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BIOPSYCHOLOGY 8e

Topics

6.1

Light Enters the Eye and Reaches the Retina

6.2

The Retina and Translation of Light into Neural Signals

6.3

From Retina to Primary Visual Cortex

6.4

Seeing Edges

6.5

Seeing Color

6.6

Cortical Mechanisms of Vision and Conscious Awareness

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 1. Research probing the components of the visual system 2. Research assessing what we see BIOPSYCHOLOGY 8e

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: 1. Particles of energy (photons) 2. Waves of electromagnetic radiation • Humans see light between 380-760 nanometers

BIOPSYCHOLOGY 8e

Experiments FIGURE 6.2: The electromagnetic spectrum and the colors associated with the wavelengths that are visible to humans

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 Pupil and the Lens FIGURE 6.3: The Human Eye

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

BIOPSYCHOLOGY 8e

The Retina and Translation of Light into Neural Signals

 The retina is in a sense “inside-out”   Light passes through several cell layers before reaching its receptors Vertical pathway – receptors > bipolar cells > retinal ganglion cells  Lateral communication  Horizontal cells  Amacrine cells

BIOPSYCHOLOGY 8e

The Retina and Translation of Light into Neural Signals FIGURE 6.5: BIOPSYCHOLOGY 8e

The Retina and Translation of Light into Neural Signals

 Blind spot: no receptors where information exits the eye  The visual system uses information from cells around the blind spot for “

completion

,” filling in the blind spot  Fovea: high acuity area at center of retina  Thinning of the ganglion cell layer reduces distortion due to cells between the pupil and the retina

BIOPSYCHOLOGY 8e

The Retina and Translation of Light into Neural Signals FIGURE 6.6: A section of the retina BIOPSYCHOLOGY 8e

Cone and Rod Vision Duplexity theory of vision –

cones and rods mediate different kinds of vision

Cones: Photopic (daytime) vision

• High-acuity color information in good lighting • Only cones are found at the fovea

Rods: Scotopic (nighttime vision)

• High-sensitivity, allowing for low-acuity vision in dim light, but lacks detail and color information • More convergence than the cone system, increasing sensitivity while decreasing acuity Copyright © Pearson Education 2011

Cone and Rod Vision FIGURE 6.8: A schematic representation of the convergence of cones and rods on retinal ganglion cells.

Cone and Rod Vision FIGURE 6.9: The distribution of cones and rods over the human retina. (Adapted from Lindsay & Norman, 1977.)

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

BIOPSYCHOLOGY 8e

Spectral Sensitivity FIGURE 6.10: Human photopic (cone) and scotopic (rod) spectral sensitivity curves.

BIOPSYCHOLOGY 8e

Eye Movement

• We continually scan the world with small and quick eye movements –

saccades

Visual Transduction: The Conversion of Light to Neural Signals

• another

Universe

of one form of energy to •

Visual transduction

– conversion of light to neural signals by visual receptors • • Pigments absorb light

Absorption spectrum

Visual Transduction Figure 6.12 The inhibitory response of rods to light

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 Copyright © Pearson Education 2011

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 Copyright © Pearson Education 2011

The M and P Channels Magnocellular layers

 Big cell bodies, bottom two layers of LGN  Particularly responsive to movement  Input primarily from rods

Parvocellular layers

Seeing Edges

 Contrast Enhancement 

Mach bands:

nonexistent stripes the visual system creates for

contrast enhancement

 Makes edges easier to see  A consequence of

inhibition lateral

Lateral Inhibition and Contrast Enhancement Figure 6.14

The illusory bands visible in this figure are often called Mach bands, although Mach used a different figure to generate them in his studies

Lateral Inhibition and Contrast Enhancement Figure 6.15

How lateral inhibition produces contrast enhancement (Adapted from Ratliff, 1972)

Receptive Fields: Neurons of the Retina-Geniculate Striate System

Similarities seen at all three levels:

   Receptive fields of foveal areas are smaller than those in the periphery Neurons’ receptive fields are circular in shape Neurons are monocular  Many neurons at each level had receptive fields with excitatory and inhibitory area Copyright © Pearson Education 2011

Receptive Fields: Neurons of the Retina-Geniculate Striate System

 Many cells have receptive fields with a center surround organization: excitatory and inhibitory regions separated by a circular boundary  Some cells are “on center” and some are “off-center”

FIGURE 6.17: The responses of an on-center cell to contrast

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  Hubel and Wiesel looked at receptive fields in cat retinal ganglion, LGN, and lower layer IV of striate cortex Copyright © Pearson Education 2011

Receptive Fields: Simple and Complex Cortical Cells



In lower layer IV of the striate cortex, neurons with circular receptive fields (as in retinal ganglion cells and LGN) are rare

  

Most neurons in V1 are either:

Simple – receptive fields are rectangular with “on” and “off” regions, or Complex – also rectangular, larger receptive fields, respond best to a particular stimulus anywhere in its receptive field Copyright © Pearson Education 2011

Receptive Fields: Simple and Complex Cortical Cells SIMPLE

• Rectangular • “On” and “off” regions, like cells in layer IV • Orientation and location sensitive • All are monocular

COMPLEX

Columnar Organization of Primary Visual Cortex

    Cells with simpler receptive fields send information on to cells with more complex receptive fields Functional vertical columns exist such that all cells in a column have the same receptive field and ocular dominance Ocular dominance columns columns changes – as you move horizontally, the dominance of the Retinotopic organization is maintained Copyright © Pearson Education 2011

Columnar Organization of Primary Visual Cortex Figure 6.19:

Plasticity of Receptive Fields of Neurons in the Visual Cortex

•

Plasticity appears to be a fundamental property of visual cortex function

–

e.g. receptive field properties depend on the scene in which the stimuli to its field are embedded

Seeing Color: Component and Opponent Processing Component theory Opponent-process (trichromatic theory) theory Proposed by Young, Proposed by Hering refined by Helmholtz

• Two different classes of cells • Three types of receptors, each encoding color and another with a different spectral sensitivity

Both theories are correct:

class encoding brightness coding of color • Each encodes two by cones seems to operate on a purely and colors that cannot appear together (reddish green or bluish yellow) Copyright © Pearson Education 2011

Seeing Color FIGURE 6.21: The absorption spectra of the three classes of cones

Color Constancy and the Retinex Theory

 Color constancy – color perception is not altered by varying reflected wavelengths  Retinex theory (Land) – color is determined by the proportion of light of different wavelengths that a surface reflects  Relative wavelengths are constant, so perception is constant   Dual-opponent color cells are sensitive to color contrast Found in cortical “blobs” Copyright © Pearson Education 2011

Color Constancy and the Retinex Theory FIGURE 6.22: The method of Land’s (1977) color-vision experiments. Subjects viewed Mondrians that were illuminated by various proportions of three different wavelengths: a short wavelength, a middle wavelength, and a long wavelength.

Cortical Mechanisms of Vision and Conscious Awareness Flow of visual information:

• Thalamic relay neurons, to • 1˚ visual cortex (striate), to • 2˚ visual cortex (prestriate), to • Visual association cortex

As visual information flows through hierarchy, receptive fields

• become larger • respond to more complex and specific stimuli

FIGURE 6.24: The visual areas of the human cerebral cortex .

Damage to Primary Visual Cortex Scotomas

• Areas of blindness in contralateral visual field due to damage to primary visual cortex • Detected by perimetry test

Completion

• Patients may be unaware of scotoma – missing details supplied by “completion”

Figure 6.26: The completion of a migrane-induced scotoma as described by Karl Lashley (1941).

Damage to Primary Visual Cortex

• •

Blindsight Response to visual stimuli

outside conscious awareness

of “seeing” Possible explanations of blindsight

Functional Areas of Secondary and Association Visual Cortex Neurons in each area respond to different visual cues, such as color, movement, or shape Lesions of each area results in specific deficits Anatomically distinct (about 12 functionally distinct areas identified so far) Retinotopically organized

Functional Areas of Secondary and Association Visual Cortex FIGURE 6.27: Some of the visual areas that have been identified in the human brain.

Dorsal and Ventral Streams Dorsal Stream:

pathway from primary visual cortex • The “where” pathway (location and movement), or • the pathway for control of behavior (e.g., reaching)

Ventral Stream:

pathway from primary visual cortex to ventral prestriate coretex to inferotemporal cortex •The “what” pathway (color and shape), or •The pathway for conscious perception of objects Copyright © Pearson Education 2011

Dorsal and Ventral Streams

Neuropsychological disorders of vision Prosopagnosia

     Inability to distinguish among faces

Most prosopagnosic’s recognition deficits are not limited to faces Prosopagnosia is associated with damage to the ventral stream between the occipital and temporal lobes Prosopagnosics may be able to recognize faces in the absence of conscious awareness

Neuropsychological disorders of vision Akinetopsia

 Deficiency in the ability to see movement progress in a normal smooth fashion  Can be induced by a high dose of certain antidepressants  Associated with damage to the middle temporal (MT) area of the cortex Copyright © Pearson Education 2011

Simulate: The eye and retina Simulate: The visual pathways from retina to visual cortex

Note: To view the MyPsychLab assets, please make sure you are connected to the internet and have a browser opened and logged into www.mypsychlab.com

Acknowledgements Slide template template

3 4 ch06 image 3 4 5 6, 7, 16, 17, 37, 38 6, 18, 37 7 8 12-Sep 9, 11 10 12 14 15 17 19, 34 19

Image Description

lightning background texture eye puzzle globe eye dark night bright day Electromagnetic spectrum colored smoke head - woman Figure 6.4

binoculars dreamlike background eye Figure 6.5

Figure 6.6

Figure 6.8

Figure 6.9

Figure 6.10

brain Figure 6.11

Image Source

© istockphoto.com/Soubrette © istockphoto.com/Hedda Gjerpen © istockphoto.com/Tyler Stalman © istockphoto.com/G ü nay Mutlu © istockphoto.com/Tyler Stalman © iStockphoto.com/Soubrette © iStockphoto.com/Online Creative Media Figure 6.2 Pinel 8e, p. 133 © istockphoto.com/Wolfgang Amri © istockphoto.com/Angel Herrero de Frutos Pinel 8e, p. 134 © iStockphoto.com/Alex Staroseltsev © istockphoto.com/Emre Yildiz © istockphoto.com/Tyler Stalman Pinel 8e, p. 136 Pinel 8e, p. 137 Pinel 8e, p. 138 Pinel 8e, p. 139 Pinel 8e, p. 140 © istockphoto.com/Stephen Kirklys Pinel 8e, p. 141 Copyright © Pearson Education 2011

Acknowledgements

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Slide

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Image Description

Figure 6.12

Figure 6.13

piano Figure 6.14

Figure 6.15

neuron Figure 6.17

Figure 6.19

Paint cans Figure 6.21

Figure 6.22

Figure 6.24

Figure 6.26

Figure 6.27

Figure 6.28

Figure 6.29

Paper cut out men Figure 6.30

laptop table and wall

Image Source

Pinel 8e, p. 142 Pinel 8e, p. 143 © iStockphoto.com/Christian Waadt Pinel 8e, p. 144 Pinel 8e, p. 145 © istockphoto.com/ktsimage Pinel 8e, p. 147 Pinel 8e, p. 148 istockphoto.com/Amanda Rohde Pinel 8e, p. 152 Pinel 8e, p. 153 Pinel 8e, p. 154 Pinel 8e, p. 156 Pinel 8e, p. 157 Pinel 8e, p. 158 Pinel 8e, p. 159 istockphoto.com/twentyfourworks Pinel 8e, p. 161 ©istockphoto.com/CostinT ©istockphoto.com/David Clark Copyright © Pearson Education 2011

2605_lect6.ppt

Transcript 2605_lect6.ppt

Topics

What Do We See?

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

retinotopic

Similarities seen at all three levels:

In lower layer IV of the striate cortex, neurons with circular receptive fields (as in retinal ganglion cells and LGN) are rare

Most neurons in V1 are either:

Directory