Common Refractive, Eye-Teaming, and Perception Terminologies Used In Optometry
Refractive Conditions
Emmetropia: emmetropic eyes are eyes where all light rays are focused on the retina, resulting in good vision and no need for correction.
Myopia: more commonly known as short-sightedness, myopia affects an individual’s ability to see objects in the distance. Myopic eyes are generally too long or too powerful. This is typically compensated with negative corrective lenses.
Hyperopia: also known as long-sightedness, hyperopia impairs an individual’s ability to see objects placed at near. Hyperopic eyes tend to be shorter than normal and focus light rays in front of the retina. It is usually compensated with positive corrective lenses.
Astigmatism: This ametropia is found in individuals with oval-shaped eyes. Practitioners can sometimes describe the eye as being shaped like a “footy” instead of completely round. Astigmatism results in ghosting of images and blurry vision. It is often correct with toric lenses, where the power is different in each direction to compensate for the shape of the eye.
Eye Teaming and Focusing
Divergence: the ability of the eyes to turn away from one another and away from the nose. This usually requires the ability to relax.
Convergence: the inward movement of the eyes towards each other and in towards the nose. Near visual tasks generally require the eyes to turn in slightly to allow adequate focusing of near objects.
Accommodation: the crystalline lens inside the eye has the ability to bulge forward and backwards to allow clarity of objects placed at different distances. The lens thickens over time, and tends to become lens flexible with increasing age. On the other hand, young patients have an exceptional ability to accommodate, as the lens is thinner and very flexible.
Binocular Vision (BV) Disorders and Ocular Motility Skills
Convergence Excess (CE): Individuals suffering from CE will often have the ability to go “cross-eyed” voluntarily, and over-turn their eyes excessively towards the nose when focusing on something up close. This can cause eye strain, headaches, and or blurry vision when performing near tasks.
Convergence Insufficiency (CI): CI can be seen in individuals where both eyes cannot turn in enough to focus on objects at near. The eyes have a tendency to sit slightly outwards instead of turning slightly inwards when performing near visual tasks.
Accommodation Excess: this BV disorder is commonly found in patients where the lens and eye muscle overwork to make images or words clear. Individuals suffering from AE will often report blurry vision when looking at near objects, or eye strain and headaches.
Accommodation Insufficiency (AI): AI is noticeable when the accommodation system fails and the eye muscles are unable to work hard enough in order to form a clear image on the retina. Similarly to AE, individuals suffering from AI can also report blurrier vision when observing objects at near.
Divergence Excess (DE): DE means that the eyes turn outward more than necessary when looking in the distance, but are able to work together efficiently to focus on near objects. This may appear as one or both eyes wandering to the side spontaneously, or when a child is tired or sick. For individuals, this may cause double vision or poor depth-perception in the distance.
Components of Visual Perception
Visual Discrimination (VD): This skill refers to the ability to distinguish visual images. VD can be used when differentiating two objects from one another, or when establishing differences between patterns or colours. VD is particularly useful when appreciating subtle differences between words, which is an important skill that can be used when learning how to read and write.
Examples
Confusing letters and numbers
Losing sense of place when reading aloud
Trouble identifying details or determining differences
Visual Memory (VM): VM is the ability to create a mental image from a symbol, an object, a shape or a form, to then recall that image in the future. A fully functional visual system will allow the VM process to occur automatically.
Examples
Difficulties solving mental mathematical equations
Decreased ability to copy text and images from a board
Slower than average reading and spelling skills
Difficulties in recognizing letters or numbers
Confusing letters or words
Trouble finding letters/numbers on a keyboard or calculator
Visual-Spatial Relationship (VSR): This skill is particularly used when forming a relationship between objects seen in space, and to form an awareness of their position towards one another.
Examples
Inability to write within a provided space on a piece of paper
Mixing the direction of letters or numbers (6 instead of 9)
Visual Form Constancy (VFC): VFC is the understanding that shapes, forms and objects stay the same in different sizes or positions. E.g. The letter ‘B’ is always ‘B’ in different words, fonts and sizes.
Examples
Difficulty finding missing items quickly
Inability to recognise familiar objects in places or photos
Difficulty using building blocks or completing puzzles
Difficulty reading unusual fonts
Visual Sequential Memory (VSM): This skill is defined as the ability to remember and recall objects in the correct order or in a sequence. VSM is a crucial skill required when learning to read, write, and when performing mathematical equations.
Examples
Reading ‘felt’ as ‘left’ or ‘cat’ as ‘act’, jumbling between letters when spelling words
Missing or adding letters into words
Difficulty recognising and remembering patterns
Difficulty remembering objects in the correct order
Visual Figure/Ground (VFG): This skill is particularly useful when an individual is required to filter-out irrelevant visual information in a crowded context, in order to only retain the relevant information.
Examples
Difficulties concentrating on books with small print or with many words on a page
Difficulties copying notes from a board
Difficulties scanning for relevant information in a text or choosing details from images
Visual Closure (VC): VC is the ability to form a complete mental image when given incomplete information or a partial view. This skill allows individuals to read text fluently and to be able to complete sentences without reading every word on a shown line. For example, reading similar words and predicting the end of a word without having the full word written out.
Examples
Difficulties finding something if it is partially hidden
Finding completing a puzzle difficult
Requiring extra time to sound out words that are already known
Ocular Motility Skills
Ocular motor skills refer to the movements and control of the eyes. These skills are crucial for a variety of everyday tasks, including reading. Proper ocular motor skills allow a person to accurately track a moving object (pursuit movement), quickly shift their gaze between two points (saccadic movement), and maintain a stable focus on a stationary object (fixation).
These skills are essential for reading because they allow the reader to follow the words on a page smoothly, without losing their place or experiencing discomfort. Problems with ocular motor skills can lead to difficulties with reading, including reading fatigue, difficulty with reading comprehension, and a reduced reading speed.
