What is "stimulus"

Stimulus \Stim"u*lus\, n.; pl. Stimuli. [L., for stigmulus, akin to L. instigare to stimulate. See Instigare, Stick, v. t.]

1. A goad; hence, something that rouses the mind or spirits; an incentive; as, the hope of gain is a powerful stimulus to labor and action.

2. That which excites or produces a temporary increase of vital action, either in the whole organism or in any of its parts; especially (Physiol.), any substance or agent capable of evoking the activity of a nerve or irritable muscle, or capable of producing an impression upon a sensory organ or more particularly upon its specific end organ. Note: Of the stimuli applied to the sensory apparatus, physiologists distinguish two kinds:

   1. Homologous stimuli, which act only upon the end organ, and for whose action the sense organs are especially adapted, as the rods and cones of the retina for the vibrations of the either.

   2. Heterologous stimuli, which are mechanical, chemical, electrical, etc., and act upon the nervous elements of the sensory apparatus along their entire course, producing, for example, the flash of light beheld when the eye is struck.
      --Landois & Stirling.

plural stimuli, 1680s, originally as a medical term, "something that goads a lazy organ" (often the male member), from a modern use of Latin stimulus "a goad, a pointed stick," figuratively "a sting, a pang; incitement, spur," from PIE *sti- "point, prick, pierce" (see stick (v.)). General sense of "something that excites or arouses the mind or spirit" is from 1791. Psychological sense is first recorded 1894.

n. (rfc-sense) Anything that may have an impact or influence on a system.

1. n. any stimulating information or event; acts to arouse action [syn: stimulation, stimulant, input]

2. [also: stimuli (pl)]

A stimulus is something that causes a physiological response:

• Stimulation
  • Stimulus (physiology), something external that influences an activity
  • Stimulus (psychology), a concept in behaviorism and perception

It may also refer to:

• Input to a system in other fields
• Economic stimulus
  • For government spending as stimulus, see Fiscal policy
  • For an increase in money designed to speed growth, see Monetary policy
  • For general information about economic stimulus, see Stimulus (economic)

In physiology, a stimulus (plural stimuli) is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it normally elicits or influences a reflex via stimulus transduction. These sensory receptors can receive information from outside the body, as in touch receptors found in the skin or light receptors in the eye, as well as from inside the body, as in chemoreceptors and mechanorceptors. An internal stimulus is often the first component of a homeostatic control system. External stimuli are capable of producing systemic responses throughout the body, as in the fight-or-flight response. In order for a stimulus to be detected with high probability, its level must exceed the absolute threshold; if a signal does reach threshold, the information is transmitted to the central nervous system (CNS), where it is integrated and a decision on how to react is made. Although stimuli commonly cause the body to respond, it is the CNS that finally determines whether a signal causes a reaction or not.

In psychology, a stimulus is an energy change (such as light or sound) which is registered by the senses. In behaviorism and related stimulus–response theories, a stimulus constitutes the basis for behavior, whereas it constitutes the basis for perception in perceptual psychology. In this context, a distinction is made between the distal stimulus (the external, perceived object) and the proximal stimulus (the stimulation of sensory organs).

In contemporary experimental psychology the term stimulus is usually used to describe the event or object to which a response is measured. Thus, not everything that is presented to participants qualifies as stimulus: For example, a fixation cross is not said to be a stimulus, because it merely serves to center participants' gaze at the center of the screen. Also, it is uncommon to refer to longer events (e.g. the Trier social stress test) as a stimulus, even if a response to such an event is measured.

In economics, stimulus refers to attempts to use monetary or fiscal policy (or stabilization policy in general) to stimulate the economy. Stimulus can also refer to monetary policies like lowering interest rates and quantitative easing.

Often the underlying assumption is that, due to a recession, production and hence also employment are far below their sustainable potential (see NAIRU) due to lack of demand. It is hoped that this will be corrected by increasing demand and that any adverse side effects from stimulus will be mild.

Fiscal stimulus refers to increasing government consumption or transfers or lowering taxes. Effectively this means increasing the rate of growth of public debt, except that particularly Keynesians often assume that the stimulus will cause sufficient economic growth to fill that gap partially or completely. See multiplier (economics).

Monetary stimulus refers to lowering interest rates, quantitative easing, or other ways of increasing the amount of money or credit.

For example, Milton Friedman argued that the Great Depression was caused by the fact that the Federal Reserve did not counteract the sudden reduction of money stock and velocity. Ben Bernanke argued, instead, that the problem was lack of credit, not lack of money, and hence, during the financial crisis, the Federal Reserve led by Bernanke provided additional credit, not additional liquidity (money), to stimulate the economy back on trail. Jeff Hummel has analyzed the different implications of these two conflicting explanations. President of the Federal Reserve Bank of Richmond, Jeffrey Lacker, with Renee Haltom, has criticized Bernanke's solution because "it encourages excessive risk-taking and contributes to financial instability."

It is often argued that fiscal stimulus typically increases inflation, and hence must be counteracted by a typical central bank. Hence only monetary stimulus could work. Counter-arguments say that if the production gap is high enough, the risk of inflation is low, or that in depressions inflation is too low but central banks are not able to achieve the required inflation rate without fiscal stimulus by the government.

Monetary stimulus is often considered more neutral: decreasing interest rates make additional investments profitable, but yet only the most additional investments, whereas fiscal stimulus where the government decides the investments may lead to populism or corruption. On the other hand, the government can also take externalities into account, such as how new roads or railways benefit users that do not pay for them, and choose investments that are even more beneficial although not profitable.

Typically Keynesians are particularly strongly pro-stimulus, Austrians and Rational expectations economists against, and mainstream economists between the two.