Probably not.

I had used dxm for a while and did experience some slight memory and learning problems, (Most only lasting for a few days.) but I think that those problems are now mostly if not completely gone and I've been off dxm for about 1.5-2 years.

I too used it on and off for about a year maybe a little longer, but I usually (not always) spaced my doses out to about twice a month, and they were always at least 300 mg, usually more.