Functional relationship between forward and backward digit repetition and a non-verbal analogue

It has been suggested that the forward digit span employs a verbal representation of sequences of digits, whereas the backward digit span employs a spatial representation. This was tested by comparing the two digit spans with a non-verbal analogue of the forward digit span. The latter did not explain any of the variation in the backward digit span when the forward digit span was stastically controlled. It is concluded that the two digit spans employ a common verbal representation.