Chapter 3 Introduction to Vision
Focusing Light Onto the Retina
Light: The Stimulus for Vision
Vision is based on visible light, which is a band of energy within the
electromagnetic spectrum: a continuum of electromagnetic energy that is
produced by electric charges is radiated as waves.
Wavelength: the distance between the peaks of the electromagnetic waves.
Visible Light: the energy within the electromagnetic spectrum that humans
can perceive, has wavelengths ranging from 400-700 nm, where 1 nanometer
= 10 –9 meters.
Light is reflected from objects in the environment and enters the eye through
the pupil and is focused by the cornea and lens to form sharp images of the
objects on the retina, which contains receptors for vision.
Two types of receptors: rods and cones, that contain light-sensitive
chemicals called visual pigments that react to light and trigger electrical
The signals then emerge from the back of the eye in the optic nerve, which
conducts signals toward the brain.
Light Is Focused by the Eye
Needs to be focused onto the retina, the transparent covering on the eye
accounts for 80% of the eyes focusing power.
Far away objects, the rays appear parallel, and focus at point, closer objects
make the rays on an angle.
Accommodation: Allows you to bring both near and far objects into focus,
although objects at different distances are not in focus at the same time.
Distance at which your lens can no longer adjust to bring close objects into
focus is called the near point.
o Presbyopia: near point increases, as a person gets older.
o Myopia/Nearsightedness: inability to see distant objects clearly.
(1) Refractive Myopia: which the cornea and/or lens bends at
the light too much (2) Axial Myopia: eyeball is too long.
The distance at which the spot of light becomes focused on the retina is
called the far point.
o Hyperopia/Farsightedness: can see distant objects clearly but has
trouble seeing nearby objects.
Laser-Assisted in Situ Keratomileusis (LASIK)
Transforming Light into Electricity
Visual Receptors & Transduction
Transduction is carried out by receptors (neurons specialized for receiving
environmental energy and transforming this energy into electricity)
Key part for rod transduction is the outer segment, because it is here and
the light acts to create electricity. Contains stacks of discs, containing
thousands of visual pigment molecules. Opsin: long strand of protein, which loops back and forth across the disc
membrane 7 times. The molecule retinal is attached – crucial for
transduction because it is sensitive to light.
o When a photon of light hits the retinal, it changes shape, so it is
sticking out from the opsin – isomerization.
How Does Transduction Occur?
One way to approach the problem is to study the chemistry of visual
pigments in a chemistry or physiology lab or to study the relationships.
Another way is to do psychophysical experiments in which we measure
relationship PP to provide information about underlying physiology.
Hecht’s Psychophysical Experiment: He concluded that a person can see
light if 7 rod receptors are activated, and a rod receptor can be activated by
the isomerization of just 1 visual pigment molecule.
o Determined a person’s absolute threshold for brief flash of light.
A biological chemical that in small amounts facilitates chemical reactions in
this way is called an enzyme; the sequence of reactions triggered by the
activated visual pigment molecule is called the enzyme cascade.
Pigments and Perception
Distribution of the Rods and Cones
1. One small area, fovea contains only cones. Look directly at image – on fovea.
2. Peripheral Retina: includes the entire retina outside of the fovea, contains
both rods and cones.
3. Many more rods than cones in peripheral because retina’s receptors are
located there 120 million rods: 6 million cones.
Macular Degeneration: which is most common in older people, destroys the
con-rich fovea and small area that surrounds it – creates a “blind spot” in
central vision so when a person looks at something they lose sight.
Retinitis Pigmentosa: degeneration of the retina that is passed from one
generation to the next – first attacks peripheral rod receptors and results in
poor vision in peripherals – can result in complete blindness.
Blind Spot: because of the absence of receptors – unaware od because it is
located off side of our visual field.
Dark Adaptation of the Rods and Cones
When you’re in the dark, the process of dark adaptation occurs which causes
the eye to increase its sensitivity to the dark.
Dark Adaptation Curve: a plot of how visual sensitivity changes in the dark,
beginning with when the lights are extinguished.
Measuring Dark Adaptation
Light adapt observer by exposure to light. While adapting light on, observer
indicates their sensitivity by adjusting the intensity of a test light so it can
just barely be seen.
o Light-Adapted Sensitivity: because it is measured while the eyes are