2. In an earlier lecture, imagination was
described as “the faculty of consciously
forming mental images or concepts of
that, which is not, actually, present to
the senses.” Although this statement
suggests that non-sensory information is
the substance from which imagination is
woven, it would be a mistake to dismiss
the importance of the senses to the
imagination.
3. Sensation
Much of the information we have about the
world is obtained through our senses.
Sensations are experienced as values and
intensities of light, sound, touch, odor and
taste.
Experiencing objects and space, however, are
attributable to “perception”.
4. SENSATION, then, is a biological process that
enables receptors to detect stimulus information
(sensations), and encode them to neural
information.
5. In this class we will be primarily concerned with
the sense of vision (hence Visual Imagination)
since, for most of us, it is the dominant form of
information detection.
A clear understanding of the visual process is
indispensable to designers and artists
because it allows them to present visual material
in an order and manner in which they want others
to perceive it. It also gives them insight into the
manipulation and control of the media.
6. An actor, director or scenic designer who
wants to evoke a particular response from an
audience must be conscious of what that
audience will be seeing. Some artists want to
control and manipulate not only what the
audience perceives, but also how it perceives
it..
How do you feel about that as an artist?
How do you feel about that as an audience
member?
7. Visual SENSATION deals with these three
important areas:
light discrimination (values and
intensities)
color registration
contrast - the value difference ( lightness
or darkness), between colors and black and
white
8. To understand human visual perception, it is
necessary to understand, a little bit, the
mechanics of the human eye.
10. Cornea
Sclera
Cornea
• Cornea is the clear
portion of the sclera.
• 2/3 of the refraction
takes place at the
cornea.
• The outer wall of the
eye is formed by the
hard, white sclera.
11. Iris and Pupil
• Colored iris controls
the size of the
opening (pupil)
where the light
enters.
• Pupil determines the
amount of light, like
the aperture of a
camera.
Iris
Pupil
Iris open
Dilated pupil
Iris closed
Constricted pupil
12. Lens
Ciliary muscle
Lens
Suspensory
Ligament
Transparent
Fibers
• The lens is made of
transparent fibers in a clear
membrane.
• Suspended by suspensory
ligament.
• Used as a fine focusing
mechanism by the eye;
provides 1/3 of eye’s total
refracting power.
• Non-uniform index of
refraction.
13. Accommodation
Distant object
Near object
• The suspensory ligaments attach
the lens to the ciliary muscle.
• When the muscle contracts, the
lens bulges out in the back,
decreasing its focal length.
• The process by which the lens
Relaxed muscle
changes shape to focus is called
Taut ligaments
accommodation.
Contracted muscle
Slack ligaments
14. Aqueous Humor and Vitreous
Humor
Vitreous Humor
Aqueous Humor
• Transparent
gelatinous liquid
filling the eye.
• Provides nutrients to
the cornea and eye
lens.
• Also helps maintain
the eyeball shape
with its pressure.
15. Retina
Retina
Fovea
Optic Nerve
• Retina is the photosensitive
“detector” for the eye.
• Two types of receptors in the
retina: rods for low light level,
and cones for color.
• Located at the center of the
retina, the fovea contains a
greater concentration of cones.
• Signals from the receptors
leave through the optic nerve
to the brain.
16. The brain then registers those impulses,
processes them into useful information about
what we are seeing.
17. Plexiform Layer
Fovea
Light
Plexiform Layer
Optic Nerve
• The retina is made of three layers:
– Plexiform layer is a network of
nerves which carry the signals
from the photo receptors.
– Photo receptors.
– Choroid provides nourishment to
Photo receptors
the receptors, as well as absorb
any light that didn’t get absorbed
by the photo receptors,
Choroid
18. Rods and Cones
Retina
Blind spot
Fovea
Optic Nerve
Rod
•
•
•
Highly sensitive to low light
level or scotopic conditions.
Black and white.
Dispersed in the periphery of
the retina.
Cone
• Sensitive to high light level
or photopic conditions.
• Three types of cones
responsible for color vision.
• Concentrated in the fovea.
23. Here is a brief and simplified review of the visual
process.
Visual Sensation has three basic, known,
components:
1. Image formation - Light waves transmitted from an
external object enter the eye through the cornea
and pupil.
2. Exposure control - Light waves are delivered
through the lens as a focused image
onto the retina.
3. Detection - Photoreceptor neurons in the retina
collect light from the visual stimulus and transmit
signals to a network of neurons.
25. Image Formation
The curved surfaces of the eye focus and invert the
image onto the back surface of the eye.
Notes de l'éditeur
The human eye is quite similar to a photographic camera. The cornea and the eye lens are the optical elements responsible for forming an image in the back of the eye. The iris is like the diaphragm of the camera, where the opening (or the aperture) controls the amount of light entering the eye. The retina, located at the back of the eye, is like the film, detecting the photons that entered the eye and then turning them into electrical impulses that exits out to the brain through the optic nerve. Now let us look at each part of the eye in detail.
The eye is encased in a white rigid outer wall called sclera. Sclera is the “white” of the eye. The front portion of the sclera is clear, and this is called the cornea. The cornea is responsible for 2/3 of the total refraction, since the greatest change in the index of refraction occurs between the air and the surface of the cornea. This is why you can’t see very clear under water without goggles.
Iris is muscle that contracts and dilates to determine the pupil size. Iris is brightly colored, and this is the “color” of the eye. The pupil is the opening where the light enters.
Eye lens is located just behind the iris. The eye lens is made of transparent fibers encased in a clear membrane. The lens is suspended by a suspensory ligament, which attaches to muscle known as ciliary muscle. The eye lens is quite flexible unlike a lens made out of glass. The ciliary muscle can actually change the shape of the back of the lens by flexing; and this is how the lens is used to focus between near and far objects.
In addition, the index of refraction of the lens is different from the middle of the lens to the edge of the lens.
The process in which the eye focuses is called accommodation. The muscle contracts, the shape of the lens deforms. The back of the lens bulges out to increase the power of the lens (decreasing the focal length) so that the eye can focus on a near object. When the ciliary muscle relaxes, the ligament tightens up, decreasing the curvature of the back of the lens.
The cavities in the eye are not empty. They are instead filled with transparent gelatious liquid. The space between the cornea and the eye lens is filled with the aqueous humor, and the space between the eye lens and the back of the eye is filled with vitreous humor. They provide nutrients to the cornea and the lens; since these elements must be clear, they do not have any blood vessels which is the normal mean of transporting nutrients and wastes. The “floaters” we often see are the dead red blood cells floating inside the vitreous humor.
In addition, the pressure of the humors provide to maintain the round shape of the eye.
The retina is the photosensitive detector of the eye. It consists of two types of receptors: Rods, which is sensitive to low light level, and cones, which is sensitive to high light level and also responsible for color vision. A little depression found at the center of the retina is called a fovea, where it contains a greater concentration of cones. This is where the part of the visual field which is the most sharpest.
The signals from the receptors are carried out by nerve network which leaves the eye through the optic nerve to the brain.
The retina is made of three major layers. The plexiform layer is a network of nerves which carry the signals from the photo receptors. The eye is actually constructed kind of backwards; the light must pass through several layers of cells before it can be detected by the photo receptors. The back of the photo receptors is the choroid, which provides nourishment to the receptors, as well as absorb any light that didn’t get absorbed by the photo receptors. This is so that the light doesn’t bounce off the back and get absorbed by the receptors, just like the anti-halation backing in film. The nocturnal animals like cats need to respond to low light levels, so rather than choroids, they had a reflective tapetum lucidum, where the light bounces off the back to be absorbed by the reflectors. This is why the cat’s eye reflect light in the dark.
The rods and cones are a special type of nerve cells sensitive to light. The rod is called a rod because it looks like a rod in reality. It is highly sensitive to low light level or scotopic conditions. The rod is sensitive to all the visible wavelengths, and it is dispersed in the periphery of the retina.
The cone is sensitive to high light level or photopic conditions.. The cones is concentrated in the fovea.
In addition, the eye has a large blind spot where there are no photo receptors. This is where all the nerve endings from the retina come together and exit the eye. So why is it that we don’t see a big hole straight ahead of us we see? The visual cortex, a the part of the brain responsible for processing vision, fills the hole. More on perception later.
Every photo receptor is not necessarily connected directly to the brain. If this was the case, a person would need close to 30 pounds of gray matter! (is that true>???) In order to reduce the amount of information carried to the visual cortex, the retina is cross linked by a complicated network of nerves.
When light receptors in your eyes receive light, they send a signal to your brain. A receptor receiving light also sends signals to neighboring receptors, telling them to turn down their own sensitivity to light
Stare at the tiny black dot at the center of the figure and notice that illusory dark spots appear at the other intersection of the vertical and horizontal white bands. The effect is greater in your periphery vision, where lateral inhibition acts over greater distances.
Mach bands also illustrate lateral inhibition. Even though the brightness from each gray steps, you perceive it as though the grays are lighter or darker near the borders between the grays.
The image is formed at the back of the eye using the cornea and the eye lens. The image formed is upside down and real. As we will see, the cornea is responsible for the most of the refraction of the light, while the eye lens is the fine tune used to focus between far and close objects.
