Processing of irrelevant visual motion during performance of an auditory attention task

The extent to which irrelevant perception of visual motion distracters can be modulated by manipulating auditory load in a relevant task was assessed with Positron Emission Tomography (PET) and behavioural experiments. Subjec ts performed an auditory task and ignored an irrelevant visual motion stimu lus, under two conditions. In a low load condition, subjects were asked to detect words spoken in a loud voice among words spoken in a quiet voice. wh ile in a high load condition they attempted to detect bisyllabic words amon g monosyllabic and trisyllabic words. We found that motion-related visual a reas were strongly activated by the irrelevant motion stimulus, compared to a static stimulus, under both conditions of load in the auditory task. In a second behavioural experiment, the duration of the motion after-effect wa s similarly unaffected by adaptation under low or high auditory load. These results are in clear contrast with the strong modulation of irrelevant mot ion processing by visual load, as reflected in the duration of the motion a fter effect (Section 6) and neural responses in motion-related visual areas (Rees ct al., Science, (1997) 278, 338). These findings support the claim that attentional capacity is restricted within but not between sensory moda lities, and indicate that processing of visual distracters may occur whenev er there is sufficient visual capacity to process them, despite being task- and modality-irrelevant. (C) 2001 Elsevier Science Ltd. All rights reserve d.