Adding random mechanism

To illustrate how to draw random pictures and screen location, let’s extend the experiment to a visual search paradigm, in which the stimulus words appear within a set of distractors at random screen locations.

We first design the distractor stimuli, which will be either the digits or words 1-6:

PICTURE 1 N
PICTURE 2 N
PICTURE 3 N
PICTURE 4 N
PICTURE 5 N
PICTURE 6 N

PICTURE One   N
PICTURE Two   N
PICTURE Three N
PICTURE Four N
PICTURE Five N
PICTURE Six   N

These picture definitions will be put in two different sets which will represent the distractor set we can draw from

SET Digits V
1,2,3,4,5,6
SET Words V
One,Two,Three,Four,Five,Six

To display the distractors, we define a circle with 6 positions. The target item (left or right) will be placed on one of the position and the remaining 5 positions will be filled with distractors

POSItioN Circle C 0,0 80 0 6

A circle layout can be directly defined by using the ‘C” position type. This circle will have a diameter of 8cm and 6 positions.

To display the target words on random positions we must mix the positions within the circle. This is achieved by the MixPosition (MP) trial command:

MP Circle

We will load the target word always on the position index, but since the indices are mixed this first position will always be one of the six screen locations

To fill the remaining 5 positions, we use the FillPosition (FP) command. This command loads a target word and fills all remaining positions with elements from a set

FP ? Circle 1 0 ? 6 1

The two question marks are the placeholders for the picture name of the target item and the name of the set that contains the distractor items. The target will change between ‘left’ and ;right’ and the distractor set will change between ‘Digits’ and ‘Words’

The entire trial structure is as follows

TRIAL ShowWord
MP Circle          ; Mix the positions of the circle
CS 2000          ; Clear Screen and wait for 1000 ms
RK ResKeys 2000          ; Activate Response Registration with MaxRt = 2000
FP ? Circle 1 0 ? 6 1          ; Load target and fill positions
FB ?          ; Compare the response against the correct key
    1:SP Wrong   FBpos 1 1000          ; Show ‘Wrong’ for 1s if response is wrong
    2:SP Correct FBpos 1 1000          ; Show ‘Correct’ for 1s if response is correct
    3:SP Correct FBpos 1 1000          ; Show ‘Late’ for 1s if response is correct

We have now three variable arguments. One for the target, one for the distractor set, and one for the correct key. We must therefore rewrite the block definition to introduce a 2x2 design:

We can directly define a 2x2 design for the block which will generate columns in the result file which contain the factorial coding. We must also adjust the RUNBLOCK command to run multiples of 4 to ensure that each trial is executed equally often

SesSION My First Experiment
RUNTRIAL Instruct Title Inst1 Start ; Execute trial ‘instruct’ and insert the pictures as instructions
RUNBLOCK ChoiceRT 12 ; Execute block ‘ChoiceRT’ with 10 trials in random order

Below is the final script for a choice reaction time task combined with a visual search task

FONT Big Arial 10 1 bold
FONT Std Arial 6
FONT Small Arial 4

PICTURE Title T
Choice Reaction Time Task
PICTURE Inst1 T
You will see the words ‘left’ or ‘right’

Press the corresponding response key

X-key for right

Y-key for left

PICTURE Start T
Press a response key to start the task

POSITION Top    P 0,-50
POSITION Bottom P 0,40
POSITION Center P 0,0

KEY ResKeys
X M          ; Define a Key Set consisting of’ ‘X and ‘M’

PICTURE Correct N
PICTURE Wrong    N
PICTURE Late     N

POSITION FBpos P
0,30

TRIAL Instruct
SF Big
SP ? Top
SF Std
&SP ? Center
SF Small
&SP ? Bottom
WK ResKeys
SF Std

PICTURE Left N
PICTURE Right N

PICTURE 1 N
PICTURE 2 N
PICTURE 3 N
PICTURE 4 N
PICTURE 5 N
PICTURE 6 N

PICTURE One   N
PICTURE Two   N
PICTURE Three N
PICTURE Four N
PICTURE Five N
PICTURE Six   N

SET Digits V 1,2,3,4,5,6
SET Words V One,Two,Three,Four,Five,Six

POSItioN Circle C 0,0 80 0 6

TRIAL ShowWord
MP Circle          ; Mix the positions of the circle
CS 2000          ; Clear Screen and wait for 1000 ms
RK ResKeys 2000          ; Activate Response Registration with MaxRt = 2000
FP ? Circle 1 0 ? 6 1          ; Load target and fill positions
FB ?          ; Compare the response against the correct key
    1:SP Wrong   FBpos 1 1000          ; Show ‘Wrong’ for 1s if response is wrong
    2:SP Correct FBpos 1 1000          ; Show ‘Correct’ for 1s if response is correct
    3:SP Late    FBpos 1 1000          ; Show ‘Late’ for 1s if response is correct

BLOCK ChoiceRT 2 2          ; define a 2 by 2 design
1 ShowWord Left   Digits X          ; Target is ‘Left’, Distractor is Digits
1 ShowWord Right Digits M          ; Target is ‘Right’, Distractor is Digits
1 ShowWord Left   Words X          ; Target is ‘Left’, Distractor is Words
1 ShowWord Right Words M          ; Target is ‘Right’, Distractor is Words

SesSION My First Experiment
RUNTRIAL Instruct Title Inst1 Start          ; Execute trial ‘instruct’ and insert the pictures as instructions
RUNBLOCK ChoiceRT 12           ; Execute block ‘ChoiceRT’ with 10 trials in random order