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Chapter 6

Chapter 6 Textbook - PSYC2410 Fall - Choleris

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University of Guelph
PSYC 2410
Elena Choleris

PSYC 2410 – Chapter 6 The visual system – how we see Understanding the visual system requires in integration of two types of research: 1) research that probes the visual system with sophisticated neuroanatomical, neurochemical, and neurophysiological techniques and 2) research that focuses on the assessment of what we see. 6.1 Light Enters the Eye and Reaches the Retina -9  Light: waves of electromagnetic energy that are between 280 and 760 nanometers (10 m) in length. o Wavelength: perception of colour o Intensity: brightness  The Pupil and the Lens o Irises: donut shaped bands of contractile tissue which give our eyes their characteristic colour o Light enters the eye through the pupil which adjusts in size to compromise between sensitivity and acuity. o When the pupils are constricted the image falling on each retina is sharper and has a greater depth of focus (a greater range of depths are simultaneously kept in focus on the retinas). o When illumination is low pupil dilates to let in more light, sacrificing acuity and depth of focus. o Lens: behind the pupil. Focuses incoming light on the retina. It is adjusted by the ciliary muscles. When we focus on a distance object the lens is flattened; on a near object it is cylindrical, its nature shape, which increases its ability to refract (bend) light to bring the object into sharper focus. o Accommodation: process of adjusting the configuration of the lenses to bring images into focus on the retina  Eye Position and Binocular Disparity o The movements of your eyes are coordinated so that each point in your visual word is projected to corresponding points on your two retinas. To do this, your eyes must converge. Convergence is greatest when inspecting things that are close. o Binocular disparity: the difference in the position of the same image on the two retinas is greater for close objects than for distant objects, therefore your visual system can use the degree of binocular disparity to construct one 3D perception from two 2D retinal images. 6.2 The Retina and Translation of Light into Neural Signals  Retina is composed of 5 layers of different types of neurons (which also have subtypes): they communicated both chemically via synapses and electrically via gap junctions. o Receptors: o Horizontal cells: specialized for lateral communication (across major channels of sensory input) o Bipolar cells o Amacrine cells: specialized for lateral communication (across major channels of sensory input) o Retinal ganglion cells  Two problems: o Light has to pass through to first 4 layers to get to the receptor layer o When all the retinal ganglion cell axon’s converge to form the optic nerve, they create a blind spot. o Fovea: an indentation, about 0.33 cm in diameter at the center of the retina, specialized for high-acuity vision. No o rods, only cones. Density of rods reaches a max. at 20 from the center of the fovea. o Completion: visual system uses information provided by the receptors around the blind spot to fill in the haps in your retinal images. Completion is a fundamental visual system function.  When you look at an object, visual system extracts key information about its edges and their location and conducts it to the cortex, where a perception of the entire object is created from that partial information. o Surface interpolation: process by which we perceive surfaces; the visual system extracts information about edges and from it infers the appearance of large surfaces.  Cone and Rod Vision o Duplexity theory: PSYC 2410 – Chapter 6  Photopic vision (cone mediated): predominates in good lighting and provides high-acuity coloured perceptions of the world. Only a few converge on each retinal ganglion cell to receive input from only a few cones. Less ambiguity, because less convergence, so the brain knows from what cells the signal came from.  Scotopic vision (rod mediated): more sensitive, predominates in dim illumination where there is not enough light to reliably excite the cones. Scotopic vision lacks detail and colour of photopic vision. The output of several hundred rods converge on a single retinal ganglion cell. The effects of dim light simultaneously stimulating many rods can summate to influence the firing of the retinal ganglion cell onto which the output of the stimulated rods converges, whereas the effects of the same dim light applied to a sheet of cones cannot summate to the same degree, and the retinal ganglion cells may not respond at all to the light. When a retinal ganglion cell receives input from hundreds of rods changes it firing, the brain has no way of knowing which portion of the rods contributed to the change, therefore less acuity. More rods in the nasal hemiretina (the half of each retina next to the nose) than in the temporal hemiretina (the half next to the temples).  Spectral Sensitivity o Lights of the same intensity but of different wavelengths can differ in brightness because our visual systems are of equally sensitive to all wavelengths in the visible spectrum. o Spectral sensitivity curve: a graph of the relative brightness of lights of the same intensity presented at different wavelengths o Animals and humans with both cones and rods have two spectral sensitivity curves:  Photopic spectral sensitivity curve: can be determined by having subjects judge the relative brightness of different wavelengths of light shone on the fovea. Under photopic conditions the VS is maximally sensitive to wavelengths of ~ 560 nm.  Scotopic spectral sensitivity curve: can be determined by asking subjects to judge the relative brightness of different wavelengths of light shone on the periphery of the retina at an intensity too low to activate the few peripheral cones that are located there. Under scotopic conditions the VS is maximally sensitive to 500 nm. o Purkinje effect: due to difference in photopic and scotopic spectral sensitivity, an interesting visual effect can be observed during the transition from photopic to scotopic. Before dusk: red and yellow flowers very bright, blue follows not bright. At night: blue more bright, red and yellow darker gray. o Eye Movement: the eyes continually scan the visual field and our visual perception at any instant is a summation of recent visual information. Due to this temporal intergration that the world doesn’t disappear when we blink. Even when we fix on an object, our eyes move continuously.  Fixational eye movements: tremor, drifts and saccades.  Visual transduction: The Conversion of Light to Neural Signals o Visual transduction: conversion of light to neural signals by the visual receptors o Rhodopsin: red pigment extracted from rods. When continuously exposed to intense light it becomes bleached and loses its ability to absorb light, but regains its redness and light-absorbing capacity when put in the day.  The degree to which rhodopsin absorbs lights of different wavelengths is related to the ability of humans and other animals with rods to detect the presence of different wavelengths of light under scotopic conditions.  Rhodopsin is a G protein-coupled receptor that responds to light rather than to neurotransmitter molecules. It initiates a cascade of intracellular chemical events when it is activated.  In the dark, sodium channels are partially open keeping the rods slightly depolarized allowing a steady flow of excitatory glutamate neurotransmitter molecules to emanate from them.  When bleached, the resulting cascade of intracellular chemical events closes the sodium channels, hyperpolarizes the rods and reduces the release of glutamate: signals transmitted through neural system inhibition. PSYC 2410 – Chapter 6 6.3 From Retina to Primary Visual Cortex Most thoroughly studied visual pathways are:  Retina-geniculate-striate pathways: conduct signals from each retina to the primary visual cortex (PVC) or striate cortex via the lateral geniculate nuclei of the thalamus. ~90% of axons of retinal ganglion cells become part of the retina- geniculate-striate pathways. No other sensory system has such a predominant pair (left and right) of pathways to the cortex. o All signals from the left visual field reach the right primary visual cortex, either ipsilaterally from the temporal hemiretina of the right eye or contralaterally (via the optic chiasm) from the nasal hemiretina of the left eye, and that the opposite is true of all signals from the right visual field. o Lateral geniculate nucleus has six layers and each layer of each nucleus receives input from all parts of the contralateral visual field of one eye. o Each lateral geniculate nucleus receives visual input only from the contralateral visual field; three layers receive input from one eye and three from the other. o Most of the lateral geniculate neurons that project to the primary visual cortex terminate in the lower part of cortical layer IV, producing a characteristic stripe or striation when viewed in cross section – striate cortex.  Retinotopic Organization o Retinotopic: each level of the system is organized like a map of the retina  Has a disproportionate representation of the fovea; although its small, a relatively large proportion of the MVC (about 25%) is dedicated to the analysis of its input.  The M and P Channels o At least 2 parallel channels of communication flow through each lateral geniculate nucleus:  One channels runs through the top 4 layers called parvocellular layers (P layers) composed of neurons with small cell bodies. Parvocellular layers are particularly responsive to colour, to fine pattern details, and to stationary or slowly moving objects. Majority of input from Cones.  Other channels runs through the bottom two layers are called the magnocellular layers (M layers) composed of neurons with large cell bodies. Magnocellular neurons are responsive to movement. Majority of input from Rods.  The magnocellular and parvocellular neurons project to different sites in the lower part of layer 4 of the striate cortex. In turn, these M and P portions of lower layer 4 project to different parts of visual cortex. 6.4 Seeing Edges Edges are the most informative features of any visual display because they define the extent and position of the various objects in it. Visual edge: the place where two different areas of a visual image meet. It is thus really the perception of a contrast between 2 adjacent areas of the visual field.  Lateral Inhibition and Contract Enhancement o Nonexistent stripes of brightness and darkness running adjacent to the edges are called Mach bands; they enhance the contrast at each edge and make the edge easier to see. o Contrast enhancement: every edge we look at is high-lighted for us by the contrast-enhancing mechanisms of our NS. Our perception of edges is better than the real thing. o Horseshoe crab eyes & studies:  Ideal for certain types of neurophysiological research, they are composed of very large receptors, called ommatidia each with its own large axon interconnected by a lateral neural network.  When single ommatidium is illuminated, it fires at a rate that is proportional to the intensity of the light striking it; more intense lights produce more firing. When a receptor fires, it inhibits its neighbors via the lateral neural network; this inhibition is called lateral inhibition because it spreads laterally across the array of receptors. The amount of lateral inhibition produced by a receptor is greatest wen the receptor is most intensely illuminated, the inhibition has its greatest effect on the receptor’s immediate neighbors. PSYC 2410 – Chapter 6  Receptors near the edge fire more intensely on the left side of the edge than the other receptors, and then the receptor on the right side of the edge fires less intensely than the other receptors on the right side of the edge that are farther from the edge.  Receptive Fields of Visual neurons o Hubel and Wiesel’s influential method for studying single neurons:  Tip of microelectrode is positioned near a single neuron in the part of the visual system that is under investigation. During testing, eye movements are blocked by paralyzing the eye muscles, and the images on a screen in front of the subject are focused sharply on the retina by an adjustable lens. Then must identify receptive field: the area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron.  The final step in the method is to record the responses of the neuron to various stimuli within its receptive field in order to characterize the types of stimuli that most influence its activity. Then the electrode is advanced slightly and the entire process of identifying and characterizing the receptive field properties is repeated for another neuron, etc. start with neurons near the receptors and gradually work up through higher and higher levels of the system in an effort to understand the increasing complexity of the neural responses at each level.  Receptive Fields: neurons of the Retina-Geniculate-Striate System o Hubel and Wiesel (1979) began their studies of visual system neurons by recording from the three levels of the retina-geniculate-striate system: first retinal ganglion cells, then lateral geniculate neurons and then striate neurons of lower layer 4, the terminus of the system. Little difference through the levels. Four commonalties:  At each level, the receptive fields in the foveal area of the retina were smaller than those at the periphery; this is consistent with the fact that the fovea mediates high acuity vision.  All the neurons had receptiv
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