because I noticed the beginning of my mental degradation after using DXM.
It could just be something called "getting older". When you're at school you're constantly learning and your mind is getting plenty of exercise. After school that slackens off and unless you make the effort to use your memory then it fades.
Apparantly the best exercise for your brain is doing lots of simple sums like "8 x3". Get a book on improving your memory and with 6 months of practise you'll have a better memory than 99% of the population who have never taken DXM.
But you will have to ask yourself the question "does having a good memory warrant the effort it takes to maintain one"? For most people the answer is "No".