Vision Flashcards
• How far one sees is dependent on
how far light travels before it strikes one’s eyes.
Arab philosophy a thousand years ago reasoned that if you
saw by sending out sight rays, they couldn’t get to the stars
that fast. Then he demonstrated that light rays bounce off an
object in all directions, but you see only those rays that reflect off the object and strike your retina
• Sensation:
registration by the sensory organ (eyes) of a physical stimuli from the environment (different form perception)
• Sensory info is influenced by past experience
• Perception
subjective interpretation of sensations by the brain
• Perception of vision is not in the eyes; it’s in the brain
• Law of specific nerve energies
states that activity by a particular nerve always conveys the same type of information to the brain – so brain codes info in terms of which neurons are active.
• The brain processes info depending on which neurons are active (And strength of activity) at any given point.
• Example: impulses in one neuron indicate light; impulses in another neuron indicate sound – the brain somehow interprets action potentials coming from specific neurons as e.g. sound.
= Each of our senses has specialized receptors that are sensitive to a particular kind of energy
Transduction of light (how does light enter the eye)
- Enters the eye through an opening in the center of the iris called the pupil
- Light is focused by the lens (adjustable) and the cornea (not adjustable) onto the rear surface of the eye known as the retina, which is lined with visual receptors (cones = process color and high accuracy, rods process less precise, more periphery, info)
- The left side of the world strikes the right side of the retina and vice versa
- From above strikes, the bottom half of the retina and vice versa
- The inversion of the image poses no problem for the nervous system. Remember, the visual system does not duplicate the image. It codes it by various kinds of neuronal activity.
Cornea = clear, outer covering.
Light comes in –> bent by cornea, goes in, bent again by lens we get an inverted and flipped image. (with respect to left-right and up-down)
Common Refractive Errors
- Myopia (nearsightedness) light falls short of the retina
- Hyperopia (farsightedness) light falls beyond the retina
In normal eyes lens figure light directly onto retina.
Myopia = image formed BEFORE retina = unclear image. (e.g. from elongated eyeball, or having too much of a curve on cornea) – can’t focus on far objects
Hyperopia = can’t focus on near points of object – eyeball may be too short, or lens is too flat (often case in older adults, lens looses elasticity)
Myopia
(nearsightedness) light falls short of the retina
Myopia = image formed BEFORE retina = unclear image. (e.g. from elongated eyeball, or having too much of a curve on cornea) – can’t focus on far objects
Hyperopia
(farsightedness) light falls beyond the retina
Hyperopia = can’t focus on near points of object – eyeball may be too short, or lens is too flat (often case in older adults, lens looses elasticity)
Route within retina (how is info processed in retina)
Receptors (back of eye) bipolar cells (closer to center of eye) (amacrine cells, get info from bipolar, send to other bipolar, refine input to ganglion cells so e.g. responding to specific shapes, directions, etc) ganglion cell ganglion axons forms optic nerve (optic nerve leaves at the “blind spot” = not receptors = blind spot (but everything in blind spot in one eye is visible to the other eye)) travel back to brain
(all cells between light and receptors are transparent, so light can passe through them.
Photoreceptors (the pink neurons)
• Located at the back of the eye
• Respond to light
• Sends signals to other cells closer to the eye
• Photoreceptors converts light to electrical signals.
Bipolar cell:
• receives input from photoreceptors
Horizontal cell:
• links photoreceptors and bipolar cells
The horizontal cells make inhibitory contact onto
bipolar cells, which in turn make synapses onto amacrine cells
and ganglion cells. All these cells are within the eyeball.
Amacrine cell:
• links bipolar cells and ganglion cells
Retinal ganglion cell:
• Their axons gives rise to the optic nerve
Optic nerve
Optic Nerve
• Axons of ganglion cells exit through the back of the eye and travel to the brain
• The point at which it leaves is called the blind spot
• it contains no receptors
Blind spot
where optic nerve leaves eye
no receptors = blind
but everything in blind spot in one eye is visible to the other eye = not ACTUALLY blind in practice.
Fovea
- Central portion of the retina and allows for acute and detailed vision
- Packed tightly with receptors known as cones
- (nearly free of any ganglion axons and blood vessels) = nearly unimpeded vision (due to tight packing of receptors)
- Each receptor attaches to a single bipolar cell and a single ganglion cell known as a midget ganglion cell – each cone has direct route to brain (midget ganglion cells provide 70 percent of input to brain – your vision is dominated by the fovea)
- Each cone in the fovea has a direct line to the brain which allows the registering of the exact location of input
The Periphery of the Retina
- Greater number of receptors called rods (about 20 to 1 to cones – higher for species active at night)
- Detailed vision is less in peripheral vision
- Toward the periphery of the retina, more and more receptors converge onto bipolar and ganglion cells. –> result, the brain cannot detect the exact location or shape of a peripheral light source.
- In the periphery, your ability to detect detail is limited by interference from other nearby objects
- Allows for the greater perception of much fainter light in peripheral vision
Photoreceptors
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation) into signals that can stimulate biological processes. hey use the photopigment rhodopsin or a related molecule.
Rods and cones exist
Both rods and cones contain photopigments, chemicals
that release energy when struck by light. Photopigments
consist of 11-cis-retinal (a derivative of vitamin A) bound to
proteins called opsins, which modify the photopigments’ sensitivity
to different wavelengths of light. Light converts 11-cisretinal
to all-trans-retinal, thus releasing energy that activates
second messengers within the cell.
(The light is absorbed in this
process. It does not continue to bounce around the eye.)
Rods: • More numerous than cones • Abundant in periphery • Sensitive to low levels of light (dim light) – not for bright light because bright light bleaches them. • Used mainly for night vision • One type of pigment only
Cones:
• Highly responsive to bright light
• Specialized for color and high visual acuity
• In the fovea only (OBS! Google says there are cones outside fovea (mostly “blue” cones)
• From book: abundant in and near fovea
• Three types of pigment
Rods
A photoreceptor
Rods: • More numerous than cones • Abundant in periphery • Sensitive to low levels of light (dim light) – not for bright light because bright light bleaches them. • Used mainly for night vision • One type of pigment only
Cones
Cones:
• Highly responsive to bright light
• Specialized for color and high visual acuity
• In the fovea only (OBS! Google says there are cones outside fovea (mostly “blue” cones)
• From book: abundant in and near fovea
• Three types of pigment
Color Vision
- Visible light is a portion of the electromagnetic spectrum
- The perception of color depends on the wavelength of light
- Humans perceive wavelengths between 400-700 nanometers (nm)
- Infrared goggles allow us to see longer waves.
Color Vision Theories
- The specificity of color perception depends on specific receptors within the eye
- Two major interpretations of color vision
- Trichromatic theory/Young-Helmholtz theory
- Opponent-process theory
Trichromatic theory/Young-Helmholtz theory
- Young realized color vision needs explanation that is biological, beyond physics of light. (later revised by Helmholtz)
- Color perception occurs through the relative rates of response by three kinds of cones
- Short-wavelength (blue)
- Medium-wavelength (green)
- Long-wavelength (red)
- The ratio of activity across the three types of cones determines the color
- More intense light increases the activity of all three cones without much change in their ratio of responses. = light appears brighter, but with same color.
- The nervous system determines the color of the light by comparing the responses of different types of cones.
- We have fewer blue cones, but approx same amount of red and green cones
Although the short-wavelength (blue) cones are about evenly distributed across the retina, the other two kinds are distributed haphazardly, with big differences among individuals
In the retina’s periphery, cones are so scarce that you have no useful color vision
Opponent-Process Theory
we perceive in terms of opposites.
That is, the brain has a mechanism that perceives
color on a continuum from red to green, another from yellow to blue, and another from white to black. After you
stare at one color in one location long enough, you fatigue that
response and swing to the opposite.
Part of the explanation for this process pertains to the
connections within the retina. For example, imagine a bipolar
cell that receives excitation from a short-wavelength cone and
inhibition from long- and medium-wavelength cones. It increases
its activity in response to short-wavelength (blue) light
and decreases it in response to yellowish light. After prolonged
exposure to blue light, the fatigued cell decreases its response.
Because a low level of response by that cell usually means yellow,
you perceive yellow.
• Proposes we perceive color at level of visual cortex, and not at level of bipolar cells in the eye.
Limitations of Color Vision Theories
- Both the opponent-process and trichromatic theory have limitations
- Ex. Color constancy, the ability to recognize color despite changes in lighting
- Retinex theory: cortex compares information from various parts of the retina to determine the brightness and color for each area – the classic picture with dark/light grey, that are really the same tone
- = visual perception requires reasoning and inference, not just retinal stimulation.
• Retinex theory:
cortex compares information from various parts of the retina to determine the brightness and color for each area - can explain Color constancy, the ability to recognize color despite changes in lighting
Color Vision Deficiency/ color blindness
impairment in perceiving color differences (complete color blindness, only perceiving black and white, is rare) – many animals have 4 types of color cones, so in that sense, all humans are color deficient.
• Gene responsible is contained on the X chromosome = because men only have one X chromosome = more men than women have deficiency.
• Caused by either the lack of a type of cone or a cone that 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
• In monkeys, it has been shown that by “adding” the lacking type of cone, monkey can adapt and learn to see the lacking color = brain is adaptive!
• Some women have 4 types of color cones – can finer distinguish between certain colors
Mammalian visual system
Receptors (back of eye) –> bipolar cells (closer to center of eye) –> (amacrine cells, get info from bipolar, send to other bipolar, refine input to ganglion cells so e.g. responding to specific shapes, directions, etc) –> ganglion cell –> ganglion axons forms optic nerve (optic nerve leaves at the “blind spot” = not receptors = blind spot (but everything in blind spot in one eye is visible to the other eye))–> travel back to brain.
–> Optic chiasm
• Junction of the optic nerves from each eye
• Axons from the nasal (inside) half of each retina cross over to the opposite (contralateral) side of the brain.
• Axons from the temporal (outer) half of each retina remain on the same (ipsilateral) side of the brain.
• Information from the left visual field goes to the right side of the brain; information from the right visual field goes to the left side of the brain.
–> most ganglion axons go to LGN + small number to superior colliculus And other areas (e.g. hypothalamus (part controlling waking-sleeping schedule)
–> from LGN, to other parts of thalamus + visual cortex
Lateral inhibition
lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction.
Lateral inhibition is important for many functions in the
nervous system. In olfaction, a strong stimulus can suppress
the response to another one that follows slightly after it, because
of inhibition in the olfactory bulb
Lateral inhibition heightens contrast. When light falls on a surface, the bipolars just inside the border are most excited, and those outside the border respond
the least.
= The response of cells in the visual system depends upon the net result of excitatory and inhibitory messages it receives
Receptive Fields
Part of the visual field that either excites or inhibits a cell in the visual system of the brain
A rod or cone has a tiny receptive field in space to which it is
sensitive. One or more receptors connect to a bipolar cell,
with a receptive field that is the sum of the receptive fields of
all those rods or cones connected to it (including both excitatory
and inhibitory connections). Several bipolar cells report
to a ganglion cell, which therefore has a still larger receptive field.
- Ganglion cells have centre-surround receptive field (meaning the center of the field can be excitatory and donut-shaped surround is inhibitory, or vice versa)
- Ganglion cells of primates generally fall into three categories
- Parvocellular (parvo = small, small neurons and small receptive field) neurons – in fovea, small cell body/receptive field, good for color + visual detail (due to their small structure)
- Magnocellular neurons (found throughout retina) – large cell body/receptive field, light & movement – highly sensitive to overall patterns and movement (mainly for rods)
- Koniocellular – small, throughout retina, several functions, terminate in many locations
- Ganglion cells axon form optic nerve, goes to LGN (most) – LGN cells have similar receptive fields to ganglion cells.
The different types of ganglion cells and their receptive fields
- Ganglion cells have centre-surround receptive field (meaning the center of the field can be excitatory and donut-shaped surround is inhibitory, or vice versa)
- Ganglion cells of primates generally fall into three categories
- Parvocellular (parvo = small, small neurons and small receptive field) neurons – in fovea, small cell body/receptive field, good for color + visual detail (due to their small structure)
- Magnocellular neurons (found throughout retina) – large cell body/receptive field, light & movement – highly sensitive to overall patterns and movement (mainly for rods)
- Koniocellular – small, throughout retina, several functions, terminate in many locations
- Ganglion cells axon form optic nerve, goes to LGN (most) – LGN cells have similar receptive fields to ganglion cells.
Lateral Geniculate Nucleus
- 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
- Similar receptive field to ganglion cells:
- An excitatory or inhibitory central portion and a surrounding ring of the opposite effect
Primary Visual Cortex (V1)
- Receives information from the lateral geniculate nucleus and is the area responsible for the first stage of visual processing
- Also called striate cortex (because of “striped” surface)
- If you imagine seeing something, activity increases in area V1 in a pattern similar to what happens when you actually see that object + for visual illusions, the area activates for what you THINK you see, rather than what it really is.
- Damage to V1 show blindsight: an ability to respond to visual stimuli that they report not seeing (e.g. identifying emotions)
- Possible explanations:
- Some amount of healthy tissue left in V1, supporting vision.
- Or connections from thalamus to other areas, such as temporal cortex. Likely this one!
- If damage to V1, no conscious vision perception. = some importance to consciousness.
- Various types of cells in the visual cortex
- Simple cells
- Complex cells
- End-stopped/hypercomplex cells
Simple Cells
Hubel and Weisel – cat experiment – They quickly realized that the cell was responding to the edge of the slide. It had a bar-shaped receptive field, rather than a circular receptive field like cells in the retina and lateral geniculate.
- Receptive field has 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
- The receptive field is typically rectangular
Complex Cells
- Located in either V1 or 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 moving stimulus
- do not respond to the exact location of a stimulus. A complex cell responds to a pattern of light in a particular orientation (e.g., a vertical bar) anywhere within its large receptive field. Most complex cells respond most strongly to a stimulus moving in a particular direction—for example, a vertical bar moving horizontally.
- A cell that responds to a stimulus in only one location is a simple cell. One that responds equally throughout a large area is a complex cell.
Hypercomplex/endstopped cells
- Like complex cells, maximally responsive to moving bars
- Also have a strong inhibitory area at one end of the receptive field
End-stopped, or hypercomplex, cells resemble complex
cells with one exception: An end-stopped cell has a
strong inhibitory area at one end of its bar-shaped receptive
field. The cell responds to a bar-shaped pattern of light
anywhere in its broad receptive field, provided the bar does
not extend beyond a certain point
Columnar Organization of the Visual Cortex
- Cells are grouped together in columns perpendicular to the surface
- Cells within a given column process similar information
- Cells within one column has preference for specific stimuli (e.g. a bar of light at 45 degrees), where if you record from cells from multiple columns, they would respond most actively to different stimuli.
The existence of columns indicates that the various layers of the cerebral cortex communicate richly with one another, instead of being independent
Visual Cortex Cells as Feature Detectors
- Feature detectors are neurons whose response indicate the presence of a particular feature/stimuli
- Prolonged exposure to a given visual feature decreases sensitivity to that feature = supports idea of feature detectors – as if it fatigues the detector.
- HOWEVER, it is not only the stimulus that shapes perception, it is also your expectations. = excitation of feature detectors is not enough to explain vision.
- Top town processing also influences perception
retinal disparity
discrepancy between what left and right eye sees
Stereoscopic Depth Perception
To perceive distance, brain compares slightly different inputs from the two eyes (most neurons in visual cortex respond to both eyes (corresponding areas in both eyes)) = stereoscopic depth perception
Strabismus
eyes do not point in the same direction (usually develops in childhood – aka Lazy eye)
• Cortical cell strengthens connections with only one eye
• Impairments of stereoscopic depth perception (because you are not getting the retinal disparity)
• Can be helped by patching good eye = forces usage of bad eye (children usually just one of the eyes)
what happens if Early Exposure to a Limited Array of Patterns
= • Cells becoming responsive to only that pattern
• Astigmatism: blurring of vision for lines in one direction
• caused by an asymmetric curvature of the eyes
Long-Term Consequences of Impaired Infant Vision
People born with cataracts (cloudy spots on the lens) but had them removed = vision can be restored slowly, but
• Difficulty in recognizing objects
• One guy – difficulty at telling faces (male-female, sad-happy)
• Unable to tell that components are part of a whole
However, some aspects of vision never fully recovered.
Their acuity (ability to see detail) remained poor, and
their motion perception and depth perception never reached normal levels.
Parallel Processing in the Visual Cortex
- One part of your brain sees its shape, another sees color, another detects location, and another perceives movement
- Secondary visual cortex (area V2) receives information from area V1, processes information further, and sends it to other areas.
- Info is transferred between V2 and V1 in reciprocal manner (back and forth)
Ventral and Dorsal Streams
- The ventral stream refers to the path that goes through temporal cortex
- The “what” path – temporal cortex
- Specialized for identifying and recognizing objects
- Damage: can’t identify objects (visual-form agnosia)
- The dorsal stream refers to the visual path in the parietal cortex
- The “how” path – parietal cortex
- Important for visually guided movements
- Damage: don’t know where an object is in space (optic ataxia)
Although the distinction between ventral and dorsal
pathways is useful, we should not overstate it. Ordinarily you
use both systems in coordination with each other
Detailed Analysis of Shape
The further you get in the visual processing stream, the larger the visual fields get, and the more specialized the functions get (from V1 checking bars of light/simpler shapes, V2 might detect complex shapes, textures, etc)
–> inferior temporal lobe: e.g. Fusiform face area = processing faces.
Most objects don’t have specific response areas – but three types of objects do produce specific responses. One part of the parahippocampal cortex (next to the hippocampus) responds strongly to pictures of places, and not so strongly to anything else. Part of the fusiform gyrus of the inferior temporal cortex, especially in the right hemisphere, responds to faces. And an area close to this face area responds more strongly to bodies than to anything else.
Visual agnosia
an inability to recognize objects
despite otherwise satisfactory vision. It is a common result
from damage in the temporal cortex. They can often describe parts of the object, but not identify the object.
Recognizing Faces
- Face recognition occurs relatively soon after birth
- Facial recognition continues to develop gradually into adolescence
- Prosopagnosia: inability to recognize face
- Damage to (or difficulty developing) fusiform gyrus and inferior temporal cortex
- They can read – so visual acuity is not the problem, it is something specific to faces.
- Some people better than average at recognizing faces = may have more connections between fusiform gyrus and occipital cortex.
- Is fusiform gyrus devoted to faces? – possibly no: fusiform activity to objects of particular interest (e.g. pokemon cards) – but often fusiform cells activite more for faces than anything else.
The occipital
face area responds strongly to parts of a face, such as the
eyes and mouth (Arcurio, Gold, & James, 2012). The fusiform
gyrus responds strongly to a face viewed from any angle, as
well as line drawings and anything else that looks like a face
Motion Perception
Involves a variety of brain areas in all four lobes of the cerebral cortex
• The middle-temporal cortex (MT/V5) responds to a stimulus moving in a particular direction
• Medial superior temporal cortex (MST) respond to expansion, contraction, or rotation of a visual stimulus
• Receive input from the magnocellular path; color-insensitive
MT
and MST neurons enable you to distinguish between the result
of eye movements and the result of object movements (e.g. if we turn our head, world is still perceived as stationary, even if input on retina shifts)
- Motion blindness: inability to determine the direction, speed and whether objects are moving
- Likely caused by damage in area MT
- Patient DM – can’t see cars moving, pouring too much coffee because it looks frozen mid-air
Saccades
- Voluntary eye movement
- Decrease in the activity of the visual cortex during quick eye movements
- Neural activity and blood flow decrease 75 milliseconds before and during eye movements in MT
