Under laboratory conditions tonic immo bility is usually elicited by the application of a brief, standardized period of manual restraint. With chickens the typical procedure involves holding the bird down on its right side with its feet extended and applying gentle but sufficient pressure to preclude escape. Such restraint is ap plied for a fixed interval of time (e.g., 15 s.), after which the experimenter slowly releases the bird. If the subject fails to show the response, manual restraint is reapplied up to a predeter
TONIC IMMOBILITY 777
Tonic immobility (also known as “animal hyp nosis”) is a state of profound, but temporary and easily reversible, motor inhibition that is ex hibited by many different species and is trig gered by a brief period of physical restraint. (The accompanying figure depicts a chicken exhibiting the tonic immobility reaction). De pending upon the testing situation and the spe cies, this peculiar catatonic-like response can last from a few seconds to an hour or more. Although the published literature on tonic im mobility spans over 300 years (see Maser & Gallup, 1977), it is only within the last several
mined number of times (e.g., five). The duration of the response is measured from the time the experimenter releases the bird until the time it shows a spontaneous righting response and gets back on its feet. In addition to the duration of immobility as a measure of the response mag nitude, the other index that is often reported involves a frequency count of the number of times the animal had to be held down in or der to elicit the response, which is used as a measure of susceptibility. In practice any sudden change in stimulation (e.g., a loud noise or an abrupt movement by the experimenter) will cause the response to terminate prematurely, so animals are typically tested by themselves in rooms without distractions. Gallup and Rager (1996) provide a detailed discussion and cri tique of the various methods of response initia tion and measurement strategies that can be employed to quantify different dimensions of tonic immobility.