Pop-out search tasks include a target that differs in one dimension from a group of homogeneous non-target items. A dimension is a categorical feature of a stimulus such as its colour (i.e. a red target among green non-targets), its shape (a square target among circle non-targets) or its orientation (a vertically presented target among horizontal non-targets). Response times in pop-out searches are generally faster to targets when the colour of both targets and distractors remains the same throughout trials, and slower when these colours are switched during trials. A conjunctive search involves non-target stimuli that have more than one dimension in common with the target stimulus. For example, when a target is a green circle in a conjunctive search, non-targets (distractors) could be red circles and green squares. The target will share one dimension in common with one set of non-targets (i.e. shape) and another dimension in common with the other non-target group (i.e. colour). If target and distractor features are the same over consecutive trials, response times are faster than when these dimensions are not repeated. The "dimension-weighting account" of visual selection states that there is a limit to the attentional weight that can be allocated to a particular dimension of an object at any one time. The dimensions of stimuli perceived as important to an observer are allocated more attentional weight (i.e. a target in a visual search), resulting in faster detection times. If a target dimension is known in advance, this can increase the saliency signals of the target. On the other hand, if the target dimension is unknown, attentional weight has to be shifted to the target dimension. When target dimensions remain the same across trials there is no change in attentional weight required, resulting in faster reaction times (intertrial facilitation). The priming of pop-out hypothesis suggests performance in a visual search task involving a pop-out target can be affected by the search history of specific target features in previous trials. If target and distractor features are repeated in subsequent trials, reaction times will be faster than if these features change across trials. The hypothesis proposes the repetition of a target used in a preceding trial makes its pop-out features more salient to an observer and therefore increases the likelihood the observer will attend to it. The episodic retrieval model suggests reduced response times in intertrial priming are due to the observers' retrieval of episodic memories relevant to the task.