Course: PSYC*1000 (DE)
Professor: Harvey Marmurek
Schedule: Summer, 2012
Textbook: Psychology Tenth Edition in Modules authored by David G. Myers
Textbook ISBN: 9781464102615
Module 18: Vision and Perceptual Organization and Interpretation
Vision Our eyes receive light energy and transduce (transform) it into neural messages that our brain then
processes into what we consciously see.
What is the energy that we see as visible light, and how does the eye transform light energy into neural
The Stimulus Input: Light Energy
o What we see as visible light is but a thin slice of the whole spectrum of electromagnetic energy,
ranging from imperceptibly short gamma waves to the long wave of radio transmission. Other
organisms are sensitive to differing portions of the spectrum. Bees cannot see what we perceive as
red but can see ultraviolet light.
o Two physical characteristics of light help determine our sensory experience of them.
Lights wavelength distance from one wave peak to the next determines hue
Intensity amount of energy in light waves (determined by a waves amplitude or height)
o Light enters the eye through the cornea (which protects the eye and bends light to provide focus).
o Light then passes through the pupil (a small adjustable opening).
o Controlling the size of the pupil is the iris (a coloured muscle that dilates or constricts in response to
light intensity and even to inner emotions).
o Behind the pupil is the lens that focuses incoming light rays into an image on the retina (a
multi-layered tissue on the eyeballs sensitive inner surface). The lens focuses the rays by changing
its curvature in a process called accommodation.
o Leonardo da Vinci = eyes watery fluids bend the light rays, reinverting the image to the upright
position as it reaches the retina.
o 1604 Johannes Kepler showed that the retina does receive upside-down images of the world
o eventually, the answer became clear the retina does not see a whole image. Rather, its millions
of receptor cells convert particles of light energy into neural impulses and forward those to the
brain. There, the impulses are reassembled into a perceived, upright-seeming image.
o If you could follow a single light-energy particle to the back of your eye, you would first make your
way through the retinas outer layer of cells to its buried receptor cells, the rods and cones. There,
you would see the light energy trigger chemical changes that would spark neural signals, activating
near bipolar cells. The bipolar cells in turn would activate the neighbouring ganglion cells, whose
axons twine together like the strands of rope to form the optic nerve. The nerve will carry the
information to your brain, where your thalamus stands ready to distribute the information. The optic
nerve can send nearly 1 million messages at once through its nearly 1 million ganglion fibers. (The
auditory nerve, which enables hearing, carries much less information through its mere 30,000
fibers.) We pay a small price for this eye-to-brain highway. Where the optic nerve leaves the eye,
there are no receptor cells creating a blind spot.
o Rods and cones differ in where theyre found in what they do.
Cones cluster in and around the fovea, the retinas area of central focus. Many have their
own hotline to the brain: Each one transits to a single bipolar cell that helps relay the
cones individual message to the visual cortex, which devotes a large area to input from the
fovea. These direct connections preserve the cones precise information, making them
better able to detect fine detail.
Rods have no such hotline; they share bipolar cells with other rods, sending combined
messages. Cones enable you to perceive colour. Rods enable black-and-white vision.
Receptors in the Human Eye: Rod-Shaped Rods and Cone-Shaped Cones
Number 6 million 120 million
Location in retina Centre Periphery
Sensitivity in dim light Low High
Colour sensitivity High Low
Detail sensitivity High Low
Some nocturnal animals, such as toads, mice, rats, and bats, have impressive night vision thanks to having many
more rods than cones in their retinas. These creatures probably have very poor colour vision.
Cats are also able to open their pupils much wider than we can, which allows more light into their eyes so they can
see better at night.
Visual Information Processing
How do the eye and the brain process visual information?
Pathway from the eyes to the visual cortex ganglion axons forming the optic nerve run to the thalamus, where they
synapse with neurons that run to the visual cortex.
At the entry level, information processing begins in the retinas neural layers, which are actually brain tissue
that migrated to the eye during fetal development. These layers dont just pass along electrical impulses; they also
help to encode and analyze sensory information. The third neural layer in a frogs eye, for example, contains bug
detector cells that fire only in response to moving fly-like stimuli.
After processing by your retinas nearly 130 million receptor rods and cones, information travels to your
bipolar cells, then to your million or so ganglion cells, and to your brain. Any given retinal area relays its information
to a corresponding location in the visual cortex, in the occipital lobe at the back of your brain.
The same sensitivity that enables retinal cells to fire messages can lead them to misfire. Your retinal cells
are so responsive that even pressure triggers them. But your brain interprets their firing as light. Moreover, it
interprets the light as coming from he left the normal direction of light that activates the right side of the r