Behavioral verification of associative learning in whiskers-related fear conditioning in mice

Fear-conditioning is one of the most widely used paradigms in attempts to unravel the processes and mechanisms underlying learning and plasticity. In most of the Pavlovian conditioning paradigms auditory stimulus is used as a conditioned stimulus (CS), but conditioning can be accomplished also to tactile CS. The whisker-to-barrel tactile system in mice offers convenient way to investigate the brain pathways and mechanisms of learning, and plasticity of the brain cortex. To support a claim that an animal learns during conditioning session and that the plastic changes are associative in nature, objective measures of behavior are necessary. Multiple types of conditioned responses can develop, depending on the training situation, CS and unconditioned stimulus (UCS) characteristics. These include physiological responses, such as salivation, heart rate, galvanic skin reaction, and also behavioral responses, such as startle reflex potentiation or suppression of the ongoing behavior. When studying learning with the whisker system in behaving mice, stimulation of individual whiskers in a well-controlled manner may require animal restrain with a disadvantage of only limited behavioral responses observed. Stimulation of whiskers in the neck-restraining apparatus evokes head movements. When whiskers stimulation (CS) is paired with an aversive UCS during conditioning, the head movements decrease in the course of the training. This reaction, called minifreezing, resembles freezing response, frequently used behavioral measure, however applicable only in freely moving animals. This article will review experimental evidences confirming that minifreezing is a relevant index of association formation between the neutral CS and the and the aversive UCS.