Spatial breakdown in spatial construction: Evidence from eye fixations in children with Williams syndrome
Introduction
Breakdown in human spatial organization can occur under a wide variety of pathological conditions, including lesions to the adult brain (Behrmann, 1999), fetal or environmental insult to the developing brain (Diamond, 1998; Stiles, 1998; Stiles-Davis, Kritchevsky, & Bellugi, 1988) and genetic deficit, such as Turner syndrome (Rovet & Buchanan, 1999) or Williams syndrome (Bellugi, Mills, Jernigan, Hickock, & Galaburda, 1999; Mervis, Morris, Bertrand, & Robinson, 1999). Although the circumstances of breakdown and the nature of ensuing impairment are clearly important in their own right, they can also shed light on the nature of normal cognitive architecture. One way this can occur is through observation of unusual dissociations between cognitive functions, which can shed light on the nature of cognitive architecture. Another way is through observation of unusual interactions between cognitive functions, which can shed light on the degree to which cognitive impairment can drive new solutions to cognitive tasks.
In this paper, we use this framework to examine spatial breakdown in children with Williams syndrome. We focus on a micro-analysis of one task that has been widely used to document spatial deficits in people with Williams syndrome, as well as those resulting from other kinds of brain damage in children (Vicari, Stiles, Stern, & Resca, 1998) and adults (Akshoomoff, Delis, & Kiefner, 1989; Caplan & Caffery, 1992; Ivry & Robertson, 1998). In this “block construction” task, people are shown a model pattern and are asked to create a replica of it in an adjacent “copy” space, using individual blocks that they can select from a larger set (see Fig. 1). Versions of the task can be found in a variety of intelligence tests, including the Wechsler Adult Intelligence Scale (WAIS-R; Wechsler, 1981) and the Differential Ability Scales (Elliott, 1990). Profoundly poor performance is diagnostic of a spatial deficit. Yet the detailed nature of the deficit that leads to poor performance in these tasks has eluded our understanding, largely because the task is quite complex, drawing on a number of possibly different capacities. These may include perception and representation of the spatial relationships within and between blocks in the model, spatial working memory, and executive processes, which guide activities such as the observers’ search for information in the model, their search for individual blocks, and any attempts to correct their copy.
In our paper, we build on an analysis of this task that has been offered to explain the mechanisms of normal performance among adults who are not spatially impaired (Ballard, Hayhoe, Pook, & Rao, 1997). We argue that the block construction task is remarkably complex, and that consequently, breakdown in performance can occur at any of a number of steps along the way. Despite this complexity, we also argue that the spatial breakdown among children with Williams syndrome can be well understood by separately considering different requirements of the task. In particular, we present evidence showing that (a) the critical breakdown in WS occurs in the spatial representations within and between individual blocks of the model, and that, by contrast, (b) the executive processes that guide problem solving are relatively intact. This evidence shows that the two processes are separable. However, we also show that (c) these two classes of capacity interact in such a way as to produce increasingly impoverished performance as the puzzles become more complex. The latter kind of interaction is consistent with theories of “cascading” processes (e.g., Thelen & Smith, 1994), in which one aspect of breakdown can set the stage for changes in other processes, both within a task and over the course of development. We suggest that our analysis provides an important framework for yielding insights about normal spatial representation, as well as the nature of spatial breakdown in Williams syndrome and other cases of spatial deficit.
Section snippets
Williams syndrome and spatial deficit in the block construction task
Williams syndrome (WS) is due to a rare genetic defect (1 in 20,000 births) whose unusual cognitive profile has recently attracted considerable attention from cognitive and neuroscientists: Individuals with WS have a unique profile of profound spatial deficit together with relatively spared language capacities (Bellugi, Bihrle, Neville, Doherty, & Jerigan, 1992; Bellugi, Wang, & Jernigan, 1994; Mervis et al., 1999). The syndrome is caused by a hemizygous submicroscopic deletion of chromosome
The role of eye fixations in separating causes of impairment
How might we separate the two possible sources of error? Adapting Ballard’s analysis, we propose to do so by examining the sequence of eye fixations as children carry out the task. Suppose, for example, that WS and control children are comparable in the accuracy of their representations of the identity and location of individual model blocks but that WS children sometimes fail to look at the model prior to a drop. This could occur because of faulty executive processes, which would normally
Experiment 1
In this experiment, we tested children and adults on a full, standard version of the Block Construction task, adapting materials from the Pattern Construction Sub-test of the DAS. We designed a computerized version of the task, in which children could use a mouse to move individual blocks from the parts area to the copy area. We examined their eye fixations while they carried out the task, as well as various key aspects of their performance.
Participants. Participants included 8 children with
Performance data
Performance on the puzzles was evaluated using two measures: (1) the percentage of individual block placements or “drops” that were correct and (2) percentage of correctly solved puzzles. These measures were entered into a mixed model analysis of variance with group (WS, control children, and adult) as the between-subjects factor and format (segmented vs. standard) as the within-subjects factor. Separate analyses were performed on simple and complex puzzles and included the relevant display
Discussion
We distinguished between two broad classes of processes that are important to success in block construction: Executive processes, which sequence the gathering of information from the model, the search of the parts area, the placing of parts in the copy, and the initiation of error–repair sequences, and spatial representational processes which extract and store the identity and locations of individual blocks. The bulk of the evidence suggests that the root of the WS deficit lies in faulty
Experiment 2
In this experiment, we tested children’s capacity to represent identity and location while reducing the executive processes that would be required to solve the whole puzzle. If we are correct in assuming that faulty spatial representations are responsible for the WS spatial deficit, then similar deficits should be observed in these experiments. In Experiment 2A, we evaluated children’s ability to encode the identity of a single cued block in the model and find the matching block in the parts
General discussion
In the present set of experiments, we sought to use evidence from eye fixations to gain insight into the nature of spatial breakdown in children with Williams syndrome when they attempt to reconstruct visual patterns. We assumed that it should be possible to separate two distinct sources of breakdown: Impaired spatial representations (including the spatial structure of individual blocks and spatial relationships among blocks) and impaired executive processes (including sequencing of fixations
Acknowledgements
This work was supported in part by Research Grant No. 12-FY-99-670 from the March of Dimes Birth Defects Foundation, SBR-9808585 and SBR-0196314 from the National Science Foundation, and 1 R55 NS37923 from the National Institutes of Neurological Disorders and Stroke. The authors thank Laura Carlson for comments on an earlier draft of this paper and Andrea Zukowski for help with all aspects of this work. We are also grateful to the children who participated in these studies, their parents, and
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