Eleven participants had their brains scanned while they judged whether faces on a computer screen were happy or not. Unbeknown to the participants, each of these visible faces was actually preceded by a subliminally presented fearful face, either straight ahead or in the periphery.
The striking finding was that a peripherally presented fearful face led to much quicker activation of brain regions known to be involved in emotion processing. Specifically, a peripherally presented fearful face was followed by increased activation in the right anterior fronto-medial region - including the famous amygdala - within just 130ms. By contrast, a fearful face presented straight on triggered activity in these emotional-processing centres only after 210ms.
Bayle's team think that fearful stimuli seen out of the corner of the eye are processed more quickly because of the preponderance of so-called 'magnocellular' receptors in the eye's periphery. These feed into the magnocellular visual pathway, known for its fast and dirty processing, which routes subcortically via the superior-colliculus. In contrast, stimuli viewed straight ahead in our full attentional glare are preferentially processed by the so-called parvocelluar pathway, which is more thorough and travels rather more leisurely via the visual cortex at the back of the brain.
The researchers concluded: 'An adaptive advantage is conferred by the fast automatic detection of potential threat outside the focus of attention, as danger in the external world mostly appears in the peripheral vision, requiring a rapid behavioural reaction before conscious control.'
Bayle DJ, Henaff MA, & Krolak-Salmon P (2009). Unconsciously perceived fear in peripheral vision alerts the limbic system: a MEG study. PloS one, 4 (12) PMID: 20011048
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