Modified ANT design

The modified ANT, which is central to the CATFieLD study, probes the efficiency of the three attentional networks discussed in chapter 1; alerting, orienting and executive

conflict. As with the original ANT (described in Chapter 1, section 1.5.1), to measure the alerting and orienting effects there were three cueing conditions: no cue (baseline), neutral cue (temporally informative), and spatial cue (temporally and spatially informative). The executive conflict effect was probed using congruent and incongruent target stimuli.

The rationale for modifying the original ANT design was to create a version of the task (described in section 2.5.1) suitable for probing attentional function in dementia cohorts using electrophysiological and neuroimaging approaches. As discussed in chapter 1 (section 1.5), our ANT was modelled on the version of the ANT described by Fan et al. (2007) (as opposed to an earlier version of the ANT (Fan et al., 2002)), as this version was modified to optimise the task for use with neurophysiological recordings. The timings of our modified ANT (see section 2.5.1) were largely comparable to the version of the ANT used by Fan et al. (2007); the duration of the presentation of the cueing stimuli was the same for both versions of the task (200 ms). Whilst our task incorporated an additional level of executive conflict complexity relative to the original ANT (see section 2.5.1), the majority of the design modifications were made for pragmatic purposes:

1) Given that elderly individuals tend to have reduced visual acuity relative to young adults (Sjostrand et al., 2011), and young adults were used to validate the original ANT (Fan et al., 2002), stimuli for our modified version were increased in size in comparison to the original. Furthermore, in order to optimise the task design for individuals with poor visual acuity, arrowheads were used for targets (depicted in Figure 2.3) as opposed to arrows which were used in the original task, and flashing boxes were used for the cues (see section 2.5.1) as opposed to asterisks.

2) The timings of our modified task are discussed in detail in section 2.5.1. Relative to the ANT described in the Fan et al (2007) study, the mean cue-target interval of our task (1800 ms, Figure 2.3) was longer than that of the original (mean 550 ms), and the target was presented on screen for longer (for 3000 ms as opposed to 2000 ms). These modifications were made in order to account for slower cognitive processing speed exhibited by elderly individuals relative to young adults (Hoogendam et al., 2014).

2.5.1 Task design

The computerised task was programmed by Dr Michael Firbank using the Cogent MATLAB toolbox (http://www.vislab.ucl.ac.uk/cogent_2000.php). The design of the modified ANT is depicted in Figure 2.3. There were 8 runs of the task, each comprising 36 trials. Throughout the task a central fixation cross and three boxes were present on the LCD screen. During each trial, one of three possible cues (no cue, neutral cue, spatial cue) was presented for 200 ms. During the presentation of a neutral cue the central box flashed, in the spatial cue condition one of the boxes either above or below the central fixation flashed (indicating the box in which a subsequent target would appear). In the no cue condition the boxes remained unchanged. Following the disappearance of the cue, a target comprising four arrowheads in a row (horizontal spacing of 0.48 degrees) was presented in either the box above or below the central box. The time between the disappearance of the cue and the onset of the target was one of the following exponentially distributed times: 700, 770, 850, 960, 1080, 1240, 1430, 1660, 1940, 2300, 2700, 3200 ms. The target stimuli were either congruent or incongruent; congruent targets comprised arrowheads which were all pointing in the same direction (left or right), whereas for incongruent target stimuli one arrowhead was pointing in the opposite direction to the other arrowheads. The incongruent arrowhead appeared either on the end of the row (incongruent easy) or as one of the middle two arrowheads (incongruent hard). The easy incongruent target condition had three congruent arrows in a row (unilateral flanker effect), whereas the hard incongruent condition had only two (bilateral flanker effect), and therefore provided greater conflict. The target remained on screen until the participant responded, or until 3000 ms had elapsed. The time between the onset of the target and the onset of the next trial cue was one of the following: 4300, 4500, 4750, 5000, 5350, 5700, 6100, 6400, 6800, 7200, 7700, 8300 ms, each occurred 3 times during each run (in a random order). During each run, the 9 trial types (3 cue by 3 target conditions) were presented in a predetermined counterbalanced order; each cue appeared 12 times, there were 18 congruent trials and 18 incongruent trials (equally split between incongruent easy and hard).

Figure 2.3.Design of the modified Attention Network Test (ANT).

Three cue conditions were presented (no cue, neutral cue, spatial cue). Two target conditions were presented (congruent and incongruent). The display consisted of a central fixation cross and three white boxes against a black background. In cue-present conditions, one of the three boxes flashed for 200 ms. During the neutral cue condition the central box flashed. During the spatial cue condition one of the boxes either above or below the central fixation flashed to cue the participant to the position of the subsequent target. Target stimuli consisted of a row of four white arrowheads pointing leftward or rightward which presented randomly in the box above or below the fixation cross. The arrowheads either all pointed in the same direction (congruent) or in opposite directions (incongruent); this defined the executive conflict component of the ANT. The objective of the task was to identify the direction in which the majority of arrowheads were facing (the points of the arrowheads) by squeezing a right hand or left hand air pressure bulb as quickly as possible.