N165 Quiz 2

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

Lens

Situated behind the iris of the eye, it focuses light entering the eye onto the retina

Sclera

White part of the eye (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. It appears black because light rays entering the pupil are 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

Fovea

The part of the retina where vision is most acute and color vision is best. Prevalent cone photoreceptors.

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 axons of the retinal ganglion cells exit the retina and form the optic nerve. Due to lack of photoreceptors on the optic disc, the corresponding part of the field of vision is invisible. Our brain '“fills-in” the blind spot with estimates of expected visual information based on surrounding detail.

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. Function of photoreceptor cell: convert light energy of photon into a form of energy communicable to the nervous system and usable to the organism (signal transduction).

Types of photoreceptor cells

Rod and Cones, with 3 different subtypes of cones

Rods

Photoreceptor cells located outside of the fovea, highly sensitive to light and responsible for scotopic (low-light) vision. Contributes to visual motion detection, but poor visual acuity and does not differentiate between colors

Cones

Photoreceptor cells concentrated in the fovea, but more sparsely extend into the periphery. Responsible for high acuity vision and color vision, but takes more light to activate (photopic vision). 3 types of cones: S-cones (blue), M-cones (green), and L-cones (red), each most responsive to different wavelengths of light. The combination of inputs from different cone types through opponent processing produces color vision

Photoreceptor proteins

Light-sensitive protein molecules involved in sensing

Opsin

Type of photosensitive pigment found in photoreceptors: rhodopsin in rods & photopsin (3 types) in cones & melanopsin in melanopsin-containing retinal ganglion cells.

Retinal ganglion cells

Cells in the retina that receive input from modulatory neurons (gets 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).

Opponent-processing color vision theory

Color vision theory that color is processed in 3 different opponent channels created by specific wiring together of cone photoreceptors and retinal ganglion cells:

• red (L cone) vs. green (M cone)

• blue (S cone) vs. yellow (L+M cone),

• dark vs. bright (red/L+green/M+blue/S) —> comparison produces luminance

Optic nerve

Composed of axons of retinal ganglion cells that leave the retina and head back towards the optic chiasm in the brain with carried visual information. The reason for a blind spot, because no photoreceptors exist where the optic nerve exits the eye.

Parvocellular pathway

Visual processing stream that pools over fewer receptors. Midget retinal ganglion cells (which are involved) have a sustained response and are involved in processing color, fine details, textures, and depth.

Magnocellular pathway

Visual processing stream that pools over many receptors. Parasol retinal ganglion cells (which are involved) fire in bursts and useful for detecting motion.

Koniocelluar pathway

Visual processing stream that receive 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

AKA intrinsically photosensitive retinal ganglion cells, retinal ganglion cell which can directly absorb light. Contains photopigment melanopsin, allowing them to function like photoreceptors that transduce light for the rod and cone retinal ganglion cells. Large dendritic tree, contributing to 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 optic nerves cross in the brain, allowing information from left visual field and right visual field to be separated and directed to the appropriate contralateral hemisphere.

Thalamus

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 on their way to the optic radiations and primary visual cortex.

Optic radiations

Nerve pathway along the visual processing stream from the LGN to the primary visual cortex

Primary sensory cortex

First location in the cortex that receives input from the peripheral sensory receptors — the retina.

Primary visual cortex (V1)

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. AKA striate cortex due to visual 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 older age due to sunlight (UV) exposure

Retinal colorblindness

Inability to correctly see colors due to mutations in photoreceptors

Monochromacy

A form of congenital achromatopsia (color blindness) arising from problems in the retina.

Rod monochromacy

Cone photoreceptors present yet non-functional. The cones cannot absorb light and patient only replies on rod vision (sees in gray with low visual acuity).

Cone monochromacy

Patient has one functioning cone type. Color vision is restricted to about 100 colors (rather than ~10 million). Blue-cone monochromacy rare, but slightly more common than L/M monochromacy.

Dichromacy

Color vision disorder where one type of cone is absent or non-functioning

Protanopia

(L) photoreceptor 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

form of retinal colorblindness where either the green cones are missing completely or respond like red cones. More common in males than females.

Blue-yellow colorblindness

Form of retinal colorblindness where people confuse blue with green and yellow with violet. Rare and not sex-linked

Anomalous trichromacy

Patients with types of anomalous trichromacy (protanomaly, deuteranomaly, or tritanomaly) are trichromats, but the color matches differ significantly 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

Tritanomaly

blue-yellow discrimination altered

Ishihara Plates

38 colored plates used to test those with red-green color blindness

Tetrachromats

Women with 4 types of cones: can see ~100 million colors rather than ~10 million. Occurs with inheritance of 2 different L-cone alleles, which codes for an L-cone photopigment with small mutation that makes it absorb a slightly different wavelength of light than the other allele.

Scotoma

Area of impaired or lost vision in the visual field. Can arise from damage anywhere along the visual pathway from the retina to primary visual cortex (V1).

Unilateral field loss

Loss of an entire eye’s vision due to tumor or trauma that results from the disconnection of 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 in both eyes, due to damage to the optic chiasm

Binasal hemianopsia

Blindness in the outer halves of the visual field in both eyes, due to damage to uncrossed fibers (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 of the opposite hemisphere of cortex.

Sensation

First stage in the functioning of the senses, starting with information at the 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 primary visual cortex. This pathway is involved in perception for action.

Ventral visual pathway

Made up of multiple visual areas, involved in perception for recognition

Cortical magnification

Property of sensory and motor system where one part of a topographical representation is relatively larger than the rest.

• produces region with higher acuity (better sensitivity) in the magnified region

• describes how many neurons in an area of the visual cortex are responsible for processing a stimulus of a given size, as function of visual field location.

• Increased number of neurons devoted to process central vision —> central vision more sensitive than peripheral vision

• due to densely packed cones at fovea and midget retinal ganglion cells of parvocellular pathway

Blindsight

A phenomenon where people who are perceptually blind demonstrates response to visual stimuli

Visual agnosia

A neurological disorder where patients suffer from inability to recognize and identify objects, features of objects or scenes, faces or persons despite having knowledge of the characteristic of the objects, scenes, faces or persons.

• 2 types: apperceptive and associative

Apperceptive visual agnosia

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

Disorder where visual object recognition is impaired but elementary visual perception is more or less preserved. Object agnosia.

Visual object agnosia

Disorder where 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, drawing objects from memory, and non-visual object recognition.

Fusiform face area

Bilateral visual processing area thought to be specialized for face processing. Damage can cause face perception deficits.

Prosopagnosia

Disorder where faces cannot be recognized, but other forms of object recognition are unimpaired.

Apperceptive type (prosopagnosia)

Problems with recognizing a face vs. other objects (can’t tell by vision alone whether something is a face or not)

Associative (amnesic) type prosopagnosia

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. Commonly seen in schizophrenia, dementia, and brain trauma. May arise from an abnormal emotional response to faces —> due to disconnection between temporal and limbic cortex.

Fregoli syndrome

Delusional belief that different people are a single person who changes appearance or is in disguise. Due to damage to left frontal and right temporoparietal regions, possible a disconnection between hemispheres.

• may inaccurately recall places, objects, and events —> theory that associations among stored memories may be messed up

Simultagnosia

Deficit in scene perception, normal visual fields and normal lower-level (elementary) visual perception. May be due to disruption of specific process or representational structure.

Dorsal simultagnosia

A deficit in scene perception where patient can only perceive one stimulus at a time

Ventral simultagnosia

A deficit in scene perception where the patient can see multiple objects, but cannot recognize them

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.

Hemiachromatopsia

Loss of color vision restricted to ½ of the visual field. The rest of the visual field has normal color vision.

Transient achromatopsia

Temporary loss of color vision in any part of the visual field, usually from a TIA

Akinetopsia

The inability to perceive motion that arises from damage to V5/MT — area of cortex responsible for visual motion. Can be caused by damage such as stroke and trauma