I am conducting a classical conditioning experiment in which three colored squares are presented to participants in the computer screen. Two of these pictures are paired with a mild electrical shock in approximately 1/3 of the trials, and the third is never paired with the shocks. I would like picture presentation to be random, but there has been times in which the same stimulus appeared three times in a roll during the experimental session. In my experiment, a stimulus cannot appear more than two times in a roll. I know it is possibile to randomize trials in a block once per participant, once per group or every time the block is presented, but I could not change this parameter.
Can anyone help me with this?