In a study like the ERP CORE P3b experiment, there are many different combinations of stimuli (5 letters) and task instructions (which of the 5 letters was the target). Some care is therefore needed to come up with a scheme for representing each possible stimulus/task combination as a separate event code using integers between 1 and 255. Table 6.1 shows our scheme. We used the tens place in the event code to indicate which letter was the target and the ones place to indicate what the current letter was. For example, event code 23 indicates that B was the target letter in the current block (because 23 has a 2 in the tens place) and that the letter C was presented on the current trial (because 23 has a 3 in the ones place). In this scheme, the Rare category is coded as event codes 11, 22, 33, 44, and 55, and the Frequent category is coded as the other combinations. We also used 201 and 202 as the event codes that were generated when the participant pressed a response button, with 201 for a correct response and 202 as an incorrect response.
Table 6.1. Event code scheme for the ERP CORE visual oddball P3b experiment.
|
Event Code
|
Target Letter for Current Block
|
Tens place
|
Current Stimulus
|
Ones place
|
|
11
|
A
|
1
|
A
|
1
|
|
21
|
B
|
2
|
A
|
1
|
|
31
|
C
|
3
|
A
|
1
|
|
41
|
D
|
4
|
A
|
1
|
|
51
|
E
|
5
|
A
|
1
|
|
12
|
A
|
1
|
B
|
2
|
|
22
|
B
|
2
|
B
|
2
|
|
32
|
C
|
3
|
B
|
2
|
|
42
|
D
|
4
|
B
|
2
|
|
52
|
E
|
5
|
B
|
2
|
|
13
|
A
|
1
|
C
|
3
|
|
23
|
B
|
2
|
C
|
3
|
|
33
|
C
|
3
|
C
|
3
|
|
43
|
D
|
4
|
C
|
3
|
|
53
|
E
|
5
|
C
|
3
|
|
14
|
A
|
1
|
D
|
4
|
|
24
|
B
|
2
|
D
|
4
|
|
34
|
C
|
3
|
D
|
4
|
|
44
|
D
|
4
|
D
|
4
|
|
54
|
E
|
5
|
D
|
4
|
|
15
|
A
|
1
|
E
|
5
|
|
25
|
B
|
2
|
E
|
5
|
|
35
|
C
|
3
|
E
|
5
|
|
45
|
D
|
4
|
E
|
5
|
|
55
|
E
|
5
|
E
|
5
|