Top-down and bottom-up

Top-down and bottom-up are strategies of information processing and knowledge ordering,in system theories (see systemics).

This vocabulary is employed in neuroscience and psychology. The study of visual attention provides an example. If your attention is drawn to a flower in a field, it may be simply that the flower is more visually salient than the surrounding field. The information which caused you to attend to the flower came to you in a bottom-up fashion -- your attention was not contingent upon knowledge of the flower; the outside stimulus was sufficient on its own.

Contrast this situation with one in which you are looking for a flower. You have a representation of what you are looking for. When you see the object you are looking for, it is salient. This is an example of the use of top-down information.

Parsing
Parsing is the process of analyzing an input sequence (such as that read from a file or a keyboard) in order to determine its grammatical structure. This method is used in the analysis of both natural languages and computer languages, as in a compiler.

Bottom-up parsing is a strategy for analyzing unknown data relationships that attempts to identify the most fundamental units first, and then to infer higher-order structures from them. Top-down parsers, on the other hand, hypothesize general parse tree structures and then consider whether the known fundamental structures are compatible with the hypothesis. See Top-down parsing and Bottom-up parsing.

Ecological
In ecology, top down control referes to when a top predator controls the structure/population dynamics of the ecosystem. The classic example is of kelp forest ecosystems. In such ecosystems, sea otters are a keystone predator. They prey on urchins which in turn eat kelp. When otters are removed, urchin populations grow and reduce the kelp forest creating urchin barrens. In other words, such ecosystems are not controlled by productivity of the kelp but rather a top predator.

Bottom up control in ecosystems refers to ecosystems in which the nutrient supply and productivity and type of primary producers (plants and phytoplankton) control the ecosystem structure. An example would be how plankton populations are controlled by the availability of nutrients. Plankton populations tend to be higher and more complex in areas were upwelling brings nutrients to the surface.

There are many different examples of these concepts. It is not uncommon for populations to be influenced by both types of control.