Abstract
Several studies have suggested that it is possible to direct attention in 3-D space (e.g., Nakayama & Silverman, 1986). If attention is directed similarly in depth as in 2-D space, it is possible that preknowledge of a target's depth plane not only help to direct spatial attention to a relevant location in space, but it also enhances the sensory quality of the stimulus representations. Previously we have shown that attention in 3-D does not enhance the sensory quality of stimuli, and thus is different from attention in 2-D (Puhakka, Häkkinen & Laarni 2002).
To further explore these questions, three search experiments were carried out in which observers searched for a digit target among letter distractors. Probability of the target locating in a particular depth plane was varied between blocks. In Experiment 1 and 3 half the characters were at one depth plane and the other half at the other depth plane; in Experiment 2 only one character was at a different depth plane. In the first two experiments the exposure duration was set to 200 ms, which is considered to be too short for vergence eye movements. In the third experiment display remained visible until a response was made.
Probability manipulations had no effect on performance when the exposure duration of the search display was 200 ms. Even when only one character was at a different depth plane, observers were still not able to use the preknowledge of target location in searching for the target. Preknowledge had a clear effect on performance only when the vergence eye shifts were possible.
Results suggest that when display remains visible long enough and there is time to allocate attention to multiple items, the performance improves. It is not clear whether attending to a depth plane enhances the sensory quality of items located at that depth plane or whether these items are only prioritized (see Moore & Egeth, 1998).