Nikita Thomas, PhD Student in School of Optometry and Vision Sciences
We make thousands of eye movements every day; some of which we are aware of and others which are subconscious actions that have been programmed into our brain’s visual control centres over millions of years of evolution. All visually-related behaviours, including eye movements, are processed by the visual system, which takes up about 50% of the brain’s cortex (surface of the brain). The phrase “the eyes are a window to the soul” is certainly well-known in its metaphorical sense, however, eye movements in particular, do provide a unique window to understanding how our brains work.
Advances in eye tracking technology have allowed us to identify the smallest and most rapid eye movements. One such eye movement is a very fast, “ballistic” movement, known as a saccade. They can be either conscious or subconscious and their purpose is to act as “jump” eye movements from one object to another, so that the centre of our retina (where vision is best) is always pointed at the target we wish to look at. It is estimated that we make approximately 100,000 saccades per day. Other eye movements, such as fixations, allow us to keep our gaze fixed on a particular target for a certain period of time, in order to enable us to gather more details from the object in question.
While reading, we carry out a very specific pattern of eye movement behaviour. Although reading is a learned skill, the eye movements we use to read are not taught. Saccades are used to move across a line of words, whereas fixations of a shorter duration are made on familiar words and fixations of a longer duration are made on more complicated words. The average length of fixations while reading is 225 milliseconds. In English, most of the saccades we make are to the right of the text, however up to 10-15% of saccades are made backwards (to the left) when we have difficulty understanding a word or portion of text.
The average reading speed of an adult is 300 words per minute (equivalent to 0.2 seconds per word). However, reading speed can be increased with practice. Our visual system is actually able to identify information up to 14-15 letters ahead of the word that is currently being read. With training you can change your eye movements to make fewer saccades on a line of words, while gathering as much information from a single word and those around it to achieve ‘speed reading’. However, as your reading speed quickens, comprehension can decrease, meaning there is a trade-off between speed and being able to read a text accurately.
- Social Communication
When conversing with others we partake in a refined ‘dance’ of eye movements, in which we look at someone more often when we are listening than when we are talking ourselves. This keeps a conversation flowing by sending hidden signals to the other person as to when it is and isn’t their time to speak. There is evidence to suggest that deliberately “looking into space” while you are talking may aid thinking and help to retrieve your memories, however, quick glances back to the listener usually form part of the conversation routine to ensure that they are still paying attention (and to let them know that you are checking!).
As eye movements play such an important role in these interactions, people who have certain eye disorders, such as strabismus (also known as a “lazy eye”), can be adversely affected in social situations. The same is true of another eye condition, called nystagmus, where the eyes continuously and visibly move throughout the entirety of a person’s life and are never able to be completely focused on the object/other person in question.
A complex interplay of head and eye movements is responsible for orientating our path through an area. For example, when making large eye movements quickly while walking, we will usually use our head rather than our eyes. When moving through unknown terrain, we tend to always look two steps ahead to predict the ground we are about to step on approximately one second later. Slow eye movements downwards are used to track the oncoming path, followed by fast saccades upwards to locate a new point.
An astonishing correlation between eye movements and sports has recently been revealed that shows just how essential these movements are to progressing as an amateur or even as a professional athlete. Research has found that the better the golf player is, for example, the longer they will hold their eye steady, or fixate, on a ball before and during a strike. This phenomenon, dubbed “quiet eye”, is more than the simple idea of ‘keeping your eye on the ball’, as the durations of these fixations need to be extremely precise in order to achieve a high percentage of accuracy in a given sport. In basketball, quiet eye is translated into fixating on the hoop’s rim for slightly longer before performing a free throw, and in football, it means holding your gaze longer in the top right or left corner of the goal before taking a free kick or penalty.
Most professional athletes may not have had explicit training in the quiet eye technique and will not have even made a conscious decision to change their eye movements, but will have picked it up implicitly and often describe it as a “natural thing”. In any case, when using quiet eye, professional athletes can hold their gaze for up to 62% longer than novices. Quiet eye can also be learned, with a university basketball team one of the first subjects of this training. The players used quiet eye to practice their free throws and their accuracy improved by 22% over the next two seasons, compared to an 8% improvement by a control group of players.
- Interacting with objects
Research has shown that your eyes make saccades towards the next required object in a sequence of actions before you have even finished the previous action. For example, while pouring milk into a cup of tea, your eyes may look towards the sugar bowl before you’ve even finished pouring, in order to prepare for your next action. Certain fixations, known as “look ahead” fixations, guide our eyes to non-essential items in the environment, in order to encode object positions into our memories for use at a later point. In the tea-making example, your eyes might subconsciously jump to other aspects of the kitchen scene not involved in the tea-making process, such as the curtains, oven and cupboards, so to program into your memory where these items are for later use.
In reality, eye movements are embedded into almost everything we do. Although some eye movements are conscious, being aware of those which are subconscious gives us a fascinating look into how we perceive the world, how others perceive us and how we perform complex actions.