Localization of the cerebellar cortical zone mediating acquisition of eyeblink conditioning in rats.

Delay eyeblink conditioning is established by paired presentations of a conditioned stimulus (CS) such as a tone or light and an unconditioned stimulus (US) that elicits eyelid closure before training. The CS and US inputs converge on Purkinje cells in the cerebellar cortex. The cerebellar cortex plays a substantial role in acquisition of delay eyeblink conditioning in rabbits and rodents, but the specific area of the cortex that is necessary for acquisition in rodents has not been identified. A recent study identified an eyeblink microzone in the mouse cerebellar cortex at the base of the primary fissure (Heiney, Kim, Augustine, & Medina, 2014). There is no evidence that the cortex in this eyeblink microzone plays a role in rodent eyeblink conditioning but it is a good candidate region. Experiment 1 examined the effects of unilateral (ipsilateral to the US) lesions of lobule HVI, the lateral anterior lobe, or the base of the primary fissure on eyeblink conditioning in rats. Lesions of either the anterior lobe or lobule HVI impaired acquisition, but lesions of the base of the primary fissure produced the largest deficit. Experiment 2 used reversible inactivation with muscimol to demonstrate that inactivation of the putative eyeblink microzone severely impaired acquisition and had only a modest effect on retention of eyeblink conditioning. The findings indicate that the base of the primary fissure is the critical zone of the cerebellar cortex for acquisition of eyeblink conditioning in rats.