Chapter 5: Vision

Modified: 2025-08-24 1:11 PM CDST

In this chapter we begin our exploration of human and animal sensory systems. This chapter covers vision. To start, watch this video.

Module 5.1 Visual Coding (p. 147)

• How far one sees is dependent on how far light travels before it strikes one’s eyes

• Perception of vision is not in the eyes; it’s in the brain

• Intromissive not Extromissive

General Principles of Perception

• Each of our senses has specialized receptors that are sensitive to a particular kind of energy

• Law of specific nerve energies states that activity by a particular nerve always conveys the same type of information to the brain

• Example: impulses in one neuron indicate light; impulses in another neuron indicate sound

The Eye and Its Connections to the Brain

• Light:

  • Enters the eye through an opening in the center of the iris called the pupil

  • Is focused by the lens and the cornea onto the rear surface of the eye known as the retina, which is lined with visual receptors

  • The left side of the world strikes the right side of the retina and vice versa

  • From above strikes, the bottom half of the retina and vice versa

Route Within the Retina—Bipolar Cells

• Cells, located closer to the center of the eye, that receive messages from visual receptors at the back of the eye

• These cells send messages to ganglion cells that are even closer to the center of the eye

  • The axons of ganglion cells join one another to form the optic nerve that travels to the brain

Route Within the Retina— Amacrine Cells

• Additional cells that receive information from bipolar cells and send it to other bipolar, ganglion, or amacrine cells

• Control the ability of the ganglion cells to respond to shapes, movements, or other specific aspects of visual stimuli

Cross-Section of a Vertebrate Eye (p. 149)

Visual Path Within the Eye (p. 151)

The Optic Nerve

• Consists of the axons of ganglion cells that band together and exit through the back of the eye and travel to the brain

• Leaves the back of the eye; the point at which it leaves is called the blind spot because it contains no receptors

Illustration of the Blind Spot (p. 151)

Close your left eye and focus your right eye on the o in the top part. Move the page toward you and away, noticing what happens to the x. At a distance of about 10 inches (25 cm), the x disappears. Now repeat this procedure with the bottom part. At that same distance, what do you see?

The Fovea and the Periphery of the Retina

• Is the central portion of the retina and allows for acute and detailed vision

  • Packed tightly with receptors

  • Nearly free of ganglion axons and blood vessels

• Each receptor (almost exclusively cones) in the fovea attaches to a single bipolar cell and a single ganglion cell known as a midget ganglion cell

• Each cone in the fovea has a direct line to the brain which allows the registering of the exact location of input

• Our vision is dominated by what we see in the fovea

The Placement of Receptors on the Retina (p. 151)

One owlet has turned its head almost upside down to look up. Birds of prey have many receptors on the upper half of the retina, enabling them to see down in great detail during flight. But they see objects above themselves poorly, unless they turn their heads. Take another look at the prairie falcon at the start of this chapter. It is not a one-eyed bird; it is a bird that has tilted its head. Do you now understand why?

• In the periphery of the retina, a greater number of receptors (primarily rods) converge into ganglion and bipolar cells

  • Detailed vision is less in peripheral vision

  • Allows for the greater perception of much fainter light in peripheral vision

Convergence of Input onto Bipolar Cells (p. 152)

In the fovea, each bipolar cell receives excitation from just one cone (and inhibition from a few surrounding cones), and relays its information to a single midget ganglion cell. In the periphery, input from many rods converges onto each bipolar cell, resulting in higher sensitivity to faint light and low sensitivity to spatial location. Notice that the resolution is higher for the cones (smaller spots) and less for the rods (bigger spots). TV screens exploit the same principle: the more and smaller dots, the better.

• Highly adaptive

• Examples:

  • Predatory birds have a greater density of receptors on the top of the eye

  • Rats have a greater density on the bottom of the eye

The Difference Between Foveal and Peripheral Vision (p. 152)

Visual Receptors: Rods and Cones (p. 152)

• The vertebrate retina consists of two kinds of receptors

  • Rods: most abundant in the periphery of the eye and respond to faint light (120 million per retina)

  • Cones: most abundant in and around the fovea (6 million per retina)

  • Essential for color vision and more useful in bright light

  • Purkinje effect: a PERCEPTUAL effect noticed when ambient light is perceptible by both rods and cones simultaneously. Nice explanation HERE

  • Presbyopia: As you get older (40 years +) your lens hardens and it becomes more difficult to focus on close objects. The reason is the muscles on either side of the lens are not strong enough to focus. (The lens focuses by become thinner (for far vision) or thicker (for near vision)

  • Rods and Cones

• Though cones are outnumbered, they provide about 90 percent of the brain’s input

• On average, 120 million rods and 6 million cones converge onto 1 million axons in the optic nerve

• The ratio of rods to cones is higher in species that are more active at dim light

Structure of Rods and Cones (p. 153)

(a) Diagram of a rod and a cone. (b) Photo of rods and a cone, produced with a scanning electron microscope. Magnification x 7000.

Photopigments

• Chemicals contained by both rods and cones that release energy when struck by light

  • Consist of 11-cis-retinal bound to proteins called opsins

  • Light energy converts 11-cis-retinal quickly into all-trans-retinal

  • Light is thus absorbed and energy is released that activates second messengers within the cell

• Night vision

  • Takes 30 to 40 minutes

  • Bleaching of photopigments stops

  • Faster in red light (e.g., war movies)

  • Graph of process, night vision improves as lines go down, top line is for cones, bottom for rods

  • Dark Adaptation

   

Color Vision (p. 153)

• Visible light is a small portion of the electromagnetic spectrum

• The perception of color is dependent upon the wavelength of the light

• “Visible” wavelengths are dependent upon the species’ receptors

• Humans perceive wavelengths between 400 and 700 nanometers (nm)

Visible Light on the Electromagnetic Spectrum (p. 154)

• Roy G. Biv is mnemonic to remember the order of colors of visible light.

• Infrared is another word for heat and pit viper snakes can detect heat differences using the pits under their eyes to catch warm-blooded prey in total darkness.

• Honeybee eyes can detect ultraviolet light so their perception of flowers is different from ours.

Specificity of Color Vision

• Depends on specific receptors within the eye

• Two major interpretations of color vision

  • Trichromatic theory/Young-Helmholtz theory

  • Opponent-process theory

Trichromatic Theory (p. 154)

• Color perception occurs through the relative rates of response by three kinds of cones

  • Short-wavelength

  •  Medium-wavelength

  •  Long-wavelength

• Each cone responds to a broad range of wavelengths, but some more than others

• The ratio of activity across the three types of cones determines the color

• More intense light increases the brightness of the color but does not change the ratio

• Three kinds of cones are unevenly distributed

Wavelength-Sensitivity Functions (p. 155)

Distribution of Cones in Two Human Retinas (from 13th ed.)

Investigators artificially colored these images of cones from two people’s retinas, indicating short-wavelength cones with blue, medium wavelength cones with green, and long-wavelength cones with red. Note the difference between the two people, the scarcity of short-wavelength cones, and the patchiness of the distributions.

The Opponent-Process Theory (p. 155)

• Suggests that we perceive color in terms of paired opposites

• The brain has a mechanism that perceives color on a continuum from red to green and another from yellow to blue

  • A possible mechanism for the theory is that bipolar cells are excited by one set of wavelengths and inhibited by another

An Afterimage as an Effect of Context (p. 156)

Stare at the x under bright light for a minute and then look at a white surface. Many people report an alternation between two afterimages, one of them based on the illusion of a red square.

Limitations of Color Vision Theories

• Both the opponent-process and trichromatic theory have limitations

  • Color constancy, the ability to recognize color despite changes in lighting, is not easily explained by these theories

• Retinex theory suggests the cortex compares information from various parts of the retina to determine the brightness and color for each area

The Context of Color Perception (p. 157)

After removal of the context, squares that appeared blue on the left or yellow on the right now appear gray.

Brightness Constancy (p. 157)

In the center of this figure, do you see a gray object above and a white object below? Place a finger over the border between them and then compare the objects.

Visual Constancies (read)

Stimulus Factors in Perception (read)

Color Vision Deficiency (p. 158)

• An impairment in perceiving color differences

  • Gene responsible is contained on the X chromosome

  • Caused by either the lack of a type of cone or a cone that has abnormal properties

  • Most common form is difficulty distinguishing between red and green

    • Results from the long- and medium-wavelength cones having the same photopigment

Module 5.2 Visual Processing in the Brain (p. 161)

• Senses, such as vision, provide psychological experiences

• Neuroscientists have developed a relatively detailed understanding of vision

• Understanding the mechanisms of vision provides a model of what it means to explain something in biological terms

   

An Overview of the Mammalian Visual System (p. 161)

• Rods and cones of the retina make synaptic contact with horizontal cells and bipolar cells

• Horizontal cells are cells in the eye that make inhibitory contact onto bipolar cells

• Bipolar cells make synapses onto amacrine cells and ganglion cells

• Different cells are specialized for different visual functions

• Ganglion cell axons form the optic nerve

• The optic chiasm is the place where the two optic nerves leaving the eye meet

• In humans, half of the axons from each eye cross to the other side of the brain

• Most ganglion cell axons go to the lateral geniculate nucleus, a smaller amount to the superior colliculus, and fewer to other areas

The Vertebrate Retina (p. 161)

The top of the figure is the back of the retina. The optic nerve fibers group together and exit through the back of the retina, in the “blind spot” of the eye.

The Path of Visual Input (p. 162)

a) one path to thalamus, another to superior colliculus, b) retinotropic organization of retinal axons.

Lateral Inhibition in the Retina

• Sharpens contrasts to emphasize the borders of objects

• The reduction of activity in one neuron by activity in neighboring neurons

• The response of cells in the visual system depends upon the net result of excitatory and inhibitory messages it receives

An Illustration of Lateral Inhibition (p. 163)

Try to "catch" the dots that appear in the periphery of your visual field. You cannot.

Processing in the Retina (p. 162)

• The lateral geniculate nucleus (LGN)

  • Part of the thalamus

  • Specialized for visual perception

  • Destination for most ganglion cell axons

  • Sends axons to other parts of the thalamus and to the visual areas of the occipital cortex

• The cortex and thalamus constantly feed information back and forth to each other

Further Processing (p. 163)

• The receptive field refers to the part of the visual field that either excites or inhibits a cell in the visual system of the brain

• For a receptor, the receptive field is the point in space from which light strikes it

• For other visual cells, receptive fields are derived from the visual field of cells that either excite or inhibit

  • Example: ganglion cells converge to form the receptive field of the next level of cells

Receptive Fields (p. 164)

Primate Receptive Fields

• Ganglion cells of primates generally fall into three categories

  • Parvocellular neurons

  • Magnocellular neurons

  • Koniocellular neurons

Parvocellular Neurons

• Mostly located in or near the fovea

• Have smaller cell bodies and small receptive fields

• Highly sensitive to detect color and visual detail

Magnocellular Neurons

• Distributed evenly throughout the retina

• Have larger cell bodies and visual fields

• Highly sensitive to large overall pattern and moving stimuli

Koniocellular Neurons

• Have small cell bodies

• Found throughout the retina

• Have several functions, and their axons terminate in many different places

Characteristics of Receptive Fields

• Cells of the lateral geniculate have a receptive field similar to those of ganglion cells:

  • An excitatory or inhibitory central portion and a surrounding ring of the opposite effect

The Primary Visual Cortex (1 of 2)

• The primary visual cortex (area V1) receives information from the lateral geniculate nucleus and is the area responsible for the first stage of visual processing

• Some people with damage to V1 show blindsight: an ability to respond to visual stimuli that they report not seeing

• Hubel and Weisel (1959, 1998) distinguished various types of cells in the visual cortex

  • Simple cells

  • Complex cells

  • End-stopped/hypercomplex cells

  • Beyond this level of analysis it is not possible to link specific cortical cells or regions to particular visual stimuli

Simple Cells

• Fixed excitatory and inhibitory zones

• The more light that shines in the excitatory zone, the more the cell responds

• The more in the inhibitory zone, the less the cell responds

• Bar-shaped or edge-shaped receptive fields with vertical and horizontal orientations outnumbering diagonal ones

Complex Cells

• Located in either V1or V2

• Have large receptive field that can not be mapped into fixed excitatory or inhibitory zones

• Responds to a pattern of light in a particular orientation and most strongly to a moving stimulus

Simple and Complex Receptive Fields (p. 166)

The Receptive Field of a Complex Cell (p. 168)

End-Stopped or Hypercomplex Cells

• Similar to complex cells but with a strong inhibitory area at one end of its bar shaped receptive field

• Respond to a bar-shaped pattern of light anywhere in its large receptive field, provided the bar does not extend beyond a certain point

The Receptive Field of an End-Stopped Cell (p. 167)

The cell responds to a bar in a particular orientation (in this case horizontal) anywhere in its receptive field, provided the bar does not extend into a strongly inhibitory area.

Cells in the Primary Visual Cortex (p. 168)

Columnar Organization of the Visual Cortex

• In the visual cortex, cells are grouped together in columns perpendicular to the surface

• Cells within a given column process similar information

  • Respond either mostly to the right or left eye, or respond to both eyes equally

  • Do not consistently fire at the same time

When an electrode passes perpendicular to the surface of the cortex (first part of line A), it encounters a sequence of neurons responsive to the same orientation of a stimulus. (The colored lines show the preferred stimulus orientation for each cell.) When an electrode passes across columns (B, or second part of A), it encounters neurons responsive to different orientations. Column borders are drawn hereto illustrate the point; no such borders are visible in the real cortex.

Are Visual Cortex Cells Feature Detectors? (p. 169)

• Cells in the visual cortex may be feature detectors, neurons whose response indicate the presence of a particular feature/stimuli

• Prolonged exposure to a given visual feature decreases sensitivity to that feature

Development of the Visual Cortex (p. 169)

• Animal studies have greatly contributed to the understanding of the development of vision

• Early lack of stimulation of one eye: leads to synapses in the visual cortex becoming gradually unresponsive to input from that eye

• Early lack of stimulation of both eyes: cortical responses become sluggish but do not cause blindness

Critical Periods in Development

• Sensitive/critical periods are periods of time during the lifespan when experiences have a particularly strong/enduring effect

  • Ends with the onset of chemicals that inhibit axonal sprouting

  • Changes that occur during critical period require both excitation and inhibition of some neurons

• Cortical plasticity is greatest in early life, but never ends

Stereoscopic Depth Perception

• A method of perceiving distance in which the brain compares slightly different inputs from the two eyes

  • Relies on retinal disparity or the discrepancy between what the left and the right eye sees

  • Demo: With both eyes open, hold a pencil or pen upright and line it up with an object at least 10" away. Now, with your other hand cover one eye and then the other. What happened? You should have seen the object jump when one eye was covered. Why? Ordinarily, one eye is dominant. When you covered your dominant eye you were able to see the image from the non-dominant eye. Your dominant eye is the one that DID NOT jump compared to your two-eyed original view.

  • The ability of cortical neurons to adjust their connections to detect retinal disparity is shaped through experience

Strabismus

• A condition in which the eyes do not point in the same direction

  • Usually develops in childhood

  • Also known as “lazy eye”

• If two eyes carry unrelated messages, cortical cell strengthens connections with only one eye

• Development of stereoscopic depth perception is impaired

Strabismus Examples (p. 170)

Early Exposure to a Limited Array of Patterns (p. 170)

• Leads to nearly all of the visual cortex cells becoming responsive to only that pattern

• Astigmatism refers to a blurring of vision for lines in one direction caused by an asymmetric curvature of the eyes

  • 70 percent of infants have astigmatism

Restricting Early Visual Experience (p. 171)

For a few hours a day, the kitten wears goggles that show just one stimulus, such as horizontal stripes or diagonal stripes. For the rest of the day, the kitten stays with its mother in a dark room without the mask.

An Informal Test for Astigmatism (p. 171)

Do the lines in one direction look darker or sharper than the other lines do? If you wear corrective lenses, try this demonstration both with and without your lenses. (My implanted lenses correct for a great deal of astigmatism in each eye and each eye has its own, unique pattern.)

Impaired Infant Vision and Long-Term Consequences (p. 171)

• Study of people born with cataracts but had them removed indicate that vision can be restored gradually, but problems persist

  • Difficulty in recognizing objects

  • Unable to tell that components are part of a whole

  • Best prognosis is for children whose vision problems are corrected early in life

Module 5.3 Specialized Visual Processes (p. 176)

• Neuroscientists have identified many different brain areas that contribute to vision in different ways

• One part of your brain sees its shape, another sees color, another detects location, and another perceives movement

• The secondary visual cortex (area V2) receives information from area V1, processes information further, and sends it to other areas

• Information is transferred between area V1 and V2 in a reciprocal nature

The Ventral and Dorsal Streams

• The ventral stream refers to the path that goes through temporal cortex

  • The “what” path

  • Specialized for identifying and recognizing objects

• The dorsal stream refers to the visual path in the parietal cortex

  • The “how” path

  • Important for visually guided movements

• Normal behavior makes use of both pathways in collaboration

• Damaging either stream will produce different deficits

  • Ventral stream damage: can see where objects are but cannot identify them

  • Dorsal stream damage: can identify objects but not know where they are

Shape Perception (p. 177)

• Receptive fields become larger and more specialized as visual information goes from simple cells to the complex cells and then to other brain areas

• The inferior temporal cortex contains cells that respond selectively to complex shapes but are insensitive to distinctions that are critical to other cells

• Cells in this cortex respond to identifiable objects

Transformations of a Drawing (p. 177)

In the inferior temporal cortex, cells that respond strongly to the original respond about the same to the contrast reversal and mirror image but not to the figure–ground reversal. Note that the figure–ground reversal resembles the original in terms of the pattern of light and darkness, but it is not perceived as the same object.

Visual Agnosia

• The inability to recognize objects despite satisfactory vision

  • Caused by damage to the pattern pathway usually in the temporal cortex

Face Recognition—The Fusiform Gyrus (p. 178)

During facial recognition many cells in fusiform gyrus are active.

Recognizing Faces

• Face recognition occurs relatively soon after birth

  • Newborns show strong preference for a right-side-up face and support idea of a built-in face recognition system

• Facial recognition continues to develop gradually into adolescence

Amount of Time Infants Spend Looking at Patterns (p. 179)

How Infants Divided Their Attention Between Faces (p. 180)

A right-side-up face drew more attention than an upside-down one, regardless of whether the faces were realistic (left pair) or distorted (central pair). They divided their attention about equally between two right-side-up faces (right pair), even though one was realistic and the other was distorted.

Prosopagnosia

• The impaired ability to recognize faces

  • Occurs after damage to the fusiform gyrus of the inferior temporal cortex

  • The fusiform gyrus responds much more strongly to faces than anything else

Motion Perception (p. 180)

• Involves a variety of brain areas in all four lobes of the cerebral cortex

• The middle-temporal cortex (MT/V5) responds to a stimulus moving in a particular direction

• Cells in the dorsal part of the medial superior temporal cortex (MST) respond to expansion, contraction, or rotation of a visual stimulus

• Both receive input from the magnocellular path; color-insensitive

• Demo [find]

Stimuli That Excite the Dorsal Part of Area MST (p. 181)

Cells here respond if a whole scene expands, contracts, or rotates. That is, they respond if the observer moves forward or backward or tilts his or her head.

Motion Blindness

• The inability to determine the direction, speed and whether objects are moving

  • Likely caused by damage in area MT

• Some people are blind except for the ability to detect which direction something is moving

• Area MT probably gets some visual input despite significant damage to area V1

Saccades

• Several mechanisms prevent confusion or blurring of images during eye movements

  • Saccades are a decrease in the activity of the visual cortex during quick eye movements

  • Neural activity and blood flow decrease 75 milliseconds before and during eye movements

Illusions (not in text)

• Muller-Lyer and Muller-Lyer (solution) Look at both. This is a powerful illusion.
• Devil's Pitchfork: Another powerful illusion. Count the tines on the left and their base on the right.
• Reversible Illusions
  • Old Woman and Young Woman: These two are reversible illusions. It's impossible to see them both at the same time.

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