Difficulties with Matlab12b function imshow()

Difficulties with Matlab12b function imshow()

I'm using MATLAB for image processing and I came across a code with the
instruction:
imshow(pixel_labels,[]);
when executed it give a binary image.
I have check the manual of the function on Mathworks.com, the most similar
used mode is
imshow(I,[low,high]);
but they don't say a thing about the case where that array is empty ([])
I tried to remove it:
imshow(pixel_labels);
but all I see is a white board. I would like to know what is happening in
the first use case (imshow(pixel_labels,[])), I hope from there I will
understand why I get a white board in the last use case.