Image processing by pixel range

[Rishi Bedi]

Hi,
I'd like to use Octave to process an image from my FireWire webcam --
essentially, what I want to do is convert the image to black and white
and then test the color of a small range of pixels (they'll be in the
same place for every image, and will either be 100% black or 100%
white). I'm going to use im2bw for the conversion, but I was trying to
figure out what to do for the second part. Is there a way to use the
bwarea function on only a specified x,y range? -- or, if not, what
would you suggest to accomplish this?
Thanks!