UN MÉTODO DE VISUALIZACIÓN, ESTUDIO Y MEDIDA DE ABERRACIONES

The TMT (Reitan and Davison, 1974) is a commonly used neuropsychological instrument which assesses multiple cognitive processes (Lezak et al., 2004). The test comprises two parts, the first section, referred to as Trails A or the TMT part A, requires the subject to join a series of numbered circles in increasing numerical order as quickly as possible. This part of the test assesses visual search (O’Rourke et al, 2011) and processing speed. Part B of the test, referred to as Trails B or TMT part B, requires the subject to join a sequence of circles containing numbers and letters, alternating between numbers and letters, in increasing numerical/alphabetical order. This part of the assessment is a measure of EF, particularly visual search and set switching (O’Rourke et al., 2011). The most commonly used scoring system is the time taken to complete each part in seconds (Ashendorf et al., 2008). Other scoring methods which take into account the number of errors, ratio scores (Giovagnoli et al., 1996) and the difference in score between part A and B have also been investigated (Arbuthnott and Frank, 2000). Arbuthnott and Frank (2000) stated that subtraction of the time to complete part A from part B removes the motor component of TMT part A and gives a better index of EF. The time to completion still remains the most common scoring method, as even cognitively

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normal participants are known to make errors on this test (Ashendorf et al., 2008).

Some studies have noted main effects of age (Coffey et al., 2001; Tombaugh, 2004) and education (Horton and Roberts, 2001) on this assessment. Hammar and Ardal (2009) found significant effects of depression on TMT score, whereby, depressed individuals took longer to complete both parts of the test. This is an important consideration for AD research as depression is a common co morbid condition in both MCI and AD (Overshott and Burns, 2004). Symptoms of depression can present as a cognitive impairment, highlighting the need to check for depressive symptoms before the interpretation of neuropsychological test scores.

Research has shown that the score on TMT part B may be useful in predicting the likelihood of progression from aMCI to AD. Chen et al., (2000) reported that TMT part B was sensitive to group differences of pre symptomatic AD patients and healthy controls. Further, a combination of delayed word recall scores and TMT part B score were the optimal set of cognitive measures used to predict conversion. Likewise, Ewers et al., (2012) reported that the TMT part B has significant value for the prediction of AD development in MCI cases. In their sample of 81 AD patients and 101 cognitively normal elderly individuals, TMT part B score was one of the best single predictors of subsequent conversion to AD. Similar findings were also found by Samtani et al., (2012).

Brown et al., (2011) found that the processing speed component of TMT part A correlated with greater functional impairment in 394 aMCI participants in comparison with 229 cognitively normal participants and 193 mild AD patients. From their study of 42 MCI participants, Chapman et al., (2011), suggest that measures of speeded EF, such as the TMT part B, may serve as an early detection tool as they are sensitive to conversion from MCI to AD. Impairments in rapid switching between sets may be useful as a potential screening method. Scores recorded for the purpose of the experiments of this thesis will be the time in seconds taken to complete each part of the test.

72 4.6 Dual Tasks (DT)

The DT is a pen and paper test of EF. Participants are given an A4 piece of paper depicting a string of connected boxes and are first timed for one minute to put a cross in each box following the direction of the string (Baddeley et al., 1997). Participants then repeat the task but are simultaneously required to remember and repeat strings of numbers based on their digit span score (see section 4.3.7). They are, therefore, required to place a cross in each box at the same time as repeating aloud strings of digits. The scores used in the evaluation of test performance are the difference in the number of boxes crossed between the first and second times (DT T1-T2) and the dual task ratio (DTR) which is calculated as the ratio of boxes crossed to the number of digit strings remembered.

Generally it is expected that performance will decline when two tasks are performed simultaneously due to the larger cognitive demand and the requirement of dividing attention between the two tasks (Nebes et al., 2001). Baddeley et al., (1986) and (1991) stated that in AD patients there is a clear deficit when completing two tasks simultaneously, an effect which is not common in normal aging (Della Sala and Logie, 2001). This is likely to be a result of a deficit in the central executive component of working memory (Baddeley et al., 1991). This would suggest that both AD and MCI participants would be impaired on the DT (Lonie et al., 2009). There are conflicting results surrounding the time of onset of divided attention impairments in AD. Both Baddeley et al., (2001) and Logie et al., (2004) report impaired DT performance in AD patients, even in the early stages of the disease. Furthermore, Baddeley et al., (1991) report that the size of the dual task deficit increases as the disease progresses. Lonie et al., (2009), however, report that in aMCI, there is no impairment of dual task performance in comparison to healthy non demented elderly participants. Likewise, there was no impairment in early AD participants either. Early AD patients were, however, impaired on the TMT part B, an alternative assessment of divided attention which has a speeded component (discussed in the previous section 4.3.3). Further, Greene et al., (1995) reported that dual task impairment occurs only in the later stages of AD when MMSE score falls below 23 out of 30.

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The discrepancies between these studies may be due to slight differences in the methodology employed. For example, differences in the time used to complete each stage, Lonie et al., (2009) used a total time of 90 seconds whereas Baddeley et al., (2001) used a longer duration of 120 seconds. It is difficult to make direct comparisons between the data sets. There may have been differences in disease severity between participants groups. The results of these studies, therefore, suggest that the DT is not sufficiently sensitive for the detection of early AD (Lonie et al., 2009) but maybe useful alongside another assessment.