This section introduces characteristics in the non-verbal cognitive domains in individuals with PWS, including intelligence, memory, visual and auditory processing, and higher-order processing.
2.2.1 Intelligence
Depressed intellectual functioning was one of the defining characteristics of PWS (Dunn, 1968; Prader, Labhart & Willi, 1956; Zellweger, Schneider &
Johannsson, 1968). PWS is usually associated with mental retardation (50-70 % of cases) or low-normal intelligence (Curfs & Fryns, 1992; Holm, 1981). From the 57 studies reviewed and summarized by Curfs and Fryns in 1992 on 575 individuals of different ages and of different nationalities, the distribution of IQ scores is as follows:
Normal 4.9%, Borderline 27.8%, Mild Mental Retardation 34.4%, Moderate Mental Retardation 27.3%, Severe to Profound Retardation 5.6%. Gender was not a
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significant factor that influenced the distribution of the IQ scores. In addition to the severity of mental retardation of PWS, stability of cognitive functioning has also been investigated. Previous studies demonstrated that IQ scores in PWS declined with age (Crnic, Sulzbacher, Snow & Holm, 1980; Dunn, 1968; Stein, Hutt, Spitz & Hollander, 1993); however, there was little subsequent study that could verify this claim.
Therefore, the current consensus is that the cognitive trajectories in PWS appear to be stable over time as opposed to other genetic disorders affecting intelligence, such as Fragile X and Down syndromes (Dykens, Hodapp, Walsh & Nash, 1992; Greenswag, 1987).
2.2.2 Memory
It has been found that short-term memory is relatively weak in children with PWS (Warren & Hunt, 1981). In Warren & Hunt‟s study, children with PWS were matched in age and IQ with children without PWS but with mental retardation of unknown etiology. They were asked to perform a series of short-term memory tasks as well as a long term memory task of phonological code retrieval. The children with PWS performed as well as the ones without PWS on the long-term retrieval task, but they performed worse on the short-term memory tasks: they lost more information that had been learned over time. Consistent with the previous findings, Conners, Rosenquist, Atwell and Klinger (2000) also found that long-term memory is superior to short-term memory in the PWS group, but not in the control group (nine age- and IQ-matched adults without PWS).
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2.2.3 Visual and Auditory Processing
Visual processing is superior to auditory processing in children with PWS (see Baker & Leland, 1967; Conners, Rosenquist, Atwell & Klinger, 2000 for a different view). Cognitive skills, such as visual perception, organization, and puzzle-solving abilities, have been noted as relative advantages in some people with PWS (Dykens, 2002). Taylor and Caldwell (1983) suggested that PWS participants performed superiorly to those overall-IQ-matched obese control participants in picture
completion, object assembly, and block design, on Wechsler Adult Intelligence Scale (WAIS). Curfs, Wiegers, Sommers, Borghgraef and Fryns (1991) also showed that some individuals with PWS possess the capability to recognize and evaluate figural relations greater than would be expected in a similar block design on the Wechsler Intelligence Scale for Children (WISC), as evidenced by 9 of 26 PWS children in their study. Corroborating previous findings, an event-related potential study by Stauder, Brinkman and Curfs (2002) revealed that although the short term memory is impaired in PWS patients, their visual modality is more preserved (i.e. less affected) than their auditory one.
2.2.4 Higher-order Processing
PWS patients show deficits in higher-order cognitive processing, such as abstract thinking, executive functions, and metacognitive abilities (Whitman & Thompson, 2006). Metacognition abilities are defined as “thinking about thinking” or “a higher level (usually a regulatory level) that controls or dominates thinking”. Simply put, it is concerned about the awareness or analysis of one's own learning or thinking processes or it is also paraphrased as the use of executive processes in overseeing and regulating cognitive processes. Many affected individuals show difficulties changing viewpoints (i.e. obstinacy, inflexibility or rigidity of thinking) even when they are proven wrong,
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have problems generalizing (i.e. poor at generalizing from one situation to another to see cross-situational commonalities or similitude), and have impulsive behavior without elaborate (sophisticated or fully-planned) thinking (Sullivan &
Tager-Flusberg, 2000; Tager-Flusberg & Sullivan, 2000).
These abstract-thinking difficulties and metacognitive malfunctioning traits lead to academic and social functioning inabilities, which further impact on the
mobilization of executive functions, the use of memory, visuomotor skills, language, and objective judgment among PWS individuals (Whitman & Thompson, 2006). In fact, both Holm (1981) and Sulzbacher, Crnic, and Snow (1981) suggested that, based on the results from independent parent surveys, reading development is close to normal, while math and number concepts are relatively weak in PWS patients.
Dykens, Hodapp, Walsh and Nash (1992) also reported that reading and math are slightly differentiated, with math almost a year delayed of reading. As to reading specifically, PWS patients show relative strengths in written language skills, which include vocabulary knowledge and reading decoding. Good reading decoding ability may result from the visual spatial skills of individuals with PWS that have been reported as a relative strength (Dykens, Hodapp, Walsh, and Nash, 1992). However, some studies also show that reading can be weaker than math (Conners, Rosenquist, Atwell & Klinger, 2000). Roof, Stone, MacLean, Feurer, Thompson and Butler (2000) studied different genetic subtypes of PWS and found that patients with a deletion on average have significantly lower verbal IQ scores than those with uniparental disomy (UPD). Plus, for those with a deletion, performance IQ exceeds verbal IQ while for those with UPD the opposite pattern is manifested, further suggesting that those with a deletion may have relatively greater visual-perceptual-spatial abilities, while those with UPD may have relatively greater verbal skills. This could account for the reported phenomenon that those with UPD do substantially greater on reading and
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spelling than do those with a deletion. As for math achievement, little difference was noted between the two genotype groups on arithmetic skills, with both uniformly impoverished (Butler, Bittel, Kibiryeva, Talebizadeh & Thompson, 2004; Whittington, Holland, Webb, Butler, Clarke & Boer, 2004).