On the unconscious subcortical origin of human fear.

Consistent with the hypothesis that the amygdala is central to fear activation, brain imaging studies show that fear stimuli activate the amygdala, even when conscious recognition is prevented by backward masking. The bulk of the data suggest that the amygdala can be activated from potentially accessible but unattended fear stimuli. Activation of the amygdala facilitates low level visual processing. Several lines of evidence suggest that activation of the amygdala is mediated by a subcortical pathway. Thus, according to data from patients with lesions in the primary visual cortex, the amygdala can be activated in the absence of cortical processing. There is considerable support for the hypothesis that visual stimuli can access the amygdala via a pathway that includes the superior colliculus and the pulvinar nucleus of the thalamus. These data are consistent with an evolutionary argument, focusing of the role of snakes as a predator on primates.