how to assign a range of pixel for pixel intensity

I have a 578x50 image. I want to process a range of pixel intensities let say pixel 386 until 409 for column and pixel 2 until 49 for row. When I assign that range of pixels like this:

for c = 386:409
for r = 2:49

    central_pixel(r,c) = I(r,c);
    centralPixel = double(central_pixel);
    if centralPixel(r,c) >= 0
    central_pixel(r,c) = I(r,c);
    centralPixel = double(centralPixel);
    else
    centralPixel = NaN;
    end
  end
  end

the output of c = 409 and r = 49 in workspace. While the output of centralPixel display pixel intensity for 386th until 409th pixel while 0 for 1st until 385th pixel. How can I fix this? I just want to display only pixel intensity of 386th until 409th pixel for column and 2nd until 49th pixel for row. Please help. Thanks in advance