Topic 3: Neural Processing in Vision Flashcards
Wavelength
for light energy, the distance between one peak of a light wave and the next peak
Visible Light
the band of electromagnetic energy that activates the visual system and that, therefore, can be perceived
for humans, visible light has wavelengths between 400 and 700 nanometers
Eyes
the eyeball and its contents, which include focusing elements, the retina, and supporting structures
Pupil
the opening through which light reflected from objects in the environment enters the eye
Cornea
the transparent focusing element of the eye that is the first structure through which light passes as it enters the eye
the cornea is the eye’s major focusing element
Lens
the transparent focusing element of the eye through which light passes after passing through the cornea and the aqueous humor
the len’s change in shape to focus at different distances is called accommodation
Retina
a complex network of cells that covers the inside back of the eye
these cells include the receptors, which generate an electrical signal in response to light, as well as the horizontal, bipolar, amacrine, and ganglion cells
Photoreceptors
the receptors for vision
Rods
a cylinder-shaped receptor in the retina that is responsible for vision at low levels of illumination
Cones
cone-shaped receptors in the retina that are primarily responsible for vision in high levels of illumination and for color vision and detail vision
Outer Segments
part of the rod and cone visual receptors that contains light-sensitive visual pigment molecules
Visual Pigments
a light-sensitive molecule contained in the rod and cone outer segments
the reaction of this molecule to light results in the generation of an electrical response in the receptors
Optic Nerve
bundle of nerve fibers that carry impulses from the retina to the lateral geniculate nucleus and other structures
each optic nerve contains about 1 million ganglion cell fibers
Fovea
a small area in the human retina that contains only cone receptors
the fovea is located on the line of sight, so that when a person looks at an object, the center of its image falls on the fovea
Peripheral Retina
the area of retina outside the fovea
Macular Degeneration
a clinical condition that causes degeneration of the macula, an area of the retina that includes the fovea and a small surrounding area
Retina Pigmentosa
a retinal disease that causes a gradual loss of vision, beginning in the peripheral retina
Blind Spot
the small area where the optic nerve leaves the back of the eye
there are no visual receptors in this area, so small images falling directly on the blind spot cannot be seen
Accommodation
in vision, bringing objects located at different distances into focus by changing the shape of the lens
Refractive Errors
errors that can affect the ability of the cornea and/or lens to focus incoming light onto the retina
Presbyopia
the inability of the eye to accommodate due to a hardening of the lens and a weakening of the ciliary muscles
it occurs as people get older
Myopia
an inability to see distant objects clearly
also called nearsightedness
Refractive Myopia
myopia (nearsightedness) in which the cornea and/or the lens bend the light too much
Axial Myopia
myopia (nearsightedness) in which the eyeball is too long
Hyperopia
a condition causing poor vision in which people can see objects that are far away but do not see near object clearly
also called farsightedness
Isomerization
change in the shape of the retinal part of the visual pigment molecule that occurs when the molecule absorbs a quantum of light
isomerization triggers the enzyme cascade that results in transduction from light energy to electrical energy in the retinal receptors
Dark Adaptation
visual adaptation that occurs in the dark, during which the sensitivity to light increases
this increase in sensitivity is associated with regeneration of the rod and cone visual pigments
Dark Adaptation Curve
the function that traces the time course of the increase in visual sensitivity that occurs during dark adaptation
Light-Adapted Sensitivity
the sensitivity of the eye when in the light-adapted state, usually taken as the starting point for the dark adaptation curve because it is the sensitivity of the eye just before the lights are turned off
Dark-Adapted Sensitivity
the sensitivity of the eye after it has completely adapted to the dark
Rod Monochromats
a person who has a retina in which the only functioning receptors are rods
Rod-Cone Break
the point on the dark adaptation curve at which vision shifts from cone vision to rod vision
Visual Pigment Bleaching
the change in the color of a visual pigment that occurs when visual pigment molecules are isomerized by exposure to light
Visual Pigment Regeneration
occurs after the visual pigment’s two components (opsin and retinal) have become separated due to the action of light
regeneration, which occurs in the dark, involves a rejoining of these two components to reform the visual pigment molecule
the process depends on enzymes located in the pigment epithelium
Detached Retina
a condition in which the retina is detached from the back of the eye
Spectral Sensitivity
the sensitivity of visual receptors to different parts of the visual spectrum
Spectral Sensitivity Curve
the function relating a subject’s sensitivity to light to the wavelength of the light
the spectral sensitivity curves for rod and cone vision indicate that the rods and cones are maximally sensitive at 500 nm and 560 nm, respectively
Monochromatic Light
light that contains only a single wavelength
Cone Spectral Sensitivity
a plot of visual sensitivity versus wavelength for cone vision, often measured by presenting a small spot of light to the fovea, which contains only cones
can also be measured when the eye is light adapted, so cones are the most sensitive receptors
Rod Spectral Sensitivity Curve
the curve plotting visual sensitivity versus wavelength for rod vision
this function is typically measured when the eye is dark adapted by a test light presented to the peripheral retina
Purkinje Shift
the shift from cone spectral sensitivity that takes place during dark adaptation
Absorption Spectrum
a plot of the amount of light absorbed by a visual pigment versus the wavelength of light
Neural Circuits
a number of neurons that are connected by synapses
Bipolar Cells
a retinal neuron that receives inputs from the visual receptors and sends signals to the retinal ganglion cells
Ganglion Cells
a neuron in the retina that receives inputs from bipolar and amacrine cells
the axons of the ganglion cells are the nerve fibers that travel out of the eye in the optic nerve
Horizontal Cells
a neuron that transmits signals laterally across the retina
horizontal cells synapse with receptors and bipolar cells
Amacrine Cells
a neuron that transmits signals laterally in the retina
amacrine cells synapse with bipolar cells and ganglion cells
Neural Convergence
synapsing of a number of neurons onto one neuron
Visual Acuity
the ability to resolve small details
Receptive Field
a neuron’s receptive field is the area on the receptor surface (the retina for vision; the skin for touch) that, when stimulated, affects the firing of that neuron
Center-Surround Receptive Fields
a receptive field that has center-surround organization
Excitatory Area
area of a receptive field that is associated with excitation
stimulation of this area causes an increase in the rate of nerve firing
Inhibitory Area
area of a receptive field that is associated with inhibition
stimulation of this area causes a decrease in the rate of nerve firing
Center-Surround Antagonism
the competition between the center and surround regions of a center-surround receptive field, caused by the fact that one is excitatory and the other is inhibitory stimulating center and surround areas simultaneously decreases responding of the neuron, compared to stimulating the excitatory area alone
Lateral Inhibition
inhibition that is transmitted laterally across a nerve circuit; in the retina, lateral inhibition is transmitted by the horizontal and amacrine cells
Edge Enhancement
an increase in perceived contrast at borders between regions of the visual field
Chevreul Illusion
occurs when areas of different lightness are positioned adjacent to one another to create a border
the illusion is the perception of a light band on the light side of the border and a dark band on the dark side of the border, even though these bands do not exist in the intensity distribution
Mach Bands
light and dark bands perceived at light-dark borders
Preferential Looking (PL) Technique
a technique used to measure perception in infants
two stimuli are presented, and the infant’s looking behavior is monitored for the amount of time the infant spends viewing the stimulus
Visual Evoked Potential
an electrical response to visual stimulation recorded by the placement of disk electrodes on the back of the head
this potential reflects the activity of a large population of neurons in the visual cortex
How was the Snellen chart used to measure visual acuity?
measures foveal acuity only
normal is 20/20 vision (what you can see at 20 feet vs. distance for a normal person to see)
20/200 (or worse) is legally blind
what is the unit of measurement?
How do optometrists measure visual acuity?
reciprocal of focal length (m) of corrective lens
negative = concave lens (for nearsightedness)
positive = convex lens (for farsightedness)
What is visual angle?
size of retinal image in degrees
with 20/20 vision, details of 1’ can be resolved (size of a quarter at the distance of a football field)
What is hyperacuity?
resolution of details of 10” or less of vernier gratings (exceeds resolution of receptors)
cone spacing in fovea = 12” (1 um)
expected resolution = 24” (theoretical limit)
hyperacuity may be due to complex neural processing in the visual cortex
How does retinal position affect acuity?
fovea has the greatest acuity
high (cone) receptor density
low spatial summation (convergence of a number of receptors to a single neuron)
cortical magnification factor: gives millimeters of cortex per degree of visual angle, as a function of retinal eccentricity
What is myopia?
image focused in middle of the eyeball (nearsightedness)
What is hyperopia?
images focused behind retina (farsightedness)
What is astigmatism?
cornea is not spherical, but asymmetrically curved (like a football), causing multiple focal points
What is chromatic aberration?
different wavelengths focus at different points
What is spherical abberration?
light rays focus at different points depending on how far from center they pass through a lens
smaller pupil minimizes this
What is diffraction?
light waves bend around obstacles in their path or through a slit; affects different wavelengths to different extents
larger pupil minimizes this
optimum pupil trade off size = 2.4 mm
What is radiance?
radiant power from a light source
unit: lumen = light produced by a standard candle (“candela”)
e.g. 1 lm = 1.46 mW
What is illuminance?
amount of light falling on a surface
unit: lux = 1 lumen per square metre of area (lm/m^2)
e.g. daylight = 10,000 lux, full moon = 0.1 lux
What is luminance?
amount of light reflected from a surface
unit: nit = 1 candela per square metre of area (cd/m^2)
e.g. LCD monitor = 260 nits, CRT moniter = 150 units
What is reflectance?
proportion of light reflected from a surface
unit: percent (%) or “albedo” = (luminance/illuminance) x 100
e.g. white paper = 90%, black paper = 10%
What is brightness?
perceptual impression of intensity of light source
psychological counterpart to radiance
What is lightness?
perceptual impression of surface “greyness”
psychological counterpart to reflectance
How does dark/light adaptation affect brightness perception?
being in a dark room then moving to a bright room seems brighter before you are habituated
How does retinal locus affect brightness perception?
threshold lower in the periphery (due to greater rod convergence)
How does wavelength affect brightness perception?
match standard color with comparisons based on brightness; get absorption spectrum
repeat under different illuminations (photopic vs. scotopic)
during dark adaptation, we shift from using cones to rods
result is Purkinje shift: peak sensitivity changes to shorter (bluer) wavelengths
How does time and area affect brightness perception?
brightness affected by duration and retinal size of stimulus
What is the optic nerve?
axons of retinal ganglion cells
exits back of the eye where there are no receptors (optic disc), resulting in a blind spot
retina begins processing visual information: ~126 million receptors, but only 1 million nerve fibers
What is the receptive field?
area on the retina that, in response to a stimulus, influences the firing of a neuron
typical (ganglion cell) receptive field is centre-surround
this formation is due to a pattern of connectivity between many receptors and a single ganglion cell
What is lateral inhibition?
some cells, when activated (e.g., by the presence of a stimulus), decrease the activity of adjacent cells
due to the release of inhibitory neurotransmitter
What is the Chevreul illusion?
each band is uniform shade of grey, but seems to darken near a lighter band, and lighten near a darker band
adjacent receptors believed to inhibit neighboring receptors
difference in luminance at border exaggerated by lateral inhibition
What is simultaneous contrast?
central squares appear different shades
explained by lateral inhibition: receptors activated by larger surrounding square inhibit receptors in smaller central square
What is the Benary cross illusion?
both triangles should receive equal lateral inhibition, but seem different shades
What is White’s illusion?
rectangle A should receive little lateral inhibition (and seem lighter), but it seems darker
rectangle B should receive a lot of lateral inhibition (but seem darker), but it seems lighter
explained in terms of “belongingness”: appearance of an areas is influenced by the surroundings to which it seems to belong
suggests higher-order processing instead of retinal mechanism
What is spatial frequency?
how a stimulus changes over space (cycles per degree of visual angle)
one cycle = one dark bar + one light bar
higher intensity regions produce peaks, lower intensity areas correspond to troughs
What is Fourier analysis?
simplified scene description in terms of a set of sine waves
result: mathematical expression describing the visual scene in terms of sine waves
better than taking inventory of the activity of all ~126 million receptors
allows us to investigate commonalities in the processing of visual information
What is the contrast sensitivity function?
describes ability of a system to preserve contrast and spatial frequency information after it has been encoded
What is the contrast ratio?
based on physical measures of light (L = luminance)
however, apparent contrast is affected by spatial frequency: wide black bars appear darker than narrower bars, wide white bars appear lighter than narrower bars
What are the steps of creating a contrast sensitivity function?
- present observer with a grating of black & white bars of a certain spatial frequency
- change contrast between the black and white bars until observer no longer perceives stimulus as lines
- change spatial frequency and repeat
Is a contrast sensitivity function monolithic or comprised of “channels”?
- adapt to a spatial frequency of 7.5 cycles/degree
- remeasure the contrast sensitivity function
suggests contrast sensitivity function is comprised of a series of spatial frequency channels
What did the Maffei & Fiorentini (1973) study demonstrate about contrast sensitivity function?
each simple cell responds best to a narrow range of spatial frequencies
adaptation effects are caused by neural fatigue
