The brain processes fearful faces more quickly when seen out of the corner of the eye than when viewed straight on. Dimitri Bayle and colleagues, who made their finding using magnetoencephalography (MEG) brain scanning, believe this bias has probably evolved because threats are more likely to come from side-on.
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