The previous blog talked about reflexive blinking which occurs automatically to protect the eyes and is controlled by centers in lower parts of the brain - brainstem and pons. We also discussed spontaneous blinking, controlled largely by brain structures called the basal ganglia, using the neurotransmitter dopamine. The dopamine system develops gradually through adolescence, and that is when blink rate increases to adult levels. Both these blink types typically occur outside our control or awareness. There is also conscious/voluntary control of blinking arising in the frontal cortex of the brain. We can use this to blink more, as in a flirtatious eye flutter, or to suppress blinking when posing for photos or in a staring contest.
In addition to humans, many animals blink. Based partly on studies of mudskippers – a type of amphibious fish that blink only when on land - scientists think that blinking evolved to keep the eyes moist in land animals when they transitioned out of the oceans 375 million years ago. Fish, because water keeps their eyes moist, do not blink. Aquatic mammals do blink, but infrequently, as they protect their eyes with thick jelly-like tears. Insects, because of their eye structure, do not blink. Birds and sharks blink using a ‘third eyelid’ – the nictitating membrane (NM) which moves
from one side of the eye to the other. Reptiles may have the most varied spectrum of blink patterns. Most blink and have a NM; turtles blink one eye at a time; snakes do not blink; chameleons and geckos use their tongues to lick and keep their eyes moist instead of blinking! (check out this video of a gecko: https://rangerrick.org/rr_videos/geckoeyelick/)
Many mammals, including dogs and cats, have a NM. When cats blink, their eyelids close partially, and the NM moves quickly across the eye. The cat NM is thin enough to see through so the blink does not interrupt vision, possibly giving an advantage when hunting prey. In addition, cats use their eyelids to communicate, such as a slow blink to show trust and calm. Dogs also can convey intentions with blinking. They deliberately slow blink to one another as a pacifying or submissive gesture to the dog receiving the blink, in order to avoid conflict.
Blinking has been studied extensively in primates as well. It appears that species active in daytime have increased spontaneous blink rates vs nocturnal species. Additional data show that tree-living primates blink less and for shorter duration than ground-living species, likely because rapidly navigating complex tree arbors requires minimizing interruption to visual attention. The most important correlate of higher blink rate, however, seems to be group size. Researchers suggest that spontaneous eye-blinks acquired a role in social communication, similar to grooming, to adapt to complex social living during primate evolution.
In humans, there are several social and emotional correlates to blinking, and voluntary blink control as well as alterations of spontaneous blink rate can be observed in social settings. Studies show that during social discourse, blinking acts a nonverbal clue. For example, when an explanation is requested, a longer blink from the listener may signify understanding, and leads the speaker to shorten their answer. On the other hand, psychological arousal during interactions - such as with stress, anxiety, excitement or heightened attention - can be recognized as a flurry of rapid eye blinks. This may be why the deliberate eyelash flutter came to represent flirtatious or sexual interest. The social role of blinking is also evident during conversation, as people start to blink their eyes in sync with each other, especially during pauses in speech. This behavior holds even when watching an actor speaking on screen. However, individuals with autism, who struggle with facial social cues, do not synchronize their blinking with others in this manner.
Another interesting fact about blinking is that, contrary to common belief, during deception, blinking is inhibited, and we make more - not less - eye contact when trying to deceive. One explanation may be that because of the widespread myth of making less eye contact when lying, we compensate by making more, and avoid blinking in an effort to appear honest.
A final testament to the importance of the humble eye blink is that alterations in blink rate have diagnostic value in several neuropsychiatric disorders, presumed to result from abnormal dopaminergic function. Increased blink rates in schizophrenia, Tardive dyskinesia or Tourette's syndrome are associated with enhanced dopamine function, whereas Parkinson's disease is associated with reduced dopamine function and decreased blink rate.
All that in a simple blink of an eye! I will never fail to be awed by the mind blowing complexity of living beings, their brains and their behavior. In the words of Chaim Potok, “… a blink of an eye in itself is nothing; but the eye that blinks, that is something.”
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Post by: Nadia Fike
Read more: 1. Homke, P, et al. Eye blinks are perceived as communicative signals in human face--to-face interaction. PLOS One, 2018; 13 (12). 2. Tada, H, et al. Eye-Blink Behaviors in 71 Species of Primates. PLOS One. 2013; 8(5}.