The results show response times to the tone as a function of how soon after the letter the tone was presented (stimulus onset asynchrony, or ‘SOA’). The responses in the chart are for trials where both responses were reported correctly.
Response times to the letter were unaffected by how closely the tone followed the presentation of the letter. You can see the line for ‘Letters’ is relatively flat. This is because the letter task is always started before the presentation of the tone, so the same amount of attention is available at all stimulus onset asynchronies (SOAs). Responses to the tone, however, get much faster with increased SOAs. This is because of dual-task interference; at short SOAs, attention is not available to complete the tone task, the letter task must be completed first. At longer SOAs, when the letter response has been made before the tone is presented, response times are much faster because the participant can give full attention to the tone task as soon as it is presented. This effect has been replicated over and over by cognitive psychologists using a variety of different stimuli and tasks. It has been found whenever a choice response has to be made on both tasks.