Saturday, August 10, 211:16 AM
Module 6.1 Visual Coding
1. Be able to describe the parts of the eye and its connections to the brain.
2. Understand the process by which three types of cones, and the neurons they connect with, can produce a rich spectrum of perceived color.
3. Understand the trade-off between acuity for detail and sensitivity to dim light.
Module 6.2 The Neural Basis of Visual Perception
1. Understand the concept of receptive fields and how they change from the retina to the various areas of the visual cortex.
2. Understand the inputs to the dorsal and ventral streams of cortical processing and what each pathway analyzes.
3. Know the contributions of areas V1, V2, and inferior temporal cortex to shape perception.
4. Be able to describe the brain areas that process color and motion.
Module 6.3 Visual Development
1. Be able to describe the effects of early experiences and of visual deprivation on the development of the visual system.
1) Each sensory neuron conveys a particular type of experience. For example, anything that stimulates the optic nerve is perceived as light.
2) Vertebrate vision depends on two kinds of receptors: cones, which contribute to color vision, and rods, which do not.
3) Every cell in the visual system has a receptive field, an area of the visual world that can excite or inhibit it.
4) After visual information reaches the brain, concurrent pathways analyze different aspects, such as shape, color, and movement.
5) Neurons of the visual system establish approximately correct connections and properties through chemical gradients that are present before birth.
However, visual experience can fine-tune or alter those properties, especially early in life.
6.1 Visual Coding
General Principles of Perception
• How far we see depends on how far light travels (we see because light strikes our eyes)
○ Light rays bounce off any object in all directions, but we only see those rays that strike the retina perpendicularly
• Law of specific nerve energies-- each nerve always conveys the same kind of information to the brain
• The strength of a stimulus determines the amount of receptor cell's depolarization or hyperpolarization
○ The amplitude (amount) of the receptor's response determines how many action potentials the next set of neurons sends and their timing
○ Much of sensory coding depends on the frequency of firing
• From Neuronal Activity to Perception
○ The brain's activity does not duplicate the objects that you see, but rather codes them in various kinds of neuronal activity
The Eye and Its Connections to the Brain
• Light enters the eye though the pupil-- an opening in the center of the iris
• It is focused by the lens (adjustable) and cornea (not adjustable) and projected onto the retina, the rear surface of the eye, which is lined with visual
• Light from the ride side of the world strikes the right half of the retina, and vice versa
Textbook Notes Page 1 • Light from the ride side of the world strikes the right half of the retina, and vice versa
• Light from above strikes the bottom half of the retina, and vice versa
• Route Within the Retina
○ In the vertebrate retina, messages go from receptors at the back of the eye to bipolar cells, located closer to the center of the eye
○ The bipolar cells send their messages to ganglion cells, located still closer to the center of the eye
The ganglion cells' axons join together and travel back to the brain
○ Additional cells called amacrine cells get information from bipolar cells and send it to other bipolar cells, other amacrine cells, and ganglion
○ The ganglion cell axons form the optic nerve, which exits through the back of the eye
The point at which it leaves is the blind spot because it has no receptors
• Fovea and Periphery of the Retina
○ Fovea-- a tiny area specialized for acute, detailed vision
○ The ganglion cells in the fovea of humans and other primates are called midget ganglion cells because each is small and responds to just a single
○ Foveal vision has better acuity (sensitivity to detail), and peripheral vision has better sensitivity to dim light
Visual Receptors: Rods and Cones
• Two types of receptors in the retina:
1) Rods are abundant in the periphery of the human retina and respond to faint light but are not useful in daylight because bright light bleaches
2) Cones are abundant in and near the fovea, are less active in dim light, more useful in bright light, and essential for color vision
• Rods outnumber cones by about 10 to 1, though cones provide about 90% of the brain's input
○ In the fovea, each cone has its own line to the brain
○ In the periphery (mostly cones), each receptor shares a line with tens or hundreds of others
• Both rods and cones contain photopigments, chemicals that release energy when struck by light
○ Light converts one photopigment (11-cis-retinal) to another (all-trans-retinal), thus releasing energy that actives second messengers within the
• In the human visual system, the shortest visible wavelengths are perceived as violet; progressively longer wavelengths are perceived as blue, green,
yellow, orange, and red
• The Trichromatic (Young-Helmholtz Theory)
○ Young proposed that we perceive color by comparing the responses across a few types of receptors, each of which was sensitive to a different
range of wavelengths
○ According to this theory, we perceive color through the relative rates of response by three kinds of cones (short-, medium-, and long-wavelength
cone types), each kind maximally sensitive to a different set of wavelengths
Textbook Notes Page 2 ○
○ When all three types of cones are equally active, we see white or grey
○ The nervous system determines the color and brightness of light by comparing the ratio of responses of different types of cones
Animals with only one type of cone, such as much, are color blind)
○ Long- and medium-wavelength cones are far more abundant than short-wavelength (blue) cones
○ There is no useful color vision in the periphery
○ Visual field-- the part of the world that you see
• The Opponent-Process Theory
○ Negative color afterimage-- a replacement of the red you had been staring at with green, green with red, yellow and blue with each other, and
black and white with each other
○ Opponent-process theory-- we perceive color in terms of opposites (e.g., the brain has a mechanism that perceives color on a continuum from
red to green, another from yellow to blue, and another from white to black)
○ This theory cannot explain why some afterimages are not the opposite color they are expected to be (e.g., if staring at a white circle on a green
background, the afterimage of the white circle may appear green instead of grey or black)
• The Retinex Theory
○ The trichromatic theory and the opponent-process theory cannot easily explain color constancy-- the ability to recognize colors despite changes
○ A certain wavelength of light can appear as several different colors depending on the background
Similarly, our perception of brightness of an object requires comparing it with other objects
○ To account for color and brightness constancy, Edwin Land proposed the retinex theory-- the cortex compares information from various parts of
the retina to determine the brightness and color for each area
○ Visual perception requires a reasoning process, not just retinal stimulation (i.e., color depends on what our brains do with incoming light; it is
not a property of light itself)
• Color Vision Deficiency
○ Color vision deficiency (color blindness)-- discovered by psychologists in the 1600s
○ Caused by a lack of one or two types of cones (or three types of cones, but one is abnormal)
○ Red-green color deficiency is the most common (in which people have difficulty distinguishing red from green because their long- and medium-
wavelength cones have the same photopigment instead of different ones)
○ People with Four Cone Types
Some women have more than 3 kinds of cones
The gene controlling the long-wavelength (red) cone receptor varies, causing slight differences in which wavelength produces the
These women draw slightly finer color distinctions than other people do
Textbook Notes Page 3 6.2 Neural Basis of Visual Perception
An Overview of the Mammalian Visual System
• Motion blindness-- some people with otherwise satisfactory vision fail to see an object that is moving
• The rods and cones of the retina make synapses with bipolar cells and horizontal cells-- cells that make inhibitory contact onto bipolar cells, which in
turn make synapses onto amacrine cells and ganglion cells
○ All these cells are within the eyeball
• Most ganglion cells go to the lateral geniculate nucleus (part of the thalamus), which in turn sends axons to other parts of the thalamus and the
Processing in the Retina
• The wiring diagram enables cells in your eye and brain to identify important patterns (such as the edges of objects)
• Lateral inhibition is the retina's way of sharpening contrasts to emphasize the borders of objects
• Rods and cones have spontaneous levels of activity, and light striking them decreases their output (i.e., they have inhibitory synapses onto the
bipolar cells, and therefore, light decreases their inhibitory output)
• The response of horizontal cells decay over distance
• Lateral inhibition-- the reduction of activity in one neuron by activity in neighboring neurons
○ The main function of lateral inhibition is to heighten contrast
Textbook Notes Page 4 ○ The main function of lateral inhibition is to heighten contrast
Pathways to the Lateral Geniculate and Beyond
• Each cell in the visual system of the brain has what we call a receptive field, which is part of the visual field that excites or inhibits it
• The receptive field has an excitatory or inhibitory central portion and a surrounding ring with the opposite effect
• 3 categories of ganglion cells in primates:
1) Parvocellular neurons-- neurons mostly in or near the fovea with small cell bodies and small receptive fields