Working Memory and Concurrent Tasks

In a memory span experiment, participants’ recall of numbers, letters, and shapes was tested. In the first condition, the average number of items recalled was 8.2 for numbers, 7.1 for letters, and 5.4 for shapes. In a second condition, participants performed the same recall task while simultaneously tapping a rhythm. Their scores dropped to 6.1 for numbers, 4.2 for letters, and 5.1 for shapes. The rhythmic tapping task most likely interfered with which component of working memory?