You can create a vector calculating the difference between the two, and then find the first index where it is larger than a specific threshold:
MyDifference = abs(acti(:,2)-phs0(:,2)); % calculate the absolute error between corresponding elements.
MyThreshold = 1; % or whatever number you want.
find(Mydifference>=Mythreshold,1) %find first difference that is bigger than MyThreshold
Theoretically you could use:
find(Mydifference~=0,1)
But in your attachments the values aren't exactly zero so you would need to determine what difference is to big for you.
