Local and global processing by persons with Williams syndrome: the case of visuo-constructive tasks
In Fayasse, M. & Thibaut, J.-P. (2002) Local and global processing by persons with Williams syndrome: the case of visuo-constructive tasks. Journal of Cognitive Education and Psychology, 2, 266-282.
Michèle Fayasse Jean-Pierre Thibaut
University of Liège
Michèle Fayasse Jean-Pierre Thibaut 5, boulevard du rectorat 4000 Liège. Belgium email :
[email protected] [email protected]
Abstract Williams syndrome (WS) is a rare genetically based neurodevelopmental disorder resulting in mild to moderate mental retardation. People with WS are known for their particular weakness in visuo-spatial construction. In a block-design task and a jigsaw-puzzle task, we compared WS persons with normally developing children matched on mental age. Three hypotheses were contrasted: (a) the standard local hypothesis maintaining that WS persons are biased toward local processing and have a deficit in processing the global level of stimuli (Bellugi & al., 1994); (b) the disengaging from the global level hypothesis which states that they have difficulties disengaging from global configurations when local processing is required (Pani & al., 1999); and a new hypothesis (c) the disengaging from the local level hypothesis which states that they have difficulties to disengage from salient local features when global processing is required. The third hypothesis is compatible with most of the observations regarding visuoconstructive problems in WS. In the last part, we propose an interpretation of WS persons problems in terms of executive functions.
Williams syndrome is a rare genetically based developmental disorder resulting in mild to moderate mental retardation. Interest in the Williams syndrome results from the uneven cognitive profile associated the syndrome. People with Williams syndrome show relative strengths in expressive language relative to their IQ, whereas it is agreed that they show pronounced deficits in visuo-spatial functioning relative to IQ (Mervis, Morris, Bertrand, & Robinson, 1999; Bellugi, Lichtenberger, Jones, Lai, & St-Georges, 2000; see also Mervis, Robinson, Bertrand, Morris, Klein-Tasman, & Armstrong, 2000; Fayasse & Thibaut, submitted). Compared with participants with mental retardation of mixed etiologies, Down syndrome, or normally developing individuals, persons with Williams syndrome perform typically lower on visuo-constructive tasks. Using the Wechsler-based Block Design test, Bellugi, Wang, and Jernigan (1994) showed that WSs have difficulties organizing blocks into a global and coherent configuration, even for simple 2 by 2 squares. The authors suggested that the visuo-spatial construction deficit had his source in a weakness in global processing. In another example, when requested to copy displays that have global and local structure such as a letter (e.g. an H) composed of small letters (e.g. S’s), people with Williams syndrome are more likely to reproduce the small letters (S’s) accurately whereas the global letter (the H) is lost in the drawing (see figure 1) (see also Birhle, Bellugi, Delis & Marks, 1989, for similar data).
Figure 1. Examples of hierarchical stimuli models and copies of these models by WS persons.
WS persons also have difficulties to identify incomplete familiar objects embedded in a distracting background (Crisco, Dobbs & Mulhern, 1988). Studies of the drawing skills of individuals with Williams syndrome have considered both the ability to copy geometric figures and to draw or copy objects such as people or houses (Bertrand, Mervis & Eisenberg, 1997). When subjects with WS are asked to draw, either in a copy task or in a free-drawing task, they typically produce fragmented details whereas the drawing displays poor cohesion and lacks overall organization (see figure 2). One explanation of the WS difficulties in various constructive tasks would be in terms of perceptual impairment. However, data available do not seem to confirm this hypothesis. Farran, Jarrold and Gathercole (2001) presented the Children’s Embedded Figures Test (CEFT, Witkin & al., 1971) to WS children and typically developing control children. This task assesses the ability to locate an element (a triangle) embedded in a global image. In this task, WS persons’ results did not reveal any preference towards the stimuli components at the expense of the global
pattern (see also Pani, Mervis and Robinson, 1999, in a visual search task showing that WS processed the stimuli at a global level). Figure 2. Examples of drawings by WS children. Geometric patterns Models
“ Draw a House ” (drawing from memory)
Copy from WS subject
The first example is a geometrical figure copy task. The second one is an object drawing example.
Fayasse (2002) asked WS children and normally developing children matched on mental age to match two sets of Kanizsa figures (Kanizsa, 1976) (see figure 3). Each stimulus from one set of stimuli could be matched with a stimulus of a second set, either on the basis of the emergent shape (e.g., two emergent triangles) or on the basis of the elements used to build each Kanizsa figure (e.g. angular, rounded, etc.). Of course, the stimulus globally similar to the target was different from the stimulus locally similar to the same target. If WS people were biased towards local processing compared with their controls, they should match the stimuli according to their local elements more often than controls. The results revealed no significant difference between the two groups, which suggests that WSs perceptually
process the stimuli in the same way as their controls but could also compare them on the basis of the global shape. Figure 3. “ Kanisza ” stimuli: perceptual global or local mapping in WS? Target stimulus
Local mapping
Global mapping
Up to this point, two hypotheses explain the WS people difficulties in spatial reconstruction. The first one states that WS people have a deficit in processing the global level of stimuli (Bellugi & al., 1994) whereas the second one maintains that WS people deficit is to shift from global to local processing (Pani & al., 1999).
Experiment 1: The block-design task In a recent experiment, Thibaut & Fayasse (submitted) have designed a block-design task to disentangle the role of local and global processing in a visuo-spatial constructive task. Participants had to reproduce a target pattern with blocks from the Wechsler-based Block Design subtest. The models were squares that had to be reproduced as a 2x2 block design. There were two types of target pattern, cued and uncued (see figure 4). For the cued patterns, the constitutive units were clearly separated on the model. They were expected to be easily matched with the corresponding pattern displayed on one block side (see models 1’ and 2’ in figure 4). For the uncued target
patterns, potential units on the model were not “underlined” on the target model (see Figure 4, models 1 and 2). A second variable was the nature of the target pattern. In the first condition (called global), target models had an emergent global pattern so that the pattern units are “ hidden ” in the global pattern (see Figure 4, model 2). In the second condition (called local), target models were composed of units which displayed units easy to notice and thus easy to match with the face of a cube (see model 1, Figure 4). In order to analyze how global and local aspects were processed as a function of type of display, two scores were calculated: a configuration score that refers to the quality of the “copy” of the entire square constructed by participants; an orientation score of the individual cubes of the constructed pattern (see also Akshoomoff & Stiles, 1996, for more details about a similar study in normally developing children). Fourteen persons with WS were compared with 28 normally developing children matched on mental age (the 10 subtests “ Mental Processes ” of the K-ABC, Kaufmann & Kaufmann, 1983). Two results are important. First, WS people were significantly poorer than their controls in reconstructing a local pattern. On the other hand, there was no difference for the global patterns (that is a significant interaction Group by Nature of pattern - local vs global). Secondly, WS people had significant difficulties in reconstructing the target square in the cued condition and had fewer difficulties in the uncued condition (that is a significant interaction Group by Type of pattern - cued vs uncued). The results obtained by WS persons cannot be explained by the two hypotheses mentioned above (global processing deficit or difficulty to disengage from the global processing). The global processing deficit did not predict a better performance in the case of global stimuli and in the uncued condition. Indeed, impaired global processing predicts poorer performance in the global conditions because they are more difficult to analyze and, thus, more difficult to integrate as wholes. On the other hand, the local hypothesis also predicts that local patterns should be
easier for the orientation scores. In fact, if parts of the stimulus are made clear in the model, a local processor should match them with the individual blocks easily.
Figure 4. Examples of stimuli used in the block design task (Thibaut & Fayasse, submitted). Model 1 Local uncued pattern
Model 1’ Local cued model
WS reconstruction
Model 2 Global uncued pattern
Model 2’ Global cued pattern
WS reconstruction
Models 1 and 1’ are local patterns presented in the uncued and cued condition. Model 2 and 2’ are global patterns presented in the same two conditions. The reconstruction of a WS subject is presented under each model.
In order to cope with these results, Thibaut and Fayasse (submitted) have proposed that WS persons have significant difficulties in disengaging from salient local parts. Highly salient components interfere with the way the global shape is processed and decrease the configuration scores in WS people. There is a larger difference between Ma-Matches and WS persons in the cued condition than in the uncued condition, but only for the configuration scores. As for the orientation of the cubes, WS people and normally developing children benefit from the individuation of the constituents in the cued condition and for the local stimuli. Indeed, as the
global shape remains available, the location and identity of each component becomes more salient with respects to the global shape. Since normally developing children have no problem of balance between global and local processing, they are able to locate the individual cubes with respect to the global form. One interpretation of this hypothesis is that, when they explore the faces of a cube, WS’s people have difficulties to match one particular face with one part of the target display. Two facts might explain their problems. First, each face of a cube can be matched with several parts of the target. Second, the faces of the cubes can be matched with different potential parts of the target. By contrast, when the target design is seen as a whole, (in the global and the uncued conditions), parts of the target stimuli are perceptually more embedded in the whole and less perceptually salient and there is less competition between each face of the cube and the several potential parts of the target display with which they can be matched. Experiment 2: Jigsaw puzzles In the following experiment, we manipulated the status of the parts with respects to the whole. Four types of jigsaw puzzles were used to manipulate the status of the pieces in terms of their semantic meaningfulness. Each piece taken separately represents a meaningful part or not. The meaningless type is the classical one in which individual parts are random cuts of the stimulus. Meaningful parts can be of several types. They can be consistent with the target stimulus or not. Consistency means that each piece corresponds to a semantic part of the stimulus. For example, one piece displays an arm and the target is a human body. However, a meaningful piece can be inconsistent with the target stimulus as when a target rabbit must be reproduced with pieces that display vegetables (e.g., the ears are carrots, the head is an apple, etc., see Figure 5.). Each constitutive piece has a meaning in one field (vegetables) but not as a part of the target (rabbit). In the last condition, each piece was a geometric shape (rectangle, etc.) and the target stimulus was composed of these geometric shapes.
Table 1: The four types of jigsaw puzzles Type of puzzle
Relation between pieces and the target parts
Type of parts
Meaningful
Consistent
Semantic parts
Meaningful
Inconsistent (concrete)
Vegetables
Meaningful
Inconsistent (geometric)
Geometric
Meaningless
Meaningless cuts
Random cuts
This condition is different from the “vegetable” condition in the sense that geometric shapes are often used as building blocks of shapes and are less associated with a particular semantic domain. This condition was included because other observations have suggested that geometric shapes are salient for WS persons. For example when asked to describe an object in terms of parts, they would mention “triangle” or “rectangle” for the roof of a house whereas normally developing children would mention a “roof”.
Predictions. The standard local hypothesis predicts a difference between groups for each type of puzzle because Williams should not be able to grasp the whole while reconstructing the puzzle; they should be unable to locate the piece they manipulate with regard to the target model they have to reproduce. Thus, the standard local predicts no difference between the four types of puzzles. The disengaging from the local hypothesis predicts no difference between WS and matched children for the semantically congruent puzzles because the global level does not interfere with the local hypothesis. Indeed, if a WS child focuses on a semantic part, his/her knowledge of the human body will guide the reconstruction process. For the inconsistent puzzles,
by contrast, difficulties with disengaging from the local should interfere with the reconstruction of the whole because there is no correspondence between the two levels. For the random cuts puzzles, as the local cues are sufficient to complete the task, WS’ performance should not differ from their performance in the congruent task. The disengaging from the global hypothesis predicts that while considering the stimulus as a whole, WS children should analyze it into parts less efficiently than MA children. As the object components are hidden in the object, it should be difficult to match the model (the whole) with the pieces. However, WS children should have no difficulty with consistent puzzle because their semantic knowledge of the whole is consistent with what they know of the parts. On the other hand, difficulties should arise with inconsistent puzzles, since the whole cannot be mapped on the parts. The worse condition, however, should be the random cuts condition because there is no direct mapping possible between the semantic description of the target model and the pieces. Indeed, a piece may display incomplete but recognizable information from two different semantic parts (e.g., part of an arm and part of the trunk). This is probably the worse situation because, starting with the whole, no part of the whole can be mapped on any provided part. Thus the hierarchy of difficulty should be semantically congruent > semantically inconsistent > random cuts. To summarize: -
the standard local hypothesis predicts a difference between groups for each type of puzzle;
- the disengaging from the local hypothesis predicts a difference between groups for the inconsistent puzzles; - the disengaging from the global hypothesis predicts a difference for the inconsistent puzzles, the worse condition for WS being the random cuts condition. Methods
Participants Eleven persons with William syndrome (5 boys, 6 girls) were included in this study, ranging in age from 8 years 11 months to 25 years 2 months (mean age = 14;9; SD=5;6).The WS participants were diagnosed by pediatric geneticists. Twenty-two normally developing children (10 girls, 12 boys) were matched individually to each WS subject, using a match-ratio of two normal children for one WS, except for one WS matched to one control. The matching was based on mental age, as assessed by the 10 subtests “Mental Processes” of the Kaufmann Assessment Battery for Children (K-ABC) (Kaufmann & Kaufmann, 1983). The normally developing children ranged from 4 years 4 months to 7 years (mean = 6;6; SD = 0;10). Materials Four types of jigsaw puzzles were created as mentioned above: semantic, vegetables, geometric and random cuts puzzles. We started with three different stimuli, a human being, a horse, and a rabbit. Each of the three stimuli was translated into the four types of puzzles (see Figure 6 for the four types of puzzles created for the rabbit). Thus, there were 12 jigsaw puzzles. In the semantic type, cuttings corresponded to natural parts of the stimulus. For example, the rabbit was segmented into legs, ears, head, body and tail. Second, in the vegetable type, the stimuli were constructed with vegetables (e.g. the rabbit made of carrots, pepper, apple, etc.). Third, for the geometric type, stimuli were made of basic geometric shapes (e.g., the rabbit made of various types of ellipse). Fourth, the random cuts puzzles were obtained by cutting the original rabbit randomly with a pair of scissors, the only restriction being to avoid very small pieces. The mean number of pieces per type of jigsaw puzzles was equivalent.
Figure 6. The four types of puzzles (semantic, vegetable, geometric and random cuts) for the rabbit.
Model
Semantic
Random cuts
Vegetables
Geometric
Rabbit
(cuts = ears, legs, etc.) Note: lines in the random cuts condition correspond to the different pieces of the puzzle Procedure For each puzzle, the pieces were presented scattered on the table. The experimenter showed the target stimulus and told the instruction to the children. For example, the model “rabbit” was shown and the children were asked to use the pieces to reconstruct the model... There was no time limit for each puzzle and each model remained displayed until the subject mentioned that he had completed the task. The puzzles were presented randomly with the additional constraint that two instances of the same stimulus (e.g., two rabbits) would not follow each other. The time necessary to complete the task was recorded for each stimulus separately.
Results Williams syndrome and their MA controls were compared on the four types of jigsaw puzzles. For both groups of participants, the percentage of correct answers was computed for each of the four types of jigsaw puzzles. A correct answer was defined as a correct location (position and orientation) of a piece. Two adjacent pieces had to touch one another. However, because of the young age of some participants and motor difficulties in WS persons, a gap of 5 millimeters between two pieces was accepted. Table 2 presents the mean percentage obtained by the two groups of participants for each type of jigsaw puzzle. A t-test revealed that Williams syndrome’s scores were significantly lower than the equivalent score obtained by the MA controls for the
geometric puzzles (t(31) = -3.87, p < .005) and the vegetables puzzles (t(31) = -3.55 , p < .005). There was no significant difference between the two groups for the random puzzles and the semantic puzzles.
Table 2. Mean percentage of correct answers (standard deviation) obtained by Williams syndrome persons (WS) and Mental Age matches (MA) in the four types of jigsaw puzzles (semantic, vegetable, geometric and random) Participants
Semantic
Vegetables
Geometric
Random
WS
73.9 (23.3)
64 (21.7)
72 (31)
61 (20.6)
MA
84.6 (16)
88.4 (14.2)
96.5 (9)
85.8 (17.7)
After a pre-analysis of the data and observations done during the experiment, we decided to separate high mental age WS (mean mental age: 6 years, 6 months) from low mental age WS (mean mental age: 5 years 3 months). The corresponding MA controls were divided in the same way. A three-way ANOVA (2 x 2 x 4 analysis of variance) with mental age (high mental age vs. low mental age) group (Williams vs. MA) as between variables and type of jigsaw puzzle (semantic vs. vegetables vs. geometric vs. random) as a within group variable was performed on the percentage of correct answers. It revealed main effects of group and age. The MA-matched performed significantly better than WS participants, F(1, 29) = 18,61, p < 0.001, high mental age significantly better than low mental age, F(1, 29) = 10,74, p < 0.005. The main effect of type of puzzle was also significant, F(3, 87) = 4.75, p < 0.01. Interestingly, there was a significant group x type of puzzle interaction, F(3, 87) = 2.75, p < 0.05. A posteriori analyses (Tukey HSD) revealed a significant difference between Williams and MA-Matched for the vegetables, the geometric and the random puzzles.
However, previous analyses suggest that high and low mental age persons with Williams syndrome have very different results in a number of tasks and behave in qualitatively different ways. This seems to be the case here, as shown by the marginally significant three way interaction between group, mental age, and type of jigsaw puzzle, F(3, 87) = 2.47, p < 0.067 (see Figure 7). As shown by this graph, low mental age Williams syndrome persons got equivalent (and poor) results for the four types of puzzles. By contrast, the results obtained for the high mental age WS revealed larger differences between high mental age Williams and their MA matched for the vegetable and geometric puzzles than for the semantic and the random cuts conditions. Figure 7: Mean performance of the four subgroups of participants for the four types of puzzles.
120 100 80
young WS
60 40 20 0 Semantic Vegetable Geometric Type of puzzle Random
Completion times A three way analysis of variance (2 x 2 x 4 ANOVA), with mental age (high vs. low) and group (WS vs. MA) as between variables and type of puzzles as a within variable was performed on the variables. It revealed a significant effect of type of puzzle, F(3,87) = 47.52, p < 0.001 and a
significant mental age effect, F(1,29) = 11.62, p < 0.005: high mental age participants did the task faster than low mental age participants. As shown in Figure 8, the conditions semantic (55.1 sec), vegetables (47.2 sec), and geometric puzzles (51 sec) were completed within the same duration whereas the random puzzles were completed in a mean time of 108.6 sec, that is more or less the double. Figure 8. Mean completion times (in sec.) for the four subtypes of stimuli and for the two groups of participants (WS and MA-matched)
120
Competion time (in sec.)
100 80
W
60 40 20 0 Semantic
Vegetable TypeGeometric of puzzles Random
Note the equivalent profile for each subgroup.
Discussion There are three main results. The first is that low mental age subjects were particularly poor achievers compared with their MA-matched group. This suggests that during the first years, their visuo-constructive capacities, that is the capacity to integrate components in a structured whole, are limited in a very general way. This is suggested, among others, by their poor performance in the semantic puzzles which reveals their incapacity to use the meaning of the
components. The second result is that the difference between high mental age WS persons and their controls is limited to the geometric and the vegetables puzzles. This is consistent with the disengaging from the local hypothesis that predicted interference between salient pieces semantically unrelated with the target model and the integration of these pieces in the whole. However, this hypothesis does not apply to the low mental age WS persons who performed (equivalently) poorly in the four conditions. During development, WS persons’ difficulties remain for the inconsistent cues, a result compatible with the idea that once they process the content of a piece they come back less easily to the global shape in which the piece must be integrated. Interestingly, they perform better when pieces have no or less semantic content (random cuts condition). These results would suggest that one central pervasive difficulty of WS persons is associated with the inhibition of a salient local level of processing in favor of a global level of processing. The third result is related with the completion times. The three conditions in which a piece could be matched directly with a part of the model were completed in the same amount of time, whereas the random cuts puzzles took more time. Most likely, this results from the use of a different strategy for the random cuts puzzle. Here, as the content of each piece cannot be mapped on the model, participants probably try to adjust the pieces according to the borders of the pieces. Is Williams syndrome a deficit in executive functioning ? The previous experiment and the summary of the cube experiment, together with Mervis et al. (1999), suggest that SW have difficulties to shift from salient global parts of stimuli to the whole and to coordinate the whole. In terms of executive functions, this might mean that WS persons have either inhibition problems (they seem to be stuck on a salient part) or flexibility problems (they cannot shift from the part to the whole, come back to another part and then back
to the whole, etc.). We tested this hypothesis (Fayasse and Thibaut, in preparation) with a number of tasks assessing inhibition and flexibility. Testing inhibition was done with the following tests: -
“day-night” test (“when I show the picture of the day you must tell me “night” and when I show the picture of the night, you have to tell “day”),
-
tapping test (“when I tap once, you tap twice, when I tap twice, you tap once”),
-
opposite hand movement test (“when I knock on the table with the fist, you knock with the palm; when I knock with the palm, you knock with the fist”),
-
pointing gesture inhibition test (“when I tell you “floor” you point to the floor, when I tell you “ceiling”, you do not point –no movement-”). Flexibility was assessed with the alternated fluency test (“give the name of animals and
clothes, first an animal, then a garment, an animal then a garment, etc.”) and with the alternated striking out test (“cross out a round then a cross, a round, a cross, etc.”). WS persons got significantly higher results in the alternate fluency test than their control. This is not surprising given that WS persons’ lexicon have been described as larger than the lexicon of the MA-matched persons. In the tapping task, the SW latency was significantly larger than MA controls’ latency. In the opposite hand movement, SW performance was also significantly below that of MA controls. These results suggest that aspects of inhibition capacities in WS persons are weaker than the equivalent processes in MA persons. This observation is consistent with the afore-mentioned hypothesis that visuo-constructive deficits in WS children are associated with inhibition. However, which dimension of inhibition is impaired is not clear at this stage. The Day-Night test and the tapping-tasks are supposed to assess the inhibition of a dominant response (Diamond & Taylor, 1996). Thus, one should expect a difference between our two groups in both tasks. However, the two tasks differ in the response modality required, that is telling the answer on the
day-night and enacting the answer on the tapping task. Given that the two groups were different on the opposite hand movement task, which also has a motor component, it is plausible that the inhibition component involved in the difference between WS and controls is inhibition of motor responses rather than inhibition of verbal components, which, in turn, is compatible with the observation that WS persons are impaired in visuo-constructive tasks (see Dempster, 1992; KippHarnishfeger, 1995, for a discussion of the notion). General discussion The present contribution contrasted three explanations of visuo-constructive problems associated with the Williams syndrome. We believe that the standard local hypothesis cannot account for all the results. By contrast, these results, especially those of the high mental age WS seem to be more compatible with the disengaging from the local hypothesis. The main difference between the two hypotheses is that the standard local hypothesis implies a weakness in global processing whereas global processing difficulties do not necessarily result from the disengaging from the local hypothesis. Indeed, the probability of global processing difficulties should increase when local components are more salient or when the requirements associated with the processing of local components are important, as when the whole stimulus is composed of a large number of components. Generally speaking, we believe that the disengaging from the local hypothesis is compatible with most of the observations regarding visuo-constructive problems. It is compatible with observations regarding WS’ drawings. For example, scattered drawings (see above for examples) result from the necessary focus on components in a stimulus copy task. Drawing is a sequential task in which one has to combine components in a whole. Thus, components are salient, by definition of the task. There are other observations compatible with the hypothesis. The difference between WS persons and their MA controls increases when the number of parts in
a task increases. For example, in a jigsaw puzzle task, we noticed that WS persons were equivalent to their controls when the number of pieces was small. WS performed significantly worse with more complex puzzles. During the construction of simple puzzles, it is easier to relate any piece with the whole and the other pieces than during the construction of more complex puzzles.
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