How can I make my threshold window only work for half my image?

How can I make my threshold window only work for half my image?

I am using Visual Basic 2010 Express, and opencv in c++, and while this
line already limits my color to track in the threshold window,
inRange(HSV,Scalar(0,1,170),Scalar(196,137,256),thresholdL);
I am trying to figure out how to also say 'only show pixels(I don't know
the proper terminology for it) that lie somewhere from 320 to 640 on the x
axis in this threshold window.'
I do have a variable that i use to check the x position, called
int xPos;
but I'm not sure how to take that and funnel the info into my threshold
window. Do I modify the line shown? is there a function for this that I
just haven't found and can insert underneath? Will I have to set up a
whole new class or something?
Thank you in advance.