Our ability to see with our eyes hardly attracts the attention that the eyes capture with social signals. First, of course, it’s the pupils that attract the curious and the health-conscious and even the prurient. But eye movements are not far behind in lurid speculation. They “give away your secrets”, “betray your thoughts”, and even act as the “mirror of the soul”.
Such correlations lack the technical fascination in figuring out why our eyes move at all, and how eye movements work together to make two eyes better than one.
Without eye movements, vision itself would be a chore, requiring head movements to replace eye movements. That’s because an image that remains stationary on the retina will soon become invisible. With eye movements, what we see is constantly refreshed; without it, things fade away. (Try the Troxler effect to get an idea of this. It’s a feature, not a bug. Find the whole nine yards here or there.)
There are other reasons for moving our eyes, but, as you have probably noticed and a Canadian military technical report will confirm (p. 4), being shaken at certain frequencies will degrade your acuity—in reading, for example.
So our eye movements are not just shaking eyeballs but organized behavior, right down to the smallest eye movements needed to resolve the finest detail and right up to the way we coordinate vision in our two eyes with conjugate eye movement. We are not chameleons, although (a) chameleon eyes are not quite independent; and (b) there was an interesting case of a human patient with independently moving eyes, mentioned by Gordon Walls in footnote 5 on p. 76.
Eye movements and gaze can be interesting out of concern for disability, too. Eye movements are a common cause of diplopia, for example. (Please note that double vision can have many causes.) Or look at the Korsakoff syndrome exhibited by some long-term alcoholics. It’s preceded by a condition called Wernicke’s encephalopathy that features a condition called ophthalmoplegia, or gaze paralysis. It’s caused by a lack of thiamine (vitamin B1), which weakens the abducens nerve (sixth cranial nerve) so that one of the extrinsic (extraocular) eye muscles doesn’t work (scroll down).
“The eyes, Chico. They never lie.” But Al Pacino was only partly right. The mechanisms of eye movement are quite diverse and complex. Dozens of research articles have documented the variations from culture to culture and even among species. (My sample is haphazard rather than definitive.)
BIO: We inherit muscles that permit four kinds of eye movements, and early in infancy we learn to use them accurately. Saccades and smooth pursuit movements are distinguished with the help of this illusion. Generally, we see whether our eyes are moving or relatively still, with the exception of saccades. We are not aware of saccadic or post-saccadic suppression because the brain fills in the missing sensation, as in the stopped-clock illusion. Sometimes filling-in relies on surrounding context, as in the blind spot, while filling-in for the stopped-clock illusion operates on expectation. In addition, saccades are really fast eye movements.
You can realize the importance of the eye muscles by considering that the fovea would be almost useless without them, so we use them very carefully. We have to fixate an object to place its image within the fovea, so control has to be precise. The eye muscles’ functions vary as change our behavior, but the movements are “programmed” well to carry out each task efficiently.
You could almost say that vision would not be useful without eye movements. But because vision is so useful, marketers, security guards, and psychologists want to track it. If we think something is worth a look, we can be pretty sure that our eyes are worth a look to someone else.
PSYCHO: Our eye movements change when we inspect something closely. They seem to halt as the eyes fixate a point in space, but even the small movements that persist, like microsaccades, ocular drift, and ocular tremor, have helpful functions. In viewing less detailed parts of a scene they prevent fading, while in highly-detailed images they enhance acuity.
Out of our two retinas the brain constructs a single (start viewing at 29 minutes), stable world, with the help of space constancy. Perhaps our view turns into a single world in secondary visual cortex, or V2. Soon after, we use our eye movements in a number of different ways suited to our different daily tasks.
When the two eyes see slightly different views of an object through conjugate eye movements, the brain is able to fuse the two retinal images into a single subjective view, often called a Cyclopean (or one-eyed) view. It then uses the difference between the two retinal images, called binocular disparity, as a depth cue called stereopsis, or stereoscopic vision.
However, when the two eyes see very different views of an object, the brain cannot fuse the two retinal images and we see two images consciously, which is diplopia.
There are often large differences between one person and another in their thresholds for diplopia. A difference that one person can fuse, another cannot. If the brain cannot fuse images over a long time because of strabismus, the result is likely to be amblyopia.
Here are some diagrams that may help if you want to take it further, composed by someone who suffers from strabismus.
In early development the brain works hard to make sense of sensory input. It becomes an interpreter of whatever the sense organs come up with. For example, the brain learns to perceive faces more quickly than you might think.
As Colin Blakemore once put it, “We program our brains by our experiences.” The cerebral cortex learns to interpret what it hears, sees, feels, and touches. If what the baby senses is only meaningless blurs or noise because of a sensory impairment, the cortex will tune neurons to interpret noise, which may delay speech development in hearing-impaired children or visual development with strabismus.
In the 1950s, Austin Riesen’s studies of sensory deprivation broke new ground in the study of developmental plasticity. Colin Blakemore carried on more specific studies in the 1960s. World wars in the 20th century brought discoveries of compensation for loss of function that were expanded in lab studies like those of Glees and Cole in 1955.
Now neuroplasticity has influenced all of neuroscience and probably provoked unrealistic expectations for brain training. At the same time we are thinking more deeply about what plasticity means. Plasticity has limits and has drawn skeptics. With practice, people with poor vision can learn to overcome or postpone their impairments to some extent.
Nevertheless, there is clear evidence of neuroplasticity during development, in infants and children as well as in the sensory deprivation studies of Riesen and Blakemore. Would you believe 700 synaptic connections are formed every second in an infant? That provides opportunity for adaptation! The environment shapes brain development. Plasticity continues through life in the form of learning.
The psychology of eye movements should lead us to consider hand-eye coordination and distractibility and stereoscopic depth perception, but those are long discussions, better suited to another post. To end on a limited yet triumphal note, here’s a way to move just one eye at a time, which eminent surgeon Charles Bell said was impossible.
SOCIAL: In another post I mentioned oral capture, which makes us attribute flavor to the mouth when it is more heavily influenced by the nose, and visual capture, which makes us hear a voice coming from the face that is moving rather than from the one our ears tell us is speaking, which is makes ventriloquism successful.
Without intending to, we pay attention to some things more than others, and occasionally to nothing much at all. The basis of that difference is three networks of nerve cells called the salience network, dorsal attention network, and the default mode network, which tend to operate in mutually exclusive or flip-flop fashion. That is, when one is active, the others aren’t. (And there’s also a ventral attention network that collaborates with the dorsal attention network.)
The salience network is responsible for telling us to look at faces, particularly ones that move. Movement of the eyes or mouth is an attention grabber. That means that we may miss what is going on away from the face, which is how stage magicians can make a living by misdirection. A magician doesn’t need to make us look far away; a few shifts of our microsaccades is enough to, um, do the trick. Not many folks want to have the tricks explained to them, but talks on the science of magic–or better, the neuroscience of magic—will pack the house.
This misdirection been going on for a long time with pickpockets, magicians, and even dentists. I haven’t even mentioned other muscles around the eye that allow us to blink instead of move our eyes, but they’re subject to misdirection in a way, too.
These researchers think blinking resembles eye movements in having been co-opted as a social signal. Might this explain the increase in blink rate when someone is flirting? And sometimes we synch our blinks in a group. Odd thing to do unless it has a social function.
There’s even research to show that blinking is a sign that we’re switching attention, too. It may give the brain a momentary break, perhaps by way of activating the default mode network. Makes it harder to view the Mona Lisa, though. Blinks are important to moviemakers, too; but what could filmmakers and magicians read in our eyes?
Maybe the theater owners are responsible. The screenwriter provides a story that the film director has to present briefly enough to allow the theater owner to turn over audiences profitably. How do they compress a drama effectively?
One clue is the audience’s blink rate. We blink more than we need to for the health of our eyes (research here). In each eyeblink we release our attention briefly and may redirect it. Film editors time their cuts to the audience’s blinking, when it’s time for them to switch attention.
Incidentally, the gaze is likely to reveal what people are paying attention to, but there’s evidence against the shifty gaze hypothesis. In fact, telling a child “Look at me when I’m talking to you” is likely to defeat its purpose. Managing the direction of kids’ eyes may achieve the opposite of what we want, even with adults.
That doesn’t mean we should disregard the eyes. Eyeblinks may be a sign of deception. We should note that eye tracking may reveal more than a shifty gaze will. Despite the varied claims of experienced investigators, here’s what the FBI really says: “The findings from these studies also have clearly indicated that no one indicator of lying exists; if so, research would have identified it by now, and almost everyone could unerringly detect when people lie.” There is no Pinocchio’s nose.
Don’t believe the claims of the Neuro-Linguistic Programming folks, either.