Coding of Visual Information in the Retina

Transcription

1 Coding of Visual Information in the Retina Coding of Light and ddark Coding of Color Le Parade, by Seurat 48 Copyright Allyn & Bacon

2 A quick comparison: Characteristic Foveal Vision Peripheral Vision Receptors Cones in the fovea Proportion of rods itself; cones and rods increases toward the mix in the surrounding periphery; the extreme area. periphery has only rods. Convergence of receptors Brightness sensitivity One or a few receptors send their input to each postsynaptic cell Useful for distinguishing g among bright lights; responds poorly to faint lights. Increasing numbers of receptors send input to each postsynaptic cell. Responds well to faint lights; less useful for making distinctions in bright light. Color vision Good (many cones) Poor (few cones) 50 Color discrimination problem Problem: how does one cell code for two types of information? A neuron can only vary its frequency of action potentials. If the cone s response indicates brightness, then it cannot signal for color. 51 2

3 The solution No single neuron can simultaneously indicate brightness and color Our perceptions must depend on patterns of responses by a number of different neurons. 52 Trichromatic Theory AKA: Young-Helmholtz Theory Thomas Young g( ( ) among the first to recognize that color required a biological explanation. He proposed that we perceive color by comparing the responses of some small number of receptors, each of which is sensitive to a different part of the range of visible wavelengths. 53 3

4 Hermann von Helmholtz Modified Young s theory We perceive color through the relative rates of response by three kinds of cones, each kind maximally sensitive to a different set of wavelengths. 54 How did he know that? Psychophysical observations He found that people could match any color by mixing appropriate amounts of just three colors. 55 4

5 Trichomatic theory: We discriminate among wavelengths by the ratio of activity across the tree types of cones. 56 5

6 Cone distribution Long and medium cones are more abundant Easier to see tiny red, yellow, and green dots. Really difficult to see tiny blue dots you perceive them to be black. Cones are distributed randomly within the retina. 58 The Opponent-Process Theory Theory proposed by Ewald Hering (19 C) Negative color afterimage Replaces green and red with each other; yellow and blue with each other; & white and black with each other. We perceive color in terms of paired opposites 59 6

7 Copyright Allyn & Bacon 2007 Copyright Allyn & Bacon

8 Copyright Allyn & Bacon 2007 The Stimulus sensory receptors A specialized neuron that detects a particular category of physical events. sensory transduction The process by which sensory stimuli are transduced into slow, graded receptor potentials. receptor potential A slow, graded electrical potential produced by a receptor cell in response to a physical stimulus. 63 8

9 The Stimulus hue One of the perceptual dimensions of color; the dominant wavelength. brightness One of the perceptual dimensions of color; intensity. saturation One of the perceptual dimension of color; purity. 64 Anatomy of the Visual System vergence movement The cooperative movement of the eyes, which ensures that the image of an object falls on identical portions of both retinas. saccadic movement The rapid, jerky movements of the eyes used in scanning a visual scene. pursuit movement The movement that the eyes make to maintain an image of a moving object on the fovea. 65 9

10 Anatomy of the Visual System accommodation Changes in the thickness of the lens of the eye, accomplished by the ciliary muscles, that focus images of near or far objects on the retina. retina The neural tissue and photoreceptive cells located on the inner surface of the posterior portion of the eye. rod One of the receptor cells of the retina; sensitive to light of low intensity. cone One of the receptor cells of the retina; maximally sensitive to one of three different wavelengths of light and hence encodes color vision. 66 Anatomy of the Visual System photoreceptor One of the receptor cells of the retina; transduces photic energy into electrical potentials. fovea The region of the retina that mediates the most acute vision of birds and higher mammals. Color-sensitive cones constitute the only type of photoreceptor found in the fovea. optic disk The location of the exit point from the retina of the fibers of the ganglion cells that form the optic nerve; responsible for the blind spot

11 Anatomy of the Visual System bipolar cell A bipolar neuron located in the middle layer of the retina, conveying information from the photoreceptors to the ganglion cells. ganglion cell A neuron located in the retina that receives visual information from bipolar cells; its axon give rise to the optic nerve. horizontal cell A neuron in the retina that interconnects adjacent photoreceptors p and the outer processes of the bipolar cells. amacrine cell A neuron in the retina that interconnects adjacent ganglion cells and the inner processes of the bipolar cells. 68 Anatomy of the Visual System lamella A layer of membrane containing photopigments; found in the rods and cones of the retina. photopigment A protein dye bonded to retinal, substance derived from vitamin A; responsible for transduction of visual information. opsin A class of protein that, together with retinal, constitutes the photopigments. retinal A chemical synthesized from vitamin A; join with opsin to form a photopigment. rhodopsin A particular opsin found in rods

12 Anatomy of the Visual System dorsal lateral geniculate nucleus A group of cell bodies within the lateral geniculate body of the thalamus; receives inputs from the retina and projects to the primary visual cortex. magnocellular layer One of the inner two layers of neurons in the dorsal lateral geniculate nucleus; transmits information necessary for the perception of form, movement, depth, and small differences in brightness to the primary visual cortex. 70 Anatomy of the Visual System parvocellular layer One of the four outer layers of neurons in the dorsal lateral geniculate nucleus; transmits information necessary for perception of color and fine details to the primary visual cortex. koniocellular layer One of the sublayers of neurons in the dorsal lateral geniculate nucleus bound to each of the magnocellular and parvocelllar layers; transmits information from short-wavelength ( blue ) cones to the primary visual cortex

13 Anatomy of the Visual System calcarine fissure A horizontal fissure on the inner surface of the posterior cerebral cortex; the location of the primary visual cortex. striate cortex The primary visual cortex. optic chiasm A cross-shaped connection between the optic nerves, located below the base of the brain, just anterior to the pituitary gland. 72 Coding of Visual Information in the Retina receptive field The portion of the visual field in which the presentation of visual stimuli will produce an alteration in the firing rate of a particular neuron. protanopia An inherited form of defective color vision in which red and green hues are confused; red cones are filled with green cone opsin. deuteranopia An inherited form of defective color vision in which red and green hues are confused; green cones are filled with red cone opsin

14 Coding of Visual Information in the Retina tritanopia An inherited form of defective color vision in which hues with short wavelengths are confused; blue cones are either lacking or faulty. negative afterimage The image seen after a portion of the retina is exposed to an intense visual stimulus; consists of colors complementary to those of the physical stimulus. complimentary colors Colors that make white or gray when mixed together 74 Charles Angrand. Hay Ricks in Normandy 75 14