Nanotechnology Top-down and bottom-up are two approaches for the manufacture of products. These terms were first applied to the field of nanotechnology by the
Foresight Institute in 1989 to distinguish between molecular manufacturing (to mass-produce large atomically precise objects) and conventional manufacturing (which can mass-produce large objects that are not atomically precise). Bottom-up approaches seek to have smaller (usually
molecular) components built up into more complex assemblies, while top-down approaches seek to create nanoscale devices by using larger, externally controlled ones to direct their assembly. Certain valuable nanostructures, such as
Silicon nanowires, can be fabricated using either approach, with processing methods selected on the basis of targeted applications. A top-down approach often uses the traditional workshop or microfabrication methods where externally controlled tools are used to cut, mill, and shape materials into the desired shape and order.
Micropatterning techniques, such as
photolithography and
inkjet printing belong to this category. Vapor treatment can be regarded as a new top-down secondary approaches to engineer nanostructures. Bottom-up approaches, in contrast, use the chemical properties of single molecules to cause single-molecule components to (a) self-organize or self-assemble into some useful conformation, or (b) rely on positional assembly. These approaches use the concepts of
molecular self-assembly and/or
molecular recognition. See also
Supramolecular chemistry. Such bottom-up approaches should, broadly speaking, be able to produce devices in parallel and much cheaper than top-down methods but could potentially be overwhelmed as the size and complexity of the desired assembly increases.
Neuroscience and psychology These terms are also employed in
cognitive sciences including
neuroscience,
cognitive neuroscience and
cognitive psychology to discuss the flow of information in processing. Typically,
sensory input is considered bottom-up, and
higher cognitive processes, which have more information from other sources, are considered top-down. A bottom-up process is characterized by an absence of higher-level direction in sensory processing, whereas a top-down process is characterized by a high level of direction of sensory processing by more cognition, such as goals or targets. Conversely, psychology defines bottom-up processing as an approach in which there is a progression from the individual elements to the whole. According to Ramskov, one proponent of bottom-up approach, Gibson, claims that it is a process that includes visual perception that needs information available from proximal stimulus produced by the distal stimulus. Theoretical synthesis also claims that bottom-up processing occurs "when a stimulus is presented long and clearly enough." Studies in task switching and response selection show that there are differences through the two types of processing. Top-down processing primarily focuses on the attention side, such as task repetition. Bottom-up processing focuses on item-based learning, such as finding the same object over and over again. Implications for understanding attentional control of response selection in conflict situations This also applies to how we structure these processing neurologically. With structuring information interfaces in our neurological processes for procedural learning. These processes were proven effective to work in our interface design. But although both top-down principles were effective in guiding interface design; they were not sufficient. They can be combined with iterative bottom-up methods to produce usable interfaces . Undergraduate (or bachelor) students are taught the basis of top-down bottom-up processing around their third year in the program. Going through four main parts of the processing when viewing it from a learning perspective. The two main definitions are that bottom-up processing is determined directly by environmental stimuli rather than the individual's knowledge and expectations. Related concepts include
field dependence,
global precedence and the
weak central coherence theory of autism.
Public health Both top-down and bottom-up approaches are used in public health. There are many examples of top-down programs, often run by governments or large
inter-governmental organizations; many of these are disease-or issue-specific, such as
HIV control or
smallpox eradication. Examples of bottom-up programs include many small NGOs set up to improve local access to healthcare. But many programs seek to combine both approaches; for instance,
guinea worm eradication, a single-disease international program currently run by the
Carter Center has involved the training of many local volunteers, boosting bottom-up capacity, as have international programs for hygiene, sanitation, and access to primary healthcare.
Ecology represents the ecosystem and its layers, the symbols represent the various limiting factors. In
ecology top-down control refers to when a top predator controls the structure or population dynamics of the
ecosystem. The interactions between these top predators and their prey are what influences lower
trophic levels. Changes in the top level of trophic levels have an inverse effect on the lower trophic levels. Top-down control can have negative effects on the surrounding ecosystem if there is a drastic change in the number of predators. 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. This reduces the diversity of the ecosystem as a whole and can have detrimental effects on all of the other organisms. In other words, such ecosystems are not controlled by productivity of the kelp, but rather, a top predator. One can see the inverse effect that top-down control has in this example; when the population of otters decreased, the population of the urchins increased. Bottom-up control in ecosystems refers to ecosystems in which the nutrient supply, productivity, and type of
primary producers (plants and phytoplankton) control the ecosystem structure. If there are not enough resources or producers in the ecosystem, there is not enough energy left for the rest of the animals in the food chain because of
biomagnification and
ecological efficiency. 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 where
upwelling brings nutrients to the surface. There are many different examples of these concepts. It is common for populations to be influenced by both types of control, and there are still debates going on as to which type of control affects food webs in certain ecosystems. == Management and organization ==