“…A third difficulty concerns "structural alteration effects", which can often be observed when two tasks are time shared and one of them has a change in processing structure, such as input or output modality or memory code. Under such circumstances a change in interference between the two tasks has been repeatedly observed although as such the difficulty of both tasks remained unaltered (Isreal, 1980;Martin, 1980;Rollins & Hendricks, 1980;Treisman & Davies, 1973;Vidulich & Wickens, 1981;Wickens et al, 1983;Harris, Owens & North, 1978;McLeod, 1977;Wickens, 1980;Friedman, Polson, Dafoe & Gaskill, 1982;McFarland & Ashton, 1978;Wickens & Sandry, 1982;Pritchard & Hendrickson, 1985). It is certainly impossible to account for such results by a simple single capacity notion.…”