Neurobio vision

light

Electromagnetic radiation described by wavelength, frequency, and amplitude. Energy is proportional to frequency.

visible light

The portion of the electromagnetic spectrum humans see; the slides give approximately 380–750 nm.

pupil

Opening through which light enters eye.

sclera

White outer part of eye.

iris

Colored part of eye, surrounds and controls pupil.

cornea

Transparent external surface of the eye, helps focus images on the retina. Its shape affects refractive power.

retina

Light-sensitive neural tissue at the back of the eye; contains photoreceptors and circuits that transmit visual signals.

ciliary body/muscle

Ciliary body adjusts the refractive power of the lens and produces fluid in the front of the eye. Helps during accomodation.

optic nerve

Bundle of retinal ganglion-cell axons carrying visual information away from the eye.

lens

Focuses light onto the retina; its refractive power is adjusted during accommodation.

vitreous humor

Gel-like material filling the large interior space of the eye behind the lens.

Emmetropia

Normal refractive condition in which light focuses on the retina.

myopia

Nearsightedness: too much refraction, so light focuses in front of the retina.

hyperopia

Farsightedness: too little refraction, so light focuses behind the retina.

visual field

The region of the environment that can be seen by an eye or both eyes.

binocular visual field

The region seen by both eyes; the overlapping portion of the two eyes' visual fields.

monocular visual field

The region seen by only one eye, outside the overlap of the two visual fields.

left visual field

The scene to the left of the point of fixation. Visual information is ultimately processed by the right hemisphere.

right visual field

The scene to the right of the point of fixation. Visual information is ultimately processed by the left hemisphere.

nasal retina

Retina half closest to the nose. Its ganglion-cell axons cross at the optic chiasm.

temporal retina

Retinal half closest to the temple. Its ganglion-cell axons remain on the same side at the optic chiasm.

ipsilateral

On the same side.

contralateral

On the opposite side.

Optic chiasm

Site where nasal retinal fibers cross; temporal retinal fibers do not cross. This routes each visual field to the opposite brain hemisphere.

5 classes of neurons (light processing)

The retina contains photoreceptors, bipolar cells, ganglion cells, horizontal cells, and amacrine cells.

photoreceptors

Rods and cones; specialized retinal neurons with light-sensitive photopigment in membranous disks.

bipolar cells

Excitatory retinal neurons linking photoreceptors to ganglion cells.

ganglion cells

Retinal output neurons; their axons form the optic nerve and transmit visual information to the brain.

photoreceptor → bipolar → ganglion

The direct pathway carrying visual information from photoreceptors through bipolar cells to ganglion cells.

horizontal cells

Receive input from photoreceptors and project to other photoreceptors and bipolar cells; help regulate center-surround responses.

amacrine cells

Receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells.

rods

Long cylindrical outer segment with many disks; highly light-sensitive; important for dim-light vision; numerous outside the fovea; high convergence reduces spatial resolution.

cones

Shorter tapered outer segment with fewer disks; less light-sensitive; mediate color vision and high visual acuity; concentrated in the fovea.

high-acuity perception

Detailed, sharp vision; highest in the fovea, where cone pathways have low convergence.

phototransduction

Conversion of light into a change in photoreceptor membrane potential through opsin signaling and changes in cGMP-gated ion channels.

11-cis-retinal

Light-sensitive molecule in photopigment; absorption of a photon changes it to trans-retinal.

rhodopsin

Photopigment in rods, made of 11-cis-retinal coupled to an opsin.

cone opsins

Opsins in cones tuned to different wavelengths and responsible for red-, green-, and blue-sensitive cone responses.

opsins

Seven-pass transmembrane proteins coupled to retinal; tune photopigment absorption of light.

cGMP levels are high (light or dark?)

Dark.

cGMP levels low (light or dark?)

Light.

fovea

Pit in the retina where outer layers are pushed aside; area of highest visual acuity and cone specialization.

color blindness

Can result from loss of genes encoding cone pigments; red- and green-cone genes are close together on the X chromosome, contributing to a common male pattern.

red cones

Cones with long-wavelength-sensitive opsins.

green cones

Cones with medium-wavelength-sensitive opsins.

blue cones

Cones with short-wavelength-sensitive opsins.

color determination

Color perception depends on the relative contributions of red-, green-, and blue-sensitive cones. Mixing red, green, and blue light can activate all three and be perceived as white.

absolute darkness

Photoreceptors are in their dark state, with high cGMP and open Na+/Ca2+ channels.

scotopic vision

Vision under dim-light conditions, primarily mediated by rods.

mesopic vision

Intermediate-light vision where rods and cones both contribute.

photopic vision

Bright-light vision, primarily mediated by cones.

visual receptive field

Region of visual space where light changes the activity or firing pattern of a retinal neuron or visual brain neuron.

ON bipolar/ganglion cells

_________ bipolar cells depolarize in response to light; _______ ganglion cells increase firing when light increases.

ON pathway

Responds to increases in light; ON ganglion cells increase discharge rate when light increases.

OFF bipolar/ganglion cells

_______ bipolar cells hyperpolarize in response to light; ______ ganglion cells increase firing when light decreases.

OFF pathway

Responds to decreases in light; OFF ganglion cells increase discharge rate when light decreases.

antagonistic center-surround

Stimulating the receptive-field center produces the opposite response from stimulating the surrounding region.

GABAergic

Neurons that use/release GABA as a neurotransmitter; horizontal and amacrine cells are identified as this.

lateral geniculate nucleus (LGN)

Largest target of retinal ganglion cells; relays visual signals to primary visual cortex.

superior colliculus

Retinal target involved in coordinating head and eye movements.

suprachiasmatic nucleus

Retinal target involved in regulating circadian rhythms.

p cells

Ganglion cells projecting to parvocellular LGN layers; smaller receptive fields, sustained responses, color signaling, and fine color/object detail.

m cells

Ganglion cells projecting to magnocellular LGN layers; larger receptive fields, transient responses, low-contrast signaling, and movement detection.

k cells

Ganglion cells projecting to koniocellular layers.

primary visual cortex

Six-layer visual cortical area receiving LGN input; located in the occipital lobe.

striate cortex

Another name for primary visual cortex (V1).

ocular dominance columns

Segregated patterns of input from the two eyes in layer 4 of primary visual cortex.

extrastriate visual areas

Visual areas outside primary visual cortex that participate in vision.

ventral stream (down)

Pathway for high-resolution vision and high-level object recognition; includes V1, V2, V3, V4, and IT.

achromatopsia

Partial or complete loss of color vision associated in the slides with damage to area V4.

dorsal stream (up)

Pathway for spatial aspects of vision and analysis of motion and speed; includes V1, V2, V3, MT, and MST.