In a room with a low light level, why can’t you see what you’re looking at directly, but if you look away you can see it in your peripheral vision?

07 Dec In a room with a low light level, why can’t you see what you’re looking at directly, but if you look away you can see it in your peripheral vision?

Because there are two different types of vision at work. Straight-ahead vision puts an image on the macula, at the geographic center of the retina. [The retina is the “film” in the camera analogy of the eye.]The macula is packed very tightly with “cone” photoreceptor cells. These can detect very fine detail (high definition) along with a vast array of color-sensitive detail. But this kind of detailed vision requires good lighting to function properly.

The other type of vision comes from the “rod” photoreceptors that predominate in the rest of the retina -the peripheral areas. Rods cannot discern much detail, but they are extremely sensitive even under very low light conditions. Presumably this developed evolutionarily to give our distant ancestors a survival advantage in the dark by alerting us to attacks from the side and behind.

So if the lighting is too low for the cones to fully sense an object straight in front of you, turning your eye slightly puts that image on the adjacent retina (outside the macula) where rods predominate. Being much more sensitive, they can “see” an object the cones in the macula would miss -but without color or much fine detail.

_Written by J. Trevor Woodhams, M.D. – Chief of Surgery, Woodhams Eye Clinic