Eye movements during visual search: the costs of choosing the optimal path.

Saccadic eye movements are usually assumed to be directed to locations containing important or useful information, but such assumptions fail to take into account that planning saccades to such locations might be too costly in terms of effort or attention required. To investigate costs of saccadic planning, subjects searched for a target letter that was contained in either one of two clusters located on either side of a central fixation target. A target was present on each trial and was more likely (probability=0.8) to appear in one cluster than the other. Probabilities were disclosed by differences in cluster intensities. The distance between each cluster and central fixation varied (60'-300'). The presentation time was limited (500 ms) to ensure that a successful search would require a wisely chosen saccadic plan. The best chance of finding the target would be to direct the first saccade to the high-probability location, but only one of the six subjects tested followed this strategy consistently. The rest (to varying degrees) preferred to aim the first saccade to the closer location, often followed by an attempted search of the remaining location. Two-location searches were unsuccessful; performance at both locations was poor due to insufficient time. Preferences for such ineffective strategies were surprising. They suggest that saccadic plans were influenced by attempts to minimize the cognitive and attentional load attached to planning and to maximize the number of new foveal views that can be acquired in a limited period of time. These strategies, though disastrous in our task, may be crucial in natural scanning, when many cognitive operations are performed at once, and the risk attached to a few errant glances at unimportant places is small.