There are curious effects of arousal on attention to brief sounds versus flashes. After arousing mild shocks tones become more detectable than flashes, and peripheral flashes become detectable than foveal flashes. Very useful in lots of areas perceptually and cognitively. Cued signals were detected with a benefit of -30 msec in a Posner experiment with two possible locations for a signal. Posterior parietal regions produce the most pronounced effects of expectation.
PET scans give similar results.
Then some is selected, in a fashion that often involves consciousness.