  The motion of an optically trapped sphere constrained by the vicinity of a
wall is investigated at times where hydrodynamic memory is significant. First,
we quantify, in bulk, the influence of confinement arising from the trapping
potential on the sphere's velocity autocorrelation function $C(t)$. Next, we
study the splitting of $C(t)$ into $C_\parallel(t)$ and $C_\perp(t)$, when the
sphere is approached towards a surface. Thereby, we monitor the crossover from
a slow $t^{-3/2}$ long-time tail, away from the wall, to a faster $t^{-5/2}$
decay, due to the subtle interplay between hydrodynamic backflow and wall
effects. Finally, we discuss the resulting asymmetric time-dependent diffusion
coefficients.
