4 Terminology

• Functionally blind means no usable vision. The learner may have some light perception with or without projection, meaning they can determine the direction of the light source. It is highly unlikely that the visual channel will be a viable means for the learner to gather information.
• Functionally blind students rely primarily on senses other than vision as channels for learning (2 EDC §56350, 2018). Specialized assessments from the TVI provide information on how students use their senses for learning.

Low vision

• Low vision is a permanent and significant loss of visual function that interferes with daily activities. Low vision may be due to reduced visual acuity, reduced visual field, or both. Low vision cannot be corrected with conventional glasses, contact lenses, surgery, or medication (Turbert & Gudgel, 2023). Per Massof and Lidoff, as cited by the American Foundation for the Blind (n.d.), low vision “is better defined in terms of function, rather than [numerical] test results” (A Functional Definition of Low Vision section).
• According to the International Statistical Classification of Diseases and Related Health Problems, 11th Edition (ICD-11), the visual acuity threshold (see below) for low vision (or “vision impairment”) is acuity that is equal to or below 20/70 (World Health Organization, 2019). Mathematically, this means that a learner would have a 3.5 times difference or worse from unimpaired visual acuity (i.e., 20/20) for a medical classification of vision impairment.
• Most low vision students use vision as the primary sensory channel for learning but may also benefit from braille instruction. Using both print and braille as learning media is commonly referred to as “dual media.” A student’s most efficient learning media are determined by the Learning Media Assessment (LMA), a specialized assessment process that is implemented by the TVI.

Legally blind

• Legal blindness is defined as visual acuity of 20/200 or less in the better eye with the best correction or a field of vision no greater than 20 degrees. This definition is the same in Canada and the United States (20 CFR § 404.1581, 1983).
• Legal blindness is more of a legal term than a functional or educational term that may be used by government or health organizations for eligibility for disability benefits, rehabilitation, vocational training, or other programs.

Vision characteristics

• Color perception is the ability to perceive color accurately.
• Visual acuity is the clarity or sharpness of vision. Typical visual acuity is measured as 20/20. Someone with 20/20 vision can see clearly at 20 feet what should normally be seen at that distance. If someone has 20/200 vision, they must be as close as 20 feet to see what a person with 20/20 vision can see at 200 feet (10x closer).
• Vision may be missing in part of the visual field. The visual field is the entire area that can be seen when the eyes are focused on a single point. More information can be found in the chapter on Visual Field Loss.
• Contrast sensitivity is the ability to distinguish shades of a color from its foreground to its background.
• Stereopsis is the ability to use both eyes together (Dhar et al., 2023). From a central point of fixation, the visual system processes two different images. The ability to integrate these inputs so that one image is perceived results in depth perception.

Ocular vs. cortical/cerebral visual impairment (CVI)

• Ocular visual impairment is an uncorrectable disruption to visual sensory input caused by some injury or insult to one or more parts of the eye’s anatomy.
• Neurological visual impairment is the most common cause of visual impairment in children in the economically developed world (e.g., 40-48% of visually impaired children in the UK; Gorrie et al., 2019). It is also called cortical or cerebral visual impairment (CVI). CVI results from damage to the brain’s visual system structures that are not directly involved in sensation (i.e., the eye). Instead, CVI results from damage to the neurological structures responsible for encoding, transmitting, decoding, and interpreting visual information in the brain (Martín et al., 2016). It is most common in children with neurological injury or syndromes. More information can be found in the chapter on CVI.
• A child may have both ocular and cortical/cerebral VI.

Braille

• Unified English Braille (UEB) is the standard braille used across the English-speaking world, including both Canada and the United States.
• Braille can be written symbol by symbol (alphabetic or uncontracted) or used with a system of contractions (i.e., one symbol for common words or word endings). Thus, UEB has alphabetic, logographic, and syllabic components as a writing system.
  • Traditionally, braille has been referred to as a “code,” but recent scholarship more accurately refers to braille as a tactile orthographic (or writing) system (Englebretson et al., 2024).
• In 2013, 137 braille writing systems were in place around the world, representing 133 languages. See the World Braille Usage, Third Edition (Perkins et al., 2013) for more information.
• It is generally estimated that 10% of school-aged youth with visual impairment read braille fluently according to US data (National Federation of the Blind, 2009, p. 8). 
  • This figure should interpreted with caution as estimates of braille literacy rates are frequently based on extrapolations from national eligibility data and not on a count of students using braille as a learning medium (Sheffield et al., 2022).

More than meets the eye

For more information about these terms and a general introduction to working with blind and low vision students, please watch this 42-minute video from the California School for the Blind (McKerracher & Green, 2020).

References

American Foundation for the Blind. (n.d.). Low vision and legal blindness terms and descriptions. https://www.afb.org/blindness-and-low-vision/eye-conditions/low-vision-and-legal-blindness-terms-and-descriptions

Dhar, S. K., Banerjee, S., Al-Zubidi, N., Vannadil, H., Mortensen, P. W., & Albreiki, D. (Ed.). (2023). Stereopsis and tests for stereopsis. EyeWiki. https://eyewiki.aao.org/Stereopsis_and_Tests_for_Stereopsis#Definition

Englebretson, R., Holbrook, M. C., Treiman, R., & Fischer-Baum, S. (2024). The primacy of morphology in English Braille spelling: An analysis of bridging contractions. Morphology, 34(2), 125-150. https://doi.org/10.1007/s11525-023-09413-8

Gorrie, F., Goodall, K., Rush, R., & Ravenscroft, J. (2019). Towards population screening for Cerebral Visual Impairment: Validity of the Five Questions and the CVI Questionnaire. PLoS ONE, 14(3): e0214290. https://doi.org/10.1371/journal.pone.0214290

Individualized Education Program for Visually Impaired Pupils, 2 EDC §56350. (2018). https://leginfo.legislature.ca.gov/faces/codes_displayText.xhtml?lawCode=EDC&division=4.&title=2.&part=30.&chapter=4.&article=3.5

Martín, M. B., Santos-Lozano, A., Martín-Hernández, J., López-Miguel, A., Maldonado, M., Baladrón, C., Bauer, C. M., & Merabet, L. B. (2016). Cerebral versus ocular visual impairment: The impact on developmental neuroplasticity. Frontiers in Psychology, 7, 1958. https://doi.org/10.3389/fpsyg.2016.01958

McKerracher, A., & Green, M. [California School for the Blind]. (2020, January 13). More than meets the eye [Video]. YouTube. https://youtu.be/2Jz4r8fdj_k

Meaning of blindness as defined in the law, 20 CFR § 404.1581 (1983). https://www.ssa.gov/OP_Home/cfr20/404/404-1581.htm  

National Federation of the Blind. (2009). The braille literacy crisis in America: Facing the truth, reversing the trend, empowering the blind. https://nfb.org/images/nfb/documents/pdf/braille_literacy_report_web.pdf

Perkins, International Council on English Braille, & National Library Services for the Blind and Physically Handicapped, Library of Congress. (2013). World braille usage (3rd ed.). https://www.perkins.org/wp-content/uploads/2021/07/world-braille-usage-third-edition.pdf 

Sheffield, R. M., D’Andrea, F. M., Morash, V., & Chatfield, S. (2022). How many Braille readers? Policy, politics, and perception. Journal of Visual Impairment & Blindness, 116(1), 14-25. https://doi.org/10.1177/0145482X211071125

Turbert, D., & Gudgel, D. (2023). What is low vision? American Academy of Opthalmology. https://www.aao.org/eye-health/diseases/low-vision

World Health Organization. (2019). 9D90 Vision impairment including blindness. In International statistical classification of diseases and related health problems (11th ed.). https://icd.who.int/browse/2024-01/mms/en#1103667651