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Vision: from Eye to Brain (Chap 3, Part B)

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Vision: from Eye to Brain (Chap 3, Part B) Vision: From Eye to Brain (Chap 3, Part B) Lecture 7 Jonathan Pillow Sensation & Perception (PSY 345 / NEU 325) Princeton University, Spring 2015 1 more “channels”: spatial frequency channels spatial frequency: the number of cycles of a grating per unit of visual angle (usually specified in degrees) • think of it as: # of bars per unit length low frequency intermediate high frequency 2 Why sine gratings? • Provide useful decomposition of images Technical term: Fourier decomposition 3 Fourier decomposition • mathematical decomposition of an image (or sound) into sine waves. reconstruction: “image” 1 sine wave 2 sine waves 3 sine waves 4 sine waves 4 “Fourier Decomposition” theory of V1 claim: role of V1 is to do “Fourier decomposition”, i.e., break images down into a sum of sine waves • Summation of two spatial sine waves • any pattern can be broken down into a sum of sine waves 5 Fourier decomposition • mathematical decomposition of an image (or sound) into sine waves. Original image High Frequencies Low Frequencies 6 original low medium high 7 Retinal Ganglion Cells: tuned to spatial frequency Response of a ganglion cell to sine gratings of different frequencies 8 The contrast sensitivity function Human contrast sensitivity illustration of this sensitivity 9 Image Illustrating Spatial Frequency Channels 10 Image Illustrating Spatial Frequency Channels 11 If it is hard to tell who this famous person is, try squinting or defocusing “Lincoln illusion” Harmon & Jules 1973 12 “Gala Contemplating the Mediterranean Sea, which at 30 meters becomes the portrait of Abraham Lincoln (Homage to Rothko)” - Salvador Dali (1976) 13 “Gala Contemplating the Mediterranean Sea, which at 30 meters becomes the portrait of Abraham Lincoln (Homage to Rothko)” - Salvador Dali (1976) 14 lateral geniculate nucleus (LGN): one on each side of the brain • this is where axons of retinal ganglion cells synapse Organization: • represents contralateral visual field • segregated into eye- specific layers • segregated into M and P layers Ipsilateral: Referring to the same side of the body Contralateral: Referring to the opposite side of the body 15 Primary Visual Cortex • Striate cortex: known as primary visual cortex, or V1 • “Primary visual cortex” = first place in cortex where visual information is processed (Previous two stages: retina and LGN are pre-cortical) 16 Receptive Fields: monocular vs. binocular - LGN cells: responds to one eye or the other, never both LGN V1 - V1 cells: can respond to input from both eyes • By the time information gets to V1, inputs from both eyes have been combined (but V1 neurons still tend to have a preferred eye - they spike more to input from one eye) 17 Topography: mapping of objects in space onto the visual cortex • contralateral representation - each visual field (L/R) represented in opposite hemisphere • cortical magnification - unequal representation of fovea vs. periphery in cortex - a misnomer, because “magnification” already present in retina (that is, the amount of space in cortex for each part of the visual field is given by the number of fibers coming in from LGN) 18 Acuity in V1 Visual acuity declines in an orderly fashion with eccentricity—distance from the fovea (in deg) 19 V1 receptive fields: elongated regions of space Major change in representation: • Circular receptive fields (retina & LGN) replaced by elongated “stripe” receptive fields in cortex • Has ~ 200 million cells! • (vs. 1 million Retinal Ganglion Cells) 20 Orientation tuning: • neurons in V1 respond more to bars of certain orientations • response rate falls off with difference from preferred orientation “preferred orientation” 21 Receptive Fields in V1 Many cortical cells respond especially well to: • Moving lines • Bars • Edges • Gratings • Direction of motion Ocular dominance: • Cells in V1 tend to have a “preferred eye” (i.e., respond better to inputs from one eye than the other) 22 Simple vs. Complex Cells 23 Receptive Fields in V1 Cells in V1 respond best to bars of light rather than to spots of light • “simple” cells: prefer bars of light, or prefer bars of dark • “complex” cells: respond to both bars of light and dark [Hubel & Weisel movie] 24 Column: a vertical arrangement of neurons • ocular dominance • orientation column: column: for particular for a particular location in location in cortex, neurons cortex, neurons have same have same preferred eye preferred orientation 25 Hypercolumn - contains all possible columns • Hypercolumn: 1-mm block of V1 containing “all the machinery necessary to look after everything the visual cortex is responsible for, in a certain small part of the visual world” (Hubel, 1982 • Each hypercolumn contains a full set of columns - has cells responding to every possible orientation, and inputs from left right eyes 26 web demos receptive fields http://sites.sinauer.com/wolfe4e/wa03.04.html columns http://sites.sinauer.com/wolfe4e/wa03.05.html 27 Adaptation 28 Adaptation: the Psychologist’s Electrode “tilt after-effect” 29 Adaptation: the Psychologist’s Electrode “tilt after-effect” • perceptual illusion of tilt, provided by adapting to a pattern of a given orientation • supports idea that the human visual system contains individual neurons selective for different orientations 30 Adaptation: the Psychologist’s Electrode Adaptation: the diminishing response of a sense organ to a sustained stimulus • An important method for deactivating groups of neurons without surgery • Allows selective temporary “knock out” of group of neurons by activating them strongly 31 Effects of adaptation on population response and perception Before Adaptation 0 degree stimulus unadapted population resp to 0 deg Stimulus presented = 32 Effects of adaptation on population response and perception Then adapt to 20º Before Adaptation unadapted population resp to 0 deg Stimulus presented = 33 Selective adaptation alters neural responses and perception After Adaptation perceptual effect of adaptation is repulsion away from the adapter Stimulus presented = 34 Selective Adaptation: The Psychologist’s Electrode Selective adaptation for spatial frequency: Evidence that human visual system contains neurons selective for spatial frequency 35 Adaptation that is specific to spatial frequency (SF) 1. adapt 2. test 3. percept 36 Adaptation that is specific to spatial frequency (SF) 1. adapt 2. test 3. percept 37 Adaptation that is specific to spatial frequency (SF) 1. adapt 2. test 3. percept 38 Adaptation that is specific to spatial frequency AND orientation 1. adapt 2. test 3. No adaptive percept 39 Adaptation that is specific to spatial frequency AND orientation 1. adapt 2. test 3. No adaptive percept 40 Adaptation that is specific to spatial frequency AND orientation 1. adapt 2. test 3. No adaptive percept 41 Selective Adaptation: The Psychologist’s Electrode Orthodox viewpoint: • If you can observe a particular type of adaptive after-effect, there is a certain neuron in the brain that is selective (or tuned) for that property THUS (for example): There are no neurons tuned for spatial frequency across all orientations, because adaptation is orientation specific. 42 Selective Adaptation: The Psychologist’s Electrode adapting spatial freq contrast sensitivity after threshold increases near width of “channels” adaptation to a sine the adapted frequency that contribute to wave with a frequency contrast sensitivity of 7 cycles/degree. 43 Selective Adaptation: The Psychologist’s Electrode adapting spatial freq Therefore: • adaptation reveals separate channels devoted to orientation and spatial frequencies • width of adaptive effect reveals the width of the channel 44 The Development of Spatial Vision • how can you study the vision of infants who can’t yet speak? 1. preferential-looking paradigm - infants prefer to look at more complex stimuli 45 The Development of Spatial Vision • how can you study the vision of infants who can’t yet speak? 2. visually evoked potentials (VEP) - measure brain’s electrical activity directly 46 The Development of Spatial Vision young children: not very sensitive to high spatial frequencies • Visual system is still developing ! Cones and rods are still developing and taking final shape ! Retinal ganglion cells are still migrating and growing connections with the fovea ! The fovea itself has not fully developed until about 4 years of age 47 Summary • early visual pathway: retina -> LGN -> V1 • “contralateral” representations in visual pathway • visual acuity (vs. sensitivity) • spatial frequency channels • Fourier analysis • spatial frequency sensitivity & tuning • V1 receptive fields, orientation tuning • Hubel & Weisel experiments • simple vs. complex cells • cortical magnification • cortical columns, hypercolumns • adaptation 48.
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