The First Steps in Vision: From Light to Neural Signals

Flashcards relating to the first steps in vision.

What is a photon?

A quantum of visible light demonstrating both particle and wave properties.

List five ways that light interacts with a surface

Light can be absorbed, scattered, reflected, transmitted, or refracted.

Define 'absorbed' in the context of light

Energy (e.g., light) that is taken up and not transmitted at all.

Define 'scattered' in the context of light

Energy that is dispersed in an irregular fashion.

Define 'reflected' in the context of light

Energy that is redirected when it strikes a surface, usually back to its point of origin.

Define 'transmitted' in the context of light

Energy that is passed on through a surface (when it is neither reflected nor absorbed by the surface).

Define 'refracted' in the context of light

Energy that is altered as it passes into another medium.

What is the cornea?

The transparent 'window' into the eyeball.

What is the aqueous humor?

The watery fluid in the anterior chamber of the eye.

What is the crystalline lens?

The lens inside the eye that focuses light onto the back of the eye.

What is the pupil?

The dark circular opening at the center of the iris where light enters the eye.

What is the iris?

The colored part of the eye, a muscular diaphragm, that regulates light entering the eye by expanding and contracting the pupil.

What is the vitreous humor?

The transparent fluid that fills the large chamber in the posterior part of the eye.

What is the retina?

A light-sensitive membrane in the back of the eye that contains rods and cones.

What is accommodation in the context of the human eye?

The process in which the lens changes its shape, thus altering its refractive power.

What is presbyopia?

Age-related loss of accommodation, making it difficult to focus on near objects.

What is emmetropia?

The condition of no refractive error.

What is myopia?

Nearsightedness; light is focused in front of the retina, and distant objects cannot be seen sharply.

What is hyperopia?

Farsightedness; light is focused behind the retina, and near objects cannot be seen sharply.

What is astigmatism?

Unequal curving of one or more of the refractive surfaces of the eye, usually the cornea.

What are photoreceptors?

Cells in the retina that initially transduce light energy into neural energy; named for their shapes (rods and cones).

What are rods?

Photoreceptors specialized for night vision, responding well in low luminance conditions but not processing color.

What are cones?

Photoreceptors specialized for daytime vision, fine visual acuity, and color, responding best in high luminance conditions.

What is the optic disc?

The location where ganglion cell axons leave the retina, also known as the blind spot.

What cells do photoreceptors signal?

Horizontal and bipolar cells.

What cells are bipolar cells connected to?

Amacrine cells and ganglion cells.

What accounts for very poor color vision in your periphery?

Cones process color; rods do not. The distribution of rods and cones is not constant over the retina.

What is the standard way to measure retinal size?

Degrees of visual angle

How does the pupil contribute to light and dark adaptation?

Pupil dilation controls the amount of light entering the eye.

How does photopigment contribute to light and dark adaptation?

Control how much light the photoreceptors process

What is age-related macular degeneration (AMD)?

A disease associated with aging that affects the macula, gradually destroying sharp central vision.

What is the macula?

The central part of the retina containing the fovea.

What is a scotoma?

A blind spot in the visual field, often caused by AMD.

What is Retinitis pigmentosa (RP)?

A family of hereditary diseases that involves the progressive death of photoreceptors and degeneration of the pigment epithelium.

What is the process of photoactivation?

When light hits a photoreceptor.

What two parts make up a photoreceptor?

Chromophore and opsin.

What is an opsin?

A protein in photoreceptors that determine the wavelength of light to which the photoreceptor responds.

Name photoreceptors that monitor light levels

Photoreceptors containing melanopsin

What is unique to ganglion cells' firing?

Ganglion cells fire in an all-or-none fashion rather than in graded potentials.

In what light do cones work best?

Photopic (high-illumination) situations

In what light do rods work best?

Scotopic (low-illumination) situations.

How is the retina's horizontal pathway structured?

Horizontal cells and amacrine cells.

What is the purpose of horizontal cells?

They are responsible for lateral inhibition, which creates the center-surround receptive field structure of retinal ganglion cells.

What is the purpose of amacrine cells?

Contrast enhancement and temporal sensitivity (detecting light patterns that change over time).

How is the retina's vertical pathway structured?

Photoreceptors, bipolar cells, and ganglion cells.

What are the two types of bipolar cells?

Diffuse and midget.

What is unique to M ganglion cells?

Connects to the magnocellular pathway.

What is unique to P ganglion cells?

Connects to the parvocellular pathway.

What is the receptive field?

The region on the retina in which stimuli influence a neuron’s firing rate.

Describe how ON-center ganglion cells are excited and inhibited.

Excited by light that falls on their center and inhibited by light that falls in their surround.

Describe how OFF-center ganglion cells are excited and inhibited.

Inhibited when light falls in their center and excited when light falls in their surround.

ON-center and OFF-center retinal ganglion cells are most sensitive to what?

Differences in intensity of light between center and surround and are relatively unaffected by average intensity.

P ganglion cells, revisited

Small receptive fields, high acuity, work best in high luminance situations, sustained firing. Provide information mainly about the contrast in the retinal image

M ganglion cells, revisited

Large receptive fields, low acuity, work best in low luminance situations, burst firing. Provide information about how an image changes over time

What is unique to Intrinsically photo sensitive retinal ganglion cells (ipRGCs) in the developing retina?

ipRGCs respond to light, but they receive no input from rods or cones. Send light signals to the developing brain, as early as in the second trimester. Babies in the womb can detect light long before they can see images

Has evolution optimized the vision system to detect a single photon?

Yes.