Methods

Twelve healthy young participants (6 males and 6 females, mean age=24.7, SD=6.9), 12 healthy young participants (4 males and 8 females, mean age=22.4, SD=3.0), and another 12 healthy young participants (3 males and 9 females, mean age=24.3, SD=5.8) were allocated to the encoding time 250ms, 500ms, and 2500ms conditions by the researcher, respectively.

Although the sexes of participants were not balanced, previous studies in our lab suggested that there is no sex difference while executing the experiment task (Gallagher et al. 2012, unpublished data). The majority of participants were Newcastle University students while a very small number were employees of either the university or a company. All received a participation fee of £10 after they had completed the study. Participants were all right-handed and had normal or corrected-to-normal vision. A consent form was provided before the experiment started. The research was approved by the Faculty of Medical Sciences Ethics Committee in Newcastle University.

2.2.2.2 Stimuli

The encoding and the response image consisted of four small squares of the same colour (red) on a computer screen. The four squares were allocated to four quadrants (left, top-right, bottom-left, and bottom-right) and two of them would be assigned as “reference” and

“target” items. Only one type of spatial relation was manipulated between reference and target whilst the other squares remained at the same positions. The target was allocated to each quadrant equally often with the reference item assigned to one of the other possible

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locations. The position of the target could be moved away from the reference item in three different shift sizes (15mm, 20mm, and 25mm), either vertically or horizontally.

On categorical-change trials, the spatial relation of the target and the reference locations was changed categorically (e.g. from left to right or from up to down) while its categorical spatial relations with the other two squares remained the same. On the other hand, on the coordinate-change trials only the distance between the target and the reference item was manipulated. For example, the target could be moved 15mm, 20mm, or 25mm to the right of the reference only if the target was originally placed on the right of the reference on the encoding image. Meanwhile, the spatial relations between the target and the other two squares remained the same. Figure 2-1 illustrates examples of categorical change, coordinate change and the same trials.

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Figure 2-1. Example stimuli and a trial procedure for CATCOORD task. The oval indicates the locations of the reference (i.e. the top-left square) and the target (i.e. the bottom-left square). For the

categorical change in this example, the target has moved from the left (in the encoding image) to the right of the reference. The coordinate change presents a difference in distance between the

reference and the target yet the target remains on the left. The same shows no changes between the encoding image and the response image. The encoding image is the to-be-remembered image with different durations (250ms, 500ms, and 2500ms). Each participant only experienced one of the encoding times and the three different retention intervals (500ms, 2000ms, and 5000ms).

2.2.2.3 Design

The study was a mixed design as each participant experienced the three retention intervals (500ms, 2000ms, and 5000ms) along with one of the encoding times (250ms, 500ms, 2000ms). In order to avoid any possible confounding caused by fatigue, participants were separated into three groups in accordance with encoding times in a between-subjects design. There were 12 categorical-change trials (4 trials for each of the three shifts), 12 coordinate-change trials (each shift containing 4 trials), and 12 same trials in one block. In the same trials, the four squares remained at the same positions in both encoding and response images. Participants went through three blocks with each retention interval and

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each of the 36 trials was randomised within a block. Overall, each participant performed nine blocks, three for each retention interval. A short break was provided between blocks. In order to minimise possible perceptual/physical carry over effects caused by different orders of the three retention intervals, participants always performed the shortest retention interval followed by the medium retention interval, and the longest maintenance time was performed last. Depending on the encoding time that a participant was assigned, the experiment durations were from 40 minutes to 70 minutes.

2.2.2.4 Procedure

Each participant was tested individually in a quiet room. Stimuli presentation and response collection were completed using E-Prime software, (Psychology Software Tools, Inc., Sharpsburg, PA). During the practice session, participants experienced 6 practice trials with feedback to ensure that they understood the experiment procedure and the instructions.

Participants then completed the formal experiment session. An experimental trial began with a blank screen (500ms) and then a fixation point “+” at the centre of the computer screen for another 500ms. The encoding image was then shown for either 250ms, 500ms, or 2500ms, followed by a blank retention interval for either 500ms, 2000ms, or 5000ms. The response image was then displayed. Participants were asked to make a same/different judgment of the two successive images during the display of the response image (maximum 3000ms) (see Figure 2-1 for an illustration). Participants were instructed to use their left or right index fingers to press the key “z” or “m” on the keyboard; the assignment of key to response was counterbalanced over participants. The next trial started as soon as participants made a response. Both accuracy and reaction times were recorded using E-Prime software.

2.2.2.5 Data analysis

Two types of analysis were included. The first analysis addressed the effects of the three experimental conditions: categorical, coordinate and same spatial relations, and whether the manipulation of retention interval and encoding time would influence performance on these spatial judgments. A 3 (condition: categorical vs. coordinate vs. same condition) × 3

(retention interval: 500ms vs. 2000 vs. 5000ms) within-subject repeated measures combined with a between-subject variable (encoding time: 250ms vs. 500ms vs. 2500ms) analysis of

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variance (ANOVA) was performed. The results focus on interactions between condition and retention interval, condition and encoding time, and main effects of condition, retention interval, and encoding time.

The second analysis aimed to observe whether manipulations of shift size, retention interval, and encoding time would influence performance for the two spatial relations. A 2 (spatial relation: categorical vs. coordinate relation) × 3 (retention interval: 500ms vs. 2000ms vs.

5000ms) × 3 (shift size: 15mm vs. 20mm vs. 25mm) within-subject repeated measures combined with a between-subject variable (encoding time: 250ms vs. 500ms vs. 2500ms) ANOVA was performed. Note that the same condition was excluded in this analysis since there were no spatial shifts for objects in the same trials. The results will report the

interaction between spatial relation and shift size. Interactions between spatial relation and retention interval, and spatial relation and encoding time will be mentioned only briefly since they would have been reported in the first analysis. Moreover, main effects of spatial relation, shift size, retention interval, and encoding time will be reported.

The results of other interactions in the first and second analysis will be shown in the Appendix.