First, keep in mind the fundamentals of neurologic diagnosis.
Focal disease causes focal deficits.
History and sometimes basic laboratory tests will discriminate among these last three. The mental status examination is discussed elsewhere in this manual and will not be repeated here. The main features that distinguish organic from psychiatric causes of mental reactions are the level of alertness and orientation. Orientation is the more useful of the two, clinically because the patient with depressed alertness is already obviously organic. Then determine the patient's ability to attend and concentrate with recall of digits or repetition of a sequence of gestures.