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Vision 1

TRANSDUCTION ¢ Senses are transducers

� Change one form of energy into another � Light, sound, pressure, etc. into – What?

¢ Action potentials! ¢ Sensory codes

� Frequency code – encodes information about intensity

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VISION: PHYSICAL STIMULUS ¢ Stimulus = Electromagnetic Radiation (Light) ¢ Human sensitivity = 380-760 nm wavelength ¢ Length of one cycle = quality (hue) ¢ Amplitude of light wave = intensity (brightness)

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OUTLINE

¢ The Eye ¢ Color Vision ¢ Visual Coding ¢ Vision and the Brain ¢ Blindsight

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•  bends light inward ………………. •  controls how much light goes in ... •  focuses image …………………… •  contains sensory receptors ………. •  sensory receptors ………………... •  has largest concentration of cones..

•  Cornea •  Iris •  Lens •  Retina •  Rods and cones •  Fovea

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Photochemical reaction: Light causes release of neurotransmitters

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¢ Light travels: � Bipolar cells

and horizontal cells

� Ganglion cells and amacrine cells

� Optic nerve to the brain

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RETINAL RECEPTORS ¢ Visual receptors (rods/cones) à bipolar cells à ganglion cells � The axons of ganglion cells join one

another to form the optic nerve that travels to the brain.

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BLIND SPOT 9

OUTLINE

¢ The Eye ¢ Color Vision ¢ Visual Coding ¢ Vision and the Brain ¢ Blindsight

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VISUAL CODING AND THE RETINAL RECEPTORS ¢  The vertebrate retina consist of two

kind of receptors: 1.  Rods - most abundant in the

periphery of the eye and respond to faint light. (120 million per retina)

2.  Cones - most abundant in and around the fovea. (6 million per retina) ¢  Essential for color vision & more

useful in bright light.

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WAVELENGTH SENSITIVITY 1 Rod photopigment & 3 Cone photopigments

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TRICHROMATIC THEORY

¢ Color perception occurs through the relative rates of response by three kinds of cones. � Short wavelength, medium-wavelength,

long-wavelength. ¢ Each cone is maximally sensitive to a

different set of wavelengths.

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TRICHROMATIC THEORY

¢ 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 and thus does not change the perception of the color itself.

¢ Incomplete theory of color vision. � Example: negative color afterimage

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+ +

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VISUAL CODING AND THE RETINAL RECEPTORS ¢ The opponent-process theory - we

perceive color in terms of paired opposites.

¢ The brain has a mechanism that perceives color on a continuum: �  from red to green �  another from yellow to blue.

¢ Possible mechanism: �  bipolar cells are excited by one set of

wavelengths and inhibited by another.

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18 ¢ Short wavelength

light (blue) � Excites bipolar

AND inhibits it through horizontal cell

� Excitation is stronger and wins

¢ Red, Green, Yellow �  Inhibit bipolar cell

through horizontal cell

� Yellow inhibits the most

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¢  Suppose a bipolar cell receives excitatory input from medium-wavelength cones and inhibitory input from all three kinds of cones.

¢ When it is highly excited, what color would one see?

¢ When it is inhibited, what color perception would result?

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OUTLINE

¢ The Eye ¢ Color Vision ¢ Visual Coding ¢ Vision and the Brain ¢ Blindsight

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FREQUENCY DETERMINES INTENSITY 21

Hubel and Wiesel: Individual neurons respond to very specific stimuli

Response to a 30-degree angled bar

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OUTLINE

¢ The Eye ¢ Color Vision ¢ Visual Coding ¢ Vision and the Brain ¢ Blindsight

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THE NEURAL BASIS OF VISUAL PERCEPTION ¢ The lateral geniculate nucleus is 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.

� Cortex and thalamus feed information back and forth to each other.

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THE NEURAL BASIS OF VISUAL PERCEPTION

¢ The ventral stream refers to the most magnocellular visual paths in the temporal cortex. � Specialized for identifying and recognizing

objects. ¢  The dorsal stream refers to the visual

path in the parietal cortex. � Helps the motor system to find objects and

move towards them.

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Is there a “grandmother” neuron in the brain?

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OUTLINE

¢ The Eye ¢ Color Vision ¢ Visual Coding ¢ Vision and the Brain ¢ Blindsight

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BLINDSIGHT ¢ Weiskrantz et al., 1974 ¢ Patient D.B. had severe

headaches ¢ Surgical removal of

arteriovenous malformation in occipital lobe

¢ Produced scotoma: partial loss of vision in a part of the visual field

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CONNECTIONS OF VISUAL SYSTEM

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D.B’S SCOTOMA

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VISION TEST FOR D.B. ¢ DB denied vision in

scotoma ¢ Verified by standard

vision tests ¢ DB instructed to “guess” and point to target

¢ Astonished! ¢ Repeatedly said he

saw “nothing at all”

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BLINDSIGHT ¢ Is there an explanation? ¢ Is there a separate conscious

and unconscious visual pathway?

¢ Thalamic (subcortical) visual areas � Lateral geniculate � Superior colliculus

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CONNECTIONS OF VISUAL SYSTEM

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THOUGHT EXPERIMENT ¢ You are a neurologist ¢ You have a patient come in who says that they cannot see.

¢ What tests would you do? ¢ What are all of the possible causes?

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IN CLASS ACTIVITY: VISION PROBLEMS

¢ Light not getting into eye or focusing (problem with cornea, pupil, iris, lens)

¢ Problem detecting light rays and transferring to action potentials (damaged rods and cones/retina/fovea, bipolar, ganglion cells)

¢ Problem transmitting that information to the brain (damaged optic nerve, damaged optic chiasm, damaged primary visual cortex [blindsight])

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FOR NEXT TIME… ¢  Read Ch. 7 Other Senses ¢  Data and Lab Report Intro & Methods due Tuesday at 8am

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