Pinel basics ch04

Copyright © 2007 by Allyn and Bacon

Chapter 4The Visual System

How We See

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What you see is not necessarily what you get Somehow a distorted and upside-

down 2D retinal image is transformed into the 3D world we perceive

2 types of research needed to study vision Research probing the components of the

visual system Research assessing what we see


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 Photons of energyWaves of electromagnetic radiation

Humans see light between 380-760 nanometers in length


Light enters the eye

Wavelength – perception of color Intensity – perception of brightness Light enters the eye through the pupil

whose size is regulated by the irises Sensitivity – the ability to see when light

is dim Acuity – the ability to see details


Light enters the eye

Lens – focuses light on the retina Ciliary muscles adjust the shape of

the lens as needed Accommodation – the process of

adjusting the lens to bring images into focus


Binocular cues

Convergence – eyes must turn slightly inward when objects are close

Binocular disparity – difference between the images on the 2 retinas

Both are greater when objects are close – provides brain with distance information


The Retina

Built inside-outLight passes through several cell layers

before reaching its receptors Vertical pathway – receptors > bipolar

cells > retinal ganglion cells Lateral communication

Horizontal cellsAmacrine cells


The Retina

No receptors where information exits the eyeCreates the blind spot

FoveaAt the center of the retina, high acuityReduced light distortion


Cone and Rod Vision

Duplexity theory of vision – cones and rod mediate different kinds of vision

Cones – photopic visionHigh-acuity in good lighting

Rods – scotopic visionHigh-sensitivity, allowing for low-acuity

vision in dim light


Cone and Rod Vision

More convergence in rod system, increasing sensitivity while decreasing acuity

Which receptors are found at the fovea?

Cones


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


The Conversion of Light to Neural Signals Transduction –conversion of one

form of energy to another Visual transduction – light energy to

neural signals by visual receptors Pigments absorb light Absorption spectrum determines

spectral sensitivity


Rhodopsin

The pigment found in rods, a G-protein-linked receptor that responds to light

In the dark, sodium channels are partially open Rods depolarized

When light strikes, sodium channels close Rods hyperpolarize in response to light


Retina-geniculate-striate pathway

~90% of axons of retinal ganglion cells Information from the left visual field of

each eye projects to the right lateral geniculate nucleus (LGN) and vice versa

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

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


The M and P Channels

Magnocellular layers (M layers)Big cell bodies, bottom 2 layers of LGNParticularly responsive to movement Input primarily from rods

Parvocellular layers (P layers)Small cell bodies, top 4 layers of LGNColor, detail, and still or slow objects Input primarily from cones


The M and P Channels

M layers – movement, rods P layers – color and detail, 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


Lateral Inhibition and Contrast Enhancement Visual system detects change Mach bands – nonexistent stripes

that visual system creates to enhance the contrast and make edges easier to see – an example of contrast enhancement

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

Hubel and Wiesel looked at receptive fields in cat retinal ganglion, LGN, and lower layer IV of striate cortex


Receptive Fields of Visual Neurons

Similarities seen at all 3 levels:Receptive fields of foveal areas smaller than

those in the peripheryCircular receptive fieldsMonocularMany had an excitatory area and an inhibitory

area separated by a circular boundary


Receptive Fields in Striate Cortex

Neurons of lower layer IV are an exception – circular receptive fields (as in retinal ganglion cells and LGN)

Most neurons in V1 are eitherSimple – receptive fields are rectangular with

“on” and “off” regionsComplex – also rectangular, larger receptive

fields, respond best to a particular stimulus anywhere in its receptive field


Receptive Fields in Striate Cortex

SIMPLE Rectangular “on” and “off” regions,

like cells in layer IV Orientation and

location sensitive All are monocular

COMPLEX Rectangular Larger receptive fields Do not have static

“on” and “off” regions Not location sensitive Motion sensitive Many are binocular


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 – as you move horizontally, the dominance of the columns changes

Retinotopic organization is maintained


Seeing Color – 2 Theories

Trichromatic theory (component theory)

Proposed by Young, refined by Helmholtz

3 types of receptors, each with a different spectral sensitivity



Opponent-process theory Hering 2 different classes of cells encoding

color, and another class encoding brightness

Each encodes two complementary color perceptions



Both are correct – coding of color by cones seems to operate on a purely component basis, opponent processing of color is seen at all subsequent levels


Color Constancy and the Retinex Theory Color constancy – color perception is not

altered by varying reflected wavelenths Retinex theory – color is determined by

the proportion of light of different wavelengths that a surface reflects

Relative wavelengths are constant, so perception is constant


Visual Cortex

Primary – receives most of its input from the LGN

Secondary – receives most of its input from primary visual cortex

Visual association cortex – receives input from secondary visual cortex and other secondary sensory systems


Scotomas: Completion

Damage to an area of primary visual cortex produces a scotoma, an area of blindness

Completion prevents many patients of from being aware of their deficit – the mind fills in the blanks


Prosopagnosia

Visual agnosia for faces Agnosia – a failure of recognition How is it that you can have an agnosia

for a particular type of information? What does this say about how information is organized in the brain?

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