Unseen fearful faces facilitate visual discrimination in the intact field

Implicit visual processing of emotional stimuli has been widely investigated since the classical studies on affective blindsight, in which patients with primary visual cortex lesions showed discriminatory abilities for unseen emotional stimuli in the absence of awareness. In addition, more recent evidence from hemianopic patients showed response facilitation and enhanced early visual encoding of seen faces, only when fearful faces were presented concurrently in the blind field. However, it is still unclear whether unseen fearful faces specifically facilitate visual processing of facial stimuli, or whether the facilitatory effect constitutes an adaptive mechanism prioritizing the visual analysis of any stimulus. To test this question, we tested a group of hemianopic patients who perform at chance in forced-choice discrimination tasks of stimuli in the blind field. Patients performed a go/no-go task in which they were asked to discriminate simple visual stimuli (Gabor patches) presented in their intact field, while fearful, happy and neutral faces were concurrently presented in the blind field. The results showed a reduction in response times to the Gabor patches presented in the intact field, when fearful faces were concurrently presented in the blind field, but only in patients with left hemispheric lesions. No facilitatory effect was observed in patients with right hemispheric lesions. These results suggest that unseen fearful faces are implicitly processed and can facilitate the visual analysis of simple visual stimuli presented in the intact field. This effect might be subserved by activity in the spared colliculo-amygdala-extrastriate pathway that promotes efficient visual analysis of the environment and rapid execution of defensive responses. Such a facilitation is observed only in patients with left lesions, favouring the hypothesis that the right hemisphere mediates implicit visual processing of fear signals.