Materialspecific Impairment Of Memory

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Temple (1992b) described Dr S., an intelligent and highly educated lady, who had sustained no neurological injury or disease and was in good health. However, throughout her life she had experienced severe difficulties with memory for faces and visual patterns. Another member of her family is also reported to have similar difficulties. Dr S. had high academic attainments, reflecting good semantic memory. She also had a well-developed vocabulary. Further, her episodic memory for the stories in the Wechsler Memory Battery was very good. However, she had a severe impairment in memory for faces, houses, buildings and visual patterns. The impairment in memory for faces produced a specific form of developmental prosopagnosia. The pattern of her difficulties on face processing was interpreted in relation to the model of Bruce & Young (1986). In relation to this model, it was suggested that Dr S. had specific difficulty in retaining or making use of the face recognition units needed to activate person identity information. Since verbal memory is good, she thus has a material-specific impairment in memory affecting visuospatial material. This focal impairment has been a lifelong deficit coexisting with a successful professional career.

Bishop et al. (2000) described material-specific memory impairments in Turner's syndrome (TS). TS is a genetic disorder in females, in which the second X chromosome is absent or abnormal. Intelligence spans the normal range but there is a set of specific learning difficulties. Cerebral glucose metabolism studies of brain activation and MRI studies suggest bilateral, parietal and occipital lobe involvement in TS (Clark et al., 1989; Elliott et al., 1996; Murphy et al., 1993; Reiss et al., 1993). However, Murphy et al.'s (1993) study also indicated bilateral involvement of the hippocampi. In a MRI study of monozygotic twins discordant for TS, Reiss et al. (1993) concurred with the parieto-occipital locus on the right with a parietal-perisylvian locus on the left. However, in addition, they found marked discrepancies between the twins in right prefrontal areas.

Impairment in visual memory has been documented in a number of studies (Alexander et al., 1966; Clark et al., 1989; Lahood & Bacon, 1985; Lewandowski et al., 1985; Riess et al., 1993; Rovet & Netley, 1981; Silbert et al., 1977; Waber, 1979). Pennington et al. (1985) reported either a verbal or a nonverbal memory deficit in 8/10 cases of TS. Surprisingly, since some theoretical interpretations of TS have implicated the right hemisphere, Pennington et al. (1985) found that 7/10 cases had episodic verbal memory impairments, whilst impairment in nonverbal episodic memory was found in only 4/10.

However, Bishop et al. (2000) argue that the pattern of memory performance varied, depending upon whether the single X came from the mother (Xm) or the father (Xp). She reported that for immediate verbal recall, both TS groups obtained significantly higher scores than control boys, and the Xp group also outscored control girls. This result is consistent with a number of studies indicating elevated verbal skills in TS (Shaffer, 1962; Lahood & Bacon, 1985; Temple, 1996, 2002; Temple & Carney, 1996). However, in relation to the elevated immediate recall level, the Xm group showed enhanced verbal forgetting. Thus, although both immediate and delayed recall scores were in the normal range, Bishop et al. (2000) argued that the relationship between the two was abnormal, with the 45Xm group showing elevated immediate recall but then disproportionate forgetting with delay. In a visual memory task, both groups performed poorly on the initial copy of a design, and both were significantly impaired on delayed recall of the design. However, Bishop et al. (2000) argued that the Xp group, who appeared marginally better than the Xm on the initial copy, had nevertheless forgotten a disproportionate amount with delay interpreted as enhanced spatial forgetting.

As Maurer (1992) pointed out, many learning disabilities can be recast as disorders of material-specific memory. Exploring this idea within the framework of developmental cognitive neuropsychology provides a number of examples. Temple (1986,1995) described the case of a developmental anomic, John, who had particular difficulty in naming animals in comparison to indoor objects. The impairment with animals was pervasive across tasks and at least part of his difficulty appeared to arise from a failure to establish effective semantic representations for animals, and may therefore be thought of as a selective impairment of the semantic system and thereby memory.

The reading disorder, surface dyslexia (e.g. Coltheart et al., 1983; Castles & Coltheart, 1993, 1996), is characterized by relatively good phonological reading skills and ability to read nonwords but poor development of the lexico-semantic reading route employed in the reading of irregular words and the reading of highly familiar established words. This difficulty could be characterized as a difficulty in developing the memory that enables word recognition. The surface dyslexic fails to recognize words that would be recognized with ease by his peers.

Similarly, in surface dysgraphia, there can be well-developed phonological spelling systems associated with using the sound of the word to analytically determine its spelling. In contrast, there is difficulty with the memory for spelling patterns, which is essential for the spelling of irregular words in English and for spelling the many words for which the representation of the vowel is ambiguous with multiple possibilities.

Number fact dyscalculia (Temple, 1991, 1994, 1997b) is a selective impairment which affects the development of factual knowledge about arithmetic and tables, with intact development of number processing and knowledge of arithmetic procedures. This could be considered a material-specific impairment of memory for arithmetical facts. Other aspects of semantic memory can be quite normal (Temple, 1997a).

However, formulating these selective disorders as material-specific impairments of memory is, to some extent, simply redescribing their problems without addressing anything more fundamental. It may be more constructive to continue to discuss such disorders in relation to the cognitive domain within which they occur.

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