Previous studies have shown that simple reaction times (RTs) to a visual target are facilitated when the target occurs at a location expected by an observer and are slowed when the target occurs at the mirror-symmetric location contralateral to the expectancy. The present 7 experiments, with 8 observers, examined the spatial extent of this attention effect by inducing Ss to expect the target at 1 location and introducing occasional probe flashes at other locations throughout the visual field. RTs to these probes were equivalent to those obtained at the expected location so long as the probe was in the same hemifield as the S's expectancy. Conversely, RTs to probes in the hemifield opposite the expectancy generated uniformly slower response times. These results were obtained when the expected location varied in eccentricity from 2| to 16| along the horizontal meridian. In addition, when the expected and unexpected locations were within the same hemifield, no expectancy effects were observed. Under these conditions, the frequently used metaphor that directed visual attention operates like a spatially restricted "beam" appears inaccurate. Implications for current views of directed attention are considered.
This may partially explain the "peripheral effect" found in PC3 of Young & Angell. If the experimental participants kept their heads predominantly in a forward-facing direction, the central visual detection task could occur in either hemifield, but the peripheral event detection task would always occur on the left. Since all experimental tasks are performed in the right visual hemifield, it can be expected that visual attention will be directed to that side more often than to the left. The inhibitory effect for onset detection in the hemifield opposite to current visual attention would therefore cause RTs for the peripheral visual detection task to be greater than RTs for the central visual detection task. Future experiments could test this hypothesis by using a peripheral event detection task in the same visual hemifield as the secondary task, in addition to a central or opposite-hemifield event detection task.
The relevance for naturalistic driving is less clear. It is reasonable to argue that actual road events will occur on both sides of the car with approximately equal probability. In that case, the inhibitory effect on the left would be balanced by a priming effect on the right. For the "React" type of interruption, we care primarily about the worst-case RT which would occur on the left. To reduce this worst-case situation, it might be beneficial to use a centrally-placed display for secondary tasks. However, this would also potentially result in more undesirable, involuntary "Notify" interruptions, as explained in the notes for Müller 1989.