1 Vision 2 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 1
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) 3 OUTLINE The Eye Color Vision Visual Coding Vision and the Brain Blindsight 4 2
5 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 Photochemical reaction: Light causes release of 6 neurotransmitters 3
7 Light travels: Bipolar cells and horizontal cells Ganglion cells and amacrine cells Optic nerve to the brain 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. 8 4
9 BLIND SPOT OUTLINE 10 The Eye Color Vision Visual Coding Vision and the Brain Blindsight 5
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. 11 12 6
WAVELENGTH SENSITIVITY 1 Rod photopigment & 3 Cone photopigments 13 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. 14 7
15 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 16 + + 8
VISUAL CODING AND THE RETINAL RECEPTORS 17 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. 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 18 9
Suppose a bipolar cell receives excitatory input from mediumwavelength 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? 19 OUTLINE 20 The Eye Color Vision Visual Coding Vision and the Brain Blindsight 10
FREQUENCY DETERMINES INTENSITY 21 Hubel and Wiesel: Individual neurons respond to very specific stimuli 22 Response to a 30-degree angled bar 11
OUTLINE 23 The Eye Color Vision Visual Coding Vision and the Brain Blindsight 24 12
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27 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. 28 14
THE NEURAL BASIS OF VISUAL PERCEPTION 29 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. 30 15
Is there a grandmother neuron in the brain? 31 OUTLINE 32 The Eye Color Vision Visual Coding Vision and the Brain Blindsight 16
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 33 CONNECTIONS OF VISUAL SYSTEM 34 17
35 D.B S SCOTOMA 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 36 18
BLINDSIGHT Is there an explanation? Is there a separate conscious and unconscious visual pathway? Thalamic (subcortical) visual areas Lateral geniculate Superior colliculus 37 CONNECTIONS OF VISUAL SYSTEM 38 19
39 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? 40 20
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]) 41 FOR NEXT TIME Read Ch. 7 Other Senses Data and Lab Report Intro & Methods due Tuesday at 8am 42 21