Visual field
the entire area or field of view that can be seen when an eye is fixed straight at a point on space. Descriptions of the visual field include: vertical meridian - line dividing the field of view into left/right halves; horizontal meridian - line dividing field of view into top and bottom halves.
Hemifield
'half the visual field,' typically refers to left and right halves only (not top/bottom)
Quarterfield
one quarter or quadrant of the visual field,' defined by the quadrant created by the vertical and horizontal meridian lines
Cornea
the transparent dome-shaped anterior portion of the outer covering of the eye, affects light rays that are coming in, thickness will move or bend light rays, light passes through here first
Lens
situated behind the iris of the eye, it focuses light entering the eye onto the retina (upside-down)
Sclera
the white part of the eye that, with the cornea, forms the protective outer covering of the eye
Iris
the colored portion of the eye, a muscular diaphragm that controls the size of the pupil, which in turn controls the amount of light that enters the eye
Pupil
the hole located in the center of the iris of the eye that allows light to strike the retina; appears black because light rays entering the pupil are either absorbed by the tissues inside the eye
Retina
the back of the eyeball, considered a part of the brain, where light hits the photoreceptive cells and visual information begins being processed. pick up light info and transmit to the the brain (part of CNS) no recovery if damaged
Fovea
the part of the retina, where vision is most acute and color vision is best; cone photoreceptors are most prevalent here (mix of rods and cones outside the fovea)
Blind spot
the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on the optic disc of the retina where the axons of the retinal ganglion cells exit the retina and form the optic nerve. Because there are no photoreceptor cells to detect light on the optic disc, the corresponding part of the field of vision is invisible. Some process in our brains "fills-in" the blind spot with estimates of expected visual info based on surrounding detail and information from the other eye, so we do not normally perceive the blind spot. (no cones or rods)
Photoreceptor Cells
Cells that line the back of the retina and have parts that change shape when they are hit with a photon, allowing them to detect light in a certain part of the visual field. The overall function of the photoreceptor cell is to convert the light energy of the photon into a form of energy communicable to the nervous system and readily usable to the organism: This conversion is called signal transduction. Humans have two main types, rods and cones, and there are three different subtypes of cones.
Rods
Photoreceptor cells that are located outside the fovea (periphery). They are highly sensitive to light and thus are responsible for low-light (scotopic) vision, like under starlight. Rods also contribute to visual motion detection, but have poor visual acuity. They also do not differentiate between colors. non-functional in bright light
Cones
Photoreceptor cells that are concentrated in the fovea (center), but also more sparsely extend into the periphery. They are responsible for high acuity vision, but take more photons of light to activate (good for daytime - photopic - vision). There are three types, each most responsive to different wavelengths of light (long (L), middle (M) and short (S) -wavelength cones, corresponding to maximal absorption of red, green, and blue light, respectively). The combination of inputs from different cone types though opponent processing (see below) produces for color vision.
Photoreceptor Protein
light-sensitive protein molecules involved in the sensing and response to light in a variety of organisms by undergoing a structural change when they absorb light. This structural change opens ion channels, which causes a change in the graded potential (ion flow) of the photoreceptor (in other words, causes the photoreceptor cell to signal that light has been detected).
Opsins
type of photosensitive pigment proteins found in photoreceptors: e.g., rhodopsin in rods and photopsin in cones (3 types are in cones, making up the L,M, and S cone types), and melanopsin in the melanopsin-containing retinal ganglion cells (also called intrinsically photosensitive retinal ganglion cells) shape of it is what affects at what wavelength the Photoreceptor responds to. They have a refractory period
Retinal Ganglion Cells
Cells in the retina that receive input from modulatory neurons (which get input from photoreceptor cells) and transmit the information down the optic nerve to the brain. Primary types of retinal ganglion cells are the midget cells (parvocellular pathway), parasol cells (magnocellular pathway), and small bi-stratified cells (koniocellular pathway). An additional light-absorbing type is the melanopsin-containing retinal ganglion cells
Opponent-processing color vision theory
color vision theory that color is processed in 2 different opponency channels created by specific wiring together of cone photoreceptors and retinal ganglion cells: red (L cone) vs. green (M cone)o blue (S cone) vs. yellow (L+M cone), dark vs. bright (red/L+green/M+blue/S) => comparison produces luminance
Optic nerve
composed of the axons of the retinal ganglion cells that leave the retina and head back towards the optic chiasm in the brain, taking with them visual information. This nerve is the reason humans have a blind spot, because no photoreceptive cells exist where the optic nerve exits the eye. (optic nerve -> optic chiasm -> optic tract -> LGN of thalamus -> optic radiation -> V1)
Parvocellular pathway
A visual processing stream that pools over fewer receptors. The cells involved (midget retinal ganglion cells) have a sustained response and are involved in processing color, fine details, textures, and depth.
Magnocellular pathway
A visual processing stream that pools over many receptors, whose retinal ganglion cells (parasol retinal ganglion cells) fire in bursts and are useful for detecting motion.
Koniocellular pathway
A visual processing stream that gets S-cone input only (from small bi-stratified cells), processing low acuity visual information, and innervating V1 and extrastriate cortex.
Melanopsin-Containing Retinal Ganglion Cells
(also called intrinsically photosensitive retinal ganglion cells) are a recently discovered type of retinal ganglion cell that can directly absorb light. They contain the photopigment melanopsin (related to melanin), which allows them to function like the photoreceptors that transduce light for the rod and cone retinal ganglion cells. These ganglion cells have a very large dendritic tree, which contributes to the role of these cells in signaling gross changes in light intensity. This information is sent to subcortical structures and is thought to be used in circadian rhythms and pupil constriction.
Optic chiasm
Where the optic nerves cross in the brain, allowing information from the left visual field (from both eyes) and right visual field (from both eyes) to be separated and directed to the appropriate contralateral hemisphere.
Thalamus
a part of the brain involved in relaying sensory information from sensory organs to processing areas of the cerebral cortex
Lateral Geniculate Nucleus (LGN)
a part of the thalamus where the visual processing streams pass through areas of the cerebral cortex (groups of cell bodies in the CNS) - first synapse outside the retina
Optic Radiations
nerve pathway along the visual processing stream from the LGN to the primary visual cortex
Primary Sensory Cortex
refers to the first location in the cortex that receives inputs from the peripheral sensory receptors (in this case the retina) first binocular cells created here, respond best to orientated lines
Primary Visual Cortex (V1)
The first area in the brain where visual information is processed at a low level. Visual information flows into here from the retina and flows to higher levels of visual processing (V2, V3, etc.) that do increasingly complex visual processing. V1 is also called striate cortex due to the visible stripe of inputs from retina to layer 4 of V1.
Cataract
an opacity in the lens that blocks light from reaching the retina; often occurs in order age due to sunlight exposure
Retinal colorblindness
an inability to correctly see colors due to mutations in photoreceptors (e.g., medium-length cones mutated to respond like long-length cones)
Monochromacy
a form of congenital achromatopsia (color blindness) arising from problems in the retina
Rod monochromacy
rod monochromats are people whose cone photoreceptors are present in the retina but are completely non-functional. The cones cannot absorb light, and therefore the patient relies only on rod vision (sees in black and white with low visual acuity).
Cone monochromacy
patient has one functioning cone type. Color vision is restricted to about 100 colors (rather than our normal ~10 million). Blue-cone monochromacy is rare, but slightly more common than L/M-cone monochromacy
Dichromacy
color vision disorder in which one type of cone is absent or non- functioning
Protanopia
(L) photoreceptors deficit that causes Red-Green Colorblindness
Deuteranopia
(M) photoreceptors deficit that causes Red-Green Colorblindness
Tritanopia
S) photoreceptors deficit that causes Blue-Yellow Colorblindness
Red-green colorblindness
A form of retinal colorblindness where either the green cones are missing completely or respond like red cones. It is much more common in males than females.
Blue-yellow colorblindness
A form of retinal colorblindness where people confuse blue with green and yellow with violet. It is very rare (roughly 1/10,000) and not sex- linked (autosomal)
Anomalous trichromacy
Patients with types of anomalous trichromacy (protanomaly, deuteranomaly, or tritanomaly) are trichromats, but the color matches they make significantly differ from normal.
Protanomaly
L-cone (red) spectrum shifted closer to M-cone (green) spectrum
Deuteranomaly
M-cone (green) spectrum shifted closer to L-cone (red)spectrum - most common type
Tritanomaly
blue-yellow discrimination altered
Ishihara Plates
38 colored plates used to test those with red-green color blindness
Tetrachromats
women who have 4 types of cones, which likely allows them to see about 100 million colors (rather than our normal ~10 million). The 4th type of cone occurs when one woman inherits two different L-cone alleles (gene subtype), each of which codes for an L-cone photopigment with a small mutation that makes it absorb a slightly different wavelength of light than the other allele. Due to a process called X-chromosome inactivation (in every female cell, one of the X chromosomes is randomly inactivated), each retinal L-cone cell may randomly express one L-cone allele or the other. Fascinatingly, the opponency system of our color vision can incorporate the two slightly different L-cones as individual photoreceptor types. I would have assumed that our brain would not be able to change to accommodate the new input type, that the two L-cone types would just be grouped together as L-cone info. Instead, our brains can instead make a more complex opponency system that allows tetrachromats to see more colors, as L- cone type 1 now can be compared to L-cone type 2, and so on, just like L-cone vs. M- cone comparisons in trichromats (normal human vision).
Scotoma
An area of impaired or lost vision in the visual field. A scotoma can arise from from damage anywhere along the visual pathway from the retina to primary visual cortex (V1). Beyond V1, more specialized types of visual disorders arise (e.g., visual object agnosia). The following terms describe different extents of a scotoma that can occur due to very specific locations of damage:
Unilateral field loss
loss of an entire eye's vision due to tumor or trauma that results from the disconnection of the optic nerve
Hemianopsia
blindness in one half of the visual field in one or both eyes
Bitemporal hemianopsia
blindness in the outer halves of the visual field inboth eyes, due to damage to the optic chiasm (tumors are often the culprit)
Binasal hemianopsia
blindness in the middle halves of the visual field in both eyes, due to damage to uncrossed fibers (often due to calcification of carotid arteries; also associated with hydrocephalus)
Homonymous hemianopsia
blindness in the same hemisphere of the visual field in both eyes, due to damage to the opposite hemisphere of cortex (often from stroke or trauma). 'Right homonymous hemianopsia' refers to the loss of the right hemifield of vision in each eye from damage to left V1.
Sensation
The first stage in the functioning of the senses, starting with information atthe peripheral sensory receptors
Perception
the process of recognizing, organizing, and interpreting sensory information
Dorsal Visual Pathway
made up of multiple visual areas, it is one of the two main visual processing streams after visual cortex; this pathway is involved in perception and action
Ventral Visual Pathway
made up of multiple visual areas, it is one of the main visual processing streams after primary visual cortex; this pathway is involved in perception for recognition
Cortical Magnification
is a property of sensory and motor systems in which one part of a topographical representation is relatively larger than the rest, producing a region with higher acuity (better sensitivity) in the magnified region. In the visual system, cortical magnification describes how many neurons in an area of the visual cortex are 'responsible' for processing a stimulus of a given size, as a function of visual field location. In the center of the visual field, corresponding to the center of the fovea of the retina, a very large number of neurons process information from a small region of the visual field. If the same stimulus is seen in the periphery of the visual field (i.e. away from the center), it would be processed by a much smaller number of neurons. The increased number of neurons devoted to processing central vision helps make our central vision more sensitive than our peripheral vision. The magnification of central (e.g., foveal) is achieved in several steps along the visual pathway, starting in the fovea with densely packed cones and the midget retinal ganglion cells of the parvocellular pathway and continuing to the large region of cortex that receives information from the central vision. Other examples of cortical magnification include the expansion of the face and hand representations in the somatosensory and motor cortical regions. These body parts have sensitive touch and excellent motor control.
Blindsight
a phenomenon where people who are perceptually blind demonstrate response to visual stimuli because only part of their visual system is impaired (damage to V1)
Visual Agnosia
disorder in which the patient suffers from the inability to recognize and identify objects, features of objects or scenes, faces or persons despite having knowledge of the characteristics of the objects, scenes, faces or persons. This condition can be loosely divided into two types that differ by severity: apperceptive and associative.
Apperceptive Visual Agnosia
a disorder characterized by inability to name, copy, or recognize visually presented objects; shape perception and figure-ground segregation is impaired, but basic visual functions and object identification based on non-visual cues are preserved
Associative Visual Agnosia
A disorder in which visual object recognition is impaired (e.g. naming of visually presented objects, categorization, matching by function), but elementary visual perception is more or less preserved. This is how object agnosia is typically described, as this is the more common type.
Visual object agnosia
A disorder in which visual object recognition is impaired (e.g. naming of visually presented objects, categorization, matching by function), but elementary visual perception is more or less preserved, e.g., matching and copying of visually presented forms and objects, drawing objects from memory, and non-visual object recognition.
Fusiform Face Area
A bilateral visual processing area that is thought to be specialized for face processing (with some controversy--some authors argue that it is specialized for detailed visuospatial processing, not necessarily just face processing). Damage to this region can cause face perception deficits.
Prosopagnosia
A disorder in which faces cannot be recognized, but other forms of object recognition are unimpaired.
Apperceptive type:
problems with recognizing a face vs. other objects (can't tell by vision alone whether something is a face or not)
Associative (amnesic) type:
problems with recognizing familiar faces (can tell it's a face, but not whose face it is)
Capgras Syndrome
The delusional belief that an acquaintance has been replaced by an identical-looking imposter. It is one of the delusional misidentification syndromes more commonly seen in schizophrenia, dementia, and brain trauma. May arise from an abnormal emotional response to faces disconnect between temporal and limbic cortex; possible example of a really high-order face processing issue
Fregoli Syndrome
The delusional belief that different people are in fact a single person who changes appearance or is in disguise, generally viewed with paranoia (that the "shapeshifting" person is out to get them). This is another rare delusional misidentification syndrome. Appears to arise from damage to left frontal and right temporoparietal regions, possibly due to a disconnection between hemispheres, that affects high-order face processing. A person with the Fregoli delusion may also inaccurately recall places, objects, and events, leading to the theory that associations among stored memories may be messed up - with the image of one face (or place/object/event) ending up being improperly associated with another one.
Simultagnosia
a deficit in scene perception, with a normal visual fields and normal lower-level (elementary) visual perception. Several theories have been proposed to account for simultagnosic symptoms: some focus on the disruption of a specific process, such as the speed of attentional processing, others focus on the disruption of a representational structure
Dorsal Simultagnosia
a deficit in scene perception where the patient can only perceive one stimulus at a time (more severe than ventral)
Ventral Simultagnosia
a deficit in scene perception where the patient can see multiple objects, but cannot recognize them (can navigate and count, but cannot read)
Central/Cerebral Achromatopsia
(Cortical color blindness) - an impairment of color vision in the entire visual field that arises from cortical lesions on the ventral surface of the temporal-occipital lobes. Loss of color vision in the full visual field is rare, as lesions would need to affect color processing in both hemispheres.
Hemiachromatopsia
Loss of color vision is restricted to one half of the visual field. The rest of the visual field has normal color vision. (Color vision can even be lost for just one quarter of the visual field.)
Transient Achromatopsia
temporary loss of color vision in any part of the visual field, usually from a TIA (transient ischemic attack)
Akinetopsia
the inability to perceive motion that arises from damage to V5/MT - the area of cortex responsible for visual motion; patients experience a strobe-light effect of vision. It can be caused by damage such as stroke, trauma and rarely from some antidepressants.