2.4 Perception Summary
Problem 1
A Keen Eye
1. Light is reflected from an object into the eye.
2. Light is focused to form an image of that object on the retina.
3. Light is absorbed by the visual pigment molecules (rods and cones).
4. Chemical reactions in the outer segments of rods and cones transduce the light into
electrical signals.
5. Electrical signals travel through the retina, they interact, excite and inhibit some. They
eventually reach the ganglion cells.
6. These signals are processed and sent out with the optic nerves.
Light
• Electromagnetic spectrum: a continuum of
electromagnetic energy that is produced by
electric changes and is radiated as waves.
• Vision is based on visible light, which is a band
of energy within the electromagnetic spectrum.
• Wavelength: the energy that is in the
electromagnetic spectrum. (The distance
between the peaks of the electromagnetic waves.)
• Wavelength of visible light is associated with the different colors of the spectrum.
• Visible light: the energy within the electromagnetic spectrum that humans can perceive.
(400-700 nm)
• Light: consists of photons.
• Photon: smallest possible packet of light energy.
Eye
• Vision begins in the eye.
• Light is reflected from objects in the environment and then enters the eye through the
pupil.
• Light is focused by the cornea
and lens to form sharp
images of the object on the
retina.
• The cornea accounts for
about 80% of the eyes
focusing power, but it can't
adjust its focus.
• Lens supplies the remaining 20% of the eye focus and can change its shape to adjust the
eyes for stimuli that are located at different distances.
• Retina contains the receptors for vision.
o There are 2 types of visual receptors.
, o They both contain light-sensitive chemicals (visual pigments)
o These visual pigments located on the visual receptors react to light and trigger
electrical signals.
o They consist the retina.
1. Rods
2. Cones
• Later on, those electrical signals emerge in the optic nerve, which conducts signals
toward the brain.
Objects
• If the object is located more than 20 feet away → light rays are parallel and they focus
on top of the retina.
• If the object is closer → light rays are not parallel, they have an angle. They meet outside
of the retina. However, the light is stopped by the back of the eye (accommodation)
o Accommodation
▪ Brings the focus point on to the retina and increases the focusing power.
▪ Ciliary muscles at the front of the eye tighten and increase the curvature of the
lens so it gets thicker.
▪ This increased curvature bends the light Rays passing through the lens to pull the
focus point back on top of the retina.
▪ Accommodation helps you to bring both near and far objects into focus, although
different distances are not in focus at the same time.
▪ Near point: The distance at which your lens can no longer adjust to bring close
objects into focus.
▪ Far point: The distance at which the spot of light becomes focused on the retina.
▪ Presbyopia: The distance of the near point increases as a person gets older. This
occurs because lens hardens with age and the ciliary muscles becomes weaker.
Myopia and nearsightedness
• Myopic eye: brings parallel rays of light into focus at a point in front of the retina.
• Formed image is blurred.
Occurs because
1. Refractive myopia: the cornea and the lens bend too much.
2. Axial myopia: eyeball is too long. So that the image falls before the retina.
Can solve the problem if
1. Move the stimulus closer. This pushes the focus point farther back on top of the
retina, creating the far point.
2. Corrective glasses or contact lenses. Lenses bend incoming light so that it is focused
as if it were at the far point.
3. Lasers or surgeries to change the shape of cornea. (LASIK surgery)
Hyperopia or farsightedness
, • People can see distant objects clearly but has trouble seeing nearby objects.
• The focus point for parallel rays of light is located behind the retina.
• The eyeball is short.
• More accommodation is required to return the focus point to the retina.
• Can cause headaches.
• Corrective glasses are required to bring the focus point back onto the retina.
Transforming light into electricity
• Vision occurs not in the retina, but in the brain.
• Before the brain can create a vision, the light on the retina must be transformed into
electricity.
The Visual Receptors and Transduction
• Transduction: the transformation of light into electricity.
• Transduction is carried out by receptors (rods and cones).
• Rods and cones act similarly during transduction, so we will
only consider rods.
Rods
• Rod outer segments contain stacks of discs.
• Each disc contains thousands of visual pigment molecules.
• Each molecule is a long strand of molecule called opsin,
which loops back and forth across the disc membrane
seven times.
• Retinal: is attached to the visual pigment molecule’s disc
membrane. It is crucial for transduction and is sensitive to
light.
Steps of transduction
1. Light-sensitive visual pigment molecule in a receptor absorbs one photon (light).
2. Molecule isomerizes itself and starts a chain reaction that leads to activation of the
receptor. (enzyme cascade)
• Isomerization: when the light hits the retinal, the visual pigment molecule changes it shape
and it sticks out from the opsin.
• Before the light is absorbed, the retinal is next to the opsin. When a photon of light hits the
retinal it sticks out from the opsin.
• Isomerization causes transduction.
Hecht’s Psychophysical Experiment
• Aim: how many visual pigment molecules are needed to be isomerized for a person to see.
• Found that a person could detect a flash of light that contained 100 photons.
• A person sees a flash of light when only 7 visual pigment molecules are isomerized.
• A rod receptor can be activated by the isomerization of just one visual pigment molecule.
, Photoreceptors
Rods & Cones
• Fovea: only contains cones.
• Peripheral retina: all of the retina outside of fovea. Contains both rodes and cones.
• There are many more rods than cones in the peripheral retina.
Problems in photoreceptors & disorders
• Macular regeneration: destroys the cone-rich fovea and a small nearby area. This creates a
blind spot in vision, so when a person looks directly at something he/she loses sight of it.
• Retinitis pigmentosa: degeneration of retina that is passed from one generation to the
next. It attacks the peripheral rod receptors and causes poor vision in peripheral visual
fields. In severe cases the foveal cone receptors are also attacked resulting in a complete
blindness.
• Blind spot: a spot in retina, where the optic nerve leaves the eye has no receptors. Blind
spot is located off to the side of our visual field, so it is hard to detect. There are no
receptors there because the ganglion cell fibers flow into the optic nerve.
o Blind spot is located off to the side of the visual field where objects are not in sharp
focus. This is why the blind spot is hard to detect.
o The brain “fills in” the place with a perception that matches the surrounding pattern to
avoid us from detecting the blind spot.
Cones & better detail vision
• Scanning: to see enough details to recognize a face you need to focus the image of the face
on your fovea.
• Good visual acuity. They result in better detail vision than rods.
• Cones have better visual acuity (detail vision) because they less convergence.
Measuring cone adaptation
• Test light is small enough so that the image only falls within the fovea.
• Cones are more sensitive to light at the beginning of dark adaptation so they control our
vision during the initial rapid phase.
Rods
• Rods are more sensitive to light than cones because they have more convergence.
• To measure dark adaptation of rods we need to observe people who have no cones.
• Rod monochromats: people who have no cones due to a genetic defect.
• Rods have greater convergence.
3 types of cones
1. S-Cones: short wavelength rods. Absorbs light best at about 419 nm.
2. M-Cones: medium wavelength rods. Absorbs light best at about 531 nm.
3. L-Cones: large wavelength rods. Absorbs light best at about 558 nm.
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