Secondary task modality, expectancy, and the measurement of attentional capacity.

Previous studies have obtained conflicting evidence regarding the effect of probe modality when a secondary probe task is used to measure attentional demands of a primary letter-matching task. The present experiments found no difference in the shape of the probe reaction time functions for auditory and visual probes as long as probe modality was known. When probes "unexpectedly" occurred in the modality that was different from the majority of the probes, differences were obtained. Only the unexpected auditory function changed, shape relative to the others, with reaction time to probes that occurred early in the matching task sequence being increased. The results are interpreted as supporting a role of attentional capacity in the activation of structures that accept relevant sensory input.