Paradigm

Every forth trial would begin with a cue that indicated whether subjects would need to complete a temporal, spatial or feature conjunction judgement during the upcoming trial (see Figure 2.2). During dual task trials the cue would indicate which two of the aforementioned tasks the participant would need to complete (Figure 2.5).

INSERT FIG 2.2 HERE separate page .

Figure 2.2 A. An example trial from the ‘what’, ‘where’ and ‘when’ conditions. The cue indicates the task that the subject will complete for the next four trials until a new cue appears. The red and green boxes represent patches of moving dots; presented as red or green squares for clarity. B. The temporal structure of a trial. This example trial is for a single task. The cue is shown initially and is valid for four trials; the following 3 trials are preceded by a cue.

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The cue presented at the beginning of trial 1 was valid for 4 trials, indicating that the subject would perform the same task 4 times before the presentation of a different cue. The cue always switched to ensure that participants never performed more than 4 repetitions of each condition during a scanning block. As shown in Figure 2.2, the cue was immediately followed by a centrally presented white fixation cross. At 1500ms two square patches of moving dots would appear. The coherence of the dots was dependent on the individual threshold-level; in each patch there was a density of 50 dots per patch; each patch was 50mm x 50mm and the coherent dots moved at 0.6 degrees per second. One patch of dots would appear above the fixation cross and the other below. The fixation cross could change from white to yellow at any time, in sync with the screen refresh rate of 16.7ms. Each patch could appear shifted to the left or the right, relative to the centre of the fixation cross. The amount in which the patch was shifted was set in accordance with the individual psychophysical thresholds; this also established the ease of the task. The temporal sequence of a single task trial is summarised below in Figure 2.3.

Figure 2.3 The timeline of a single task trial. The dual task trial would be identical apart from the added question at the end; extending the total time of a trial to 9000ms.

The single task trials could begin with the cue of either: "What?" "Where?" or "When?". These cues indicated a feature conjunction task, a spatial discrimination task or a temporal order task respectively. The colour of the stimulus that the participant is asked to attend to is counterbalanced across the block. The accompanying questions to these cues were as follows:

 Where? - Red: Left or Right? – This asked the participant whether the red patch of dots were more to the left or right of the central fixation cross.

 When? - Green: Before or After? – This asked the participant whether the green patch of dots appeared before or after the central fixation-cross changed from white to yellow.

 What? - Red: Up or Down? – This asked the participant whether the red patch of dots consisted of dots moving mostly up or mostly down.

The participant responded with either a left or a right button press using their index or middle finger, respectively, in response to the question displayed on the screen. If the answer on the left was correct, the index finger was used to respond with a left button press and if the correct answer was on the right then the middle finger was used to press the right button. Participants had 2000ms to respond. The first response was the only response that was recorded. Single task trials would last for 7s (Figure2.2), whereas dual task trials would last for 9s (Fig 2.4). Due to necessary jittering between trials, each participant had an unpredictable break of 2-8s in length. This was calculated in increments of 500ms and occurred after each trial. Each experimental block consisted of 48 trials; including 8 per condition, and a total of 6 experimental blocks were completed per participant. Each experimental run lasted for approximately 9 minutes.

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Similarly to the single task conditions, the dual task trials could consist of either the cue: “What & Where?” “Where & When?” or “When & What?” and the accompanying questions at the end of the trial were as follows. Question 1 (Q1) and question 2 (Q2) were presented separately for 2000ms each:

 What & where? Q1: Green: Up or Down? Q2: Green: Left or Right?

 Where & when? Q1: Red: Left or Right? Q2: Red: Before or After?

 When & what? Q1: Green: Before or After? Q2: Green: Up or Down?

Each participant completed a brief colour calibration to ensure the red and green dots were of equal luminance. This was carried out using a MATLAB script that changed the luminosity of the dots on screen when the mouse was moved left or -right on the desk in-front of the subject. When the subject felt the luminosity was matched between the red and green dots they pressed the left mouse button and the two colour codes were recorded. This was important to ensure that no particular set of dots attracted more attention than the other.

Thresholding of behavioural task

Thresholding consisted of a 90-minute session in which participants’ accuracy threshold was set to 70%. Using a method of constant stimuli, this was completed offline, in a behavioural testing lab. In order to assess the subjects’ threshold, participants undertook 100 trials in each condition (T, S, C, ST, SC, and CT). Rest periods were offered at the completion of each block. In order to threshold the temporal task, the time between the change of the colour of the fixation cross and the onset of the dots was either lengthened or shortened making a temporal order judgement easier or more difficult. The spatial task varied the degree to which the squares of moving dots were shifted over the centre of fixation (see Figure 2.5) below. The feature conjunction task, in which subjects had to decide whether the red or green dots were moving up or down, were a mixture of coherently moving dots, either moving up or down at 0.6 degrees per second or replotted at 63hz in order to give the impression of incoherent noise. The degree of coherent dots with incoherent dots varied during the thresholding

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session between 100% coherence where all dots moved in one direction to 0% coherence where all the dots appeared to move incoherently.

Figure 2.5 shows examples of ‘hard’ and an ‘easy’ spatial trials, respectively. The more central the dots were positioned, the harder the subject found the

judgement of left or right. However in an easy trial this was more obvious.

Once all data had been collected we then calculated the ideal stimulus parameters that would produce 70% accuracy across all conditions. The calculated threshold was then set for that participant and would dictate the parameters of the stimulus displayed in the MR scanner during the participant’s scanning session.