Fear learning through the two visual systems, a commentary on: “A parvalbumin-positive excitatory visual pathway to trigger fear responses in mice”

Citation: Yuan T-F and Su H (2015) Fear learning through the two visual systems, a commentary on: " A parvalbumin-positive excitatory visual pathway to trigger fear responses in mice. " Front. Neural Circuits 9:56. A commentary on BRAIN CIRCUITS. A parvalbumin-positive excitatory visual pathway to trigger fear responses in mice. The theory of two visual systems was firstly proposed in 1960s, to explain the distinct neural mechanisms underlying visual discrimination and localization in rodents and frogs. A recent study demonstrated that fear learning could be transmitted either through visual cortex or simply superior colliculus to the amygdala. Early brain lesion studies found that tectal lesion is associated with the dysfunction of object localization and the visual cortical lesion is associated with failure in pattern discrimination (Schneider, 1969; Ingle, 1973). Similar idea was proposed for human visual system as well: the two streams hypothesis (Botez, 1975; Milner and Goodale, 2008). It is believed that the dorsal visual system receives whole retinal inputs in fast transmission manner, contributing to visually guided behaviors; while the ventral visual system creates imagery with spatial details, and is highly relevant to visual consciousness. The traditional view suggested that fear cues are evaluated by visual cortex and then transmitted to the amygdala, through the visual thalamus. However, it is then realized that healthy human subjects could detect " unseen " fearful cues through subcortical connections between right amygdala, pulvinar, and superior colliculus (SC) (Morris et al., 1999; Tamietto and de Gelder, 2010). This highlighted the possibility of " non-conscious " or cortex-independent processing of visual fear cues. The hypothesis is further proved on human patients with cortical blindness, which showed intact fear learning to the visual cue (Hamm et al., 2003). These evidences argued for the presence of subcortical pathway in visual cue dependent-fear learning. Lesions of lateral geniculate body (LG) and lateral posterior nucleus (LP, pulvinar like structure in rodent) of the thalamus together, but not respectively, led to impairment of visual cue-paired fear conditioning (Shi and Davis, 2001). Yet, the exact neural circuits underlying the visual fear guided behavior was unknown. A recent study employed optogenetic dissection of such circuits to answer the question. One previous study investigated the innate defensive response of mice to overhead looming stimuli as the fear behavior paradigm (Wei et al., 2015). When the authors silenced a group of