Interactions between search mechanisms in conjunction search.

We report the results of a technique designed to measure interactions between different visual search processes. We interrupted pop-out search before it produced a detection response, by adding extra distractors to the display so that a target initially defined by a single feature difference (e.g., a yellow horizontal line among yellow vertical lines) could then only be found on the basis of the conjunction of two features (a yellow horizontal line among yellow vertical lines and pink horizontal lines; difficult search). This technique has been used to measure the duration of the perceptual components of pop-out search, independent of over-all response time, for targets presented among different sets of distractors. In addition, when pop-out failed because it was interrupted, past work has shown that it nevertheless provided useful information to the processes responsible for difficult search. That is, partial pop-out assisted difficult search, when extra distractors made search difficult because the target was between the two types of distractors in the relevant feature space (Olds, Cowan, & Jolicoeur, 2000a,b,c). The present results demonstrate that partial pop-out also assists difficult search when difficult search is a conjunction search, and therefore these interactions may occur at a stage where information from different feature dimensions is combined.

Tracking visual search over space and time.

Visual perception consists of early preattentive processing and subsequent attention-demanding processing. Most researchers implicitly treat preattentive processing as a domain-dependent, indivisible stage. We show, however, by interrupting preattentive visual processing of color before its completion, that it can be dissected both temporally and spatially. The experiment depends on changing easy (preattentive) selection into difficult (attention-demanding) selection. We show that although the mechanism subserving preattentive selection completes processing as early as 200 msec after stimulus onset, partial selection information is available well before completion. Furthermore, partial selection occurs first at locations near fixation, spreading radially outward as processing proceeds.

The time-course of pop-out search.

Olds, Cowan and Jolicoeur [2000. Tracking visual search over space and time. Psychonomic Bulletin & Review (in press)] interrupted pop-out search by adding distractors to a display after a delay. They analyzed the response time distributions from conditions with different delays for interruption and showed that when pop-out search fails, its partially completed computations can be used to assist other, slower search processes. This paper demonstrates that expectancies, numbers of items and colors in the display, and color onsets do not explain those results. Finally, an experiment in which the target was moved mid-trial demonstrates that partial pop-out assists difficult search by indicating something about where the target is, or where the target is not.

Partial orientation pop-out helps difficult search for orientation.

We interrupted pop-out search before it produced a detection response by adding extra distractors to the search display. We show that when pop-out for an orientation target fails because of this interruption, it nevertheless provides useful information to the processes responsible for difficult search. That is, partial pop-out assists difficult search. This interaction has also been found for color stimuli (Olds, Cowan, & Jolicoeur, 2000a, 2000b). These results indicate that interactions and/or overlap between the mechanisms responsible for pop-out and the mechanisms responsible for difficult search may be quite general in early visual selection.

Stimulus-determined discrimination mechanisms for color search.

Visual attention can be goal driven, stimulus driven, or a combination of the two. Here we report evidence for an unexpectedly stimulus-driven component of visual search for a target defined by color. Observers demonstrated a surprisingly cost-free ability to incorporate multiple classifiers in search for a target of one color from among distractors of other colors. A target color was presented among distractors that could change from trial to trial (intermixed presentation) or that remained constant across all trials in a block (blocked presentation). For blocked presentation, a single search classifier (a mechanism that segregates the target from distractors in color space) could be adopted, whereas for intermixed presentation different classifiers had to be used when the distractor colors changed. The benefit of blocked presentation was very small, suggesting that the appropriate classifier was determined very quickly in trials for which the classifier changed. The results suggest that the stimulus-driven activation of an appropriate stimulus classifier can be very efficient.

Linearity across spatial frequency in object recognition.

In three experiments, we measured recognition as a function of exposure duration for three kinds of images of common objects: component images containing mainly low-spatial-frequency information, components containing mainly high-spatial-frequency information, and compound images created by summing the components. Our data were well fit by a model with a linear first stage in which the sums of the responses to the component images equalled the responses to the compound images. Our data were less well fit by a model in which the component responses combined by probability summation. These results support linear filter accounts of complex pattern recognition.