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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

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

Lecture 7 Jonathan Pillow Sensation & Perception (PSY 345 / NEU 325) Princeton University, Spring 2015

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more “channels”: spatial frequency channels

spatial frequency: the number of cycles of a grating per unit

  • f visual angle (usually specified in degrees)
  • think of it as: # of bars per unit length

low frequency intermediate high frequency

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Why sine gratings?

  • Provide useful decomposition of images

Technical term: Fourier decomposition

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  • mathematical decomposition of an image (or sound)

into sine waves.

Fourier decomposition

“image” 1 sine wave reconstruction: 2 sine waves 3 sine waves 4 sine waves

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“Fourier Decomposition” theory of V1

  • Summation of two spatial sine

waves

  • any pattern can be broken

down into a sum of sine waves claim: role of V1 is to do “Fourier decomposition”, i.e., break images down into a sum of sine waves

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  • mathematical decomposition of an image (or sound)

into sine waves.

Fourier decomposition

Original image High Frequencies Low Frequencies

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  • riginal

low medium high

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Retinal Ganglion Cells: tuned to spatial frequency

Response of a ganglion cell to sine gratings of different frequencies

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The contrast sensitivity function

Human contrast sensitivity illustration of this sensitivity

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Image Illustrating Spatial Frequency Channels

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Image Illustrating Spatial Frequency Channels

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If it is hard to tell who this famous person is, try squinting or defocusing “Lincoln illusion” Harmon & Jules 1973

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“Gala Contemplating the Mediterranean Sea, which at 30 meters becomes the portrait of Abraham Lincoln (Homage to Rothko)”

  • Salvador Dali (1976)

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  • Salvador Dali (1976)

“Gala Contemplating the Mediterranean Sea, which at 30 meters becomes the portrait of Abraham Lincoln (Homage to Rothko)”

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Ipsilateral: Referring to the same side of the body Contralateral: Referring to the

  • pposite side of the

body

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

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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)

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Receptive Fields: monocular vs. binocular

  • LGN cells: responds to one eye or

the other, never both

  • 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)

V1 LGN

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Topography: mapping of objects in space onto the visual cortex

  • cortical magnification
  • unequal representation of

fovea vs. periphery in cortex

  • a misnomer, because

“magnification” already present in retina

  • contralateral representation
  • each visual field (L/R) represented in
  • pposite hemisphere

(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)

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Acuity in V1

Visual acuity declines in an orderly fashion with eccentricity—distance from the fovea (in deg)

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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)

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Orientation tuning:

  • neurons in

V1 respond more to bars of certain orientations

  • response rate falls off with difference from preferred orientation

“preferred orientation”

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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)

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Simple vs. Complex Cells

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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]

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  • orientation column:

for a particular location in cortex, neurons have same preferred orientation

Column: a vertical arrangement of neurons

  • ocular dominance

column: for particular location in cortex, neurons have same preferred eye

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  • Hypercolumn: 1-mm block
  • f

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

Hypercolumn - contains all possible columns

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web demos

receptive fields http://sites.sinauer.com/wolfe4e/wa03.04.html columns http://sites.sinauer.com/wolfe4e/wa03.05.html

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Adaptation

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“tilt after-effect”

Adaptation: the Psychologist’s Electrode

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“tilt after-effect”

  • perceptual illusion of

tilt, provided by adapting to a pattern

  • f a given orientation
  • supports idea that the

human visual system contains individual neurons selective for different orientations

Adaptation: the Psychologist’s Electrode

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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

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Effects of adaptation on population response and perception

Stimulus presented = Before Adaptation unadapted population resp to 0 deg 0 degree stimulus

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Effects of adaptation on population response and perception

Stimulus presented = Then adapt to 20º Before Adaptation unadapted population resp to 0 deg

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Selective adaptation alters neural responses and perception

Stimulus presented = After Adaptation perceptual effect of adaptation is repulsion away from the adapter

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Selective adaptation for spatial frequency: Evidence that human visual system contains neurons selective for spatial frequency

Selective Adaptation: The Psychologist’s Electrode

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Adaptation that is specific to spatial frequency (SF)

  • 1. adapt
  • 2. test
  • 3. percept

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Adaptation that is specific to spatial frequency (SF)

  • 1. adapt
  • 2. test
  • 3. percept

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Adaptation that is specific to spatial frequency (SF)

  • 1. adapt
  • 2. test
  • 3. percept

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Adaptation that is specific to spatial frequency AND orientation

  • 1. adapt
  • 2. test
  • 3. No adaptive percept

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Adaptation that is specific to spatial frequency AND orientation

  • 1. adapt
  • 2. test
  • 3. No adaptive percept

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Adaptation that is specific to spatial frequency AND orientation

  • 1. adapt
  • 2. test
  • 3. No adaptive percept

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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

Selective Adaptation: The Psychologist’s Electrode

THUS (for example): There are no neurons tuned for spatial frequency across all

  • rientations, because adaptation is orientation specific.

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Selective Adaptation: The Psychologist’s Electrode

width of “channels” that contribute to contrast sensitivity

adapting spatial freq

contrast sensitivity after adaptation to a sine wave with a frequency

  • f 7 cycles/degree.

threshold increases near the adapted frequency

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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

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The Development of Spatial Vision

  • 1. preferential-looking paradigm
  • infants prefer to look at more complex stimuli
  • how can you study the vision of infants who can’t yet speak?

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The Development of Spatial Vision

  • 2. visually evoked potentials (VEP)
  • measure brain’s electrical activity directly
  • how can you study the vision of infants who can’t yet speak?

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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

The Development of Spatial Vision

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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

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