Download The Visual System

The Visual System
Dr. Kline
FSU
What stimuli are required for
vision?

Light- which can be thought of as discrete
particles (photons) or traveling waves.
Human Visible Spectrum

Humans can detect waves of energy traveling
through space between 380 & 760 nanometers).

Wavelengths outside this spectrum are
undetectable to the human eye.

Some organisms do detect wavelengths outside
our visible spectrum.

E.g., Rattlesnakes detect in infrared!;
 bees detect ultraviolet light.
Electromagnetic Radiation
What are the 2 properties of light that
influence visual perception?

1. Wavelength is associated with our
perception of color.

2. Intensity is associated with our
perception of “brightness.”
Reflectance of light

Light is reflected off of the surface of
objects & to the eye.

Light energy is converted into neural energy
& then processed by the brain.
Parts of the eye
 Outer Layer
 1. Sclera: White fibrous layer

2. Cornea: the clear protruding structure in
the front of the eye that is curved.

Bends light rays & is responsible for 7080% of eye’s focusing ability.
Cornea
Middle Layer

1. Choroid: vascularized layer that provides
nutrition for retinal cells. Is located between the
retina & sclera.

2. Pigment epithileum: a black pigment found
between the choroid & retina.

**Traps photons from light to prevent scattering
of photons along the retina, which reduces
distortion.
Middle layer (contd.)

3. Iris: smooth ring of muscle with a central
opening (pupil). Gives us our eye color!

4. Pupil: Pupil changes in size depending on
intensity of light.
-Intense light, small constricted pupils
-Dim light, dilated pupils

5. Lens: focuses light on retina (convex).
-lens is round (nearby objects)
-lens is flatter for distant objects
Inner layer

1. Retina: contains receptor cells needed for
neural processing of light.
A. Fovea: indentation on retina.
-fine discrimination; colors & detail.
B. periphery: area on either side of fovea of
retina.
-detection of light

2. Optic disk: place where axons exit eye forming
optic nerve.
The Photoreceptors: Rods & Cones
Cones-specialized for color vision & detail (fovea).
Rods-sensitive to light (periphery)

126 million receptors total!
 120 million are rods & 6 million are cones


sensitivity to light enhanced by “convergence
ratio.” Many rods converge on a single retinal
ganglion cell than do cones.
Rods-big receptive fields
 cones-small receptive fields
How does visual information
get from the eye to the brain to
be processed?

Two vision pathways:

Geniculostriate & Tectopulvinar
Visual Pathways

1. Geniculostriate pathway optic chiasm----LGN---Primary Visual cortex
**involved in patter perception, color vision**

2. Tectopulvinar pathway optic chiasm---superior colliculus---Lateral
 posterior pulvinar---PVC
**detection of light; spatial orientation**
Primary Visual Cortex
How do our eyes adapt to the dark?

The photopic & scotopic systems adjust
their sensitivity as a function of time in the
dark.

The cones become more sensitive during
the first 5-10 minutes after being in the
dark.

Rods continue becoming more sensitive
over period of 20-30 minutes.
How is a dark adaptation experiment
conducted?

Step 1: The subject (S) is exposed to bright light of a
known intensity.

Step 2: S is then placed in total darkness & asked to
detect a spot of light (controlled luminance).

Step 3: S’s detection threshold is plotted as a
function of time spent in dark.
We have three cone wavelengths

1. Short wavelength: peaks at 419 nm (blues).

2. Medium wavelength: peaks at 531 nm (greens).

3. Long wavelength: peaks at 558 nm (reds).

The primary colors are blue, green, & red
What are the cell’s in the retina?
What is relationship between activity
in the retina & the brain?

An electrode is inserted into various parts of
the visual system (retina, cortex) of an
animal.

The cell’s activity in response to the
presentation of visual stimuli (lights, bars,
complex images) to the animal’s retina is
recorded.
What is a receptive field of retinal
ganglion cells?

The receptive field for these cells is the
region of the retina that, when stimulated
excites or inhibits the cell’s firing pattern.
Retinal ganglion cells
Kuffler (1953) presented spots of light to retina cells
in the cat & recorded their responses.

The cells have a Concentric circle configuration!
 usually called center-surround cells

On-center, off-surround cell has an “excitatory
center,” & “inhibitory surround”

Off-center, on-surround cell has an “inhibitory
center” & “excitatory surround”
Kuffler’s Cat
Sensation and Perception - sensory2.ppt © 2001 Laura Snodgrass, Ph.D.
Are receptive fields of cortical cells like
those of retinal ganglion cells?

No!!

Our visual cortex cells respond to more
complex stimuli (e.g., bars of light).
Hubel & Wiesel (1950s & 60s)

Recorded cortical cells in the visual cortex
of cats in response to visual images they
presented to the cat’s retina.

They found three types of cells with
different receptive fields (bar detectors).
Receptive Fields of cortical neurons—
Primary Visual cortex

1. Simple Cells
--respond to points of light or bars of light in a
particular orientation

2. Complex cells
--respond to bars of light in a particular orientation
moving in a specific direction.
3. Hypercomplex Cells:
respond to bars of light in a particular orientation,
moving in a specific direction, & of a specific line
length.
Simple Cells
Complex Cells
What is the organization of the visual
cortex?

Hubel & Wiesel found that the visual cortex is
organized into columns.

Location specific: For each place on the retina
there is a column of cells in cortex.

Two columns next to one another in the cortex
respond to stimulation of two adjacent points on
the retina.
Orientation & Ocular Dominance
columns in Primary Visual Cortex
The Visual cortex has a retinotopic
map

Visual cortex has a map of the retina’s surface.

More cortical neurons are devoted to fovea of
retina.

As fovea only has cones, they are widely mapped
on cortex’s surface.

The reason: cones allow us to see detail & color.
How do we see in color?

Two Theories:

1. Young-Helmholtz Trichromatic theory
of color vision.
The pattern of activity among the three conetypes determines the color we perceive.
The cones all respond equally to white light
(which contains all wavelengths).
Evidence for Trichromatic theory

1. We have three types of cones sensitive to
different wavelengths of light (short,
medium, & long wavelengths).

2. There are 3 types of color-blindness.

3. Approximately any color can be matched
by mixing varying amounts of red, green, &
blue light.
Additive Color mixing with lights
Results of Additive Color Mixing
Subtractive & Additive Mixing
Why mixing blue & yellow pigments,
yields the color green.
Color-blindness

Results whenever we are either missing one
of our cones or one of our cones doesn’t
work properly.
2. Opponent-Process theory

We have opposing mechanisms that allow us to
perceive colors.
Evidence for this theory:
1. Incompatible colors cannot be seen Explains why
we can’t see certain colors (reddish-green, bluishyellow) & color afterimages.
We have 3 opposing mechanisms: red-green,
yellow-blue, & black-white. These are called
complimentary colors & put together they produce
yellow or white.