Warning Signals With Rising Profiles Increase Arousal

Auditory warnings are often used to direct a user’s attention from a primary task to critical peripheral events. In the context of traffic, in-vehicle collision avoidance systems could, for example, employ spatially relevant sounds to alert the driver to the possible presence of a crossing pedestrian. This raises the question: What is an effective auditory alert in a steering environment? Ideally, such warning signals should not only arouse the driver but also result in deeper processing of the event that the driver is being alerted to. Warning signals can be designed to convey the time to contact with an approaching object (Gray, 2011). That is, sounds can rise in intensity in accordance with the physical velocity of an approaching threat. The current experiment was a manual steering task in which participants were occasionally required to recognized peripheral visual targets. These visual targets were sometimes preceded by a spatially congruent auditory warning signal. This was either a sound with constant intensity, linearly rising intensity, or non-linearly rising intensity that conveyed time-to-contact. To study the influence of warning cues on the arousal state, different features of electroencephalography (EEG) were measured. Alpha frequency, which ranges from 7.5 to 12.5 Hz, is believed to represent different cognitive processes, in particular arousal (Klimesch, 1999). That is, greater desynchronization in the alpha frequency reflects higher levels of attention as well as alertness. Our results showed a significant decrease in alpha power for sounds with rising intensity profiles, indicating increased alertness and expectancy for an event to occur. To analyze whether the increased arousal for rising sounds resulted in deeper processing of the visual target, we analyzed the event related potential P3. It is a positive component that occurs approximately 300 ms after an event and is known to be associated with recognition performance of a stimulus (Parasuraman Beatty, 1980). In other words, smaller P3 amplitudes indicate worse identification than larger amplitudes. Our results show that sounds with time-to-contact properties induced larger P3 responses to the targets that they cued compared to targets cued by constant or linearly rising sounds. This suggests that rising sounds with time-to-contact intensity profiles evoke deeper processing of the visual target and therefore result in better identification than events cued by sounds with linearly rising or constant intensity