While the pupil DOES respond to changes of light intensity, such as to a Doctor's flashlight, the biological function is related to the requirements of peripheral vision, as indicated by the many "slit" pupils in the animal kingdom. Therefore, for your experiments, you must carefully control for not only differing total illuminance from different words, but also whether some words encroach upon peripheral vision, which might result in sympathetic activation.
Another control are the same letters as in a word, but jumbled so as to not have meaning. If the jumbled have the same pupil response as unjumbled, then meaning perhaps could be eliminated as part of the mechanism.
A small pupil helps reduce blurring. Perhaps stimuli that are blurred or not might also be considered.
I think I spoke just a little too quickly. In my textbook I quoted a scientist that the % change in pupil is too small to deal with the 6 orders of magnitude change in lighting from sunlight to moonlight. This is online atww.escholarship.org http://www.escholarship.org/—6f73v7rf
Later I realized that when the pupil is dilated, the peripheral rods can be seen by the Dr. But it is painful if bright. The assumption under my comment is that the retina is uniform. Bad.
The other problem is that I am working hard on a grant due Apr5, so I haven't studied youir work in sufficient detail.
Keep up the good work.
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