This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Human night vision is not as precise as day vision. That’s why getting up barefoot in the middle of the night comes with a much higher risk of stepping on painful Lego pieces than walking along the same path during the day. I have three kids of ages twelve and under, so I know.
But the specific ways in which our night vision is worse than our day vision are surprisingly counterintuitive to most of us. I remember learning in college that night-vision is achromatic (meaning that we only see in grayscale at night) and not really believing it. It took some careful night-time observation to conclude that my professor was right: objects that were colorful during the day had no hue at night. Most shocking of all was the realization that, though I had always suffered from night-time color blindness (as all of us do), I had never been aware of my deficiency.
A recent study by Alejandro Gloriani and Alexander Schütz, from the University of Marburg, Germany, published earlier this month in Current Biology, shows that our night vision self-delusion is even more pervasive than previously thought.
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To appreciate Gloriani and Schütz’s discovery, the first thing to understand is that day and night vision rely on the activity of different types of photoreceptors (these are the retinal cells that convert light energy into electrical signals, which your brain can then process). ‘Cones’ are active during the day (or when you turn the lights on at night). ‘Rods’ are active during the night (or at very dim light levels).
During day vision, when cones are active, you have highest acuity on the very center of your gaze. This means that, to successfully remove a small splinter from your finger, you’ll want to look at that tiny region of space directly (rather than observe it, say, out of the corner of your eye). That’s because cones are packed most tightly in the very center of your retina, in a small area called the ‘fovea.’ Whatever region of visual space you point your fovea towards, you see in highest detail.
This is not so during night vision, when rods (rather than cones) are active. Whereas your fovea is packed with cones (perhaps because your fovea is packed with cones), it contains no rods. What this means is that when you get up to go to the bathroom in the middle of the night, without bothering to turn on the lights, you have a sizable hole (about the width of your thumbnail at an arm’s distance) right in the very center of your vision. The reason that you likely haven’t noticed that you are literally blind in the center of your gaze at night is that your visual system fills this gap with information from the surrounding retinal regions (which do contain rods). That is, the visual system infers the content that might be present in your central blind spot—and you’re usually none the wiser. Except, perhaps, when you step on a Lego piece that you would have totally seen during the day, that is.
The existence of a central blind spot in human night vision was well known prior to Gloriani and Schütz’s study. What they discovered is that people trust what they see with their central vision more than what they see with their peripheral vision. Both during the day and also at night. Except that you really shouldn’t trust your central vision at night—considering that you’re centrally blind then. Even so, old habits die hard, it seems, for our visual system. Participants in Gloriani and Schütz’s experiments trusted their central night vision more than their peripheral vision—even though their perceptual judgments were wrong much of the time.
The lesson here? Next time you decide to get up in the middle of the night, you may want to scan the floor a bit out of the corner of your eye, to better avoid those pesky Lego pieces. Or just turn on the lights.