Here’s another story related to Vaughan’s post of a couple of days ago about the amygdala and fear perception.
A brain imaging study reported in the journal Science  found that showing the silhouettes of fearful eyes for just 17 milliseconds was enough to increase activity in the amygdala’s of human subjects – the effect is something like just seeing the whites of someone’s eyes in the dark (as shown in the picture, along with the comparison condition – the silhouette of the eyes of someone showing a happy expression).
The two things struck me about this. The first, obviously, is how brief the exposure is. If you are shown something for 17ms you will probably be unable to tell that you’ve been shown anything at all (you might see a flash), you certainly won’t be able to tell what it is. In this study the 17ms picture of eyes was immediately followed by a picture of a normal, expressionless, face – which makes perceiving the eye-silhouettes even harder (and, indeed, none of the participants in the experiment reported that they noticed anything unusual).
But their brains did. The amygdala was already ramping up, ready to signal ‘be afraid’ to the rest of the brain. And this to something that isn’t actually scary in itself – but a social signal that there is something to be afraid of nearby. Social and emotional information is being priority-routed through the brain’s processing streams.
If your brain responds to something in 17ms then you know it is priority information – compare with the window of conscious experience (about 200 ms), or the time it would take someone to say if a coloured ball is red or green (nearer 1000 ms). And in general it is emotional information (such as fear) and things for which we have a long-evolutionary legacy of prioritising (danger, food, sex) that we can start responding to so quickly. (There’s more about all this stuff in the book, maybe start with “Subliminal Messages Are Weak And Simple”, [Hack #82]).
The other thing that struck me about the study was that the fearful eyes, are, well, more eye-like. They look more like cartoon-eyes, an exaggeration of the things that make an eye an eye (what an ethologist might call a super-normal stimulus. Like shoulder-pads are for shoulders, wonder-bras are for breasts and cartoon caricatures are for people – more ‘thing-like’ than the thing itself). I wonder if being “eyes-wide” with fear is simply a functional reaction which gives us as wide a field-of-view as possible – or if being eyes-wide with fear is a social signal that has evolved to capitalise on an existing human ability to read emotions from the eyes. We know that eyes are the most important part of our social landscape (see, for illustration, the eye-tracking picture of someone looking at a face on page 45 of the book). And we can assume that fear will be one of the more important, or at least more urgent, things to communicate. The fear signal may have evolved to take advantage of where people are already spending a lot of their time looking, and to piggyback on an existing neural-specialisation to respond to eye-like stimuli.
1. Whalen, P.J. et al (2004). Human Amygdala Responsivity to Masked Fearful Eye Whites. Science, 306, 2061. Discussed here