You feel somebody is looking at you, but you don’t know why. The explanation lies in some intriguing neuroscience and the study of a strange form of brain injury.
Something makes you turn and see someone watching you. Perhaps on a busy train, or at night, or when you’re strolling through the park. How did you know you were being watched? It can feel like an intuition which is separate from your senses, but really it demonstrates that your senses – particularly vision – can work in mysterious ways.
Intuitively, many of us might imagine that when you look at something with your eyes, signals travel to your visual cortex and then you have the conscious experience of seeing it, but the reality is far weirder.
Once information leaves our eyes it travels to at least 10 distinct brain areas, each with their own specialised functions. Many of us have heard of the visual cortex, a large region at the back of the brain which gets most attention from neuroscientists. The visual cortex supports our conscious vision, processing colour and fine detail to help produce the rich impression of the world we enjoy. But other parts of our brain are also processing different pieces of information, and these can be working away even when we don't – or can't – consciously perceive something.
The survivors of neural injury can cast some light on these mechanisms. When an accident damages the visual cortex, your vision is affected. If you lose all of your visual cortex you will lose all conscious vision, becoming what neurologists call 'cortically blind'. But, unlike if you lose your eyes, cortically blind is only mostly blind – the non-cortical visual areas can still operate. Although you can't have the subjective impression of seeing anything without a visual cortex, you can respond to things captured by your eyes that are processed by these other brain areas.
In 1974 a researcher called Larry Weiskrantz coined the term 'blindsight' for the phenomenon of patients who were still able to respond to visual stimuli despite losing all conscious vision due to destruction of the visual cortex.