  Using the activation-relaxation technique (ART), we study the nature of
relaxation events in a binary Lennard-Jones system above and below the glass
transition temperature (T_g). ART generates trajectories with almost identical
efficiency at all temperature, thus avoiding the exponential slowing down below
T_g and providing extensive sampling everywhere. Comparing these runs, we find
that the number of atoms involved in an event decreases strongly with
temperature. In particular, while in the supercooled liquid activated events
are collective, involving on average thirty atoms or more, events below T_g
involve mostly single atoms and produce minimal disturbance of the local
environment. These results confirm the interpretation and the generality of
recent NMR results by Tang et al (Nature 402, 160 (1999)).
