The patient group demonstrated considerable difficulty in the Object-Place Association task, and performed significantly worse than a matched control group on this task. This finding is consistent with previous studies such as Fowler et al. (2002), and Blackwell et al. (2004), that demonstrated this difficulty using the computerized PAL task, and provide further support for the sensitivity of this paradigm to the earliest stages of AD.
The PAL and OPAT tasks require the patient to form an associative memory for a certain object seen in a particular location. While such difficulty can be seen commonly among normal elderly people showing age-related decline in cognitive ability, the difficulty is now known to be significantly more profound among patients with early signs of dementia (Blackwell et al., 2004; Fowler et al., 2002). The entorhinal cortex (EC) and the adjacent perirhinal cortex have been shown to be important medial-temporal structures for intact associative new learning, especially for arbitrarily related items (Fowler et al., 2002; Saling, 2004; Saling, O'Shea et al., 2002). The evidence that the EC is amongst the earliest brain structures to suffer from neuronal loss in DAT provides strong support for the difficulty that these patients exhibited in the current and in previous studies in tasks that tap into the function of medial-temporal lobe structures.
The formation of an association between the objects and their location was significantly impaired in the study group. Two out of the five patients reached perfect performance in the third and fourth trials of the acquisition phase, with the remaining three failing to form the associations by the fifth trial. Examination of the groups' learning curve showed no significant improvement in performance. By contrast, excluding one control subject who reached perfect performance by the fifth trial, the other participants needed no more than three practice trials to reach perfect performance, with one participant perfectly recalling all items' locations from the first trial. As four out of the five patients couldn't remember the location of all objects, all five learning trials were administered to them. Importantly, a significant difference between the two groups was found with only the first three trials included in the analysis, suggesting that in practical terms, three trials could be reliable in indicating as sociative memory acquisition difficulties.
OPAT-delayed recall:As one would expect, as DAT subjects could not successfully form object-place associations, their ability to maintain such associations was very poor. While all but one control participants remembered all five items, DAT patients remembered between none and three objects. It could be argued that the patients' lack of ability to remember the location of the items following a delay results from their greater difficulty in forming object-place associations. While this account might be correct, it seems that the rates of forgetfulness were higher among the patient group as even the two patients who by the fifth trial reached perfect performance were unsuccessful in recalling all five associations when tested after a delay.
In the context of the present study, the question of whether the associations between items and locations per se were the critical factor in performance was not directly investigated. It could be argued that the patients recalled fewer of the objects. While this explanation is likely to have accounted for some of the difference between the groups' performance, only one patient could not remember the objects themselves on the delayed recall trial, with the remainder remembering the object's names, but pointing to the wrong boxes. Future studies are needed to examine the relationships between memory for objects, locations, and object-place associations to resolve this theoretical issue further. The current literature, however, strongly suggests that the earliest detection of AD depends on the use of an arbitrary association paradigm (Blackwell et al., 2004; Fowler et al., 2002; Swainson et al., 2001).
The OPAT used in the present study is a simple desktop version of the computerized PAL task, and in some clinical settings might be more effective especially where time constraints, equipment availability, professional knowledge and effort on behalf of patients are of paramount importance.