4.3 Attending across modalities
The preceding section raised the issue of attention operating (and to some extent failing) across two sensory modalities. By focusing on distraction we ignored the fact that sight and sound (and other senses) often convey mutually supporting information. A classic example is lip-reading. Although few of us would claim any lip-reading skills, it turns out that, particularly in noisy surroundings, we supplement our hearing considerably by watching lip movements. If attention is concerned with uniting elements of stimuli from within one sense, then we might expect it to be involved in cross-modal (i.e. across senses) feature binding too. In this section we will look briefly at one such process.
A striking example of the impact of visual lip movements upon auditory perception is found in the ventriloquism effect. This is most commonly encountered at the cinema, where the loudspeakers are situated to the side of the screen. Nevertheless, the actor's voice appears to emanate from the face on the screen, rather than from off to the side. Driver (1996) demonstrated just how powerful this effect could be. He presented participants with an auditory task that was rather like shadowing in dichotic listening (Section 1.4) – only much harder! The two messages, one of which was to be shadowed, did not go one to each ear: they both came from the same loudspeaker, and were spoken in the same voice. To give a clue as to which was to be shadowed, a TV monitor was placed just above the loudspeaker, showing the face of the person reading the to-be-shadowed message. By lip-reading, participants could cope to some extent with this difficult task. Driver then moved the monitor to the side, away from the loudspeaker. This had the effect of making the appropriate message seem to be coming from the lips. Since the other message did not get ‘moved’ in this way, the two now felt spatially separate and, although in reality the sounds had not changed, the shadowing actually became easier!
These kinds of effects have further implications at a practical level. The use of mobile telephones while driving a car has been identified as dangerous, and the danger is not limited to the case where the driver tries to hold the phone in one hand and steer with the other. If a hands-free headset is used of the type which delivers sound via an earpiece to just one ear, the caller's voice sounds as if it is coming from one side. Attending to this signal has the effect of pulling visual attention towards the lateral message, reducing the driver's responsiveness to events ahead (Spence, 2002).