Spatial maps of directed visual attention.

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 (e.g., Posner, 1978; Posner, Snyder, & Davidson, 1980). The spatial extent of this attention effect was examined by inducing subjects to expect the target at one location and introducing occasional probe flashes at other locations throughout the visual field. The results indicated that 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 subject'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 degrees to 16 degrees 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. The implications of these findings for current views of directed attention are considered.