Download Chapter 5

VISION
Images obtained from
http://dragon.uml.edu/psych/illusion.html
Detection, Transduction, Coding



Sensory receptors: specialized receptors detect
and respond to environmental stimuli
Sensory Transduction: conversion of physical
energy into a neural signal
Neural Coding: specific pattern of neural activity
that contains information about environmental
stimuli
Cross Section of the Human Eye
Retinal Circuitry
 Photoreceptors (rods and cones)
 Horizontal cells
 Bipolar cells
 Amacrine cells
 Retinal Ganglion Cells
Convergence in the Retina


Cones have better
acuity due to low
convergence.
Rods have greater
light sensitivity due
to a high
convergence.
Photoreceptors
 CONES (photopic system) 

 4 million

 concentrated in fovea

 3 photopigments

 low contrast sensitivity

 high acuity
RODS (scotopic system)
100 million
outside fovea
1 photopigment
high contrast sensitivity
low acuity
The Electromagnetic Spectrum
Physical Properties of Light



Wavelength is measured in nanometers and is
related to the perceived characteristic of hue or
color.
Intensity is related to the perceived characteristic
of brightness.
Purity refers to the number of wavelengths a
source of light contains and is related to the
perceived characteristic of saturation.
Phototransduction

Structures of the photoreceptors
Lamella—A thin membranous disc in the outer
layer of a photoreceptor.
 Photopigment—A chemical molecule in the
lamellae of the eye that absorbs light.
 Opsin—The protein component of a
photopigment.
 Retinal—The lipid component of a photopigment,
synthesized from Vitamin A.
 Rhodopsin (rosy color before light exposure)—
The photopigment in rods; consists of an opsin
and a retinal.

Phototransduction
Bleaching and Regeneration of
Visual Pigments in Rods

Insert Fig. 6.11 here
Phototransduction
 Light absorbed by
rhodopsin
 Opsin and retinal split
 Activated opsin combines
with G protein to activate
phosphodiesterase (PDE)
 PDE breaks down cGMP
to 5’-GMP
 Na+ channels close,
causing hyperpolarization
Retinal Coding
Retinal Coding


Bipolar cells connect
photoreceptors to Retinal
Ganglion Cells.
Horizontal and amacrine cells
lie parallel to the retina’s
surface.


Horizontal cells receive neural
messages from photoreceptors and
have inhibitory influences on
bipolar cells.
Amacrine cells receive neural
messages from the bipolar cells
and inhibit both bipolar and
ganglion cells.
Retinal Ganglion Cells
Retinal Ganglion Cells fire action potentials.
 RGC axons form the optic nerve.
 RGCs characterized by responses to light.

On ganglion cells: excited by bipolar cells in response
to a light stimulus.
 Off ganglion cells: excited when a light stimulus is
removed and inhibited by amacrine cells in the
presence of light.
 On-off ganglion cell: excited by both the presence
and removal of a light stimulus.

First characterized by Hartline in frogs (1938)
 Kuffler characterized RGC responses in cats (1952)

Receptive Fields

Receptive fields of visual
neurons: the region of the
visual field where light must
fall to stimulate the neuron.

For any particular visual neuron,
the location of its receptive field
depends on the locations within
the retina of the photoreceptors
that provide input to that neuron.
RGC Receptive Fields
RGC Receptive Fields
Primary Visual Pathway
Retina to Cortex
LGN Organization
 PARVOCELLULAR
 small cells
 dorsal four layers
 high spectral sensitivity
 low contrast sensitivity
 high spatial resolution
 low temporal
resolution
 MAGNOCELLULAR
 large cells
 ventral two layers
 low spectral sensitivity
 high contrast sensitivity
 low spatial resolution
 high temporal
resolution
Color Coding by the Retina

Young-Helmholtz (trichromatic) theory

Color perceptions come from a pattern of stimulation of
three sets of color receptors in the eye.
In 19th century, based merely on psychophysical
evidence I
 Modern evidence: three cone types

Theories of Color Perception

Opponent-process theory—The theory that
there are three receptor complexes operating in
opponent fashion to yield a perception of
color and brightness.

This theory explains negative afterimages.
Color Perception

Integration of Young-Helmholtz trichromatic
theory and Hering’s opponent-process theory
There are different types of cones which are
sensitive to different wavelengths as predicted by
the trichromatic theory.
 Beyond the level of photoreceptors there are
different types of ganglion cells and parvocellular
neurons of the LGN that seem to operate by
opponent-process theory.

From Trichromatic Stimulation to
Opponent-process Responding
CORTICAL
MECHANISMS OF
VISION/PERCEPTION
Receptive Fields in Visual Cortex

Simple cell—A neuron in area V1 that responds
to lines (edges) in a specific part of the visual
field having a specific orientation. If the
orientation of the line is changed, the simple cell
doesn’t fire or has a drastically reduced response.
Receptive Fields in Visual Cortex

Complex cells—found in areas V1 and V2.
These cells are sensitive to a line stimulus
oriented in a certain direction. Unlike simple
cells, the stimulus can appear in several different
locations and still activate the large receptive
field of the complex cell. Some cells respond to
line movement in a specific direction and others
respond to line movement in any direction.
Receptive Fields in Visual Cortex

Hypercomplex cells—
respond to visual stimuli
of a particular orientation
(line-tilt) within a
relatively large receptive
field. However, if the
line stimulus extends
beyond a specific point
they do not respond
(end-stopped).
Visual Cortex Organization
Columns of Cells


Ocular dominance column—A column of
cells in the visual cortex all having the same
amount of dominance of input from either
the right or left eye.
Orientation column—A column of cells in the
visual cortex all responding to the same
orientation of a line stimulus (line-tilt).
A Hypercolumn in Visual Cortex
SENSORY SYSTEM
ORGANIZATION

Hierarchical Organization


Functional Segregation


Multiple levels of analysis (e.g., primary, secondary,
association cortex)
Functionally distinct areas specializing in different kinds of
analysis (e.g., color, form, motion perception)
Parallel Processing

Simultaneous analysis of signals in different ways by multiple
parallel pathways of a neural network
Extrastriate Visual Pathways
 DORSAL STREAM
 projections from V1 to
posterior parietal cortex
 processing involved in
location of objects in space
for guiding movement
 VENTRAL STREAM
 projections from V1 to
inferior temporal lobe
 processing involved in
object recognition
Color Perception

Cerebral achromatopsia: inability to discriminate
among different hues; caused by damage to
inferior temporal cortex (V4, now called V8) of
the visual association cortex.

e.g., “The Case of the Color Blind Painter” by Oliver
Sacks
Form Perception

Perceptual problems with form recognition may
be caused by damage within the visual
association cortex even though the primary
visual pathway is intact.

Visual agnosia: an inability to identify/recognize
/name objects presented visually, despite normal
visual acuity and object identification by other senses
is otherwise normal.
Visual Agnosia

Apperceptive
Damage to ventral stream
 Despite normal visual acuity, can not
identify objects by sight; also can not draw
objects or copy pictures by sight


Associative
Ventral and dorsal streams intact;
disruption of connections between ventral
stream and verbal mechanisms
 Can copy objects or drawings by sight, but
can not do so from memory.
 Problem transferring perception to verbal
mechanisms/conscious awareness.

Visual Agnosia


Prosopagnosia: An impaired ability to recognize
specific faces visually.
Fusiform face area: The region of the
inferotemporal cortex most responsible for
recognition of faces.
Motion Perception

Akinetopsia: an inability to perceive movement,
caused by damage to area V5 (also called MST) of
the visual association cortex.
Motion Perception

Balint’s syndrome: a syndrome caused by bilateral
damage to the parieto-occipital region; includes
optic ataxia, ocular apraxia, and simultanagnosia.
 optic ataxia: difficulty in reaching for objects
under visual guidance.
 ocular apraxia: difficulty in visual scanning.
 simultanagnosia: difficulty in perceiving more
than one object at a time.
Motion Perception

intraparietal sulcus (IPS)
 The end of the dorsal stream of the visual association cortex;
involved in perception of location, visual attention, and
control of eye and hand movements.