acceptable levels of noxious stimulation are those that are well tolerated and do not result in maladaptive behaviors. Use of aversive stimuli at intensities or durations that approach or exceed the animal’s pain tolerance level should generally be avoided in behavioral procedures, unless a scientific justification is provided. As discussed previously, it is important to note that the appearance of escape and avoidance behaviors may occur well before the intensity of a stimulus reaches the pain tolerance level. In such cases, these behaviors would be considered appropriate adaptive responses. It is only when the animal’s behavior is dominated by escape-avoidance attempts that the behavior becomes maladaptive, signaling unacceptable levels of pain (NRC, 1992).
At first glance, one might assume that avoidance or minimization of discomfort, distress, and pain is more problematic when aversive stimuli are used to motivate behavior than when appetitive stimuli are used. However, that is not necessarily true, especially when one considers that the efficacy of some appetitive foods and fluids depends on the introduction of a restricted schedule of access to food or water (see Chapter 3). Thus, in some situations, an aversive stimulus that does not require prior induction of a “need” state (such as contact with mild electric shock or placement in a pool of water) may actually produce less overall discomfort and distress than the combination of an appetitive stimulus with food or fluid restriction. At the same time, however, one must recognize that detection and measurement of “distress” in animals remains problematic (NRC, 2000) (see “Pain and Distress” in Chapter 2).
In some cases, an investigator’s choice of a particular appetitive or aversive stimulus will be determined by scientific reasons. For example, the choice of aversive stimulation such as exposure to electric shock or a predator could be justified by a specific scientific interest in understanding the brain mechanisms underlying behaviors motivated by fear or anxiety. In other situations, however, the scientific question may not directly dictate the choice of one type of stimulus over another. For example, the scientific goals of investigators interested in the neural bases of learning and memory or the mechanisms underlying a specific type of motor behavior might be accomplished by using a broad range of appetitive or aversive events. In situations where the scientific rationale for the choice of a particular motivational stimulus is not compelling or the IACUC is unsure whether one stimulus produces more or less overall discomfort or distress than another (e.g., mild electric shock versus a food pellet combined with food restriction), a useful strategy may be to allow the research to begin using the investigator’s preferred stimulus, but to agree in advance to a joint plan for rigorous monitoring and periodic re-evaluation by the IACUC. If apparent pain or distress is higher than expected or other adverse consequences are noted, stimulus parameters can be refined or the stimulus choice changed with approval by the IACUC. If no problems arise during the monitoring phase, the protocol may continue as originally proposed.