 |
 |
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