About kaminakiba Expertise General neuroscience questions (I know that covers a wide region, and I`ll do my best). My interest/study is on brain rhythms, behavioural neuroscience and neuropharmacology.
Experience Three university degrees in neuroscience and over five years of research experience.
I also teach undergraduate cell biology, anatomy, immunology, physiology and biochemistry in my university, so feel free to ask me questions under those areas too.
Expert: kaminakiba Date: 11/30/2005 Subject: pupil dilation due to sympathetic ANS
Question -------------------------
Followup To
Question -
Does anyone have a GOOD answer as to why the pupils dilate in response to stimulation from the sympathetic ANS? I have read various theories from the vacuous 'let in more light' through 'increase radius of visual field' to complex sociobiological 'submission behaviour' theories. I would have thought that pupil diameter was optimal in 'normal' conditions and so any deviation would reduce survival potential...
Answer -
Hi Steve,
It may be a disappointment to you, but the reason for pupil dilation or constriction is mainly for the "vacuous" homeostatic function of light intensity regulation.
You are right about the pupil being in the "optimal condition" when the surroundings are normal. Any deviations from "optimal" would be detrimental to survivial, that's why pupils do stay a certain diameter in a constant environment. However, if your pupils can't dilate to let more light in in the dark, you might fall into a hole in the ground which you would otherwise be able to see and avoid. Conversely, on a sunny day if your pupils can't constrict, you'll probably fall into a hole in the ground since too much light would be blinding you. I consider both to be pretty essential to increase your survival potnetial. It's all about adaptation, not random changes. I am not sure why you don't think this is a good answer for change in pupil diameter size, but if you don't want to take my word for it, shine a torch (not those modern high intensity LED ones, you might blind someone!) into a friend/family's eye in a shaded area. You should be able to observe the reflex (pupil constriction, also note since it's a brainstem coordinated reflex, both pupils constrict even only one receives change in input). Move the torch away, pupils dilate to allow "optimal" light intensity to the photoreceptors to allow normal vision.
As for the "sociobiological" aspect of pupil dilation and constriction (I won't go into these, but essentially I am assuming you are talking about pupil dilation to check people out and looking more cute at the same time), it does have a evolutionary function by increasing the chance of "passing your genes on" as to speak. So in a population genetics point of view, it increases the survival potential of the population (have a better chance to reproduce, that is).
I am not sure if these are good enough asnwers you were looking for as to why pupils change their diameter under different conditions. Hopefully the above would suffice.
Thanks,
SORRY- I didn't make my first question as clear as I ought to have...
I completely understand your response to why pupil size changes under circumstances of altering light intensity and also the "cuteness factor". What I DON'T get is why it dilates as a response to STRESS i.e. stimulation by the sympathetic nervous system (and, for taht matter, due to the hormone adrenaline). Thanks for taking the trouble to reply- hope you can help with the follow-up!
Steve Hood.
Answer Hi Steve,
That's no trouble at all! I would just retire from the post if I thought it was troublesome!
Well, the reason why pupils dilate during stress is somewhat similar to why pupils dilate when you find one attractive - to gather more visual information about the "situation". The more information you can gather through the most prominent sensory input (for us humans anyway), the better you would be at fight-or-flight-ing. On the other hand, one can argue the opposite action from the parasympathetic system would help you relax and rest by not letting so much light, and visual stimuli into the eye. I understand these explanations are somewhat anecdotal, but they do make evolutionary and functional sense so I am buying them for now.
