Participants visited the laboratory on two occasions. Three to five days prior to the ERP experimental session, each volunteer was trained on the exclusion task procedure
(described in section 5.4) and completed a battery of neuropsychological tests.
Neuropsychological testing involved the National Adult Reading Test (NART), the Beck Depression Inventory (BDI, Beck et al., 1961), Logical Memory I and II from the
Weschler Memory Scale – Revised (WMS-R, Weschler, 1987), and the older adults completed the Mini Mental State Examination (MMS, Folstein et al., 1975) to ensure they did not have dementia. Participants also completed four tests from the Cambridge Neuropsychological Test Automated Battery (CANTAB): Spatial Span, Spatial
Working Memory, Stockings of Cambridge, and the ID/ED (Intra Dimensional/Extra Dimensional) Set-Shifting task. The instructions given for each task was taken from manuals provided by CENES, the company responsible for the marketing of the CANTAB tests. A description of each test is provided below.
Spatial Span
The Spatial Span task is a non-executive task of spatial short-term memory capacity.
The test assesses the participant’s ability to remember the spatial locations of a sequence of squares on a touch screen computer monitor. On each trial, a display of nine white boxes is presented on the screen. Participants watch while each white box changes colour before being asked to reproduce the sequence. The sequence begins with two boxes changing colour and increases one box at a time up to a maximum of nine. The participant’s spatial span score is defined as the longest sequence that they could reproduce correctly within three attempts.
ID/ED (Intra Dimensional/Extra Dimensional) Set-Shifting
This task assesses a participant’s ability to focus attention on specific attributes of compound stimuli (intradimensional stages) and to shift attention when required to a previously irrelevant stimulus dimension (extradimensional stages). The participant is required to learn a series of discriminations in which responding to one of two stimuli is correct and the other wrong. The computer provides immediate feedback automatically.
The task involves nine stages with the participant proceeding to the next stage when a criterion of six consecutive correct responses had been attained. Failure to achieve this criterion within 50 trials results in the premature discontinuation of the test. In the first two stages, participants were tested on simple discrimination and reversal for two stimuli varying in just one dimension (irregular purple filled shapes). A second
alternative dimension was then introduced (two different white line configurations) and compound discrimination and reversal were tested. To succeed, participants must continue to respond to the previously relevant dimension (purple shapes) while ignoring the presence of the new irrelevant dimension (white lines).
At the intradimensional shift stage, novel examples of each of the two dimensions are introduced and participants must continue to respond to one of the two examples from the previously relevant dimension (purple shapes) and complete a reversal stage where they followed the other relevant example (second purple shape). The final two stages, the extradimensional shift and reversal stages, commence with the introduction of novel examples of each stimulus dimension. In order to succeed at this stage, the participant has to shift ‘response set’ to the previously irrelevant stimulus dimension (white lines).
PET scanning studies have shown that the critical ED shift stage involves regions within the anterior frontal lobe (Rogers et al., 2000). The main measure of performance on this task is the furthest stage successfully attained.
Spatial Working Memory
The Spatial Working Memory task is an executive task that assesses working memory for spatial stimuli and strategy use. The test is a self-ordered searching task (Petrides and Milner, 1982), and Positron Emission Tomography (PET) studies have suggested that it activates the dorsal and ventral prefrontal regions (Mehta et al., 2000; Owen et
al., 1996; Robbins et al., 1998). A spatial array of coloured boxes is presented on the screen. The purpose of the task is to collect blue tokens hidden inside the boxes by touching each box so it ‘opens up’ revealing what is inside. Once a blue token is found it is used to fill an empty column on the right corner of the screen. There are four sets with three boxes, four sets with four boxes, four sets with six boxes and four sets with eight boxes. Only one blue token is hidden on each trial within a set, and overall in every set, each coloured box only contains one blue token. The key instruction for participants is that once a blue token is found within a box (in any individual trial), then that box would not contain a token again. Therefore, when searching for another token on the next trial participants must remember not to search in boxes that contained blue tokens on previous trials. Consequently, a between search error (BSE) occurs when a participant returns to a box where a token has already been found on a previous trial.
A strategy score can also be derived from this task. Performance on this task can be facilitated by employing a repetitive search strategy. The optimal strategy involves searching through the boxes in the same order on each trial, while remembering not to search in boxes containing tokens on previous trials (e.g. Fray et al., 1996; Owen et al., 1990). Such a strategy may reduce the load on working memory and would,
presumably, enhance performance at all levels of task difficulty. The strategy score is calculated by counting the number of different boxes initially opened on each trial. The lower the score, therefore, the greater the use of the strategy.
Stockings of Cambridge
This task was derived from the ‘Tower of Hanoi’ task and primarily measures spatial planning, and to a lesser extent working memory and behavioural inhibition (Shallice, 1982). Several PET studies have indicated that performing this task activates the right
dorsolateral prefrontal cortex (Baker, et al., 1996; Morris et al., 1993; Owen et al., 1996). In this test, two sets of three coloured balls are presented on a touch screen monitor, each arranged in three hanging pockets. Participants are asked to copy the top pattern by moving the balls in the lower half of the screen according to specified rules (a ball underneath another ball cannot be moved; a ball cannot be moved to a place where there is no ball underneath, unless its position will be at the bottom of the pocket).
Problems should be solved in a certain minimum number of moves (two, three, four or five moves) and participants are instructed to work out the solution prior to moving any balls. There are three sets requiring two moves, three sets requiring three moves, three sets requiring four moves and three sets requiring five moves. The maximum moves allowed correspond to twice the minimum number possible plus one, or plus two in the case of ‘five move’ problems. If the maximum number of moves is exceeded the computer indicates ‘too many moves’ before beginning the next trial.
Initial and subsequent thinking times during trials are recorded to provide estimates of cognitive speed during the preparatory and execution phases of task performance. For each trial, a yoked control condition is also executed. During these ‘following’ trials, participants are instructed to execute a sequence of single moves as quickly as possible.
The ‘following’ trials are exact reproductions of the participant’s earlier planning moves. Initial and subsequent movement times in these ‘following’ trials provide estimates of motor speed. These ‘movement times’ are subtracted from the test condition times that included both ‘thinking times’ and ‘movement times’ in order to provide an estimate of planning times in the test conditions.
Due to the relationship between declining health factors and poor memory performance in older adults (Nolan and Blass, 2002; Backman et al., 2000a) participants rated their health on a 5 point scale: 1 = poor, 2 = could be better, 3 = fair, 4 = good, 5 = excellent.
A rating of 3 or above was necessary for inclusion. In addition, any volunteer with a history of neurological or psychiatric illness, epilepsy, head injury, stroke, drug or alcohol abuse, or who was taking any psychoactive medication, was excluded. Finally, years of education were equated in young and older participants.
The exclusion task procedure training session consisted of a short practice block for the young group and two short practice blocks for the older group. The older group were given an extended training session because pilot studies showed that this was necessary for them to become comfortable with the task.