Abstract: A first light emitting element directs light through a first region of tissue, a second light emitting element directs light through a second region of tissue that is spatially distinct from the first region of tissue, and a photodetector detects light traveling through the first and second regions. First and second oxygen saturation values are calculated from the amount of light detected from the first and second regions of tissue, respectively. A difference calculating unit calculates a difference value which represents how much the first oxygen saturation value differs from the second oxygen saturation value, and a comparator compares the difference value to a threshold value. A threshold indicating signal is provided when the difference value has a selected mathematical relationship (e.g., less than or equal) to the threshold value.