Understanding and measuring sustained attention is important for basic research and applications. Measurement and testing with computerised continuous performance tests is common. When studying the neural correlates of attention, such tests aim to isolate the construct of sustained attention. However, in any ecologically-valid context, sustained attention integrates with other executive functions and depends on lower level perceptual processing. Examination of these interactions is not well-served by existing options, and hence I propose a novel protocol to probe higher cognition and its neural correlates: the Primed Subjective Illusory Contour Attention Task (PSICAT). PSICAT tackles two important aspects of ecological attention: (a) successful attending may require inhibition of interference; (b) natural target stimuli are complex and often noisy. I describe a validation experiment measuring healthy participants with high-resolution electroencephalography. Behavioural and brain-imaging results validate the key hypotheses for the functionality of PSICAT. Additionally, I demonstrate the novel result that task-irrelevant incongruency of an intra-modal distractor prime can evoke stronger behavioural and neural responses than the task-relevant stimulus condition. Finally, the protocol is available as an open-source code repository at the following url, allowing researchers to reuse and adapt it to their particular requirements. https://github.com/zenBen/Kanizsa_Prime/