on the proportion of correct responseson Tests 1 and 2 of each problem. The mean proportion of correct responses as a function of Difficulty and Test Trial are presented on the left side of Fig. 4, together with the comparable data from Exp. 2. The proportion of correct responses decreased as problem difficulty increased (F(3,21) = 137.34, p < .OOl). Test 2 performance was superior to t.est I performance (F(1,7) = 5.95, p < .025). Correlated t tests revealed that the difference between tests 1 and 2 was significant for both problems 1 and 2. This interaction replicates the finding of improved performance on test 2 for problems 1 and 2 in the Constant Form condition of Exp. 2. These data are also presented in Fig. 4. Exploration strategies. Again there were three main exploration strategies employed. The proportions of each type of exploration as a funct,ion of Problem Type are presented in Table 1. The total number of observations were divided into those observing only one dimension (72%) those observing two dimensions (24%) and those unclassified (4%). Thus 727, of all responses (compared with 42% in t.he comparable Constant Form condition of Exp. 2) could be described as attempts to feel the texture differences between cues and ignore the form outline, the optimal strategy for these studies. Of the 24% of responses where an attempt was made to observe both dimensions, 78% felt texture first compared with 44% of this type of response in the Constant Form condition of Exp. 2. In Fig. 7 the probabilities of employing each of the three exploration strategies over the three trials of a problem are presented, together with the comparable data from the Constant Form condition of Exp. 2. The data for difficulty 1 and 2 are combined as are those of difficulty 3 and

4. Again it can be seen that in problem clifficulty 1 and 2 the proportion of texture only responses increases over trials, while for problem difficulty 3 and 4 little increayc over trials is apparent. This replicates and strengthens the tentative finding of Exp. 2, where it was also shown that an increase in t.he al)l)roI)riate exploration strategy occurs on trial 2 of problem dXFiculty 1 and 2, precisely where an improvement in accuracy scores over trials is albo reported. Discussio?~

The poor performance reported in Exp. 2 for the most difficult levels of psychophysically scaled cue differences was not the result of the presence of a distracting variable irrelevant dimension on some of the trials, and therefore the main interpretations of the accuracy data in Exp. 2 were supported. In the absence of a variable form dimension, accuracy of respondmg was fouud to be essentially identical to that reported for the Constant Form condition of Exp. 2. The mean proportion of correct responsesin this experiment for problem difficulties 1, 2 and 3 was .94, .81, and .60 compared with similar findings of .92, .80, and .61, respectively, for Exl). 3. In both experiments, performance was at chance for the most difficult level 4 problems. Thus, the presence of a distinctive variable form tl.mension in Esp. 2 affected performance only in the Variable Form condition and not the overall level of performance. GENERAL

I~ISCXJYSIOK

Combining the results of both .Exps. 2 and 3, it is apparent that learning is not a simple function of the psychophysically scaled discriminability of cues. This finding supports anal extends the data reported by Gliner et al. (1969), which demonstrated that dimensional dominance is also not a strict function of discriminability. Attempts to equate stimuli for use in discrimination learning tasks should establish a rating bayed directly on a learning measure rather than on psychophysical scales of perceptual difference. In both experiments reported here, performance on level 3 and level 4 problems ranged hctween 50 and 60% correct. However, these same differences had been accurately perceived 5,/B times for level 4 and 6/6 times for level 3 problems in the original scaling study. That the two measures of discriminahility are so rery different. is a question of some interest. One explanation is that the learning situation places many more dtmands on the Xs than does the scaling situation. In the scaling study the Ss were required to feel two stimuli and reach a decision concerning the degree of similarity between them. In the learning studies, the 8s were required to make this decision regarding similarity, but in addition they must codr each stimulus as

PSYCHOPHYSICALLY

sCALb?D

CUE

DIFFERENCES

301

rough or smooth, determine whether that stimulus is correct or incorrect, retain this information throughout the intertrial interval, and then repeat the discrimination of stimulus similarity or the subsequent test trial. It could be t’hat as the demands of the situation increase a far greater difference in cues is required for accurate performance. Drucker and Hagen (1969) have suggested that the ability to focus on task relevant cues does not. develop until early adolescence. Studies of tactile exploration patt’erns with children (Cirillo, Wapner, & Rand, 1967; Lehman, 1971) have supported this. However the present studies found that preschool children can learn t,o focus on task-relevant informat’ion and ignore irrelevant information by exploring only the dimension relevant for problem solution. With a constant form in Exp. 3, the percentage of responses where texture only was felt reached 72%. The comparable Constant Form condition in Exp. 2 produced 42% of these responses. The influence of the distinctive variable form dimension in Exp. 2 can again be seen in the lowered incidence of first responses to texture on these trials when the Ss did explore both dimensions (50% in Exp. 2 compared to 78% in Exp. 3). Variability on the irrelevant dimension in a tactile discrimination task interferes with the ability of preschool children to focus only on the relevant dimension (Lehman, 1971). An additional aim of the present experiments was to investigate the feasibility of using tactile exploration strategies in an (external) analogue of attention. In visual discrimination learning experiments, there is considerable evidence that performance improves as the probability of attending to the relevant dimension increases (Zeaman & House, 1963). If the haptic exploration strategies investigated here are useful analogues of visual attention responses, there should be a correspondence between the use of the various strategies and performance. That such a correspondence exists can be seen by considering two aspects of the data. First, accuracy was highest when texture only was explored. More detailed support comes from the finding (obtained in Exps. 2 and 3) that on problem types where the probability of feeling texture increased over trials, a parallel increase in accuracy was also obtained. In both experiments, the probability that the Ss would feel only texture increased from Trial 1 to Trial 2 in the Constant Form conditions for problem difficulty levels 1 and 2. In both cases, there was a corresponding increase in the percentage of correct responses. Thus, an increase in the incidence of the appropriate exploration strategy led to improved accuracy of choice responses. The suggestion is that the general technique of using the haptic modality to externalize attentional responses is a fruitful approach worthy of further refinement.

302

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SCOTT,

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URBAN0

KEFEREXCES A. function

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