  Let $n\geq2$ be a natural number. Let $M_n(\mathbb{K})$ be the ring of all $n
\times n$ matrices over a field $\mathbb{K}$. Fix natural number $k$ satisfying
$1<k\leq n$. Under a mild technical assumption over $\mathbb{K}$ we will show
that additive maps $G:M_n(\mathbb{K})\to M_n(\mathbb{K})$ such that
$[G(x),x]=0$ for every rank-$k$ matrix $x\in M_n(\mathbb{K})$ are of form
$\lambda x + \mu(x)$, where $\lambda\in Z$, $\mu:M_n(\mathbb{K})\to Z$, and $Z$
stands for the center of $M_n(\mathbb{K})$. Furthermore, we shall see an
example that there are additive maps such that $[G(x),x]=0$ for all rank-1
matrices that are not of the form $\lambda x + \mu(x)$. We will also discuss
the $m$-additive case.
