Fast, efficient detection of potential danger is critical to ensure the survival of an organism. In rodents, this ability is supported by subcortical structures that bypass slow but highly detailed cortical visual processing areas. Similarly, anatomical tracing and functional neuroimaging studies in human and non-human primates suggest that limbic structures responsible for emotional processing, such as the amygdala, receive input from the retina through subcortical structures, as well as through cortical visual areas. Whether outputs from this subcortical pathway can support perception of emotionally laden stimuli and influence cognitive-level decisions, independently of cortical input, is not known. Here we show that information delivered by the subcortical pathway to the amygdala modulates emotional processing. In Experiments 1 and 2, emotional faces were rendered invisible using an interocular suppression technique (Continuous Flash Suppression; CFS), that prevents visual information from reaching the amygdala via the geniculate-cortical pathway, favoring instead the use of subcortical structures. In these experiments, likeability judgments over novel neutral stimuli (Chinese characters) were modulated by invisible pictures of angry but not happy faces. In Experiment 3, the same emotional faces were rendered invisible through backward masking (BM). BM is a visual masking technique that allows visual information to reach inferior and ventro-temporal regions, which can then serve as afferents to the amygdala alongside subcortical afferents. In this experiment, happy and angry faces modulated the likeability judgments. This valence-specific effect fits well with the extant literature on the role of the amygdala in threat detection. It also suggests that subcortical processing may be specifically tuned to threat detection processes. The coarse information processed by the subcortical pathway from the retina to the amygdala may prevent potential threatening events from going unnoticed by enhancing arousal levels and directing attentional resources to areas of interest for further detailed geniculo-striate cortical processing.
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