Vision - Florida Atlantic University视觉-佛罗里达大西洋大学.ppt
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1、6: Vision,Biological Bases of Behavior,Sensory Systems,The brain detects events in the external environment and directs the contractions of the muscles Afferent neurons carry sensory messages to brain Efferent neurons carry motor messages to the muscles Stimulus: any energy capable of exciting a rec
2、eptor Mechanical Chemical Thermal Photonic Sensory energies are measurable (unlike ESP),6.2,Sensory Receptors,Receptors are specialized nerve cells that transduce energy into neural signals Receptors lack axons, form synapses with dendrites of other sensory neurons Receptors are “mode” specific “Law
3、 of Specific Nerve Energies”: sensory messages are carried on separate channels to different areas of the brain Receptors detect a small range of energy levels Eye: 400-700 nM Ear: 20-20,000 Hz Taste buds: specific chemicals,6.3,Visual Systems,The function of a visual system is to detect electromagn
4、etic radiation (EMR) emitted by objects Humans can detect light with a wavelength between 400-700 nM Perceived color (hue) is related to the wavelength of light Brightness is related to the intensity of the radiation Functions of vision Discriminate figure from background (food or rock?) Detect move
5、ment (predator/prey?) Detect color (adaptive value of color vision),6.4,Eye Details,An eye consists of: Aperture (pin hole, pit, or pupil) to admit light Lens that focuses light Photoreceptive elements (retina) that transduce the light stimulus,Source: http:/www.nei.nih.gov/nei/vision/vision2.htm,6.
6、5,Retina,Light passes through the pupil and is focused by the lens onto the retina at the back of the eye The retina consists of three layers of cells Ganglion cell layer Bipolar layer Photoreceptor layer: receptors in this layer transduce light The ganglion cell layer is the outermost layer and the
7、 photoreceptor layer is the innermost layer In order to reach the photoreceptor layer, light actually passes through the outer two layers of the retina,6.6,Rods and Cones,Two types of photoreceptors are located within the retina Rods: 120 million Light sensitive (not color) Found in periphery of ret
8、ina Low activation threshold Cones: 6 million Are color sensitive Found mostly in fovea The outer segments (O.S.) of a rod or a cone contain different photopigments that react to light,Source: http:/insight.med.utah.edu/Webvision /imageswv/rodcoEM.jpeg,6.7,Retinal Circuitry,6.8,Adapted from Dowling,
9、 J.E., and Boycott, B.B. Proceedings of the Royal Society of London, B., 1966, 166, 80-111.,Primary Visual Pathway,Information from each visual field crosses over at the optic chiasm and projects to the opposite side of the primary visual cortex,6.9,Visual Pathways: LGN,Signals from the ganglion cel
10、ls of the retina are sent to the thalamus via the optic nerve/tract The dorsal lateral thalamic nucleus (LGN) has 6 layers Each layer receives input from only one eye The inner 2 layers contain large cells (magnocellular) The outer 4 layers contain small cells (parvocellular) Koniocellular sublayers
11、 are ventral to each of the 6 layers Neurons of the LGN project through the optic radiations to a region of occipital cortex termed primary visual cortex (striate),6.10,Visual Pathways: LGN,6.11,Overview of the Visual Cortex,Human visual areas,6.12,Visual areas occipital lobes, posterior view,V2,V3,
12、V3a,V4v,MT+,left,right,Organization of V1,80,Upper VM,Lower VM,HM,6.14,Visual Transduction,Photopigments are located in the membrane of the outer segment of rods and cones Each pigment consists of an opsin (a protein) and retinal (a lipid) In the dark, membrane NA+ channels are open - glutamate is r
13、eleased which depolarizes the membrane Light splits the opsin and retinal apart- Activates transducin (G protein)- Activates phosphodiesterase- Reduces cGMP - closes NA+ channels The net effect of light is to hyperpolarize the retinal receptor and reduce the release of glutamate,6.15,Retinal Respons
14、es to Light,6.16,Receptive Fields,Microelectrodes can be used to record the firing activity of a single sensory neuron Sensory neurons have a background rate of firing (impulses/sec) This rate of firing can increase or decrease in response to a stimulus Receptive Field (RF): Those attributes of a st
15、imulus that will alter the firing rate of sensory cell The general pattern of the RF can be recorded at each level of a sensory system (e.g. from a peripheral sensory receptor, the thalamus, or the cortex) RF analyses can indicate the manner in which sensory information converges from level to level
16、,6.17,Ganglion Cell Receptive Fields,Ganglion cells in the retinal periphery receive input from many photoreceptors while ganglion cells in the fovea receive input from one photoreceptor The receptive fields of ganglion cells are circular with a center field and a surround field “ON-Cell”: Cell exhi
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