The behavioural signal offered by pupil dilation is something I believe has long been under-represented in optical texts. They tend to emphasise the role of the iris as a light limiting stop, and constriction as a way of reducing light input in situations of brightness, of reducing spherical aberration and of increasing depth of focus. As dilation is a sympathetic nervous response, it is not surprising that pupil dilation is indicative of moments of arousal, whether sexual, based on fear (or both, as an ex-grammar school boy like me might suggest!) or as a betrayal of a deliberate aim to deceive. I have further wondered whether the gradual reduction in pupil function over middle to late years has never evolved because the signalling function of the pupil is primarily required during younger years, where sexual attraction is paramount to propogate the next generation. Discuss.
The pupils will dilate if a person is asked a challenging question, typically a maths problem. Try this yourself. Position a friend at a slit-lamp and set them a difficult mental arithmatic problem while viewing their pupils under a constant illumination source and watch them dilate.l I read recently that a team in Germany (University of Marburg) now are developing this as a means of communicasting with someone rendered completely paralysed. By explaining to them to consider a maths problem when they mean 'yes', and therefore show a pupil dilation, but to relax when they mean 'no', rudimentary but reliable conversation might be possible in the most adverse of circumstances.
