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Chapter 6

Chapter 6- Visual System.docx

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Western University
Psychology 2221B
Derek Quinlan

Chapter 6: The Visual System What do We See?  Somehow a distorted and upside-down 2-D retinal image is transformed into the 3-D world we perceive  Two types of research needed to study vision o Research probing the components of the visual system o Research assessing what we see- sensation side how we detect stimuli and perception side is how are we going to perceive what comes out of that. - 1) Light Enters the Eye and Reaches the Retina  No species can see in the dark, but some are capable of seeing when there is little light  Light can be thought of as o Particles of energy (photons) o Waves of electromagnetic radiation  Humans see light between 380-760 nanometers  Wavelength – perception of colour  Intensity – perception of brightness The Pupil and the Lens  Light enters the eye through the pupil, whose size changes in response to changes in illumination  Sensitivity – the ability to see when light is dim  Acuity – the ability to see details  Lens – focuses light on the retina  Ciliary muscles alter the shape of the lens as needed  Accommodation – the process of adjusting the lens to bring images into focus Eye Position and Binocular Disparity  Convergence – eyes must turn slightly inward when objects are close  Binocular disparity – difference between the images on the two retinas  Both are greater when objects are close – provides brain with a 3-D image and distance information Interspecies Difference: 2) Retina and Translation of Light to Neural Signals  The retina is in a sense “inside-out” o Light passes through several cell layers before reaching its receptors  Vertical pathway – receptors > bipolar cells > retinal ganglion cells  Lateral communication o Horizontal cells o Amacrine cells o Figure 6.5: - Blind spot: no receptors where the info exits the eye o The visual system uses info from cell around the blind spot for completion, fillin in the blind spot - Fovea: High acuity area at center of the retina- rich in cones o Thinning of the ganglion cell layer reduces distortion due to cells between the pupil and retina Demonstration of your blind spot on pg. 138 of textbook: Cone and Rod Vision  Duplexity theory of vision – cones and rod mediate different kinds of vision  Cones – photopic (daytime) vision o High-acuity colour information in good lighting  Rods – scotopic (nighttime) vision o High-sensitivity, allowing for low-acuity vision in dim light, but lacks detail and colour information  More convergence in rod system, increasing sensitivity while decreasing acuity  Only cones are found at the fovea Spectral Sensitivity  Lights of the same intensity but different wavelengths may not all look as bright  A spectral sensitivity curve shows the relationship between wavelength and brightness  There are different spectral sensitivity curves for photopic (cone) vision and scotopic (rod) vision Eye Movement  We continually scan the world with small and quick eye movements – saccades  These bits of information are then integrated  Stabilize retinal image – see nothing  Visual system responds to change 3) Visual Transduction: the Conversion of Light to Neural Transduction  Transduction – conversion of one form of energy to another  Visual transduction – conversion of light to neural signals by visual receptors  Pigments absorb light  Absorption spectrum describes spectral sensitivity  Rhodopsin is the pigment found in rods  A G protein-linked receptor that responds to light rather than to neurotransmitters In the dark  Na channels remain partially open (partial depolarization), releasing glutamate When light strikes  Na channels close  Rods hyperpolarize, inhibiting glutamate release 4) From Retina to Primary Visual Cortex  The retinal-geniculate-striate pathways are about 90% of axons of retinal ganglion cells  The left hemiretina of each eye (right visual field) connects to the right lateral geniculate nucleus (LGN); the right hemiretina (left visual field) connects to the left LGN  Most LGN neurons that project to primary visual cortex (V1, striate cortex) terminate in the lower part of cortical layer IV Retinotopic Organization  Information received at adjacent portions of the retina remains adjacent in the striate cortex (retinotopic)  More cortex is devoted to areas of high acuity – like the disproportionate representation of sensitive body parts in somatosensory cortex  About 25% of primary visual cortex is dedicated to input from the fovea Retinotopy: The M and P Channels  Magnocellular layers (M layers) o Big cell bodies, bottom two layers of LGN o Particularly responsive to movement o Input primarily from rods  Parvocellular layers (P layers) o Small cell bodies, top four layers of LGN o colour, detail, and still or slow objects o Input primarily from cones  Project to slightly different areas in lower layer IV in striate cortex, M neurons just above the P neurons  Project to different parts of visual cortex beyond V1 Seeing Edges  Contrast Enhancement o Mach bands: nonexistent stripes the visual system creates for contrast enhancement o Makes edges easier to see o A consequence of lateral inhibition Receptive Fields of Visual Neurons  The area of the visual field within which it is possible for a visual stimulus to influence the firing of a given neuron  H
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