The aim of this thesis is to examine the time-course of task-irrelevant interference effects in Navon letters. In Navon stimuli, a large letter is composed of small letters and results show that participants are generally faster when the letters are compatible. In the present work, instead of responding to letter identity, participants were required to respond to the luminance of the stimuli. This luminance discrimination task was implemented both when attention was directed to the global and when it was directed to the local level. In the global level task (chapter 2), participants were asked to respond to the luminance of a Navon letter. In the local level task (chapters 3 and 4), task-irrelevance was manipulated by asking participants to respond to the luminance of a dot that was embedded in one of the local letters. Time-course of responding was manipulated by using different luminance contrasts in different experiments. The results showed that participants responded faster when the global and local letter matched. Time-course seemed to be less influential as previously predicted (chapter 5). Finally, task-irrelevant interference effects on the local level were investigated by using a choice-reaching task (chapter 6). No interference effects occurred, neither for initiation latency nor for maximum deviation of the movement trajectory. In conclusion, the findings of this thesis suggest that in a reaction-time task, task-irrelevant interference effects in Navon letters can occur. This seems to be independent of the level of attentional focus
