Why do we open our mouths when we put on mascara?
Mike Tobias
Herne Bay, Kent, UK
Advertisement
I have no idea, but I open my mouth when I put in eye drops.
@Nikicinder
via Twitter
Maybe this is an automatic connection related to surprise, and raising your eyebrows and opening your mouth lifts your eyelids to give better access to your lashes?
@DilysCluer
via Twitter
Because we are trying to look up? My husband opens his mouth when he looks upwards.
@Pete_Branscombe
via Twitter
Opening your mouth is a good way of steadying all of your face muscles for a delicate task. Alternatively, your mouth may just be mirroring what your eyes are doing.
Gillian Forrester
University of Sussex, UK
You might think there is a simple answer to this question – by opening the mouth, we stretch the facial skin, creating a taut surface around the eyes to facilitate the application of mascara.
However, it seems that these unconscious, synergistic eye and mouth movements are more than skin deep! In fact, the mouth and eyes open together during most exaggerated attempts to open the eyes. It is likely that this pairing is down to how we are wired. Although separate nerves support the movement of the eyelids and mouth, they originate in close proximity to each other in the brainstem, so, when one is activated, it may activate the other.
In addition, human behaviours don’t often occur as single body actions in isolation. Instead, we tend to function via the synchronous pairing of two or more motor movements. Take the facial expressions of fear and surprise, both characterised by wide-open eyes and mouths. We share this fear face with our primate relatives, suggesting that these eye-mouth co-activations rely on evolutionarily ancient neural wiring. Studies dating back to at least 1907 document people whose symptoms demonstrate the tight ties between the muscles of the eyes and the mouth. For instance, surgeons noted that children with photophobia (eye discomfort from bright light) would often open their mouths while straining to open their eyes.
Or take cranial synkinesis, a congenital condition where a voluntary muscle movement causes a synchronous involuntary contraction of an independently innervated muscle. Mouth and eye muscles are most commonly affected. Marcus Gunn syndrome, also known as jaw-winking syndrome, is the most common form of synkinesis, where the individual presents with a drooping eyelid – however, the eyelid will lift or open when the jaw is moved.
To date, the neuroanatomical underpinnings of synkinesis haven’t been fully determined. While older research postulates that it results from a damaged nerve, at least suggests that synchronous mouth and eye opening is a normal characteristic in a significant minority of the general population and results from naturally occurring links between the nerves responsible for chewing and raising the upper eyelids.
While the pairing of the muscle actions may be unconscious, once we tune into them, we can control them. As children, many of us stuck our tongues out when making fine-precision hand actions. However, as adults, we probably modify these behaviours – or at least when in polite company!
To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
Â鶹´«Ã½ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.