A number of cells were bidirectional, in that they did not a have a peak firing rate in a preferred direction that was at least doubled that in the opposite direction, raising the possibility that apparently reverse replay events merely reflected forward replay of neurons in the opposite direction.
Foster & Wilson Nature (06) 440:680-683
summarized less concisely
http://www.gnxp.com/blog/2006/05/now-where-was-i.php