Ppt Chap 6

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.


• 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.


• 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 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.


• 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.


• 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.


• 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.


• Each receptor in the fovea attaches to a single bipolar cell and a single ganglion cell known as a midget ganglion cell.

• Each cone in the fovea has a direct line to the brain which allows the registering of the exact location of input.


• In the periphery of the retina, a greater number of receptors converge into ganglion and bipolar cells.– Detailed vision is less in peripheral vision.– Allows for the greater perception of much

fainter light in peripheral vision.


• 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.


• 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.


• Photopigments - chemicals contained by both rods and cones that release energy when struck by light.

• Photopigments consist of 11-cis-retinal bound to proteins called opsins.

• Light energy converts 11-cis-retinal quickly into all-trans-retinal.

• Light is thus absorbed and energy is released that activates second messengers within the cell.


• 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).


• 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.


• 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.


• 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


• 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.


• Both the opponent-process and trichromatic theory have limitations.

• Color constancy, the ability to recognize color despite changes in lighting, is not easily explained by these theories.

• Retinex theory suggests the cortex compares information from various parts of the retina to determine the brightness and color for each area.– Better explains color constancy.


• 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 medium-

wavelength 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.


• Rods and cones of the retina make synaptic contact with horizontal cells and bipolar cells.

• Horizontal cells are cells in the eye that make inhibitory contact onto bipolar cells.

• Bipolar cells make synapses onto amacrine cells and ganglion cells.

• The different cells are specialized for different visual functions.


• 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 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.


• 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 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.


• Ganglion cells of primates generally fall into three categories:

1. Parvocellular neurons

2. Magnocellular neurons

3. Koniocellular neurons


• Parvocellular neurons:– mostly located in or near the fovea.– have smaller cell bodies and small

receptive fields.– connect only to the lateral geniculate

nucleus– are highly sensitive to detect color and

visual detail.


• Magnocellular neurons:– are distributed evenly throughout the

retina.– have larger cell bodies and visual fields.– mostly connect to the lateral geniculate

nucleus but also connect to other visual areas of the thalamus.

– are highly sensitive to large overall pattern and moving stimuli.


• Koniocellular neurons:– have small cell bodies.– are found throughout the retina.– connect to the lateral geniculate nucleus,

other parts of the thalamus, and the superior colliculus.


• Cells of the lateral geniculate have a receptive field similar to those of ganglion cells:– An excitatory or inhibitory central portion

and a surrounding ring of the opposite effect.

– Large or small receptive fields.


• 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 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.


• Three visual pathways in the cerebral cortex include:

1. A mostly parvocellular neuron pathway sensitive to details of shape.

2. A mostly magnocellular neuron pathway responds to movement.

3. A mixed pathway sensitive to brightness and color.


• 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.


• Hubel and Weisel (1959, 1998) distinguished various types of cells in the visual cortex:

1. Simple cells.

2. Complex cells.

3. End-stopped / hypercomplex cells.


• Simple cells:– Fixed excitatory and inhibitory zones.– The more light that shines in the excitatory

zone, the more the cell responds.– The more in the inhibitory zone, the less

the cell responds.– Bar-shaped or edge-shaped receptive

fields with vertical and horizontal orientations outnumbering diagonal ones.


• Complex cells:– Located in either V1or V2.– Have large receptive field that can not be

mapped into fixed excitatory or inhibitory zones.

– Responds to a pattern of light in a particular orientation and most strongly to a stimulus moving perpendicular to its access.


• End-stopped or hypercomplex cells:– Are similar to complex cells but with a

strong inhibitory area at one end of its bar shaped receptive field.

– Respond to a bar-shaped pattern of light anywhere in its large receptive field, provided the bar does not extend beyond a certain point.


• In the visual cortex, cells are grouped together in columns perpendicular to the surface.

• Cells within a given column process similar information. – Respond either mostly to the right or left

eye, or respond to both eyes equally.• Cells in the visual cortex may be feature

detectors, neurons whose response indicate the presence of a particular feature/ stimuli.


• Receptive fields become larger and more specialized as visual information goes from simple cells to later areas of visual processing.

• The inferior temporal cortex contains cells that respond selectively to complex shapes but are insensitive to distinctions that are critical to other cells.


• Shape constancy is the ability to recognize an object’s shape despite changes in direction or size.

• The inferior temporal neuron’s ability to ignore changes in size and direction contributes to our capacity for shape constancy.

• Damage to the pattern pathways of the cortex can lead to deficits in object recognition.


• 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.


• Color perception depends on both the parvocellular and koniocellular paths:– Area V4 may be responsible for color

constancy and visual attention.


• Motion perception involves a variety of brain areas in all four lobes of the cortex.

• The middle-temporal cortex (MT/ V5) responds to a stimulus moving in a particular direction.

• Cells in the dorsal part of the medial superior temporal cortex (MST) respond to expansion, contraction or rotation of a visual stimulus.


• 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.


• Motion blindness refers to the inability to determine the direction, speed and whether objects are moving.– Likely caused by damage in area MT.

• Some people are blind except for the ability to detect which direction something is moving.– Area MT probably gets some visual input

despite significant damage to area V1.

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


• 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.


• 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.


• 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.


• 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.


• 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


• Study of people born with cataracts but removed at age 2-6 months indicate that vision can be restored after early deprivation.

• Subtle but lingering problems persist:– People with left eye cataracts show mild

face recognition problems.– Early in life, each hemisphere of the brain

gets input almost entirely from the contralateral eye; the fusiform gyrus is located in the right hemisphere.


• Research and case studies indicate that the visual cortex is plastic but much more so early in life.– Example: Early removal of cataracts leads

to better improvement of various aspects of vision.

Ppt Chap 6

Health & Medicine

Transcript of Ppt Chap 6