Multiple sclerosis, neglect, and prosopagnosia can cause vision problems {vision, problems}. Partial or complete color-vision loss makes everything light or dark gray, and even dreams lose color.
Cornea can have different curvature radiuses at different orientations around visual axis and so be non-spherical {astigmatism}|. Unequal lens curvature causes astigmatism.
Vision can turn on and off ten times each second {cinematographic vision} [Sacks, 1970] [Sacks, 1973] [Sacks, 1984] [Sacks, 1995].
Failure to combine or fuse images from both eyes results in double vision {diplopia}.
People can see subjective sparks or light patterns {phosphene}| after deprivation, blows, eyeball pressure, or cortex stimulation.
Genetic condition causes retina degeneration {retinitis pigmentosa}| and affects night vision and peripheral vision.
People with vision in both eyes can lose ability to determine depth by binocular disparity {stereoblindness}.
Extraocular muscles, six for each eye, can fail to synchronize, so one eye converges too much or too little, or one eye turns away from the other {strabismus}|. This can reduce acuity {strabismic amblyopia} {amblyopia}, because image is not on fovea.
Left inferior parietal lobe fusiform gyrus damage causes scene to have no color and be light and dark gray {achromatopsia} [Hess et al., 1990] [Nordby, 1990].
Cone pigments can differ in frequency range or maximum-sensitivity wavelength {anomalous trichromacy}. Moderately colorblind people can have three photopigments, but two are same type: two different long-wavelength cones {deuteranomalous trichromacy}, which is more common, or two different middle-wavelength cones {protanomalous trichromacy} [Asenjo et al., 1994] [Jameson et al., 2001] [Jordan and Mollon, 1993] [Nathans, 1999].
8% of men cannot distinguish between red and green {color blindness} {colorblind} {red-green colorblindness}, but can see blue. They also cannot see colors that are light or have low saturation. Dichromats have only two cone types. Cone monochromats can lack two cone types and cannot distinguish colors well. Rod monochromats can have no cones, have complete color blindness, see only grays, and have low daylight acuity.
People can have all three cones but have one photopigment that differs from normal {color-anomalous}, so two photopigments are similar to each other. They typically have similar medium-wavelength cones and long-wavelength cones and cannot distinguish reds, oranges, yellows, and greens.
People can lack medium-wavelength cones, but have long-wavelength cones and short-wavelength cones {deuteranope}, and cannot distinguish greens, yellows, oranges, and reds.
People can lack long-wavelength cones, but have medium-wavelength cones and short-wavelength cones {protanope}, and cannot distinguish reds, oranges, yellows, and greens.
People can lack short-wavelength cones, but have medium-wavelength cones and long-wavelength cones {tritanope}, and cannot distinguish blue-greens, blues, and violets.
Brain can have wounded or infected areas {lesion, brain}. If lesion is in right hemisphere, loss is on left visual-field side {contralesional field}. If lesion is in right hemisphere, loss is on right visual-field side {ipsilesional field}.
Mediotemporal (MT) damage causes inability to detect motion {akinetopsia}.
Two brain lesions in different places typically cause different defects {double dissociation}.
Lateral-geniculate-nucleus damage causes blindness in half visual field {hemianopia} [Celesia et al., 1991].
Removing both temporal lobes makes monkeys fail to recognize objects {Klüver-Bucy syndrome, lesion}.
Visual-cortex region can have damage {scotoma}|. People do not see black or dark area, but only have no sight [Teuber et al., 1960] [Teuber, 1960].
Visual-nerve damage can cause no or reduced vision in scene regions {visual-field defect}.
People with visual-cortex scotoma can point to and differentiate between fast movements or simple objects but say they cannot see them {blindsight}|. They can perceive shapes, orientations, faces, facial expressions, motions, colors, and event onsets and offsets [Baron-Cohen, 1995] [Cowey and Stoerig, 1991] [Cowey and Stoerig, 1995] [Ffytche et al., 1996] [Holt, 1999] [Kentridge et al., 1997] [Marcel, 1986] [Marcel and Bisiach, 1988] [Marzi, 1999] [Perenin and Rossetti, 1996] [Pöppel et al., 1973] [Rossetti, 1998] [Stoerig and Barth, 2001] [Stoerig et al., 2002] [Weiskrantz, 1986] [Weiskrantz, 1996] [Weiskrantz, 1997] [Wessinger et al., 1997] [Zeki, 1995].
properties: acuity
Visual acuity decreases by two spatial-frequency octaves.
properties: amnesia
Amnesiacs with medial temporal lobe damage can use non-conscious memory.
properties: attention
Events in blind region can alter attention.
properties: color
Color sensitivity is better for red than green.
properties: contrast
Contrast discrimination is less.
properties: dark adaptation
Dark adaptation remains.
properties: face perception
People who cannot see faces can distinguish familiar and unfamiliar faces.
properties: hemianopia
Cortical-hemisphere-damage blindness affects only half visual field.
properties: motion
Complex motion detection is lost. Fast motions, onsets, and offsets can give vague awareness {blindsight type 2}.
People with blindsight can detect movement but not recognize object that moved [Morland, 1999].
properties: perception
Blindsight is not just poor vision sensitivity but has no experience [Weiskrantz, 1997].
properties: reflexes
Vision reflexes still operate.
properties: threshold
Blindsight patients do not have altered thresholds or different criteria about what it means to see [Stoerig and Cowey, 1995].
brain
Blindsight does not require functioning area V1. Vision in intact V1 fields does not cause blindsight [Weiskrantz, 1986]. Brain compensates for visual-cortex damage using midbrain, including superior colliculus, and thalamus visual maps, allowing minimal visual perception but no seeing experience. Right prefrontal cortex has more blood flow. Blindsight uses dorsal pathway and seems different for different visuomotor systems [Milner and Goodale, 1995]. Animals with area V1 damage react differently to same light or no-light stimuli in normal and blindsight regions, with reactions similar to humans, indicating that they have conscious seeing.
senses
People can perceive smells when visual cortex has damage [Weiskrantz, 1997]. People can perceive sounds when visual cortex has damage [Weiskrantz, 1997]. People with parietal lobe damage can use tactile information, though they do not feel touch {numbsense} {blind touch}.
Blindsight patients can be conscious of fast, high-contrast object movements {Riddoch phenomenon}. Retinal output for motion can go to area V5 [Barbur et al., 1993].
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Date Modified: 2022.0225