how do our eyes respond to light? why do our eyes have two different sets of receptors – rods and...

How do our eyes respond to light? Why do our eyes have two different sets of

receptors – rods and cones?

Figure 3.1 The electromagnetic spectrum, showing the wide range of energy in the environment and the small range within this spectrum, called visible light, that we can see.

Electromagnetic spectrum◦ Energy is described by wavelength.

◦ Spectrum ranges from short wavelength gamma rays to long wavelength radio waves.

◦ Visible spectrum for humans ranges from 400 to 700 nanometers.

◦ Most perceived light is reflected light of surfaces.

Figure 3.2 An image of the cup is focused on the retina, which lines the back of the eye. The close-up of the retina on the right shows the receptors and other neurons that make up the retina.

The cornea, which is fixed, accounts for about 80% of focusing.

The lens, which adjusts shape for object distance, accounts for the other 20%. ◦ Accommodation results when ciliary muscles are

tightened which causes the lens to thicken.

Light rays pass through the lens more sharply and focus near objects on retina.

Figure 6.2  Cross section of the vertebrate eyeNote how an object in the visual field produces an inverted image on the retina.

The Eye and Its Connections to the Brain

Pupil-opening in the center of the eye that

allows light to pass through

Lens-focuses the light on the retina

Retina-back surface of the eye that contains

the photoreceptors

The Fovea-point of central focus on the retina

blind spot-the point where the optic nerve

leaves the eye

(a) Object far –lens relaxed

(b) Object near- no accommodation, object out of focus

(c) Object near – lens accommodates, object in focus

The near point occurs when the lens can no longer adjust for close objects.

Presbyopia - “elder eye”◦ Distance of near point increases

◦ Due to hardening of lens and weakening of ciliary muscles

◦ Corrective lenses are needed for close activities, such as reading

Myopia or nearsightedness - vision good for near obects.

Inability to see distant objects clearly◦ Image is focused in front of retina

◦ Caused by

Refractive myopia - cornea or lens bends too much light

Axial myopia - eyeball is too long

Focusing of light by the myopic (nearsighted) eye.

(a) Distant object out of focus

(b) No problem with near object (Nearsighted)

(c) Distant object brought into focus with lens

Solutions for myopia◦ Move stimulus closer until light is focused on the

retina

Distance when light becomes focused is called the far point.

◦ Corrective lenses can also be used.

◦ LASIK surgery can also be successful.

Hyperopia or farsightedness - inability to see nearby objects clearly◦ Focus point is behind the retina.

◦ Usually caused by an eyeball that is too short

◦ Constant accommodation for nearby objects can lead to eyestrain and headaches.

Transduction First take a look at receptors we find in the

retina.

KW 8-5

KW 8-6

(a) Rod receptor showing discs in the outer segment. (b) Close-up of one disc showing one visual pigment molecule in the membrane.

Rod Receptor have outer segments, which contain:◦ Visual pigment molecules, which have two

components:

Opsin - a large protein Retinal - a light sensitive molecule

Visual transduction occurs when the retinal absorbs one photon.◦ Retinal changes it shape, called isomerization.

Figure 3.6 (a) Rod receptor showing discs in the outer segment. (b) Close-up of one disc showing one visual pigment molecule in the membrane. (c) Close-up showing how the protein opsin in one visual pigment molecule crosses the disc membrane seven times. The light-sensitive retinal molecule is attached to the opsin at the place indicated.

Figure 3.7 Model of a visual pigment molecule. The horizontal part of the model shows a tiny portion of

the huge opsin molecule near where the retinal is attached. The smaller molecule on top of the opsin is

the light-sensitive retinal. The model on the left shows the retinal molecule’s shape before it absorbs

light. The model on the right shows the retinal molecule’s shape after it absorbs light. This change in

shape is one of the steps that results in the generation of an electrical response in the receptor.

Prior physical evidence showed that it takes one photon to isomerize a pigment molecule.

Purpose of Hecht experiment - to determine how many pigment molecules need to be isomerized for a person to see

Figure 3.9 The observer in Hecht et al.’s (1942) experiment could see a spot of light containing 100 photons. Of these, 50 photons reached the retina, and 7 photons were absorbed by visual pigment molecules.

Results showed:

◦ a person can see a light if seven rod receptors are activated simultaneously.

◦ a rod receptor can be activated by the isomerization of just one visual pigment molecule.

Differences between rods and cones◦ Shape

Rods - large and cylindrical Cones - small and tapered

◦ Distribution on retina

Fovea consists solely of cones. Peripheral retina has both rods and cones.

◦ More rods than cones in periphery.

◦ Number - about 120 million rods and 5 million cones

Figure 6.2  Cross section of the vertebrate eyeNote how an object in the visual field produces an inverted image on the retina.

Figure 3.12 The distribution of rods and cones in the retina. The eye on the left indicates locations in degrees relative to the fovea. These locations are repeated along the bottom of the chart on the right. The vertical brown bar near 20 degrees indicates the place on the retina where there are no receptors because this is where the ganglion cells leave the eye to form the optic nerve.

Blind spot - place where optic nerve leaves the eye

◦ We don’t see it because:

one eye covers the blind spot of the other.

it is located at edge of the visual field.

the brain “fills in” the spot.

Figure 3.14 There are no receptors at the place where the optic nerve leaves the eye. This enables the receptor’s ganglion cell fibers to flow into the optic nerve. The absence of receptors in this area creates the blind spot.

Macular degeneration◦ Fovea and small surrounding area are destroyed

◦ Creates a “blind spot” on retina

◦ Most common in older individuals

◦ “Wet” causes vision loss due to abnormal blood vessel growth

◦ “Dry” results from atrophy of the retinal pigment, which causes vision loss through loss of photoreceptors

What about cones and color vision? Separate lecture on color vision.

rodsCones: short, medium, long

Method used in all experiments:◦ Observer is light adapted

◦ Light is turned off

◦ Once the observer is dark adapted, she adjusts the intensity of a test light until she can just see it.

Figure 3.18 Viewing conditions for a dark adaptation experiment. The image of the fixation point falls on the fovea, and the image of the test light falls in the peripheral retina.

Experiment for rods and cones:

◦ Observer looks at fixation point but pays attention to a test light to the side.

◦ Results show a dark adaptation curve: Sensitivity increases in two stages. Stage one takes place for three to four minutes. Then sensitivity levels off for seven to ten minutes

- the rod-cone break. Stage two shows increased sensitivity for another

20 to 30 minutes.

Figure 3.19 Three dark adaptation curves. The red line is the two-stage dark adaptation curve, with an initial cone branch and a later rod branch. The green line is the cone adaptation curve. The black curve is the rod adaptation curve.

Process needed for transduction:◦ Retinal molecule changes shape

◦ Opsin molecule separates

◦ The retina shows pigment bleaching.

◦ Retinal and opsin must recombine to respond to light.

◦ Cone pigment regenerates in six minutes.

◦ Rod pigment takes over 30 minutes to regenerate.

Rods and cones send signals vertically through◦ bipolar cells.◦ ganglion cells.◦ ganglion axons.

Signals are sent horizontally ◦ between receptors by horizontal cells.◦ between bipolar and between ganglion cells by

amacrine cells.

126 million rods and cones converge to 1 million ganglion cells.

Higher convergence of rods than cones◦ Average of 120 rods to one ganglion cell◦ Average of six cones to one ganglion cell◦ Cones in fovea have one to one relation to ganglion

cells

Rods are more sensitive to light than cones.◦ Rods take less light to respond◦ Rods have greater convergence which results in

summation of the inputs of many rods into ganglion cells increasing the likelihood of response.

◦ Trade-off is that rods cannot distinguish detail

Figure 3.26 The wiring of the rods (left) and the cones (right). The dot and arrow above each receptor represents a “spot” of light that stimulates the receptor. The numbers represent the number of response units generated by the rods and the cones in response to a spot of intensity of 2.0.

All-cone foveal vision results in high visual acuity◦One-to-one wiring leads to ability to

discriminate details◦Trade-off is that cones need more light to

respond than rods

Figure 3.28 Neural circuits for the rods (left) and the cones (right). The receptors are being stimulated by two spots of light.

Cones pick up on location.

Rods do not pick up on location.

Rods Cones