Medial Geniculate Lesions Block Amygdalar and Cingulothalamic Learning-related Neuronal Activity

This study assessed the role of the thalamic medial geniculate (MG) nucleus in discriminative avoidance learning, wherein rabbits acquire a locomotory response to a tone [conditioned stimulus (CS)+] to avoid a foot shock, and they learn to ignore a different tone (CS-) not predictive of foot shock. Limbic (anterior and medial dorsal) thalamic, cingulate cortical, or amygdalar lesions severely impair acquisition, and neurons in these areas develop training-induced activity (TIA): more firing to the CS+ than to the CS-. MG neurons exhibit TIA during learning and project to the amygdala. The MG neurons may supply afferents essential for amygdalar and cingulothalamic TIA and for avoidance learning. To test this hypothesis, bilateral electrolytic or excitotoxic ibotenic acid MG nuclear lesions were induced, and multiunit recording electrodes were chronically implanted into the anterior and posterior cingulate cortex, the anterior-ventral and medial-dorsal thalamic nuclei, and the basolateral nucleus of the amygdala before training. Learning was severely impaired and TIA was abolished in all areas in rabbits with lesions. Thus learning and TIA require the integrity of the MG nucleus. Only damage in the medial MG division was significantly correlated with the learning deficit. The lesions abolished the sensory response of amygdalar neurons, and they attenuated (but did not eliminate) the sensory response of cingulothalamic neurons, suggesting the existence of extra geniculate sources of auditory transmission to the cingulothalamic areas.