How is temporal pre masking possible?
Auditory processing has a latency of around 40ms from stimulation at the cochlea to awareness in the brain. Those 40ms appear in a number of auditory cognitive effects and mark the separation between the perceived "now" and the past. Within these 40 ms the brain can group information and produce masking effects. Pre-masking is much shorter than post-masking, because of causality, but it's not necessarily 0.