Dimensional and overall similarity classifications in haptics: A developmental study

Cognitive Development, 8, 495-516 (1993)

Dimensional and Overall Similarity Classifications in Haptics: A Developmental Study Carole Berger Yvette Hatwell Laboratoire de Psychologie Experimentale de Grenoble

The developmental change from global towards dimensional classifications, usually observed in vision, was investigated in haptics with stimuli varying according to their size and roughness. Children aged 5 and 9 years old and adults were presented with a free classification task allowing either an overall similarity sorting or a dimensional sorting. In two experiments, the discriminability of the stimuli along one or both dimensions was varied. Results showed that although more overall similarity classifications were observed in children than in adults, this kind of classification was never dominant (i.e., it was never chosen more frequently than would be predicted by chance). In addition to these developmental effects, effects due to the magnitude of stimulus difference were observed: A stimulus tended to be matched with the standard especially if it was slightly different from it or if the other comparison objects were much more different from this standard. This appeared in the first experiment testing dimensional versus overall similarity matching, and in the second experiment testing dimensional size versus dimensional roughness matching. These results were discussed with reference to the characteristics of haptic exploratory procedures.

This work examines the development of dimensional versus overall similarity classifications in haptics. Its aim is to establish whether an evolution similar to that demonstrated recently in vision would also be observed in haptics. The changes studied here concern the shift occurring in vision from overall similarity (global) classifications, which are dominant in young children, to dimensional classifications characterizing older children and adults. This work has been supported by grants from the University Pierre-Mendes-Franceof Grenoble and from the Centre National de la Recherche Seientifique (LIRAn~ We are very grateful to R.L. Klatzky and L.B. Smith for their helpful comments on earlier versions of this article. Correspondence and requests for reprints should be sent to Carole Berger or Yvette Hatwell, UniversitE Pierre-Mendes-France (Grenoble II), Laboratoire de Psychologic Exl~rimentale, P.O. Box 47, 38040 Grenoble Cedex 9.

Manuscript received January 29t 1992; revision accepted August 3~ 1993

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In vision, these developmental changes were first studied in order to describe the nature of perception. Research on these problems stems from Garner's (I 974) distinction between integral and separable dimensions. According to Garner (1974), separable dimensions combine to form stimuli in which components are perceived separately. For example, the size of an object can be processed independently from its brightness. By contrast, integral dimensions (like saturation and brightness) combine to form stimuli that are perceived holistically. This means that only the global structure of the stimulus can be processed, because the subject has no access to the isolated components of the stimulus. One of the methods used by Garner (1974) to determine whether a stimulus is integral or separable is the speeded classification task, in which subjects are instructed to classify a set of stimuli on the basis of a given dimension. In the unidimensional control condition, only one dimension is varied while the other is held constant. The bidimensional experimental conditions are either "correlated" or "orthogonal." In the correlated condition, the two dimensions are varied together (in correlation), and in the orthogonal one, the two dimensions are varied independently. When the dimensions are separable, they can be processed in isolation (by an adult subject). Consequently, the irrelevant dimension is successfully filtered and the same sorting times are obtained in the three conditions. A different pattern is observed with integral dimensions. In this case, the correlated irrelevant dimension elicits a "redundancy gain" (i.e., a reduction in the classification time) whereas the orthogonal irrelevant dimension generates interferences (i.e., increases classification time) because of the difficulty of filtering. Another method proposed by Garner (1974) is the free classification task with three objects varying along two dimensions X and Y (see Figure la). Stimuli A and B share a common value on dimension X, but they are very dissimilar on dimension Y. Stimuli B and C share no common value, but are highly similar overall. Subjects are instructed to select the two objects "which go together the best." With separable stimuli, each dimension is processed independently and the sort relies on the shared dimensional value (AB classification). With integral stimuli, neither dimension can be specifically ignored or focused on; therefore, only the global structure is processed and a BC classification is elicited because it maximizes the overall (global) similarity of the stimuli. It should be noted that the similarity described here corresponds to a physical similarity (using the terms overall similarity sorts or BC sorts, we will refer to that kind of similarity throughout this article). Garner's (1974) description of integral versus separable perception has been further questioned by some authors who have suggested that, in this domain, functional factors should be considered in addition to the structural factors studled by Garner. For example, developmental research has shown that dimensions which are separable in adults (like size and brightness) are perceived holistically

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497 nB De

nA Dim X

Figure la.

Example of a triad used in Garner's (1974) free classification task.

lab

laD nC

nA Dim X

Figure lb. task.

Example of a tetrad used in Cook and Odom's (1988) free classification

by very young children. This appeared in speeded classification tasks, where redundancy gains and interferences have been observed at ages 5 and 6 years old, but not at age 9 or in adults (Shepp & Swartz, 1976). Similarly, in free classification tasks, the younger children used more overall similarity rules than dimensional ones, whereas the reverse pattern was observed in older children and adults (Smith, 1979; Smith & Kemler, 1977). These observations have been interpreted in several theorical contexts. According to the integrality-separability hypothesis (Smith & Kemler, 1977), these results stem mainly from the nonperceptual independence of the dimensions in younger children. Another explanation of the early dominance of overall similarity sorts in the free classification tasks is given by the "differential sensitivity hypothesis" of Cook and Odom (1988), which stresses the role of selective attention. According to this hypothesis, predisposed salience factors (derived

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from the perceptual system's sensitivity to the stimulus properties) and distinetiveness salience factors (derived from the magnitude of stimulus differences) are assumed to be important. Young children would elicit overall similarity responses because of their low sensitivity to the properties to be processed. Later in development, discriminative learning (such as it is described by Gibson, 1969) would take place and increase the sensitivity to these properties. As a result, dimensional responses would be favored because the critical dimensions may then be attended to. Some results support these assumptions: Using a set of triads, Aschkenasy and Odom (1982) showed that overall similarity classifications were more frequent in young children when the dimension to be processed concerned a nonsalient property, whereas dimensional classifications were more frequent with salient properties. Cook and Odom (1988) pointed out further that the use of Garner's (1974) triads was not compatible with these salience effects. They objected that in the case of a low sensitivity to the X dimension, subjects would be in a position to selectively attend only to the Y dimension. Stimuli which are closer on the basis of this latter dimension are B and C (see Figure la). Yet, prominent BC classifications could stem from a dimensional rule based on Y and not from an overall similarity rule. In order to control for this possible bias, Cook and Odom (1988) used a task in which a standard object B was to be paired with one of three comparison objects A, C, or D (see Figure lb). Subjects were instructed to look at all stimuli and to point to the comparison object that "goes with" the standard. The three comparison objects allowed either a dimensional sort on X, or a dimensional sort on Y, or an overall similarity sort. In this task, even 4-year-old children elicited more dimensional (BA or BD) than overall similarity (BC) responses if their preferred dimensions (color and size) were considered. By contrast, the reverse pattern was observed when they were tested on their less salient dimension (orientation). Although the differential sensitivity hypothesis can be considered as an important step towards understanding the meaning of classification responding (see Berger, 1992), the explanations that it provides are limited to one developmental level (4-5 years old) and to one kind of task (the free classification task). Moreover, this hypothesis does not clarify the nature of the processes determining overall similarity and dimensional responses. A more general and more precise interpretation, dealing with the nature of these processes, is found in the Weighted-Dimensions-Plus-Identity Model (Smith, 1989). In this model, which can be characterized as a model of similarity, free classification responses are supposed to be dependent on how the perceiver calculates and values the similarity between the stimuli presented. The model assumes that the dimensions are independently processed throughout development and that objects are represented in terms of their features. However, what is given at the level of immediate experience is a whole object. Given these assumptions, the model assumes that classifications are performed by calculating

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the similarity between objects across all their dimensions. What develops is how these calculations are performed. First, the differential weighting of dimensions increases with age: When calculating the perceived similarity between two objects, older children and adults tend to selectively attend to one dimension to the exclusion of the other, whereas young children attend to both dimensions. Second, the valued similarity, which is a power function of the perceived similarity, increases with age if the objects are in some way identical (identity on one dimension for example). This would result from the special status of identity, which is highly valued in older children and adults but not in young children (see Evans & Smith, 1988). Consequently, a change with age in the valued similarity between stimuli would depend on a combination of selective attention and the valuation of identity. Depending on the particular combination between these two parameters, the model can predict the "goodness of a classification" (i.e., the extent to which a given response corresponds to a good classification) as a function of the magnitude of stimulus differences. For example, when identity is highly valued and when dimension X is selectively attended to, BA part-identity classifications (see Figure 1) would correspond to a good classification across the entire range of stimulus differences if selective attention is perfect, but the goodness of this classification would be reduced for large stimulus differences if selective attention is not perfect. In our research we studied the type of sorting elicited haptically with size/roughness stimuli by children and adults presented with a free classification task similar to that used by Cook and Odom (1988). Referring to Smith's (1989) model, we also examined the influence of the magnitude of stimulus differences. This factor was varied in order to study the development of selective attention and of the valuation of identity. Although the perceptual processes involved in the picking-up of information about our environment are sometimes similar in vision and haptics (see Gibson, 1966, 1969; Hatwell, 1986), some specific haptic modes of coding information have been demonstrated in congenitally blind people (Millar, 1981, 1988), blindfolded young sighted children (Hatwell & Sayettat, 1991), and adults (Klatzky, Lederman, & Reed, 1987). These differences stem mainly from the particular characteristics of haptic scanning. Because of the restricted size of the haptic perceptual field, a great number of exploratory movements are needed in this modality (Lederman & Klatzky, 1987; Revesz, 1950). The sequential aspects of these exploratory movements could simplify the selective attention mechanisms and therefore could increase the differential weighting of dimensions. Moreover, some other differences between vision and touch could result from the two distinct haptic subsystems comprised in the hand; cutaneous (skin) and kinesthesic (muscle, tendons, joints). Some properties could be prominent because of the haptic cutaneous subsystem (this could be the case for roughness, which gives rise to a sensation considered as more or less pleasant early in development). Others properties could be prominent because of the haptic kines-

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thesic subsystem (this could be the case for size, which determines if an object can bc grasped or not). Consequently them arc some reasons for haptic classificationsto be more dimensional and correspondingly less global than visual classifications,especially in young children. The problem, therefore, is to know if overall similarity sorting would nevertheless be predominant hapticallyin young children, in spite of the particularcharacteristicsof haptic scanning, and ifthe factors affectingthis sorting would be the same as those described in vision. Recent research on haptic perception has shown that both integration of dimensions and independent processing can be observed. W h e n testing children ages 4 and 8 years with tactual dot patterns, Millar (1986) found that size discrimination was more accurate and faster when texture was correlated with it than when texture was held constant. However, the redundancy gain was asymmetric because texture discrimination was not improved when size was correlated with it. In the orthogonal condition, interference was observed in the accuracy of both size and texture judgments but not in response latencies. In adults, Reed, Lcderman, and Klatzky (1990) found on planar objects a symmetric redundancy gain with correlated size and shape dimensions: The classification based on these two correlated dimensions was performed faster than the classificationbased on one dimension only (size or shape). Classificationsbased on the texture dimension or on the hardness dimension were faster this time, when size was correlated with one of these two dimensions, than when it was held constant. However, thispattern of resultswas again asymmetric: Classification based on size was not improved when size was correlated with texture or hardness. Finally, no redundancy gain was observed on planar objects when shape was correlatedwith roughness, suggesting that adults process these dimensions independently in haptics (Klatzky, Lederman, & Reed, 1989). In children, predisposed salience effectshave been demonstrated in the haptic modality. With bidimensional stimuli, roughness was found to be more salient than shape in younger children (5-7 years) whereas shape was more salientthan roughness in older ones (Abravanel, 1970; Gliner, Pick, Pick, & Hale, 1969; Siegel & Barber, 1973). The effectsof distinctivenesswere less clearcut. W h e n the discriminability of shape or size was increased, Abravanel (1970) and Klatzky et al. (1987) found in children and adults, respectively, no related increase in dimensional classificationsalong the more distinctdimensions. How-

ever, the roughness preference disappeared in young children when the distinctiveness of this property was lowered (Gliner et al., 1969). In the research presented here, we studied the analytic/dimensional versus global/overall similarity classifications in children's and adult's haptic perception. Previous experiments in visual perception have often used size and brightness dimensions. Because haptic roughness may be considered a good substitute for visual brightness (because in both, exploration of any part of the object

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501

provides sufficient information about the dimension considered), we tested size and roughness dimensions in our experiments. We assumed that if subjects can selectively attend to the component dimensions of stimuli, and if they highly value identity, they will tend to calculate the similarity between stimuli using a strategy of weighting only one dimension. In this case, the stimuli perceived as most similar would be B and A if dimension X is weighted, and B and D if dimension Y is weighted. Yet, the frequency of dimensional part-identity (BA or BD) responding would be high. It should be noted that, in this latter case, an increase in the magnitude of stimulus difference (i.e., between the standard and the stimuli sharing a value with it) which would result in a decrease of dimensional part-identity responding would indicate that selective attention is not perfect, although it is at work. On the other hand, if the subjects cannot selectively attend to the component dimensions of stimuli (e.g., if both dimensions arc equally weighted), the frequency of dimensional partidentity (BA or BD) responding could be low. Thus, if in haptics the development of selective attention and of the valuation of identity is similar to that reported in vision, more overall similarity classifications (and correspondingly, fewer dimensional part identity classifications) should be observed in young children's haptic perception than in older children's or adults'. Alternatively, if the sequential property of haptic modality precludes holistic processing strategies early in age, the observed development of the haptic perceptual classification would be different from that observed in vision. Children ages 5 and 9 years old and adults were tested on a haptic free classification task. In it, the subjects were presented with a standard and three comparison objects and each was asked which of the three comparison objects "goes the best" with the standard. In each tetrad, two-dimensional part-identity sorts (size and roughness) and one overall similarity sort were possible (see Figures 2 and 4). The distinctiveness of the two dimensions was differently varied in two experiments. In Experiment 1, both dimensions had either a high or a low degree of distinctiveness. In Experiment 2, the distinctiveness of one dimension was increased while that of the other was decreased. EXPERIMENT 1 In this experiment, the effects of age and of the magnitude of stimulus difference were investigated. Children and adults were presented with a free classification task, similar to the task used by Cook and Odom (1988), which allowed either a size dimensional sort, a roughness dimensional sort, or an overall similarity sort. In order to test the role of the magnitude of stimulus difference, the values taken by each dimension were varied so that the distance between the standard and the comparison objects sharing a value with it was either large (3 steps) or small (2 steps).

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Method

Subjects. Sixteen children aged 5;3 (years; months) to 6;0 (M age = 5;7), 16 children aged 9;4 to 10;4 (M age = 9;9), and 16 adults (M age = 22;0) participated as subjects. Children were enrolled in kindergarten schools and third-grade classes in different suburban Grenoble schools. Adults were students from Grenoble University. Materials. The set of stimuli consisted of 16 square wooden blocks of equal thickness (1.5 cm) covered with sandpaper. The two variable dimensions were size (the length of each side of the squares were 28, 34, 41, and 48 mm) and roughness (sandpaper No. 36, 80, and 280, and smooth paper). The manufactured numbers of the sandpaper depend on the density of the grains: High density of grains (smooth surfaces) corresponds to high sandpaper numbers, and low density of grains (rough surfaces) corresponds to low sandpaper numbers. The sizes of the stimuli were chosen by reference to Lederman and Klatzky's (1987) description of the "grasping" exploratory procedure: This procedure, spontaneously used by adults when size is to be processed, consists mainly in an apprehension of the whole stimulus in a grasp. In order to allow the use of this exploratory procedure, especially when the hands of the subjects were small, small sized stimuli were presented. Even the larger stimuli could be apprehended in a single grasp by the younger children. Moreover, a pretest carried out over twenty 5-year-old and twenty 9-year-old children showed that the selected adjacent values of each variable dimension were correctly discriminated in more than 90% of the measurements. At each trial, four stimuli were fixed on a wooden rectangular board (20 x 40 cm). Their positions were specified by an isosceles triangle (base = 20 cm. sides = 13 cm) drawn in the center of the board. The standard stimulus was positioned on the vertex of the triangle. Two comparison stimuli were placed at each of the two other angles of the triangle, and the third stimulus occupied the center of the triangle basi~. The left, fight, and center position of the comparison stimuli were counterbalanced across subjects and conditions. The display was hidden by a front curtain preventing subjects from seeing the board and the stimuli. Procedure and Experimental Design. The subjects inserted their hands behind the curtain in order to e.~plore the four stimuli. They were instructed to explore the standard stimulus and then to indicate with which of the three comparison objects "it goes better." The words same, alike, and similar were never used in order to prevent any suggestion as to the type of response elicited. Two types of tetrads were tested, depending on the magnitude of differences between the stimuli. Figure 2a shows the Type 1 tetrad (Condition C1) and Figure 2b shows the Type 2 tetrad (Condition C2). In this latter condition, the differences between the

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Classifications

Roughness 4

nB

nD

~C

DA I 1

I 2

I 3

I 4

Size

Figure 2a. Example of Type 1 tetrad (Condition Cl) used in Experiment 1.

Roughness 4

t)B

DD "C

t~A

I

I

I

1

2

3

I

4

Size

Figure 2b. Example of Type 2 tetrad (Condition C2) used in Experiment 1.

standard and the two stimuli with which it shares a dimensional value (on size or on roughness) are smaller than in Condition C1. The two experimental conditions (C1 and C2) were performed by each subject in a counterbalanced order. In each task, a set of 12 tetrads was presented in a randomized order. Therefore, each subject achieved a total of 24 trials. The experimental predictions were the following: If classification responding is similar in haptics to that reported in vision, more dimensional part-identity (BA or BD) responses and fewer overall similarity (BC) responses should be observed in adults than in young children. Moreover, dimensional part-identity responses should be prominent if identity is highly valued and if subjects can selectively attend to a single dimension since the stimuli perceived as most similar should

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C. Berger and Y. Hatwell

then be B and A (selective attention to size) or B and D (selective attention to roughness). In this case, and if selective attention is not perfect, the frequency of dimensional part-identity responding could be higher in Condition C2 than in Condition C 1: Attending to one dimension should then be more difficult for high difference than for low difference between the standard and the comparison objects sharing a value with it. If, on the other hand, selective attention is perfect (and identity is highly valued) no effect of condition should be observed: The objects perceived as most similar should be B and A (perfect selective attention to dimension size) or B and D (perfect selective attention to dimension roughness) in both conditions. Results

Table 1 presents the mean number of size, roughness, and similarity classifications as a function of age and condition. The observed type of classification was entered as a two-levels factor in three separate analyses of variance (ANOVA). The three levels of this factor were: (a) the number of dimensional size versus dimensional roughness classifications in the first ANOVA, (b) the number of dimensional size versus overall similarity classifications in the second ANOVA, and (c) the number of dimensional roughness versus overall similarity classifications in the third ANOVA. The first ANOVA (3 Ages x 2 Task Orders x 2 Conditions x 2 Classification Types: Size vs. Roughness with repeated measures on the last two factors) revealed a main effect of condition, F(1, 42) = 9.11, p < .005: Higher scores were observed for the two collapsed dimensional part-identity classifications in Condition C2 than were observed in Condition C1. The Age x Classification Type interaction approached significance, F(2, 42) = 2.89, p < .07. As can be seen in Figure 3a, representing the mean number of size, roughness, and similarity classifications as a function of age, this trend means that the dimensional part-identity classifications tended to be based more on size and less on roughness in the older subjects than in the younger subjects. The second ANOVA (3 Ages x 2 Task Orders x 2 Conditions x 2 Classifica-

Table 1. Experiment 1: Mean Number of Size, Roughness, and Similarity Classifications as a Function of Age and Condition Size

Roughness

Similarity

Age

CI

C2

Cl

C2

Cl

C2

5 years 9 years

2.63 3.5 6,69

3,19 3.88 6.63

6.5 5,24 3.56

6.69 6,5 4,19

2.88 3.13 1.75

2.13 1.63 1,25

Adults Note.

The maximumnumberof similarityclassifications is 12.

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505

Mean number of

responses(max 12) 7. Qm 0

5.

Size Roughness Similarity

4,

3. 2.

I //

0

Age 5

9

Ad

Figure 3a. Mean number of size, roughness, and similarity classifications (max = 12) as a function of age in Experiment 1. Mean number of responses (max 12) 6. 5. 4.

t,-

t

Size Roughness

mr

-

Similarity

3 2

I 0 t Cl

t C2

Condition

Figure 3b. Mean number of size, roughness, and similarity classifications (max = 12) as a function of condition in Experiment 1.

tion "Pypes: Size vs. Overall Similarity with repeated measures on the last two factors) revealed a main effect of classification type, with dimensional size classifications more frequent than overall similarity classifications, F(1, 42) = 13.79, p < .001. The Age x Classification 'I~ype interaction (Figure 3a), was significant, F(2, 42) = 5.57, p < .01. This interaction means that, although the number of dimensional size classifications was always higher than the number of similarity classifications, children more frequently based their sorts on the similarity and less on the dimensional size rules than adults, F(I, 42) = 10.77, p < .01. The two groups of children did not differ. The Condition x Classification

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Type interaction was significant, F ( I , 42) = 7.72, p < .01: as represented in Figure 3b, showing the mean number of size, roughness, and similarity classifications as a function of condition, fewer similarity classifications were observed in Condition C2 than in Condition CI, F ( I , 42) = 9. I I, p < .005, and differences in size classifications were not significant. The Age • Condition • Classification "Dype did not reach significance. The third ANOVA (3 Ages • 2 Task Orders • 2 Conditions • 2 Classification 'Dypcs: Roughness vs. Similarity with repeated measures on the last two factors) revealed a main effect of age, F(2, 42) = 4. I I, p < .05, with lower scores in adults than in the two collapsed groups of children, F(I, 42) = 7.89, p < .01, which did not differ. The main effect of classification type was significant, F ( I , 42) = 18.37, p <~ .001, with more dimensional roughness classifications than similarity ones. The Condition • Classification Type interaction (Figure 3b) was significant, F ( I , 42) = 6.02, p < .02: Fewer similarity classifications were observed in Condition C2 than in Condition C I , F ( I , 42) = 9. I I, p < .005, and no significant differences were observed on roughness classifications. No Age • Classification 'I~ype or Age • Classification Type • Condition interactions were found in this ANOVA. In all three ANOVAs, task order never reached significance or interacted with any other factor. Finally, the dominance of each kind of classification was evaluated by comparing empirical data and chance level. A classification was said to be dominant if it was chosen more often than it would be predicted by chance. Dominance of the two collapsed dimensional part-identity classifications (chance level = .66) and correlatively nondominance of the similarity classifications (chance level = .33) were observed in 5-year-old children (t = 4.70, p < .002) and 9-year-old children (t = 6.54, p < .001) in Condition C2, and in adults in both Condition CI (t = 4.45, p < .002) and Condition C2 (t = 8.20, p < .001). Dominance of the similarityclassificationswas never observed. W h e n the two-dimensional part-identityclassifications(size vs. roughness) were compared, dominance of roughness was observed at 5 years of age in Condition CI (t = 2.69, p < .02) and Condition C2 (t = 2.94, p < .02). Dominance of size was observed in adults only in both Condition CI (t = 2.78, p < .02) and Condition C2 (t = 2.59, p < .05). Nine-year-old children elicitedno dimensional part-identitydominant classifications.

Discussion The results of this experiment provided some evidence that children's haptic classificationsof size and roughness dimensions are more governed by overall similarityrules than am of adults'. However, in spite of this development trend, the stimuli presented hers were classified by their separate dimensions: This conclusion was supported by the absence of dominant overall similarityclassifications, even in the youngest children. The dimensional part-identityclassifications were based more on the roughness dimension than on the size dimension in

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the youngest children, whereas the reverse pattern was observed in adults. In addition, results showed that the relative frequency of dimensional part-identity and overall similarity classifications depended on the magnitude of stimulus difference: Dimensional part-identity responding was more often elicited when the difference between the standard and the two stimuli with which it shared a dimensional value was reduced (Condition C2). These effects will be considered in more detail in the general discussion. EXPERIMENT 2 In Experiment I, we tested salience effects in order to know if they could affect the frequency of overall similarity versus dimensional part-identity (both size and roughness dimensions) classifications. However, the role of salience on the frequency of each particular kind of dimensional part-identity (size vs. roughness) classification was not studied. Experiment 2 was therefore designed to test these effects. Its purpose was to determine whether the choice of dimensional size or dimensional roughness classifications would depend on the magnitude of differences between the standard and each stimulus sharing a value with it. A classification task similar to the one used in Experiment 1 was presented to 5and 9-year-old children and to adults. In each condition, the magnitude of differences between the standard and the stimuli with which it shares a value was not the same for the size and roughness dimensions. In one condition (C1), the differences between roughness values were greater than the differences between size values. In the other condition (C2), the differences between size values were greater than the differences between roughness values. Method

Subjects. Sixteen children aged 5;4 to 6;1 (/14age = 5;7), 16 children aged 9;3 to 10;4 (M age = 9;8), and 16 adults (hi/age = 22;0) participated as subjects. Children were enrolled in kindergarten schools and third-grade classes in different suburban Grenoble schools. Adults were students from Grenoble University. Materials.

The materials were the same as those used in Experiment 1.

Procedure and Experimental Design. The general procedure was the same as in Experiment 1 but different types of tetrads were presented. Figure 4a shows the Type 1 tetrad (Condition C1) and Figure 4b shows the Type 2 tetrad (Condition C2). The experimental design was the same as in Experiment 1: Each subject w a s presented successively with the two conditions, C1 and C2, in a counterbalanced order. In Condition C1, the comparison object A sharing size with the standard object B differed by three steps from this standard on the roughness dimension, and the comparison object D sharing roughness with the standard object B

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C. BerBer and Y. Hat'well Roughness

aB

4

nD nC

nA

I

I

I

I

I

2

3

4

Size

Figure 4a. Example of Type 1 tetrad (Condition C1) used in Experiment 2. Roughness 4'

3'

aB nC

2

1

aD

aA

I

I

I

I

1

2

3

4

Siz-

Figure 4b. Example of Type 2 tetrad (Condition C2) used in Experiment 2.

differed by two steps from this standard on the size dimension. In Condition C2, the pattern was reversed: The comparison object A differed from the standard object B by 2 steps on the roughness dimension, and the comparison object D differed from the standard object B by 3 steps on the size dimension. We assume that if identity is highly valued and if subjects selectively attend to a single dimension, dimensional part-identity (BA or BD) responding should be prominent. In this case, and if selective attention to dimensions of size and roughness is not perfect, the frequency of roughness classification would increase in Condition CI as compared to Condition C2, and the frequency of size classifications would increase in Condition C2 as compared to Condition C 1. If selective attention is perfect, no effect of condition should be observed.

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Results

Table 2 presents the mean number of size, roughness, and similarity classifications as a function of age and condition. As in Experiment 1, the observed type of classification was entered as a two-levels factor in three separate ANOVAs. The first ANOVA (3 Ages x 2 Task Orders • 2 Conditions • 2 Classification Types: Size vs. Roughness with repeated measures on the last two factors) revealed an effect of age approaching significance, F(2, 42) = 2.86, p < .07. The Condition • Classification 1~ype interaction was significant, F(1, 42) = 22.90, p < .001: As can be seen in Figure 5b, representing the mean number of size, roughness, and similarity classifications as a function of condition, more size, F ( I , 42) = 10.90, p < .02, and fewer roughness, F(1, 42) = 27.49, p < .001, dimensional classifications were observed in Condition C2 than in Condition C1. The Age x Condition x Classification Type interaction was not significant. The second ANOVA (3 Ages x 2 Task Orders x 2 Conditions x 2 Classification Types: Size vs. Similarity with repeated measures on the last two factors) revealed a main effect of classification type, with dimensional size classifications more frequent than similarity classifications, F ( I , 42) = 12.49, p < .001. The main effect of condition was significant, F(1, 42) = 27.50, p < .001: Higher scores were observed in Condition C2 than in Condition CI. The Age x Classification "Dype, was significant, F(2, 42) = 5.97, p < .006: As can be seen in Figure 5a, representing the mean number of size, roughness, and similarity classifications as a function of age, this interaction means that adults more frequently based their sorts on dimensional size and less on similarity rules than did the two collapsed groups of children, F ( I , 42) = 7.25, p < .02. This interaction was not significant when the two groups of children were compared. The third ANOVA (3 Ages x 2 Task Orders x 2 Conditions x 2 Classification Types: Roughness vs. Similarity with repeated measures on the last two factors) revealed a main effect of age, F(2, 42) = 4.26, p < .02: Although the scores tended to be higher in children than in adults, and in 5-year-old children than in 9-year-old children, the orthogonal comparisons showed that these trends

Table 2. Experiment 2: Mean Number of Size, Roughness, and Similarity Classifications (max ffi 12) as a Function of Age and Condition Size Age 5 years 9 years Adults

Roughness

Similarity

Cl

C2

CI

C2

C1

C2

1.88 4.19 6.06

3.25 5.88 6.38

7.19 5.69 4.69

5.25 3.69 3.88

2.94 2.13 1.25

3.5 2.44 1.75

Note. The maximumnumberof similarityclarifications is 12.

510

C. Berger and Y. Hatwell Mean number of responses (max 12) 7. 6.

---m--

* :

5.

size Roughness Similarity

4.

3. 2. I o

//

'

,

5

9

H

' Ad

Figure 5a. Mean number of size, roughness, and similarity classifications (max = 12) as a function of age in Experiment 2.

Meannumberof responses(max 12) 654-

----4m---*

Size Roughness

9

Similarity

3. 2l-

Figure 5b.

I C1

I C2

Condition

Mean number of size, roughness, and similarity classifications (max =

12) as a function of condition in Experiment 2.

were not significant. The main effect of condition was significant, F ( I , 42) = 10.90, p < .02: Higher scores were observed in Condition C1 than in Condition C2. There was a significant Condition • Classification Type interaction (Figure 5b, F ( I , 42) = 16.38, p < .001: Fewer dimensional roughness classifications were observed in Condition C2 than in Condition C I , F ( I , 42) = 27.49, p < .001, and no differences between conditions wore observed on similarity classifications. No Age • Classification Type interaction was found in this ANOVA. Again, in all three ANOVAs, task order never reached significance or interacted with any other factor.

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As in Experiment 1, the dominance of each kind of classification was evaluated by comparing empirical data and chance level. Dominance of the two collapsed dimensional part-identity classifications (chance level = .66), and correlatively, nondominance of the overall similarity classifications (chance level = .33) was observed in 9-year-old children in Conditions C1 (t = 3.22, p < .01) and C2 (t = 2.63, p < .01), and in adults in Conditions C1 (t = 6.66, p < .002) and C2 (t = 3.54, p < .005). Results obtained from the 5-year-old children did not differ from what would be predicted by chance. When the two dimensional part-identity classification (size vs. roughness) were compared, dominance of roughness was observed in 5-year-old children in Condition CI (t = 3.90, p < .002), whereas in adults, size classification was dominant in Condition C2 (t = 2.40, p < .05). No kind of dominant dimensional part-identity classification was observed at age 9. Discussion

These results provide evidence for more holistic responding in children than in adults in haptics. But again, even in the youngest children, the stimuli used were classified by their separate dimensions because the overall similarity classifications were never dominant. Concerning the effects due to the magnitude of stimulus difference, the main conclusion of this experiment is that the kind of dimensional part-identity classification elicited depends on the distance between the standard and the comparison objects: Size classifications were more frequent in Condition C2 in which the comparison object A, sharing size with the standard, differed from this standard by two steps on roughness, whereas comparison object D, sharing roughness with the standard, differed from it by three steps on size. As concerns roughness dimensional classifications, the reverse pattern was observed: These classifications were more frequent in Condition C1 in which the comparison object A, sharing size with the standard, differed from this standard by three steps on roughness, whereas comparison object D, sharing roughness with the standard, differed from it by two steps on size. GENERAL DISCUSSION The aim of this research was to analyze the effect of age and of the magnitude of stimulus difference on the frequency of dimensional part-identity (size or roughness) versus overall similarity classifications in the haptic modality. In vision, ~a shift from similarity (holistic) to dimensional responding has been demonstrated in different studies between the ages of 2 to 3 years and 9 years. This change was described as resulting from an increase with age of selective attention and of the valuation of identity. This conclusion could be derived from an analysis of the effects due to the magnitude of stimulus difference on classification responseS,, In order to investigate such effects, we presented tetrads of stimuli in which the distance between the standard and the comparison objects was varied across

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experimental conditions. It was assumed that these variations would have no effect on classification responding if selective attention was at work and if it was perfect. Stimulus difference would, however, change the responses elicited if selective attention was not perfect. Specifically, it was assumed, if selective attention was not perfect and if identity was highly valued, that the dimensional part-identity sorts (size or roughness) would be frequently elicited, but that the rate of these sorts would be reduced when the difference between the standard and the objects sharing a value with it increased. Our results were consistent with the predictions implied by the Weighted-Dimension-Plus-Identity Model under these latter assumptions because dimensional part-identity responding was prominent, and variation of the magnitude of stimulus difference along one or both dimensions affected the type of sorting elicited in the direction just described. This effect was revealed by the comparison between Conditions C1 and C2, and it was at work both for dimensional part-identity versus overall similarity classifications in Experiment 1, and for dimensional size versus dimensional roughness classifications in Experiment 2. Concerning developmental effects, results showed that when dimensional part-identity classifications were elicited, a shift tended to occur during development from roughness preferences to size preferences. This trend was observed in Experiment 1 (where it was marginally significant). In addition, the developmental assumption concerning the relative prominance of similarity classifications in young subjects was only partially supported. Haptically, children made more similarity (and correlatively fewer dimensional) sorts than adults did. As a result, the difference between the numbers of dimensional and of similarity sorts was lower in children than in adults, especially when the dimensional criterion concerned the children's less predisposed salient dimension (size). However, even in the youngest age group, similarity classifications were always less frequent than dimensional part-identity classifications. It should be noted that the observed decrease of similarity sorts occurred haptically between the ages of 5 to 9 years and adulthood although it has been evidenced between ages 5 and 8 years in visual studies contrasting size and brightness (Smith & Kemler, 1977). Overall, these results provided support for selective attention capacities (which are not perfect) and for a high valuation of identity. Although they are at work at all developmental levels, these processes tended to increase throughout development (by the age of 5-9) because for each stimulus difference dimensional part-identity responding was higher in adults than in children. Analysis of the possible effects of haptic exploratory procedures is useful in order to understand the origin of these differential effects of age on haptic processing. As described by Lederman and Klatzky (1987), attending to the roughness of a surface (a substance property) needs an easy-to-perform optimal exploratory procedure which consists mainly of lateral movements of the fingers over the surface. On the other hand, attending to size (a structural property)

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requires a different and more costly optimal exploratory procedure consisting in global grasping, contour tracing, and length measurements. We know from previous research (Abravanel, 1968; Hatwell, Osiek, & Jeanneret, 1973; Piaget & Inhelder, 1947/1956; Zaporozhets, 1965) that perceptual haptic exploration is poor, incomplete, and often unrelated to the task in young children, whereas it is complete and systematically organized in adults. When applied to our experiments, this means that the adults' systematic exploration would lead them to perceive both the roughness and the size properties of the stimulus, but the incomplete exploratory procedures of the young children would allow them to attend only (or primarily) to roughness. More importantly, Lederman and Klatzky (1987) demonstrated that lateral movement, for which application could be limited to one part of the stimulus, is an exploratory procedure only adapted to roughness perception and that it does not lead to the perception of the structural properties of objects (it should be noted that this result is also dependent on the magnitude of the discriminations required). This means that some of our subjects (especially young children), may not have responded in the holistie mode because their exploratory procedures precluded the processing of the two dimensions of variation of the stimuli. In this case, the exploratory procedures used could have induced an extreme differential weighting of the roughness dimension whereas size was not perceived at all. In order to test this hypothesis, we videotaped 24 additional children (twelve 5-year-olds and twelve 9-year-olds) while they explored and classified the stimuli used in Experiment 1. The exploratory movements performed on each trial were analyzed. The data showed that only 13.1% of the overall similarity responses were based on the use of a lateral motion exclusively. Moreover, children's exploratory movements were often limited to this procedure, especially at age 5 (39% of the items) and less markedly at age 9 (17% of the items). Therefore, it is likely that selective attention to the roughness dimension was facilitated in the younger children by their frequent use of the lateral motion procedure. In some tetrads, this mode of scanning could have precluded holistie responding. This would explain why no Age • Classification Type interaction was observed when roughness sorts were compared to similarity sorts. On the other hand, the complex exploratory procedures required to perceive size (enclosure, static contact, and contour following procedures) have been shown by Lederman and Klatzky (1987) to provide sufficient information to discriminate roughness. Yet, with both dimensions being processed, these strategies could allow a holistic mode of responding. This hypothesis was supported by our analysis of exploratory movements because 84.1% of the overall similarity classifications were associated with enclosure, static contact, or contour following exploratory strategies. Our results showed that inasmuch as children used these procedures (this was the ease in 58% of the items at age 5 and in 82.8% of the items at age 9), their classifications were more holistic than that of adults.

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Thus, the same development trend seems to take place in the visual and in the haptic modalities, showing an increase in dimensional part-identity responding in adults. This increase with age of the dimensional part-identity responding, occurring when the exploratory strategies do not preclude a holistic processing (i.e., when both dimensions are processed), could be explained by an increase in the selective attention and in the valuation of identity. This observation provides support to the theories assuming that visual and haptic information processing rely on some of the same general perceptual laws. However, in all age groups, haptic similarity sorts were always less frequent than haptic dimensional sorts. Even in children aged 5 years old, the proportion of similarity classification was significantly below chance level. This result is inconsistent with what was observed earlier in the development of visual perception. For example, a dominance of global responding was reported in young children in speeded classification tasks (Shepp & Swartz, 1976) and in free classification tasks (Smith & Kemler, 1977). The difference between Shepp and Swartz's (1976) results and ours could be explained by the less "decisional" aspect of speeded classification tasks, as compared to free classification tasks. In Smith and Kemler's (1977) results, overall similarity responding was inferred from the frequency of BC classification (Figure la). As stated earlier (Cook & Odom, 1988), this kind of classification may be based either on overall similarity rules or on dimensional rules along the Y dimension. As the magnitude of stimulus differences were not varied in this study, the conclusions derived from it were ambiguous. It may therefore be that visual classifications in young children are actually more dimensional than it was reported by these authors. This assumption is supported by Cook and Odom's (1988) experiment based on tetrads. In it, young children made more dimensional size and color classifications than similarity classifications. However, in their study, Cook and Odom did not verify that the preference for similarity classifications was lower than chance level. In summary, there is an important difference between our results and those reported in the visual processing literature comparing size and brightness or size and color dimensions. This means that in addition to general common laws of processing, each perceptual modality may act in some specific ways determined by its particular sensory functioning. In the case of haptics, it is likely that differences in exploratory procedures am crucial and that these differences lead to more analytic and fewer holistic strategies than in vision. Further research should test this hypothesis.

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