Amygdala and subcortical vision: recognition of threat and fear

Abstract

The amygdala (Am) is a relatively voluminous gray substance, located in the depth of the ventromedial temporal lobe. The Am has diverse afferent and efferent connections throughout the neuraxis, and is involved in the modulation of neuroendocrine functions, visceral effector mechanisms, and in complex patterns of behavior: learning and memory, aggression and defense, pain modulation, reproduction, food intake, etc. A recently revealed important function of the Am is that it acts as the brain 'lighthouse' which constantly monitors the environment for stimuli which signal a threat to the organism. The data from patients with extensive lesions of the striate cortex indicate that unseen fearful and fear-conditioned faces elicit increased Am responses. Thus, also extrageniculostriate pathways are involved. A multisynaptic pathway from the retina to the Am via the superior colliculus and several thalamic nuclei was recently suggested. We here present data based on retrograde neuronal labeling that the parabigeminal nucleus emits a substantial bilateral projection to the Am. This small cholinergic nucleus (Ch8 group) in the midbrain tegmentum is a subcortical relay visual center that is reciprocally connected with the superior colliculus. We suggest the existence of a second extrageniculostriate multisynaptic connection to Am: retina - superior colliculus - parabigeminal nucleus - Am. This pathway might be very effective since all tracts listed above are bilateral. The function of the Am by the rapid response to the sources of threat before conscious detection is significantly altered by various neuropsychiatric diseases. Biomedical Reviews 2008; 19: 1-16.