Where is the light coming from?

Today’s reading is “Prior Knowledge on the Illlumination Position” by Pascal Mamassian and Ross Goutcher of the University of Glasgow (Cognition, 2001 [PDF link]).

When we see an embossed seal such as a notary stamp, how do we know which parts are convex (bumps) and which are concave (dimples)? When we look at such a seal through a microscope (or even a toilet paper roll), so that we don’t know where the illumination is coming from, we can’t tell what’s up and what’s down. This effect was first recorded in 1744, and first accurately explained in 1786 by David Rittenhouse: we assume the light is coming from over our heads; if the light isn’t coming from where we expect, then we reverse the image. This makes sense: most of the time in the natural world, that’s where the light is coming from. This preference doesn’t appear to be learned: baby chicks raised with light coming only from below still behave as if the light source is above them.

Over the course of the twentieth century, some of the researchers investigating this problem noticed that the observers in their experiments not only preferred the light source to be above their heads; they expected it to be slightly to their left as well. This phenomenon made no sense: people don’t tend to orient themselves with the sun to their left. The scientists were so certain that this finding was in error that they simply discarded results from observers who favored light from the left.

Mamassian and Goutcher suspected that we may really have a preference for light sources on the left, so they devised an experiment to test it. Consider the following two objects:

If we imagine we are in a room with very low light, coming from above, and we’re looking at the objects head-on, they will look like this:

Notice that if you turn the image on the left upside down, you see that it’s actually the identical image. What Mamassian and Goutcher did is take this figure and rotate it in increments of 15 degrees. Then they asked observers whether it looked like the wide bars or the narrow bars were projecting out from the figure. In fact, observers were looking at the same image each time, so their responses would reflect only where they perceived the light source for the image to be.

They found that observers were most consistent in their responses when the light appeared to be coming from about 25 degrees to the left of vertical. It didn’t matter whether observers were left- or right-handed; all observers had the same preference.

Mamassian and Goutcher suggest that a possible explanation for this result might be a “visual field bias.” When recognizing at people’s faces, for example, we tend to pay more attention to the right-hand side than the left. If the light source is on the observer’s left, then the right side of the person they’re looking at is illuminated.

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4 Responses to Where is the light coming from?

  1. Anne says:

    Testing for handedness seems very cool, but I have another question: IIs this connected, perhaps, to reading? Do readers of Chinese or Arabic, say, exhibit a different preference? I’ve long heard that we read images from left to right, from border to center (sprialing inward in a clockwise direction, that is) and wonder about that, too: Is that also a cultural bias?

  2. Mark Paris says:

    Hi David, I came to visit after your kind e-mail regarding my most uncivil behavior at Uncertain Principles. The illusion doesn’t work too well for me. I see ramps instead of uniformly-raised bars. Maybe I’m studying it too carefully. I find this topic interesting for another reason: atmospheric scientists who want to calculate the amount of light reflected in a given direction by a field of light cannot take a measurement from one direction and use that reflectance value for light reflected into another direction. The reflection of light from irregular surfaces is very much a function of two angles or directions, that of the incident light and that of the reflected light; that is, the reflectance requires a bidirectional reflectance distribution function. So what an observer sees depends on where the sun is relative to his eyes.

  3. Dave says:


    Yes, big time. In fact, for any two-dimensional image (which is, essentially, what is projected onto the retina), there are an infinite number of possible three-dimensional scenes that could generate it. A simple example is an ellipse. When the two-dimensional image we see is an ellipse, we could be looking at an ellipse head-on, or a circle from an angle. The perceptual system makes a number of assumptions to decrease the number of possible three-dimensional objects we might be looking at. The direction the light is coming from is just one such assumption.

    Anne: I don’t know of any cross-cultural studies of this phenomenon, but that doesn’t mean they don’t exist. I’m pretty new to this sort of psychological research.

  4. Mark Paris says:

    I meant to say light reflected by a field of clouds, not a field of light. Oh well.

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