<P> 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 utilize 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 . </P> <P> These terms are also employed in 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 (Beiderman, 19). </P> <P> According to college teaching notes written by Charles Ramskov, Rock, Neiser, and Gregory claim that top - down approach involves perception that is an active and constructive process . Additionally, it is an approach not directly given by stimulus input, but is the result of stimulus, internal hypotheses, and expectation interactions . According to Theoretical Synthesis, "when a stimulus is presented short and clarity is uncertain that gives a vague stimulus, perception becomes a top - down approach ." </P> <P> Conversely, psychology defines bottom - up processing as an approach wherein 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 ." </P>

How is top down design helpful when writing programs