Transcript Visual Coding and the Retinal Receptors

Chapter 6
Vision
Visual Coding and the Retinal
Receptors
• Each of our senses has specialized receptors
that are sensitive to a particular kind of
energy.
• Receptors for vision are sensitive to light.
• Receptors “transduce” (convert) energy into
electrochemical patterns.
Visual Coding and the Retinal
Receptors
• Law of specific nerve energies states that
activity by a particular nerve always conveys
the same type of information to the brain.
– Example: impulses in one neuron indicate
light; impulses in another neuron indicate
sound.
• The brain does not duplicate what we see.
• Which neurons respond, the amount of
response, and the timing of response
influence what we perceive.
Visual Coding and the Retinal
Receptors
• Light enters the eye through an opening in
the center of the iris called the pupil.
• Light is focused by the lens and the cornea
onto the rear surface of the eye known as the
retina.
– The retina is lined with visual receptors.
• Light from the left side of the world strikes the
right side of the retina and vice versa.
Visual Coding and the Retinal
Receptors
• Visual receptors send messages to neurons
called bipolar cells, located closer to the
center of the eye.
• Bipolar cells send messages to ganglion cells
that are even closer to the center of the eye.
– The axons of ganglion cells join one
another to form the optic nerve that travels
to the brain.
Visual Coding and the Retinal
Receptors
• Amacrine cells are additional cells that
receive information from bipolar cells and
send it to other bipolar, ganglion or amacrine
cells.
• Amacrine cells control the ability of the
ganglion cells to respond to shapes,
movements, or other specific aspects of
visual stimuli.
Visual Coding and the Retinal
Receptors
• The optic nerve consists of the axons of
ganglion cells that band together and exit
through the back of the eye and travel to the
brain.
• The point at which the optic nerve leaves the
back of the eye is called the blind spot
because it contains no receptors.
Visual Coding and the Retinal
Receptors
• The macula is the center of the human retina.
• The central portion of the macula is the fovea
and allows for acute and detailed vision.
– Packed tight with receptors.
– Nearly free of ganglion axons and blood
vessels.
Visual Coding and the Retinal
Receptors
• The arrangement of visual receptors in the
eye is highly adaptive.
– Example: Predatory birds have a greater
density of receptors on the top of the eye;
rats have a greater density on the bottom
of the eye.
Visual Coding and the Retinal
Receptors
•
The vertebrate retina consist of two kind of
receptors:
1. Rods - most abundant in the periphery of
the eye and respond to faint light. (120
million per retina)
2. Cones - most abundant in and around the
fovea. (6 million per retina)
• Essential for color vision & more useful
in bright light.
Visual Coding and the Retinal
Receptors
• The perception of color is dependent upon
the wavelength of the light.
• “Visible” wavelengths are dependent upon
the species’ receptors.
• The shortest wavelength humans can
perceive is 400 nanometers (violet).
• The longest wavelength that humans can
perceive is 700 nanometers (red).
Visual Coding and the Retinal
Receptors
•
Discrimination among colors depend upon
the combination of responses by different
neurons.
• Two major interpretations of color vision
include the following:
1. Trichromatic theory/Young-Helmholtz
theory.
2. Opponent-process theory.
Visual Coding and the Retinal
Receptors
• Trichromatic theory - Color perception occurs
through the relative rates of response by
three kinds of cones.
– Short wavelength, medium-wavelength,
long-wavelength.
• Each cone is maximally sensitive to a
different set of wavelengths.
Visual Coding and the Retinal
Receptors
• Trichromatic theory (cont.)
• The ratio of activity across the three types of
cones determines the color.
• More intense light increases the brightness of
the color but does not change the ratio and
thus does not change the perception of the
color itself.
• Incomplete theory of color vision.
– Example: negative color afterimage
Visual Coding and the Retinal
Receptors
• The opponent-process theory suggests that
we perceive color in terms of paired
opposites.
• The brain has a mechanism that perceives
color on a continuum from red to green and
another from yellow to blue.
• A possible mechanism for the theory is that
bipolar cells are excited by one set of
wavelengths and inhibited by another.
Visual Coding and the Retinal
Receptors
• Color vision deficiency is an impairment in
perceiving color differences.
• Gene responsible is contained on the X
chromosome.
• Caused by either the lack of a type of cone or
a cone has abnormal properties.
• Most common form is difficulty distinguishing
between red and green.
– Results from the long- and mediumwavelength cones having the same
photopigment.
The Neural Basis of Visual Perception
• Structure and organization of the visual
system is the same across individuals and
species.
• Quantitative differences in the eye itself can
be substantial.
– Example: Some individuals have two or
three times as many axons in the optic
nerve, allowing for greater ability to detect
faint or brief visual stimuli.
The Neural Basis of Visual Perception
• Ganglion cell axons form the optic nerve.
• The optic chiasm is the place where the two
optic nerves leaving the eye meet.
• In humans, half of the axons from each eye
cross to the other side of the brain.
• Most ganglion cell axons go to the lateral
geniculate nucleus, a smaller amount to the
superior colliculus and fewer going to other
areas.
The Neural Basis of Visual Perception
• The lateral geniculate nucleus is part of the
thalamus specialized for visual perception.
– Destination for most ganglion cell axons.
– Sends axons to other parts of the
thalamus and to the visual areas of the
occipital cortex.
– Cortex and thalamus feed information back
and forth to each other.
The Neural Basis of Visual Perception
• Lateral inhibition is the reduction of activity in
one neuron by activity in neighboring
neurons.
• The response of cells in the visual system
depends upon the net result of excitatory and
inhibitory messages it receives.
• Lateral inhibition is the retina’s way
responsible of sharpening contrasts to
emphasize the borders of objects.
The Neural Basis of Visual Perception
• The receptive field refers to the part of the
visual field that either excites or inhibits a cell
in the visual system of the brain.
• For a receptor, the receptive field is the point
in space from which light strikes it.
• For other visual cells, receptive fields are
derived from the visual field of cells that either
excite or inhibit.
– Example: ganglion cells converge to form
the receptive field of the next level of cells.
The Neural Basis of Visual Perception
• Pattern recognition in the cerebral cortex
occurs in a few places
• The primary visual cortex (area V1) receives
information from the lateral geniculate
nucleus and is the area responsible for the
first stage of visual processing.
• Some people with damage to V1 show
blindsight, an ability to respond to visual
stimuli that they report not seeing.
The Neural Basis of Visual Perception
• The secondary visual cortex (area V2)
receives information from area V1, processes
information further, and sends it to other
areas.
• Information is transferred between area V1
and V2 in a reciprocal nature.
The Neural Basis of Visual Perception
• The ventral stream refers to the most
magnocellular visual paths in the temporal
cortex.
– Specialized for identifying and recognizing
objects.
• The dorsal stream refers to the visual path in
the parietal cortex.
– Helps the motor system to find objects and
move towards them.
The Neural Basis of Visual Perception
• Visual agnosia is the inability to recognize
objects despite satisfactory vision.
– Caused by damage to the pattern pathway
usually in the temporal cortex.
• Prosopagnosia is the inability to recognize
faces.
– Occurs after damage to the fusiform gyrus
of the inferior temporal cortex.
The Neural Basis of Visual Perception
•
Several mechanisms prevent confusion or
blurring of images during eye movements.
1. Saccades are a decrease in the activity
of the visual cortex during quick eye
movements.
2. Neural activity and blood flow decrease
shortly before and during eye
movements.
Development of Vision
• Vision in newborns is functional but poorly
developed at birth:
– Face recognition occurs relatively soon
after birth (2 days)
– Show strong preference for a right-side-up
face and support idea of a built-in face
recognition system
Development of Vision
• Animal studies have greatly contributed to the
understanding of the development of vision.
• Early lack of stimulation of one eye leads to
synapses in the visual cortex becoming
gradually unresponsive to input from that eye.
• Early lack of stimulation of both eyes, cortical
responses become sluggish but do not cause
blindness.
Development of Vision
• Sensitive/critical periods are periods of time
during the lifespan when experiences have a
particularly strong and enduring effect.
• Critical period begins when GABA becomes
widely available in the brain.
• Critical period ends with the onset of
chemicals that inhibit axonal sprouting.
• Changes that occur during critical period
require both excitation and inhibition of some
neurons.
Development of Vision
• Stereoscopic depth perception is a method of
perceiving distance in which the brain
compares slightly different inputs from the two
eyes.
• Relies on retinal disparity or the discrepancy
between what the left and the right eye sees.
• The ability of cortical neurons to adjust their
connections to detect retinal disparity is
shaped through experience.
Development of Vision
• Strabismus is a condition in which the eyes
do not point in the same direction.
– Usually develops in childhood.
• Also known as “lazy eye”.
• If two eyes carry unrelated messages, cortical
cell strengthens connections with only one
eye.
• Development of stereoscopic depth
perception is impaired.
Development of Vision
• Early exposure to a limited array of patterns
leads to nearly all of the visual cortex cells
becoming responsive to only that pattern.
• Astigmatism refers to a blurring of vision for
lines in one direction caused by an
asymmetric curvature of the eyes.
– 70 % of infants
Cortical Mechanisms of Vision and
Conscious Awareness
• Flow of visual information:
Visual areas of the human cerebral cortex
– 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
Copyright © 2009 Allyn & Bacon
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”
Copyright © 2009
Damage to Primary Visual Cortex
(continued)
• Blindsight
– Response to visual stimuli without conscious
awareness of “seeing”
– Possible explanations of blindsight
• Islands of functional cells within scotoma
• Direct connections between subcortical
structures and secondary visual cortex, not
available to conscious awareness
Copyright © 2009
Functional Areas of Second 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
• Retinotopically organized
Copyright © 2009
Dorsal and Ventral Streams
• Dorsal stream: pathway from primary visual cortex to
dorsal prestriate cortex to posterior parietal cortex
– The “where” pathway (location and movement), or
– Pathway for control of behavior (e.g. reaching)
• Ventral stream: pathway from primary visual cortex to
ventral prestriate cortex to inferotemporal cortex
– The “what” pathway (color and shape), or
– Pathway for conscious perception of objects
Copyright © 2009
Prosopagnosia
• Inability to distinguish among faces
• Most prosopagnosic’s recognition deficits are not
limited to faces
• Often associated with damage to the ventral stream
• Prosopagnosics have different skin conductance
responses to familiar faces compared to unfamiliar
faces, even though they reported not recognizing
any of the faces
Copyright © 2009
Retinal Diseases
• Macular Degeneration-destruction of
photoreceptors
– Wet (blood vessels) and Dry (drusen)
Copyright © 2009
Retinitis Pigmentosa
• Progressive degeneration of photoreceptors
Copyright © 2009
Prostethic Retina
• Bioelectronic implant
• Images collected by camera hidden in
glassesdata sent to the unharmed retinal
cells then onto optic nerve
• 60 pixels (distinguish btwn light and dark)
• Artifical Retina Project Video
Copyright © 2009