Hi
I understand from your question that you need to compute the variance of part of image and then use a global threshold for finding the transition regions.
The variance of an image can be found by using the function “var”.
Variance = var(img(:));
The above line of code would calculate the variance of the input( image or a selected region of image).
For global thresholding, you can use the function “graythresh” of Image Processing Toolbox.
Here are few links that would be helpful: