© Cengage Learning 2016 Chapter 5 Vision. © Cengage Learning 2016 Sensation and Perception...

© Cengage Learning 2016 © Cengage Learning 2016

Chapter 5

Vision

© Cengage Learning 2016

Sensation and Perception

• Sensation– Ability to detect and encode information

– Raw data

– Each sense has specialized receptors sensitive to a particular kind of energy

Kolb & Whishaw, An Introduction to Brain and Behavior, Fourth

Edition - Chapter 9


Sensation and Perception

• Perception– Ability to organize and interpret information

– Subjective experience of raw data

– Law of specific nerve energies• activity by a particular nerve always conveys the

same type of information to the brain

– Perception not always reality


Perceptual Illusions


Edition - Chapter 9



Edition - Chapter 9

Vision

• Vision is our primary sensory experience

• More of the human brain is dedicated to vision than to any other sense



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Structure of the Eye

• Cornea – Clear outer covering

• Iris – Opens and closes to allow more or less light in– The hole in the iris is called the pupil.

• Lens– Focuses light– Bends to accommodate near and far objects

• Retina– Where light energy initiates neural activity


Cross Section of a Vertebrate Eye



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The Basics: Visible Light and the Structure of the Eye

• Myopia (nearsightedness)– Inability to bring distant objects into clear focus

– Focal point of light falls short of the retina

• Hyperopia (farsightedness)– Inability to focus on near objects

– Focal point of light falls beyond the retina

• Presbyopia– Common form of hyperopia seen in older

adults



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The Vertebrate Retina


Bipolar Cells

• Get info from visual receptors at the back of the eye

• send messages to ganglion cells


Amacrine Cells

• Get info from bipolar cells

• Control the ability of the ganglion cells to respond to specific aspects of visual stimuli


The Optic Nerve

• Consists of the axons of ganglion cells

• exits through the back of the eye and travel to the brain

• Blind spot (optic disc)

– Region of the retina where there are no photoreceptors

– Activity: Find Your Blind Spot!


Visual Path Within the Eye



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Retina of the Retina

• Retina– Light-sensitive surface at the back of the eye– consists of neurons and photoreceptor cells

• Convert light to action potentials• Discriminate wavelengths so we see colors

–Which is an example of a sensation? Perception?

– Images on retina is upside down and backward• Brain makes the adjustment


Our Amazing Brain

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• S1M1L4RLY, Y0UR M1ND 15 R34D1NG 7H15 4U70M471C4LLY W17H0U7 3V3N 7H1NK1NG 4B0U7 17.


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• Fovea– “pit”– Region at the center of the retina – Receptors densely packed

• Lots of cones–Each one has direct line to brain–Allows us to register exact location of

input• Clearest vision

– Needed for detail• Reading, driving


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Central Focus


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• Periphery of the retina– Less detailed vision

– Doesn’t need as much light as fovea

– Fovea = visual acuity

– Periphery = see in dim light


Acuity Across the VisualField


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Convergence of Input onto Bipolar Cells


The Arrangement of Visual Receptors

• Highly adaptive– How Animals See the World (3 min.)

https://www.youtube.com/watch?v=XbZ9xJUyIWY


Rods and Cones


Rods and Cones

• Retina has two kinds of receptors

– Rods:

• periphery of the eye

• 120 million per retina

• Sensitive to low levels of light

• night vision

• Each ganglion cell excited by many rods


– Cones• fovea only

• 6 million per retina

• responsive to bright light

• color vision and high visual acuity

• Each ganglion cell excited by a single cone

Rods and Cones


Rods and Cones

• Rods and 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

• Individual differences– Athletes and aim


Color Vision

• Visible light is a 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


Color Vision

• Why do we see in color?– Evolutionary advantage

• Detect ripe fruit

• How do we see color?– Light waves stimulate cones

– Red, green, blue


Edition - Chapter 9



Edition - Chapter 9

Color Vision

• Three Types of Cone Pigments– Most responsive to:

• 419 nm (“blue” or short wavelength)

• 531 nm (“green” or middle wavelength)

• 559 nm (“red” or long wavelength)

– There are about equal numbers of red and green cones, but fewer blue cones.


Distribution of Cones in Two Human Retinas


Color Vision

• Trichromatic Theory (1800’s)– Color vision based on three primary colors:

red, green, and blue• “RGB let’s us see!”

– Color is determined by the responses of the different cone types

– Each cone responds to some wavelengths more than others

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



Edition - Chapter 9


Color Vision


Edition - Chapter 9


Color Vision

• Trichromatic Theory

– Can explain different types of color blindness

• Examples of color-deficient vision

• 1 in 20 males, 1 in 400 females– Genes for pigments on X chromosome

– Limitation:

• Four basic colors: red, green, yellow, and blue

• Cannot explain afterimages

–Red-green; blue-yellowKolb & Whishaw, An Introduction to Brain and Behavior, Fourth

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http://www.vischeck.com/examples/


Demonstrating Afterimages


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Edition - Chapter 9

Color Vision

• Opponent-Process Theory

– Ewald Hering

– Perceive colors in terms of opposites

• Red versus green, blue versus yellow, black vs. white

– Opponent processing occurs in bipolar cells

• Cells excited by one set of wavelengths and inhibited by another

– Ex: some neurons are “turned on” by red and “turned off” by “green”


Limitations of Color Vision Theories

• Both theories have limitations– Can’t explain color constancy

• ability to recognize color stays same despite changes in lighting

• Retinex theory – Cortex compares info from different parts of

retina to determine brightness, color


The Dress


• Why do people see different colors?

• Two explanations, one paragraph each, of why people see different colors.

• One paragraph discussing explanation is most convincing to you and why.

• Please cite sources.

• 15 points possible

The Dress



Edition - Chapter 9

[Insert Fig. 9-10]


Processing of Visual Information

• 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



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Further Processing

Retinal Ganglion Cells (RGC)- Magnocellular cell (M-cell)

• Magno = large• Receives input from rods• Sensitive to light, pattern and moving stimuli

– Parvocellular cell (P-cell)• Parvo = small• Receives input from cones• Sensitive to color, fine details


The Primary Visual Cortex

• Primary visual cortex (area V1)– receives information from the LGN

– first stage of visual processing

• Damage to V1 – blindsight


5.3 Parallel Processing in the Visual Cortex

• At least 80 brain areas that contribute to vision in different ways.– Shape, color, movement, location



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Dorsal and Ventral Visual Paths

• Dorsal Visual Stream

– Pathway from occipital lobe to the parietal lobe

– The “how” pathway

• Responds to image movement

• Tell us how to act toward objects

–Ex: reaching to catch a ball


Dorsal and Ventral Visual Streams

• Ventral Visual Stream

– Pathway from occipital lobe to the temporal lobe

– The “what” pathway

• Recognition and identification

–Ex: faces


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Edition - Chapter 9



Edition - Chapter 9

• Agnosia = not knowing

• Visual-Form Agnosia

– Inability to recognize objects or drawings of objects despite having vision

– Video clip (5:00 minutes)

– Cannot recognize objects, but can:• Copy objects and draw objects from memory

• Correctly shape hands when grasping for objects, despite not recognizing those objects

Injury to the “What” Pathway

https://www.youtube.com/watch?v=ze8VVtBgK7A



Edition - Chapter 9


Injury to the “What” Pathway

• Color Agnosia (achromatopsia)– Inability to recognize colors

• Face Agnosia (prosopagnosia)– Inability to recognize faces

– Damage to fusiform gyrus

– 60 Minutes segment (12:00 minutes)

• Capras delusion– Secrets of the Mind (start at 7:30)


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https://search.yahoo.com/yhs/search?p=60+minutes+prosopagnosia&ei=UTF-8&hspart=mozilla&hsimp=yhs-002


Face Recognition – The Fusiform Gyrus



Edition - Chapter 9

Injury to the Pathway Leading to the Cortex

• Monocular Blindness– Destruction of the retina or optic nerve of one eye

– produces loss of sight in that eye

• Homonymous Hemianopia– Blindness of an entire left or right visual field

– Damage to LGN, optic tract or visual cortex

– Related to visual neglect (start at 3:30)

https://www.youtube.com/watch?v=On7jttGB7pw


Injury to the Pathway Leading to the Cortex

• Quadrantanopia– Blindness of one quadrant of the visual field

• Scotoma– Small blind spot in the visual field

– Caused by a small lesion in visual cortex

– Eyes are always moving, brain fills in gap


Edition - Chapter 9



Edition - Chapter 9

© Cengage Learning 2016 Chapter 5 Vision. © Cengage Learning 2016 Sensation and Perception...

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