Experimental paradigm. A, Affective significance was manipulated via an item's history. In this example, during an initial conditioning procedure, visual gratings paired with the auditory stimulus “one” were paired with mild shock 50% of the time, whereas those paired with the stimulus “two” were not. The figure is schematic only. B, During the detection phase, trials occurred every 16 s. Subjects were required to indicate whether or not a low-contrast grating (indicated by the dashed circle; not displayed during actual task performance) was shown on that trial. Because the task was demanding, an auditory cue was sounded 100 ms after a grating may have been shown to indicate that subjects should respond. The figure is not drawn to scale; for simplicity, noncritical parts of the trial were omitted.