Human beings are the only primates with a large, bright, and highly visible white part of the eye—which is called the sclera. Why did humans evolve to have more visible eye whites?
A new study has found that our eye whites communicate important social cues that are key to our bonding and survival at a conscious and subconscious level. The October 2014 study, “Unconscious Discrimination of Social Cues from Eye Whites in Infants,” was published in the online journal Proceedings of the National Academy of Sciences.
When you first think about how the whites of your eyes convey subconscious truths, what comes to mind? My first response would be that having bloodshot eye whites is associated with partying, and having discolored eye whites often reflects having a disease like hepatitis, which turns your eye whites yellow.
The pure whiteness of the sclera can send a conscious social cue, but it isn't the hue of your eye whites that sends unconscious social cues. Your subconscious mind picks up on the movements of the eyes and the timing of how much of your eye whites are exposed in different quadrants of the eye socket from millisecond-to-millisecond.
There are obvious ways we use our eye whites to convey blatant social cues like rolling your eyes behind someone's back or refusing to make eye contact as a sign of rejection or shame. When was the last time you recall making a blatant eye gesture as a social cue?
The Visibility and Whiteness of Our Sclera Makes Homo Sapiens Unique
As a branch of the great apes, modern homo sapiens have many unique characterizations that set us apart from other members of the hominin clade of primates.
Our more complex brains, erect posture, bipedal locomotion, ability to make and use tools with manual dexterity, and our propensity for larger societies are commonly referred to as characteristics that set us apart from other great apes. Based on new research, we can add the human sclera to this list.
The highly visible sclera of the human eye makes it easier for us to see where someone else is looking and to engage in nonverbal communication. Knowingly or not, we also use our eye whites to flirt, ignore, and let others know what we’re thinking both consciously and subconsciously.
The amount of sclera showing and how it's moving indicates a wide range of emotions, attitudes and conveys social cues. Wide-open eyes, in which you see a lot of white, implies fear or surprise. A thinner slit of exposed eye whites, such as when smiling, expresses happiness or joy and is interpreted as such.
An averted gaze, as well as direct eye contact, can mean a wide range of things depending on the circumstance. The amount of eye white, the angle and the directional speed of rapid eye movements plays a huge role in human interactions.
The October 2014 University of Virginia and Max Planck Institute study found that the ability to respond to eye cues typically begins to develop during infancy around the age of seven months.
In a press release, Tobias Grossmann, a University of Virginia developmental psychologist and one of the study's authors said, "Our study provides developmental evidence for the notion that humans possess specific brain processes that allow them to automatically respond to eye cues. This demonstrates that, like adults, infants are sensitive to eye expressions of fear and direction of focus, and that these responses operate without conscious awareness."
The researchers found that the infants' brains responded differently depending on the expression suggested by the eyes they were viewing. The infants only viewed the eye images for about 50 milliseconds—which is actually much less time than needed for an infant of this age to interpret the social cues conveyed by the sclera.
Grossmann concluded, "Their brains clearly responded to social cues conveyed through the eyes, indicating that even without conscious awareness, human infants are able to detect subtle social cues. The existence of such brain mechanisms in infants likely provides a vital foundation for the development of social interactive skills in humans."
The Cooperative Eye Hypothesis
Maintaining eye contact when interacting with another person is probably the most important rule of social engagement. Eye contact allows you to see into the window of another person’s soul to some degree and it builds trust.
We often create a conscious narrative based on the subconscious social cues that our mind is picking up by reading the saccadic rhythms, rapid eye movements, and the amount and angle of sclera exposed when communicating nonverbally with one another.
The cooperative eye hypothesis suggests that the eye's distinctive visible characteristics evolved to make it easier for humans to follow another's gaze while communicating or while working together on tasks.
The cooperative eye hypothesis was first proposed by H. Kobayashi and S. Khoshima in 2002 and was later tested by Michael Tomasello and others at the Max Planck Institute for Evolutionary Anthropology in Germany.
Interestingly, animal researchers have also found that, in the course of their domestication, dogs have also developed the ability to pick up visual cues from the sclera of humans.
What Are “Saccades?”
Saccades are the very quick, simultaneous movements made by the eye to receive visual information and shift the line of vision from one position to another. As visual information is received from the retina it is translated into spatial information and then transferred to motor centers for appropriate motor responses.
We rely on the accuracy of these movements every millisecond of our lives. During normal day-to-day conditions, you make about 3-5 saccades per second which amounts to about a half-million saccades a day.
Monitoring the speed of saccadic movements is an excellent way to objectively measure someone's level of fatigue. Recently, scientists in Europe began using a new Google glass type of device to monitor the level of fatigue in physicians working overtime by tracking their rapid eye movements.
In terms of the saccades of someone who seems hyper-alert... I was amused to watch the precise rapid eye movements of Taylor Swift as she clapped a buzzing fly between her hands during a live interview promoting her album 1989.
Someone with saccadic dysmetria produces uncontrollable eye movements including microsaccades, ocular flutter, and square wave jerks even when the eye is at rest. The cause of dysmetria is thought to be lesions in the cerebellum or lesions in the proprioceptive nerves that lead to the cerebellum. Your cerebellum is responsible for coordination of visual, spatial, and other sensory information with motor control.
The cerebellum is also essential for the automatic motor learning of the vestibulo-occular reflex (VOR) and is responsible for ensuring accurate eye movements in conjunction with head movements. Implicit motor learning in the VOR is in many ways analogous to classical eyeblink conditioning. The circuits of both are structured similarly and the molecular mechanisms work the same way.
Your Sclera, Saccades, and Cerebellum Are Intertwined
Most of us take the interpretation and projection of scleral social cues for granted since they become innate early in our childhood development. But for children with autism spectrum disorder (ASD) the ability to make eye contact or interpret the social cues held in the eyes and eye whites doesn’t come naturally.
Fortunately, neuroscientists around the globe are making huge advances towards understanding why people with ASD struggle to interact with others and the world around them.
Samuel Wang, Associate Professor of Molecular Biology at Princeton University, is doing fascinating research on information processing in the cerebellum, including: its contributions to motor learning, the cerebellar roles in cognitive and affective function, and autism spectrum disorder.
Wang and his colleagues at Princeton recently discovered that early cerebellum malfunction hinders neural development and could be a possible root of autism. In August 2014, they published this new theory in the journal Neuron.
Conclusion: The Social Cues Conveyed by Eye Whites Can Strengthen Social Connections
Healthy social and cognitive development relies on the ability of your brain to consciously and unconsciously interpret social cues held in the eye whites of others.
For children with autism, the challenges of trying to explicitly learn how to interpret thousands of saccades an hour and the social cues conveyed through eye whites are astronomical. This new research offers more insights as to why it can be so challenging for people with ASD to interact and connect with others and the environments around them.
Hopefully, these findings will lead to more research and possible interventions that will build stronger and healthier social bonds between people from all walks of life.
If you'd like to read more on related topics please check out my Psychology Today blog posts:
- "The Neuroscience of Making Eye Contact"
- "New Clues On the Inner Workings of the Unconscious Mind"
- "Autism, Purkinje Cells, and the Cerebellum Are Intertwined"