Why did you not test completion time or gave users a time-frame to complete a task and introduce a stress-level? The interference with the tapping described above is a bit vague in this context. Some people are better with ‘tapping’ than others. ‘Tapping’ comes more ‘natural’ for musicians and people with similar trained skills. There is also a significant difference if the person was left- or right-handed. In addition – does tapping with the left foot not only influence the right hemisphere of the brain? How did you distinguish those participants and circumstances? Does such ‘generic’ tapping not just interfere with a very specific component in our brain? You might not interfere the executive component of our working memory with a plain tapping. Nor does this secondary task influence performance of the visuospatial sketchpad. So what exactly did it do?