Basal Ganglia Contributions to Associative Learning: The Role of the Caudate Nucleus in Trace Eyeblink Conditioning
Eyeblink conditioning (EBC) is an associative learning task in which a behaviorally salient unconditioned stimulus (US; airpuff directed towards the eye) is repeatedly paired with a behaviorally neutral conditioned stimulus (CS; whisker stimulation) until the subject responds to the CS with a conditioned response (CR, eyelid closure). In trace EBC, there is a stimulus free period that separates the offset of the CS presentation with the onset of the US presentation. Acquisition of this form of conditioning requires forebrain structures such as the hippocampus, anterior cingulate cortex, and SI barrel cortex. The basal ganglia are a collection of brain nuclei that are intimately involved with motor planning and execution. The caudate nucleus (CN) is an input structure of the basal ganglia and receives input from forebrain regions necessary for acquiring trace EBC. In these experiments, the CN of rabbits were lesioned both before and after acquisition of trace EBC to test whether the CN is necessary for memory formation, retention, or both. In separate cohorts of rabbits, recording electrodes were implanted into the CN both before and after acquisition of trace EBC in order to determine how changes in neuronal activity might underlie learning and memory in the basal ganglia network. A final experiment is described in which a neuronal tracer was injected into the primary somatosensory (SI) cortex of two rabbits in an effort to identify future targets for study in whisker-signaled trace EBC.