A bottom-up process, broadly speaking, is a process in which lower-order mechanisms, like sensory inputs, trigger additional processing by higher-order systems. Conjunction searches run much faster when you can direct your attention to a single basic attribute of the stimuli to be scanned rather than using systematic voluntary steering of the attentional spotlight using all attributes simultaneously.

Conjunction Search: Bottom-Up Processing

The binding problem raises the question of how the brain knows that a particular set of features combine to define a single object, considering that these different features are processed in different regions of the brain. This is answered by the feature integration theory.
- Feature Integration Theory

Binding Problem

Conjunction search involves searching for an item on the basis of two or more features (ie. size and color) that together distinguish the target from distracters that may share some of the same attributes. "Where's Waldo" is a good example before you are searching for a 1. boy 2. specific outfit.

Johns Hopkins University

New York University

Attention, in addition to being used to focus attention on certain stimuli, is used to systematically scan the world, or order to find some specific item in a sea of distracters.

Using Attention to Search for Objects

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

With feature search, if the sought item is different from all the distracters in some fundamental way - your friend in a red coat, in a crowd of dark-suited businesspeople - the search is easy and the target of the search immediately "pops out."

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