Saling et al (2002) found a significant difference between a group of mild to moderate DAT patients and a control group in their ability to detect clock anomalies. In the current study however, no significant difference was found between early DAT patients and the control group on the CADT. Instead, some cases in which patients provided explanations that were limited to the clock face rather than to the conceptual level were observed. The unexpected finding however, was the patient's difficulty to correctly classify non-anomalous clocks. Compared to the control group who invariably correctly classified the valid clocks, DAT patients rejected the valid clocks frequently. This finding is not directly comparable with those obtained by Saling et al as anomalous clocks only were used in their study. In accounting for the results obtained in this study both theoretical and methodological issues are raised below.
Theoretical issues.The second hypothesis in the present study postulated that the CADT would not distinguish between the groups. The rationale was that as the deterioration of semantic memory has been found to be related to temporoparietal glucose hypometabolism (Hirono et al., 2001; Salmon et al., 1999) which becomes more evident after the transentorhinal period (Nagi et al., 1999), it is expected that in very early DAT semantic functioning is relatively intact. In light of this, the results obtained for detection of clock anomalies are in accordance with the temporal schedule proposed for hypometabolism of the temporoparietal area (Mosconi et al., 2004; Nagy et al., 1999). By contrast, clock-drawing on routine neuropsychological examinations was impaired for most of the patients and this will be elaborated on below. Further, the noted tendency of some patients to provide explanations that related to the surface level of the clock face and not to the conceptual level of the anomaly can be interprete d as early signs of a semantic system breakdown. It is possible that as semantic deterioration progresses, DAT patients first experience a difficulty in identifying the conceptual level of the anomaly, and at a later stage fail to recognize an anomaly altogether. Nevertheless, the finding that DAT patients failed in the classification of valid clocks implies the presence of a semantic impairment that may be limited to the ability to recognize valid exemplars of a category.
Traditionally, semantic memory is regarded as a sub-category of long-term memory which stores encyclopedic information about the world, such as words, facts concepts and their meaning. These are usually not attached to a time, place and context but are instead more general in their nature (Hirono et al., 2001; Spaan et al., 2003; Tulvin, 1983). Early theories on the organization of semantic knowledge emphasized a hierarchy of conceptual knowledge, and the reliance on core attributes in assigning a particular exemplar to a category (Collins & Quillian, 1969). Later models that emphasized the notion of typicality are regarded as providing more coherent explanation of the way conceptual knowledge is represented in semantic memory (Collins & Loftus, 1975; Smith, Shoben, & Rips, 1974). According to this view, the more a given exemplar shares features with a prototypical representation, the easier it is to categorize it. Although people may judge a particular exemplar as a member of a more general category on th e basis of prototypical representation, retrieval on the basis of core attributes is still possible.
It is possible that as conceptual attributes are less well represented with advancing DAT (Salmon et al., 1999), there is a growing reliance on prototypical appearances of objects and reduced ability to refer to core features when needed. Hence, the tendency to reject the valid clocks, noted by DAT patients, could be explained by the fact that some of these clocks were less prototypical in their appearance, for reasons that did not violate any core aspects of the clock face. Indeed, where the valid clocks were portrayed in an absolutely prototypical fashion, the issue did not arise. Conversely, DAT patients showed difficulty with those valid clocks that had less of a typical portrait (for example, had no numbers on it) of a clock face. By contrast, although the invalid clocks violated the core features of the category (for example, additional hands or more than an hour represented in the clock), they also greatly violated clocks typical appearance, and therefore correctly identified as invalid.
This possibility is supported by a recent series of studies by Rogers et al. (2003) that suggests that the nature of semantic breakdown starts with a weakened ability to recognize less typical examples of a category before a failure to recognize invalid examples of the category emerges (Rogers et al., 2003). Further studies are needed in order to further clarify the nature and staging of the semantic breakdown in DAT. If supported, this theory implies that an impaired ability to correctly classify non-regular examples of a category could be sensitive to amnestic MCI or very early DAT, while a failure to identify anomalies in invalid clocks could reflect a more advance stage of DAT.
Methodological issues.Importantly, there are some methodological differences between Saling et al (2002) and the present study. First, the two studies did not use the same set of anomalous clocks and it is therefore possible that the particular clocks in the two studies differed in such ways that influenced the results. Second, in the Saling et al study, the instruction given to the participants specifically said that the clocks had one anomaly, and participants were asked to point it out. By contrast, in the present study, as valid clocks were also included, the participants were asked to judge whether they think the clocks were valid or anomalous. It is possible that this difference in instruction led to different involvement of other cognitive systems (e.g. attention) in the performance. Future studies are needed in order to better evaluate differences in administration of the CADT and in order to understand which method is more likely to best reflect an underlying semantic dimension and to under stand the extent to which other cognitive systems are involved or interact with performance on this task.
In the present study, three out of four patients had an impaired performance on the Clock-Drawing Test administered as part of the routine neuropsychological examination. This is somewhat surprising in light of the relatively intact performance in detecting clock-anomalies, and contrasts with the findings from Saling et al. (2002). This discrepancy could be partly attributed to deviations from previous methodology, and additional studies are required using this paradigm.
Taken together, participants' performances on the CADT and OPAT were a predictor of their membership group. Discriminant Function Analysis that included the delayed trial of the OPAT and the valid clocks section of the CADT, classified all participants in their original groups. In other words, participants could be correctly assigned to the appropriate diagnostic group solely by means of their performance on a delayed trial for object-place associations, and their classification of the five valid clock faces.