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Chapter 10

Chapter 10 - psy290.docx

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Michael J.Dewar

Chapter 10: Vision  about one third of human cerebral cortex is devoted to visual analysis and perception  visual field – the area you can see without moving your head or eyes  only in the center of the visual field you perceive objects accurately (not notice it since move eyes rapidly as you scan the scene)  visual acuity – the sharpness of vision  each of our senses has specialized receptors that are sensitive to a particular kind of energy  receptors for vision are sensitive to light  photoreceptors – neural cells in the retina that respond to light  they transduce (convert) energy into electrochemical patterns  law of specific nerve energies – activity by a particular nerve always conveys the same type of info to the brain  impulses from one neuron indicate light, impulses from another neuron indicate sound etc.  brain does not duplicate what we see, sensory coding is determined by which neurons are active  functional segregation ?  colour  in about 8% of males and 0.5% of females the color distinction is either absent or less striking  complete color blindness is rare caused by brain lesion or absence of specialized receptors  three dimensions of colour: 1. brightness – varies from light to dark 2. hue – different colours (blue, red, yellow, etc) 3. saturation – from bright colours to grey  perception of a particular hue is not only dependent on the wavelength of light but also on: 1. intensity of illumination 2. the surrounding field 3. prior exposure to different stimulus  brightness  brightness is created by the visual system not by the amount of light reflected  lateral inhibition  phenomenon that occurs in sensory system involving cells inhibiting other cells lateral to themselves (either directly or through interneurons)  serves to sharpen perception, accentuate edges and contours – the difference between adjacent stimuli  (can be seen nicely with mach bans - the rate of firing in the two central receptor cells are more different than the rates of the two receptor cells on each side) Eye  has 2 functions 1. optical functions – capturing light and forming detailed spatial images 2. neural functions – transducing light into neural signals and processing these signals  contains  cornea  transparent outer layer of the eye, whose curvature is fixed  light travels in straight line until it reaches change in density (cornea)  cornea bends light rays responsible for forming image on the retina  refraction – bending of light rays by a change of the density of the medium, such as cornea and lens  lens  structure inside the eye  helps focus the image on the retina  the focus is adjusted by the shape of the lens  controlled by the ciliary muscles  accommodation – the process of controlling the shape of the lens to form a sharp image on the retina  for close vision – ciliary muscles tighten  lens become more rounded  for distant vision – ciliary muscles relax lens become flatter  pupil  the opening inside the colourful disk, the iris  the amount of light that enters the eye is controlled by the size of the pupil  dilation of pupil controlled by sympathetic system  constriction of pupil  controlled by parasympathetic system  belladonna – drug that blocks ACh transmission, relaxes spinncter muscle fibres dilates the pupil  morphine – constricts the pupil  extraocular muscles  three pairs of muscles that extend from the outside of the eyeball to the bony socket of the eye  controls the position and movement of the eye  retina  a thin receptive surface inside the back of the eye  light from the left side of the world strikes the right side of retina and vice versa (the image is inverted)  lined with visual receptors: 1. rods  long, narrow form  most concentrated in the periphery (edges) of the retina  respond to faint light  poor spatial resolution  motion sensitive  high convergence  120 million per retina  respond to scotopic system  works in faint light  does not respond differently to different wavelengths  highly convergent neural processing (?)  2. cones  most concentrated in and around the fovea  more useful in bright light  high spatial resolution (good visual acuity)  essential for colour vision  low convergence  6 million per retina  respond to photopic system  works in bright light  shows different sensitivity to different wavelengths colour vision  more detailed neural processing   at moderate levels of illumination both rods and cones function, some ganglion cells receive info from both types of receptors we can see over a wide range of light intensities  the arrangement of visual receptors is highly adaptive predatory birds have greater concentration of receptors on the top of the eye, rats have greater density on the bottom of the eye  structure  consists of: 1. outer segment o contains stack of disks increases the probability of capturing the quanta of light (important since only a portion of light reaches the retina, light is reflected by the eyeball, lens and fluid inside the eyeball) o since rods capture faint light more disks o place where photopigments are stored 2. inner segment o filled with mitochondia o place where photopigments are made o photopigments – chemicals contained by both rods and cones that release energy when stuck by light  rods and cones have rhodopsin photopigment  consists of 1. 11-cis-retinal –chromophore part 2. opsin – protein which determines which wavelength of light is absorbed 3. synaptic terminal  location where axons terminate at the synapse for transmission of information by release of neurotransmitters  signal transduction  rods and cones send messages to bipolar cells (closer to the center of the eye)  horizontal cells – interneurons that receive info from rods and cones send info to bipolar cells  stimulation by light induces hyperpolarization of rods and cones  photon of light comes into the outer segment of receptor (the disks)  photon is absorbed by rhodopsin  rhodopsin transfers energy to chromophore portion (11-cis-retinal)  causes flow of events result in closing of Na+ channels inside of the cell becomes more negatively charged hyperpolarization  the size of the hyperpolarizing photoreceptor potential determines the magnitude of the reduction in the release of the synaptic neurotransmitter  3 characteristics of the visual system 1. integration  at rest, receptors steadily release glutamate  light always hyperpolarizes photoreceptors release less glutamate  photoreceptors excite one group of bipolar cells and inhibit the other group by glutamate  bipolar cells send messages to ganglion cells (even closer to the center of the eye)  amacrine cells – receive info from bipolar cells send info to other bipolar and ganglion cells; control the ability of ganglion cells to respond to shapes, movements and other aspects of visual stimuli  there are two groups of bipolar cells their receptive fields differ 1. off-center bipolar cells – turning off light in the center of its receptive field receives more glutamate excites the cell depolarizes the cell 2. on-center bipolar cells – turning on light in the center of its receptive field receives less glutamate excites the cell (more glutamate inhibits  bipolar cells always depolarize ganglion cells 1. when light is turned offoff-center bipolar cells excite off-center ganglion cells 2. when light is turned on on-center bipolar cells excite on-center ganglion cells  ganglion cells the axons form the optic nerve caries info to the brain  1 million ganglion cells per retina  blind spot – the point at which the optic nerve leaves the back of the eye; it contains no receptors  ganglion cells fire action potentials  all other cells generate graded potentials  on-center and off-center ganglion cells fire nerve impulses and report “light” or “dark” to higher visual areas 2. adaptation  there major factors: 2. role of calcium – when Ca 2+ ions are made unavailable for reactions, visual system can no longer adapt to higher levels of illumination 3. the level of photopigment – photopigment is split apart by light, its two components, retinal and opsin slowly recombine ie: if you go from bright light into a dark theatre it takes several minutes until enough rhodopsin becomes available for the night vision 4. the availability of retinal chemicals for transduction – required for transduction; at low light high concentration; at brighter light low concentration more photons are needed to hyperpolarize the receptors 3. sensitivity  visual system deals with different intensities by adjusting the size of the pupil  range fractionation – receptors with different thresholds handle different intensities  macula – the center of the retina  fovea – center of macula allows for acute and detailed vision  has a dense concentration of cones, rods are absent  almost free of ganglion axons and blood vessels light reaches cones right away without having to pass through other cell layers and blood vessels  periphery - outside edges of the retina  greater number of receptors converge into ganglion and bipolar cells  detailed vision is less in the periphery  greater number of rods than cons  allows for greater perception of fainter light  from retina to the brain  signals that result from processing in the retina go to ganglion cells and then to other brain structures  ganglion cells form action potentials that are conducted along axons to send visual info to the brain  axons from the ganglion cell form the optic nerve, leaving the retina at the blind spot  optic chiasm – the place where the optic nerves partially cross  in humans, half of the axons from each side cross to the other side of the brain  axons from the half toward the nose cross over to the opposite side of the brain (nasal retina)  axons from the half toward the side of the head projects to its own side of the head (temporal retina)  optic tract – the axons from the ganglion cells after they have passed the optic chiasm  one part (80%) of axons go to LGN (nucleus in the thalamus specialized for visual perception)then go to the visual cortex of the occipital lobe (optic radiation?)  other part (20%) of axons  superior colliculus  superior colliculus  20% of optic fibres terminate here  function  concerned with where the object is – the location of object  input from M cells (10%) and K cells (10%)  reflex orienting head and eyes to visual stimuli 9control of saccadic eye movement)  respond to all stimuli regardless of shape or orientation  LGN  80% of optic fibres terminate here  function  concerned with what the object is – the identification of object  relay station between eyes and visual cortex  organizer of information  contrast enhancer  has 3 types of layers: 1. parvocellular  the four outer (dorsal) layers of LGN (3-6)  parvocellular ganglion cells  also midget or P cells  make up about 80% of ganglion cells  mostly located near or around the fovea  have small cell bodies and small receptive fields slow  connect to LGN (90% to parvocellular layers and 10% to magnocellular layers)  highly sensitive to detect color and visual detail  high spatial but low temporal resolution 2. magnocellular  the two inner (ventral) layers of LGN  mgnocellular ganglion cells  also parasol or M cells  make up about 10% of ganglion cells  distributed evenly throughout retina (1-2)  have larger cells bodies and visual fields  large axons – fast  mostly connect to LGN but also other visual areas of the thalamus  detect stimuli with low contrast and sensitive to moving stimuli  low spatial but high temporal resolution 3. kaniocecullar  divided into:  layers 1, 4 and 6 (contraleteral retinal axons)– receive input from the eye on the opposite site of the body  layers 2, 3 and 5 (ipsilateral retinal axons) – receive input from the eye on the same side  kinocellular ganglion cells  also bistratified or K cells  distributed throughout retina  have small cell bodies moderately slow  connect to LGN, other visual areas of thalamus and superior colliculus  heterogeneous responses, although mainly noted for carrying blue/ yellow colour signals  moderate spatial and temporal resolution  receptive fields  similar to those of ganglion cells but stronger surround accentuate contrast  discovered by Kuffler studying cats and Barlow studying frogs  moved small spot of light across the visual field keeping the animal’s eye still discovered that ganglion cells are concentric – have central area and a ring around it  bipolar and ganglion cells 2 types of receptive fields 1. on center/ off surround  spot of light in the center: bipolar cell – depolarizes ganglion cell – fires
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