Sunday, 28 July 2013

Pupil constrictions to photographs of the sun

Photo by Arun Kulshreshtha.

If you clicked through to this page via the Google homepage, which is almost entirely white, the amount of light your screen is pumping out will have decreased when the page loaded, because much of the above photo is blue/grey.

But your pupils will have constricted, not dilated, because the photo is an image of the sun.

That finding was reported by Binda, Pereverzeva and Murray in a paper published in the Journal of Vision in May.

The paper, which is so pithily named I couldn't think of a way to better it for the title of this blog post, is freely available to read here. Binda et al showed eight people four sets of photos presented in a random order. One set of photos was made up of pictures of the sun, and the other three sets were precisely matched to these in terms of the amount of light a screen gives off when it displays them - their luminosity. These were the controls against which the pictures of the sun were compared. The control pictures were images of the moon, scrambled versions of the sun images, and uniform grey squares with no images.

You can see the images here.

The participants' eyes were tracked as the images were randomly displayed on a screen after an interval of maximum-luminosity blank whiteness. Each image therefore caused a decrease in the amount of light being given off by the screen, regardless of what was depicted.

Binda et al found that when pictures of the moon, scrambled pictures of the sun or uniform grey squares were displayed, the viewers' pupils dilated in response to the dimming of the light, as expected. But when pictures of the sun were displayed, the viewers' pupils constricted, just as if they had looked directly at the real sun, even though the light reaching them had actually decreased.

Additional experiments ruled out some possible explanations for the finding that would have been due to weakness in the study design. Viewers' subjective impressions of the images' brightness were not responsible, nor was the effect due to the viewers focusing on the brightest parts of the sun images but not of the other images.

The study also ruled out the possibility of the effect being due to the sun images demanding less attention than the others. Although this might seem strange, the step was necessary because it is well known to researchers that tasks which require a lot of conscious thought cause the pupils to dilate - although the reason for this is itself unknown.

The effect therefore seems to be due to some subconscious phenomenon occurring in the brain: in the researchers' words it seems to be a "conditioned light-avoidance behaviour".

If you thought that pupilary reflexes were boring, I hope you've seen the light.


Binda P, Pereverzeva M, Murray SO (2013) Pupil constrictions to photographs of the sun. J Vis 13(6): 8. doi: 10.1167/13.6.8

1 comment:

  1. The Spanish painter JoaquĆ­n Sorolla was a master at achieving this effect with paint.

    Some of his paintings almost cause you to squint when looking at parts of them, despite knowing that the brightest part is only as bright as your screen (or titanium white, if seeing it in the paint)

    The spot of sunlight in this painting, is a good example: