Transcript Visual Coding and the Retinal Receptors

Chapter 6
Vision
Sensation and Perception:
Important Vocabulary Terms
• Sensation is the process
of receiving, transducing,
and coding stimulus
energy in the world.
– Stimulus energy is
physical energy, such
as light, sound, heat
– Sense organs such as
eyes, ears, skin
receive energy
Sensation and Perception:
Important Vocabulary Terms
– Reception: the absorption of physical energy
by receptors
– Transduction: the conversion of physical
energy into electrochemical energy
Sensation and Perception:
Important Vocabulary Terms
– Perception: the brain’s process of organizing and
interpreting sensory information to give it meaning
– Coding: a one-to-one communication between an
aspect of a physical stimulus and an aspect of
nervous system activity
Neuroanatomy Handout #4: The Visual System
• Sclera (A):
– Outermost layer of
eye; white, fibrous,
protective globe
• Cornea (A1): Clear
disk at front of eye;
focuses light rays onto
the receptor cells at
back of eye (retina)
• Aqueous humor (B):
Fluid in the very front
of the eyeball; refracts
light rays onto retina
Neuroanatomy Handout #4: The Visual System
• Iris (C): Colored part in
center of eye; ring of
muscles that controls
amount of light that
gets into eye
• Pupil (D): Opening in
center of eye (appears
black); constricts or
dilates with movement
of iris
Neuroanatomy Handout #4: The Visual System
• Lens (E): Along with
cornea, focuses light
onto receptor cells
• Vitreous humor (F):
Fluid filling majority of
eyeball; refracts
lightwaves
• Retina (G): lined with
visual receptor cells,
rods and cones
Neuroanatomy Handout #4: The Visual System
•
•
•
Rods (I) - most abundant in the
periphery of the eye
– 120 million per retina
– respond to faint light
Cones (J) - most abundant in
and around the fovea
– 6 million per retina
– essential for color vision,
which requires bright light
Photopigments: chemicals
released by rods and cones
when struck by light
Neuroanatomy Handout #4: The Visual System
• The fovea (G2):
central portion of
the human
retina which
allows for acute
and detailed
vision.
– Packed tight
with receptor
cells
Neuroanatomy Handout #4: The Visual System
• Rods and cones are located in
the outmost layer of the eye.
• They communicate their
messages to neurons called
bipolar cells (K) and horizontal
cells, which are located closer
to the center of the eye.
• Bipolar cells send messages
to ganglion cells (L) and
amacrine cells that are even
closer to the center of the eye.
Neuroanatomy Handout #4: The Visual System
• Axons of ganglion
cells (L1) join one
another to form the
optic nerve.
• The optic nerve (L2)
exits through the
back of the eye and
travels to the brain.
Neuroanatomy Handout #4: The Visual System
• Blind spot (G1):
The point at which
the optic nerve
leaves the back of
the eye
– it contains no
receptor cells
– It does contain
retinal veins and
arteries (H)
Visual Coding and the Retinal Receptors
• Perception of color is dependent upon the
wavelength of the light.
Visual Coding and the Retinal Receptors
• “Visible” wavelengths depend upon species’
receptors.
• Human range: 350 nanometers (violet, short
wavelength) to 700 nanometers (red, long
wavelength).
– Some species can detect ultraviolet light
Visual Coding and the Retinal Receptors
•
Discrimination among colors depends upon the
combination of responses by different neurons.
• Two major interpretations of color vision:
1. Trichromatic theory/Young-Helmholtz theory
2. Opponent-process theory
Visual Coding and the Retinal Receptors
• Trichromatic theory Color perception
occurs through the
ratio of activity across
the three types of
cones :
– short wavelength
– medium-wavelength
– long-wavelength
Visual Coding and the Retinal Receptors
• More intense light increases the brightness of
the color but does not change the ratio and thus
does not change the perception of the color
itself.
Visual Coding and the Retinal Receptors
• The opponent-process theory
(Ewald Hering) suggests that
we perceive color in terms of
paired opposites.
– white/black
– red/green
– yellow/blue
• A possible mechanism for the
theory is that bipolar cells are
excited by one set of
wavelengths and inhibited by
another.
Negative color afterimage
Visual Coding and the Retinal Receptors
The opponent-process and trichromatic theories
can’t explain:
• Color constancy, the ability to recognize color
despite changes in lighting.
• Retinex theory suggests the cortex compares
information from various parts of the retina to
determine the brightness and color for each
area.
Visual Coding and the Retinal Receptors
• Color vision deficiency:
impairment in perceiving
color differences
• X-linked trait
• Causes:
– lack of a type of cone
– cone has abnormal
properties
• Most common form:
difficulty distinguishing
between red and green
The Neural Basis of Visual Perception
• Ganglion cell axons
form the optic nerve.
• Optic chiasm: place
where the two optic
nerves meet.
• In humans, half of the
axons from each eye
cross to the other side of
the brain.
• Most axons go to the
lateral geniculate
nucleus, a smaller
amount to the superior
colliculus and fewer go to
other areas.
• Visual field: the
whole area of the
world that you
can see at a
given time
• Receptive field:
the portion of the
visual field to
which any one
neuron responds
“Eyes Right”
by Oliver Sacks
From The Man Who Mistook
His Wife for a Hat
The Neural Basis of Visual Perception
• Some people with damage to the primary visual
cortex (V1) show blindsight, an ability to
respond to visual stimuli that they report not
seeing.
The Neural Basis of Visual Perception
• Visual agnosia is the inability
to recognize objects despite
satisfactory vision.
– Caused by damage to the
pattern pathway usually in
the temporal cortex.
• Prosopagnosia is the
inability to recognize faces.
– Occurs after damage to
the fusiform gyrus of the
inferior temporal cortex.