༼ ༎ຶ ෴ ༎ຶ༽ i;ve never taken DXM, but google is your friend;
'Not everyone notes distinction between the first and second plateaus, or between the third and fourth plateaus. Others suggest that each effect of DXM has a dosage level at which it starts, and (in some cases) a dosage level at which its effects are no longer noticeable (being overpowered by other effects). Some people will disagree with this classification method, but I think this is the best way to represent DXM's effects. Both the third and fourth plateaus have significant dissociative characteristics, much like ketamine.
The most important thing to keep in mind is that the effects in different plateaus are often very different. For example, on the first plateau, DXM tends to have a stimulant effect. Upon reaching the second plateau, however, the stimulant effect may no longer be present.' - Erowid