Study Guide for PSB2000 06 Exam 2 Vision Audition Mechanical Senses Chemical Senses Vision What is the law of specific nerve energies o Which neurons respond the amount of response and the timing of response influence what we perceive o Activity by a particular nerve always conveys the same type of information to the brain o Ex Impulses in one neuron indicate light impulses in another indicate sound o The brain does not duplicate what we see Know the anatomy of the eye What are the functions of the labeled parts in the image included here o o o o Pupil the hole in the center if the eye where light passes through Size dilated or constricted determines how much light can pass Iris the circular band of muscles that controls the size of the pupil therefore controls light entry Dilates with dilator and constricts with sphincter Lens bends light to focus image on the retina passing through the eye It does this by bending itself using ciliary muscles The cornea also focuses light Fovea point of central focus most detailed vision happens here o Retina rods and cones Tissue with nerve cells and photoceptors Like an outgrowth of the brain same embryonic tissue o Blind Spot Hole in retina where the optic nerve exits the eye No photoreceptors here o Optic Nerve Cranial Nerve CN II Sends sensory info from photoreceptors to the brain o Macula the area that holds to fovea and acts as a sun blocker from harmful light When light enters through the pupil and strikes the retinal neurons in what order In other words know the retinal circuitry pictured below and how photons stimulate action potentials Receptors Bipolar Cells Ganglion Cells First to visual receptrors in the back of the eye Those sent messages to neurons called bipolar cells located closer to the center of the eye o Bipolar cells send messages to ganglion cells that are even closer to the center of the eye o The axons of ganglion cells join one another to form the optic nerve that travels to the brain Receptors rods and cones Outer Segment Inner Segment Bipolar Cells Optic Nerve Fibers o o Horizontal cells Amacrine cells Ganglion Cells How protons stimulate action potentials Without light receptors inhibit bipolar cells Light hyperpolarizes receptors Receptors stop NT release Bipolar cells are disinhibited Bipolar cells stimulate ganglion cells o By releasing an excitory NT AP down optic nerve We talked a lot about rods and cones and their difference in foveal and peripheral vision table 6 1 Also understand the Acuity Sensitivity Tradeoff Rods are abundant in the periphery of the retina and respond to faint light but are not useful in daylight because bright light bleaches them See black white bad detail but good sensitivity Cones are abundant in and near the fovea are less active in dim light more useful in bright light and essential for color vision good detail but bad sensitivity In the fovea each cone has its own line to the brain In the periphery mostly rods each receptor shares a line with tens or hundreds of others Color Vision You should know that there are photopigments comprised of retinal and opsins in rods and cones that respond to different wavelengths The exact structure of opsin molecule determines maximal sensitivity to wavelengths of light Long wavelength red light medium wavelength green light short wavelength blue light Photopigments chemicals contained by rods and cones that release energy when struck by light Consist of 11 cis retinal bound to proteins called opsins What is the Trichromatic Theory of color vision What is the Retinex Theory Opponent Process Theory Trichromatic Theory We perceive color through the relative rates of response by three kinds of cones each one maximally sensitive to a different set of wavelengths Determined by the particular opsin within a photoreceptor Retina contains equal red and green but smaller number of blue Retinex Theory The cortex compares information from various parts of the retina to determine the brightness and color for each area Better explains color constancy 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 yellow to blue and another from white to black A possible mechanism is that bipolar cells are excited by one set of wavelengths and inhibited by another Understand color and contrast constancy lateral inhibition Color constancy is the ability to recognize colors despite changes in lighting Your brain compares the color of one object with the color of another in effect subtracting the changes in wavelength in both Better explained by the Retinex theory Lateral inhibition or contrast constancy is 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 excitory and inhibitory messages it receives It s the retina s way of sharpening contrasts to emphasize the borders of objects What causes color vision deficiency For genetic reasons genes on the X chromosome some people lack one or two of the types of cones Most common is Red Green which results from long and medium having the same photopigment Protanopia deutoranapia red green color blindness red cone is filled with green opsin protanopia or green cone is filled with red opsin deuteranopia Tritanopia blue color blindness Retina lacks blue cone The receptive field of a receptor is the point in space from which light strikes the cell The receptive field of the ganglion cell is the combined receptive fields of those receptors The ganglion axons exit the as the optic nerve and then go where In other words what are brain regions involved in vision and the visual pathways through to the cortex Axons from the ganglion cells form the optic nerve which proceeds to the optic chiasm Half of the axons from each eye cross to the other side of the brain Most of the axons go to the lateral geniculate nucleus of the thalamus LGN and a few to the superior colliculus Most visual information from the LGN of the thalamus goes to the primary visual cortex in the occipital cortex also known as area V1 Although we do not know whether conscious visual perception occurs in area V1 area V1 is apparently necessary for it The cortex and thalamus feed information back and forth to each other There are correlations between cones rods and magno parvocellular cells Review PPT 10 slides 11 12 for the overview of the ganglion cells Parvocellular
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