- Partial or complete loss of color vision despite presence of normal functional cones in retina
- Similar to more common cases of color blindness, except that it is due to lesions in the brain, rather than lack of specific cones
- Area of the brain associated with this disorder is area 4 (V4), suggesting that the ventral stream of the visual system is associated with color processing

University of Colorado at Boulder

- This area receives input from blob and interblob regions of V1
- Neurons in this area have larger receptive fields and many are color and orientation specific, suggesting their prominent role in the perception of color and shapes
- Damage in this area could result in achromatopsia

Area V4: Shape and Color Perception

- Macular degeneration
- Color blindness
- Achromatopsia
- Hemianopsia/Hemianopia
- Quadrantanopia
- Photophobia
- Blepharospasm
- Visual Aura
- Strabismus
 - Nystagmus

Types of Visual Impairments

Bear, M. F., Connors, B. W., & Paradiso, M. A. (2016). Neuroscience: Exploring the brain. Wolters Kluwer. 

Neuroscience: Exploring the Brain

Achromatopsia

- Loss of a quarter of one's visual field in both eyes
- Usually caused due to a lesion past the optic chiasm 
- Can show an onset of a stroke, but is also caused by strokes and other brain traumas
- Can be categorized as either homonymous (binasal, bitemporal, upper or lower) or crossed (one upper and the other lower) as well as congruous (equal size of the defects) or incongruous (unequal size of the defects)

Quadrantanopia 

- A consistent blur in one's vision
- Known symptom of neurological disorders
- Positive aura: seeing something that is not there
- Negative aura: not seeing something that is there
- Major type of visual aura: scotoma

Visual Aura

- damage to the retina causes visual impairment
“Dry” type:
- less severe and more common
- caused by atrophy of retinal pigmented epithelium
- results in photoreceptor death
“Wet” type:
- more severe and less common
- caused by abnormal growth of retinal capillaries
- results in detachment of the retina 
- also can cause photoreceptor death

Macular degeneration

Color 'blindness' is more accurately described as a color vision deficiency. The most prevalent type is red-green color blindness, an X-linked inherited condition caused by the absence of cones that process medium-wavelength light. Because the responsible gene is on the X chromosome, and men have only one X chromosome, this deficiency is far more common in men than in women. Women possess two X chromosomes, which typically provides a functional gene if the other is mutated. This condition impairs the ability to distinguish between certain colors, especially reds and greens.

Color "blindness"

- Involuntary closure or twitching of the eyelids 
- Can be temporary but also long-term and chronic
- Can develop into severe spasms rendering patients functionally blind
- Most common in adults over 50 and in women
- Causes are variable, but most commonly due to environmental factors of anti-emetic and antipsychotic drugs and exposure and history of head trauma
- Treatments include injections (such as Botox) and surgery (myectomy - removal of muscles or nerve tissue)

Blepharospasm

- A symptom of many neuro-ophthalmic disorders and migraines
- Comes from the Greek words "photo" (light) and "phobia" (fear), directly meaning fear of light
- The painful or uncomfortable feeling when looking at light
- Referred to as the sensitivity to light
- Many causes, some of which include migraines, retinal dystrophies, and trauma to the brain
- Some researchers hypothesize that photophobia is a functional symptom without an organic basis
- Colored lenses can be used to help the symptoms of photophobia caused by posttraumatic brain inury

Photophobia

- Loss of vision or blindness in half of one's visual field
- Most commonly is permanent; can occur temporarily due to migraines
- Two major types of permanent hemianopia: heteronymous and homonymous hemianopia
- The image below represents vision of an individual without any damage to their visual fields. Use this as a reference for heteronymous and homonymous hemianopia

Hemianopsia/Hemianopia

- Misalignment of the eyes
- Typically one eye is misaligned while the other is properly aligned
- Best treated at a young age, and usually treated with special eyewear and therapies, but extreme cases may need to be fixed with eye surgery

Strabismus

- Movement of the eye that occurs involuntarily
- Can be permanent or temporary 
- Commonly seen in patients who experience dizziness, balance issues, and vertigo (sudden spinning sensation)

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