Going "upstream", from the market environment to the technology, market signals act to inform the producers of the technology about variations in relative prices. However, the extent to which technology producers can shift from more expensive to cheaper inputs, or modify technology toward the use of cheaper complement goods is bound by technical constraints. Such constraints emerge because inputs are characterized by low substitutability due to the physical and chemical limits involved in the production process. Consequently, the upstream incentives given by market signals affect only the "rate" of use of certain inputs as well as the rate of development of a trajectory but not the "direction" of technical change, which is bound by the technical constraints of production.