1) If I choose to introduce null events, should I determine the same probability occurence for this null event as for events of interest ? For exemple A:0,33 ; B:0,33 ; C(null event): 0,33 or with another probability occurence? For exemple A:0,4 ; B:0,4 ; C(null event): 0,2. Note that in fact I would have more than two events of interest but I just wanted to simplify the situation.

It's up to you - as you increase the probability of null events, your efficiency for the [1 1]

contrast will increase, but that for the [1 -1] contrast will decrease, and vice versa.

2) I believe that if I choose equal probability I could only construct a stationnary stochastic design and not a dynamic stochastic design ? Am I right?

No - to create a dynamic design, the probability of each trial-type should itself be a function

of time, rather than fixed. A fixed probability, regardless of value, will always be stationary.

3) If I’m right, which probabilities should I enter for five conditions (4 trial types + 1 null event) when I want to build a design matrix of a dynamic stochastic design with the stochastic design option (yes/no) ?

I don't know. I don't use this option.

4) My last question concern the single SOA which is asking me when I click “yes” to the design stochastic option. I don’t know why SMP ask me to enter a SOA because I’m precisely asking to him to constrcuct a serie of onsets? Is this SOA refer to the minimum SOA between events?

It is most likely referring to the minimal SOA, yes. See Friston et al (1999) for more details.

Rik