<P> The structure forming the shell of a chamber is a box large enough to easily accommodate the animal being used as a subject . (Commonly used model animals include rodents--usually lab rats--pigeons, and primates). It is often sound - proof and light - proof to avoid distracting stimuli . </P> <P> Operant chambers have at least one operandum (or "manipulandum"), and often two or more, that can automatically detect the occurrence of a behavioral response or action . Typical operanda for primates and rats are response levers; if the subject presses the lever, the opposite end moves and closes a switch that is monitored by a computer or other programmed device . Typical operanda for pigeons and other birds are response keys with a switch that closes if the bird pecks at the key with sufficient force . The other minimal requirement of a conditioning chamber is that it has a means of delivering a primary reinforcer (a reward, such as food, etc .) or unconditioned stimulus like food (usually pellets) or water . It can also register the delivery of a conditioned reinforcer, such as an LED (see Jackson & Hackenberg 1996 in the Journal of the Experimental Analysis of Behavior for example) signal as a "token". </P> <P> Despite such a simple configuration, one operandum and one feeder, it is possible to investigate many psychological phenomena . Modern operant conditioning chambers typically have many operanda, like many response levers, two or more feeders, and a variety of devices capable of generating many stimuli, including lights, sounds, music, figures, and drawings . Some configurations use an LCD panel for the computer generation of a variety of visual stimuli . </P> <P> Some operant chambers can also have electrified nets or floors so that shocks can be given to the animals; or lights of different colors that give information about when the food is available . Although the use of shock is not unheard of, approval may be needed in countries that regulate experimentation on animals . </P>

Who invented a conditioning chamber in which rats pressed levers to earn food rewards