Biology 463 - Neurobiology
Topic 10
The Eye & the Sensory
Modality of Vision
Lange
Introduction
Significance of vision
– Relationship between human eye, camera, and projector screen
– Retina – the neural portion of the sense of vision
• Photoreceptors: Converts light energy into neural activity
• Detects differences in intensity of light
• Detects differences in wavelengths of light
– Lateral geniculate nucleus (LGN)
• the primary processing center for visual information received
from the retina of the eye
• The LGN is found inside the thalamus
START: Retina – the neural portion of the sense of vision
Photoreceptors: Converts light energy into neural activity
Detects differences in intensity of light
Detects differences in wavelengths of light
END: Lateral geniculate nucleus (LGN)
the primary processing center for visual information received from the
retina of the eye
The LGN is found inside the thalamus
Properties of Light
• Light
– Electromagnetic radiation
– Wavelength
– Amplitude
Light properties:
– Energy is proportional to frequency
(e.g., gamma radiation and cool colors - high energy )
(e.g., radio waves and hot colors - low energy )
Optics
– Study of light rays and their interactions
• Reflection
– Bouncing of light rays off a surface
• Absorption
– Transfer of light energy to a particle
or surface
• Refraction
– Bending of light rays from one
medium to another
All of these factors will influence
how the retinal cells perceive
light energy.
•Near UV light that is
absorbed in the lens
changes proteins there
causing them to become
cloudy (cataracts).
•High intensity blue light
focused on the retina can
also induce a
photochemical reaction
which causes the light
receptors in the eye to lose
sensitivity. This is known
as photo bleaching and
mostly effects blue color
vision.
•Long exposure to bright
sunlight can cause
photobleaching type of
vision loss.
The Structure of the Eye
Gross Anatomy of the Eye
– Pupil: Opening where
light enters the eye
– Sclera: White of the eye
– Iris: Gives color to eyes
– Cornea: Glassy
transparent external
surface of the eye
– Optic nerve: Bundle of
axons from the retina
The Structure of the Eye
Ophthalmoscopic appearance of the eye:
Cross-sectional anatomy of the eye
Punctum Caecum - the place in the visual field that corresponds to the
lack of light-detecting photoreceptor cells on the optic disc of the retina
where the optic nerve passes through it. Often called the “Blind Spot”.
Tests for detecting the “Blind Spot” in the visual field:
Positive Blind Spot Test (Easiest test to detect/locate the blind spot.)
•
The next slide will show a standardized postive blind spot test.
•
This test will likely NOT work as a projected image on the screen due to
size. Instead, if you wish to try out this test, you will need to print out the
slide (in the 4 or 6 slide per page format).
•
Close one eye.
•
Place the paper on a tabletop or wall, and stare at the “+” while
approximately 25 cm away.
•
Move your head slowly towards the paper, and there will be a point where
the “dot” will no longer appear. That is the location of your blind spot.
Tests for detecting the “Blind Spot” in the visual field (continued):
Negative Blind Spot Test (Shows how the eye compensates for the blind spot.)
•
The next slide will show a standardized negative blind spot test.
•
This test will likely NOT work as a projected image on the screen due to
size. Instead, if you wish to try out this test, you will need to print out the
slide (in the 4 or 6 slide per page format).
•
Close one eye.
•
Place the paper on a tabletop or wall, and stare at the two bars while
approximately 25 cm away.
•
Move your head slowly towards the paper, and there will be a point where
the missing segment of the line will disappear. This is the location of your
blind spot, and shows how the brain compensates for this missing
segment by attempting to “fill in” the missing information.
Strabismus – the collective name for
misalignments of the eyes with each other
Exotropia – the condition as seen in the image
where the woman has one eye that points
outward compared to the other
Esotropia – strabismus where the misaligned
eye points inward
Strabismus that arises in childhood will result in
the condition of amblyopia (lazy eye) and
results in a loss of depth perception that often
can be permanent if not treated early enough.
Strabismus that arises in adulthood can be an
indication of a brain tumor or stroke. It can also
arise due to an accident.
In the 1960s, studies showed that
children with untreated strabismus
would often perform poorly in school.
If severe enough, the mixed signals
received by the brain would end up
resulting in the visual signal of the
straying eye to be fully ignored and
the signals from the eye no longer
being perceived in the brain.
Studies during the last five years
have also shown that children with
strabisumus of the exotropia variety
have a much higher rate of
developing psychiatric disorders.
To learn more:
Mohney et. al., 2008. Mental Illness in Young Adults Who Had Strabismus as
Children. Pediatrics. 122:1033- 1038.
Cataracts - a clouding that develops in the crystalline lens of the eye or in its
envelope, varying in degree from slight to complete opacity
Two primary forms:
Senile Cataract – most typical, seen in the
majority of cases. The lens will first yellow,
and then will tend to shrink and grow more
opaque (see image to the right)
Morgagnian Cataract – a cataract where the
lens will liquify and rupture as it grows cloudy
(see below)
How the cornea and lens are used to focus images onto the retina.
Accommodation is the ability of the lens to flex by growing flatter
(stretching out) or relaxing (becoming more bulbous) which further refines
the ability of the eye to focus.
Age related stiffening of
the lens with a resultant
loss in near vision is
called presbyopia.
Forms of visual deficits based upon
shape issues with the eye. The
corrective lens(es) are chosen
based upon what sorts of assistance
to diffraction of light is (are) needed.
George Wald 1906 – 1997
received the Nobel Prize in 1967 for discoveries
concerning the primary physiological and
chemical visual
processes in the eye
Image Formation by the Eye
The Pupillary Light Reflex
– Connections between
retina and brain stem
neurons that control
muscle around pupil
– Continuously adjusting
to different ambient
light levels
– Consensual
– Pupil similar to the
aperture of a camera
Image Formation by the Eye
• The Visual Field
– Amount of space viewed by the retina when
the eye is fixated straight ahead
• Visual Acuity
– Ability to distinguish
two nearby points
– Visual Angle:
Distances across the
retina described in
degrees
Microscopic Anatomy of the Retina
•
Direct (vertical) pathway:
– Ganglion cells

– Bipolar cells

– Photoreceptors
Microscopic Anatomy of the Retina
•
Retinal processing also
influenced lateral
connections:
– Horizontal cells
• Receive input from
photoreceptors
and project to
other
photoreceptors
and bipolar cells
– Amacrine cells
• Receive input from
bipolar cells and
project to ganglion
cells, bipolar cells,
and other
amacrine cells
Microscopic Anatomy of the Retina
•
The Laminar Organization
– Inside-out
– Light passes through ganglion and bipolar cells before reaching
photoreceptors
•
Photoreceptor Structure
– Converts
electromagnetic
radiation to neural
signals
– Three main regions
• Outer segment
containing a
variety of PLB
membrane disks
that contain
photopigments that
absorb light
• Inner segment
(containing the cell
body)
• Synaptic terminal
– Types of
photoreceptors
• Rods and cones
•
Regional Differences in Retinal Structure
– Varies from fovea to retinal
periphery
– Peripheral retina
• Higher ratio of rods to cones
• Higher ratio of photoreceptors to
ganglion cells
• More sensitive to light
Think through logically…. How
would these different patterns of
distribution of rods and cones be
related to a) scotopic (night) vision,
b) photopic (day) vision, and c)
central vision vs. peripheral
vision?
•
Regional Differences in Retinal Structure
– Cross-section of fovea: Pit in retina where outer layers are pushed
aside
• Maximizes visual acuity
– Central fovea: All cones (no rods)
• 1:1 ratio with ganglion cells
• Area of highest visual acuity
Macula Lutea - an oval-shaped highly pigmented,
yellow spot near the center of the retina of the human
eye
•
it has a diameter of around 5 mm
•
is often histologically defined as having two or
more layers of ganglion cells
•
near its center is the fovea, a small pit that
contains the largest concentration of cone
cells in the eye and is responsible for central,
high resolution vision. The macula also
•
contains the parafovea (the area surrounding
the fovea and containing both rods and
cones) and perifovea (the area that is only
cones, sometimes called the fovea centralis).
•
Because the macula is yellow in color it
absorbs excess blue and ultraviolet light that
enter the eye, and acts as a natural sunblock
Phototransduction
•
Phototransduction
in Rods
– light energy
interacts with
photopigment
to produce a
change in
membrane
potential
– analogous to
activity at Gprotein coupled
neurotransmitt
er receptor
– but causes a
decrease in
second
messenger
• Phototransduction in
Rods
– Dark current: Rod
outer segments
are depolarized in
the dark because
of steady influx of
Na+
– Photoreceptors
hyperpolarize in
response to light
• More About Phototransduction in Rods
– light activated biochemical cascade in a photoreceptor
– the consequence of this biochemical cascade is signal
amplification
Opsins are proteins and the retinal-binding visual pigments found in the
photoreceptor cells in the retinas of eyes.
Rhodopsin, also known as visual purple, is a pigment of the retina that is
responsible for both the formation of the photoreceptor cells and the first
events in the perception of light.
Conversion of Opsin into Rhodopsin:
Exposed to light, the pigment retinal immediately photobleaches, and it takes
about 30 minutes to regenerate fully in humans.
Mutations in the rhodopsin gene is a major contributor to various retinopathies such
as:
•
Retinitis Pigmentosa - the mutated gene produces proteins which are
difficult to remove. This can disrupt the intermediate filament network of
opsin and impairs the ability of the cell to degrade these non-functioning
proteins. This can lead to premature photoreceptor apoptosis.
•
X-linked Congenital Stationary Night Blindness - has two forms, complete,
known as type-1 (CSNB1), and incomplete, known as type-2 (CSNB2).
The difference between each form is a matter of severity of effect. In the
complete form (CSNB1), there are no measurable rod cell responses to
light, whereas this response is mildly measurable in the incomplete form.
Make note of the pigmented
regions where vision is lost.
Phototransduction
•
Dark and Light Adaptation
– Dark adaptation—factors
• Dilation of pupils
• Regeneration of unbleached rhodopsin
• Adjustment of functional circuitry
20–25 minutes
All-cone daytime vision
All-rod nighttime vision
Young-Helmholtz Trichromacy Theory of Color Vision - there are three
receptors in the retina that are responsible for the perception of color. One receptor is
sensitive to the color green, another to the color blue, and a third to the color red.
These three colors can then be combined to form any visible color in the spectrum.
•
Phototransduction in Cones
– Similar to rod
phototransduction
– Three different opsins
• Red, green, blue
•
Color detection
– Contributions of blue,
green, and red cones
to retinal signal
– Spectral sensitivity
Color Blindness - a genetic disorder where individuals lose the ability to
distinguish some or all variations in color vision
Deuteranopia & Deuteranomaly (forms of GREEN deficiency)
Deuteranopia and deuteranomaly are the most common forms of color-blindness.
People with these conditions have cones that are insensitive to medium wavelengths
(greens), but the end result is similar to protonopia, with the exception that reds do not
look as dark.
Deuteranomaly is the less severe of the two conditions. Individuals with deuteranomaly
cannot see reds and greens in the same way that people with full color vision can, they
are able to distinguish between shades of reds and greens relatively accurately.
full color vision
green deficiencies
Protanopia & Protanomaly (forms of RED deficiency)
Color receptors in the eyes of people with protanopia are not sensitive to long
wavelengths (the reds). Reds look more like beiges and appear to be somewhat darker
than they actually are. The greens tend to look similar to the reds.
Protanomaly is milder than protanopia, but the end result is similar. Although many
people with protanomaly can distinguish some reds and greens, they cannot do so as
easily as someone with color-normal vision, and, as with protanopia, reds tend to look
darker as well.
full color vision
red deficiencies
Tritanopia (a form of BLUE deficiency)
Note… Tritanopia is much less common than the other categories mentioned
above.
Tritanopia is insensitivity to short wavelengths (the blues). Blues and greens can
be confused, but yellows are also affected in that they can blend with blues or
appear as lighter shades of red.
full color vision
blue deficiencies
Rod Monochromacy or Achromacy (NO color vision)
This group constitutes an extremely small minority among people who are colorblind. All cones of the eye are non-functional, so the rods (receptors which can
only differentiate between light and dark) are the only available source of visual
information. Individuals with achromacy see no color at all. People with
achromacy often have poor visual acuity and have an aversion to bright light.
full color vision
complete cone deficiency
Retinal Processing
• Receptive Field: “On” and “Off” Bipolar Cells
– Receptive field: Stimulation in a small part of the visual field
changes a cell’s membrane potential
– Antagonistic center-surround receptive fields
Retinal Processing
• On-center Bipolar Cell
– Light on (less glutamate); Light off -> more glutamate
• Ganglion Cell Receptive Fields
– On-Center and Off-Center ganglion cells
– Responsive to differences in illumination
• Types of Ganglion Cells
– Appearance, connectivity, and electrophysiological
properties
• M-type (Magno) and P-type (Parvo)ganglion cells in monkey
and human retina
Retinal Output
• Color-Opponent Ganglion Cells
Retinal Output
• Parallel Processing
– Simultaneous input from two eyes
• Information from compared in cortex
– Depth and the distance of object
– Information about light and dark: ON-center and OFFcenter ganglion cells
– Different receptive fields and response properties of
retinal ganglion cells: M- and P- cells, and nonM-nonP
cells
Concluding Remarks
• Light emitted by or reflected off objects in space  imaged
onto the retina
• Transduction
– Light energy converted into membrane potentials
– Phototransduction parallels olfactory transduction
• Electrical-to-chemical-electrical signal
• Mapping of visual space onto retina cells not uniform
END.