I have been told different methods
to calculate the cut-off period for the high pass filter. The differences
occur when the block design contains some complexity.I would therefore like to
know how we should proceed in such cases like ours.
In a recent experiment, our RUN had 8 blocks (7
scans each) separated by rest periods (3 scans each). One scan lasted 5.8
seconds. There were 4 types of tasks (images to remember) in this RUN and they
were given 2 times. The tasks were counterbalanced between subjects except the
last 2 negatives blocks. Here is an example of subject 1:
Rest 3 scans
Positives 7 scans
Rest 3
scans
Neutrals 7 scans
Rest 3
scans
Symbolics 7
scans
Rest 3
scans
Positives 7 scans
Rest 3
scans
Neutrals 7 scans
Rest 3
scans
Symbolics 7
scans
Rest 3
scans
Negatives
7 scans
Rest 3
scans
Negatives 7 scans
The negatives images are at the end because they
are highly emotional and could contaminate other blocks if they are placed
before. As you can see, Positives, Neutrals, Symbolics are presented twice in
the same order.
The traditional method to calculate the cut-off
period would be to take the longest On-Off cycle, which in this case was rest (3
scans) + task (7 scans), multiply by 2 and multiply by the duration of one scan
(5.8 sec). It gives (3+7) * 2 * 5.8 sec = 116 seconds.
Someone else indicated that in our block design,
because of its nature, it should be (Rest +
Positives + Neutrals + Symbolics) times 2 times 5.8 seconds. That gives
(3+7+7+7) * 2 * 5.8sec = 278.4 sec. It is so because our longest cycle is in
fact one serie of 3 blocks presented twice (this method excludes the 2 negatives
blocks at the end of the RUN).
I am not sure if the repetition of 3 blocks
could be consider a cycle in itself. Should the simple rule of the longest
On-Off cycle be sufficient in this case to calculate the cut-off
period?
Claude Breault Ph.D.
Research Center
Cognitive Neuroscience
laboratory
Institut Universitaire de gériatrie de
Montreal
Montreal, Quebec, Canada