PS222: Eye to Brain (Retina)

How does light fall on the retina?

• The light is bouncing off of a real physical object, making a photo, basically projecting, the three-dimensional world in two dimensions on the right way.

• Upside down and left-right reversed image

Pinhole camera

teeny little pinhole, and when the light passes through it, it does exactly what is seen on the retina—an upside-down, left-right reversed image of the world, projected onto the little back surface

Retina

Light passes through the cornea, the aqueous humor, the lens, the vitreous humor, blood vessels and neural cells to reach this

Anatomy of a photoreceptor

• Two parts

• The light-sensitive portion of the cell (the part that's shaped like a rod, or shaped like a cone)—embedded in the choroid layer and sticking into the choroid layer, the retina and the cell body

Isomerization

• Disc segments inside the light-sensitive parts of photoreceptors contain pigments that are photosensitive and they respond to the presence of light

• Photons fall on the disc segments of photoreceptors which contain photopigments—before light they have a specific shape, but when stimulated with light the molecules that make up the photopigments physically change shape

• This causes a change in flow of the electrical current in the photoreceptor, resulting in a change in the photoreceptor’s signaling and disrupting the flow of neurotransmitters (glutamate) between the synaptic gap between the photoreceptor and other neural cells

How much of a reduction in activity is there in the photoreceptors?

Indicates how much light there is—more light means greater reduction in the activity of the photoreceptor

Photoreceptors: sensitivity to light

• How much light needs to be present for the photoreceptor to send a reliable signal (threshold)

• Participant is in the dark for a really, really, really long time → shown tightly controlled photons of light → measure absolute threshold for brightest light they can see

• Rods are maximally sensitive after about 30 min in the dark

• Maximum cone sensitivity: light needs to be bright for the cone to detect it

• Cones less sensitive than rods, need more photons to get a cone to signal the presence of light

Photoreceptors: response to wavelength

Three different types of cones are defined by the wavelengths they respond to—some sensitive to short, medium, long wavelengths of light—sensitivity defined by how much of a particular wavelength of light they absorb → responsible for color vision

Optic disk

Hole in the back of the retina, no photoreceptors—allows the blood vessels and axons of the retinals, neural cells in the eye, to pass out of the eye

Fovea

all cones in the eye concentrated there, middle of the macula, about 5 million total

Periphery

all rods in the eye spread across it, about 100 million total; peaking 20 degrees eccentricity outside of the phobia, and then tapering off across the retinal surface

Light has the information about objects

• Absorbance/reflectance properties of each object tell us different things about the object—the reflectance properties of objects dictate what light bounces off of their surface; we need to find where the absorbance/reflectance differs in order to perceive individual objects

• Light-dark boundaries matter because they tell us what the borders are of the objects that are in the world

What do photoreceptors tell us?

photoreceptors respond proportionally to the amount of light they pick up (and to particular wavelengths); there are millions of photoreceptors in the retina