vision that is 6/18-6/60 or a significant loss of useful visual field
worse than 20/200 or only 20 degree visual field
What is the leading cause of vision loss?
What is Glaucoma?
Pressure build up due to blockage of flow of aqueous liquid before pressure damage optic flow nerve head
can be wet (fast, more severe, somewhat
dry (slower but no treatments), is usually age‐related
and affects central vision
bacterial infection in the eye
Lateral Geniculate Nucleus
Component of the thalamus that processes visual information from the eyes and sends this information to the primary and secondary visual cortex.
LGN layered structures
6 layers, eye of origin: which eye the information is coming from, on or off receptive fields: excitatory or inhibitory, Magnocellular or parvocellular pathways
Large cell bodies that carry fast information, low spatial frequency information(large objects) moving fast, high temporal-colorblind. dorsal: WHERE and HOW pathways
Small cell bodies that slow information, high spatial frequency (more detail), low temporal-sensitive to color ventral: "WHAT" pathwayspathway projects to the color-processing cytochrome oxidase blobs in primary visual cortex
Primary Visual Cortex V1
located in posterior occipital cortez
input layers are in the middle (layer 4)
Local processing and connections to brain are in the superficial layers (2 &3)
feedback to the thalamus come deep from layer( 5&6)
V1 neurons respond best to what?
short, oriented bars
the systematic ordering of the neuronal pathway from the retina to the occipital lobe; preserves spatial relationships, so that areas of the retina correspond to spatially correlated areas in the primary visual cortex.
is a cluster of adjacent columns that includes all possible orientations and ocular dominance columns
region of the cortex that represents stimuli in the fovea is disproportionately large
Like the rest of cortex, neurons with similar response properties are clustered in columns that run throughout the depth of cortex
ocular dominance columns
Inputs from the 2 eyes are segregated here
V1 has pinwheels
Columns of neurons that with similar preferred orientations are generally next to each other, creating a spinning pattern of responses as stimuli of different orientations are presented.
created by Hermann von Helmholtz; theory of color vision based on additive color mixing; suggest that the retina contains three types of color receptors, cones: red, green, blue
Opponent process theory
This circuit is created by adding the L & M responses, then subtracting the S (blue) response., the theory that opposing retinal processes (red-green, yellow-blue, white-black) enable color vision. For example, some cells are stimulated by green and inhibited by red; others are stimulated by red and inhibited by green.
True color blindness
rare, found in rod monochromats and as a result of some brain injuries.
is more common, and results from the lack of one of the cone photo pigments about 5percent of male populatiodnd
no L (long wavelength, or red) pigment. Hard to distinguish between red and green; reds look particularly dark. (since the genes for the cone pigments are on the X chromosome)., dichromacy characterized by lowered sensitivity to long wavelengths of light resulting in an inability to distinguish red and purplish blue
no M (medium wavelength, or green) pigment. Hard to distinguish between red and green.
no S (short wavelength) pigment. Difficult to distinguish yellows, greens and blues. Very rare.
there are (at least) three things that contribute to our ability to experience an object as being the same color in spite of dramatic changes in the spectral content of the illumination (and therefore the spectral content of the light being reflected to our eyes
Prolonged exposure to chromatic colour. Adaptation bleaches your cone pigments which decreases sensitivity to certain lights.
we (consciously and unconsciously) compare all the colors in a scene to normalize our perception. Even though the true color of a patch of material might be brown (true = viewed under sunlight on a white background), it will look red if it's next to something green, or green if it's next to something red
occurs for color as well as luminance and is a low-level mechanism by which local context comparisons can be made, High-level visual processes can also be used to make this local context comparison
normalizing our perception of lightness, perceiving the same lightness for objects, even if retinal image changes; perceived lightness depends on relative luminance
photobleaching modulates the responsivity of the retina. If photobleaching did not occur, receptors that were sensitive enough to function inside would be "over-excited" outdoors. This is a form of gain control
this demonstrates how easy it is for us to perceive a gray patch as being different lightnesses
these are cues that can be perceived without stereo vision, i.e. with just one eye
stuff gets in front of stuff
things on the ground at a distance look like their base is higher
things farther away are smaller
parallel lines look like they get closer as they get farther away
if we know how big something is, it will look farther away when it seems too small, and close to us when it seems too big
far away things look hazy (this cue can be misinterpreted when hiking on a clear day!)
similar to relative size, textures look finer as they recede
Kersten's ball-in-a-box demo illustrates that shadows are a powerful cue for relative height and a depth -- as long as they're consistent with our "light from above" assumption
as we move, things at different depths cross over each other
when one thing moves in front of another, the amount of the thing in back that you can see gets deleted; when the front thing moves out of the way, there's accretion of the back object
stereo vision, depth cues, such as retinal disparity and convergence, that depend on the use of two eyes
the relative location of images of the same object on each retina.
an imaginary arc drawn through the thing the eyes are converged on, which traces out the location of all other objects in the 3D visual field that will land on the retinae with zero disparity
primary visual cortex
disparity neurons are tuned to the relative location of images of the same object on each retina. Some neurons are tuned to near
the problem of figuring out how to line up the images on each retina to perceive depth. It's a complex calculation and we're not sure how the brain does it
the misalignment of the two eyes, crossed eyes
occurs when the brain receives different images from the two eyes. This can be because of strabismus or because the optical path in one eye is much longer or shorter than the other, in which case one image will always be blurry
A law stating that the size of an afterimage depends on the distance of the surface against which the afterimage is viewed. The farther away the surface, the larger the afterimage appears.
demonstrates a situation in which size constancy breaks down because depth cues are wrong
illusion demonstrates how strongly our perception of size is affected by our perception of depth
have big inner segments and small outer segments, so visual information is under sampled.
have relatively sparse connections. The first 6 months of development witnesses a great elaboration of connections, cells in column; respond best when retina receiving response at certain angles
is awful at birth -- 20/400 vision (legally blind). Acuity is reasonable at 3 months, but still improving, better in fovea, where there are no convergence between the cones input, and the ganglion cells output.
begins at 3 mo. (evidence from vergence -- eyes track objects as they get closer, and stereo vision); infants understand that closer things are larger ~ 7 mo.
infants understand occlusion and Gestalt principle of common fate (stuff that moves together belongs together) at ~3 mo.
Contrast sensitivity function
makes good progress toward "normal" (sensitivity to high spatial frequencies increases; this is related to acuity) during the first several years of life, but does not reach adult form until ~10 years old.
(the ability to perform better on a Vernier acuity task than predicted by normal visual acuity) develops after ~10 years.
Good from Day 1
Color vision appears the same as for adults
Infants show an immediate preference for faces, in spite of horrid visual acuity.
The simplest shape is usually the right explanation for an image.
Things that look alike probably come from the same source.
Contours rarely change abruptly; curves are smooth, acute angles are rare.
Things that are close together belong together.
Things that move together belong together.
Meaningfulness or familiarity.
We cluster features into familiar patterns (e.g., faces made out of rocks or branches in an image)
Features that are circled or on the same background belong together
Connected features belong together
Features that appear and disappear at the same time belong together
Light from above
when a scene is ambiguous, our perception of shape relies on the assumption that light is coming from above and casting shadows downward.
Shape from shading
our perceptions of lighting, lightness, and shape are interrelated.
Themes for organizing the visual system
Local: within a visual area, how are selective responses organized?
Global: between visual areas, how does selectivity change?
Positron Emission Tomography. Use of small amounts of radioactive tracers, and each data point takes a long time to acquire, but this technique gives us good information about metabolic activity, or specific neurotransmitters (e.g., maps of dopamine concentrations in the brain).
Electroencephalography, measures the electric fields in the scalp that are generated by clusters of neurons that are strongly stimulated. The technique suffers from poor spatial resolution, but can detect millisecond timing differences.
magnetoencephalography. This measures the magnetic fields (perpendicular to the electric fields) that are generated by clusters of active neurons. MEG has slightly better spatial resolution (both techniques have millisecond temporal resolution), but is more difficult and much more expensive than EEG.
fMRI: functional magnetic resonance imaging.
functional MRI is the use of MRI images to detect blood flow and blood oxygenation changes in the brain, which are the result of neural activity.
colorblind neurons responding to large, fast things) tends to serve up the "where/how" pathway
color-sensitive neurons responding to small, slow things) tends to head down the "what" pathway.
Centrality of motion
this is self-evident. We constantly use motion to break camouflage, determine object boundaries, and orient attention in cluttered scenes. Yet, in spite of its centrality, motion is often ambiguous
we focus on three aspects of our experience perceiving motion in which the cues are ambiguous
The aperture problem
when we only see a bit of a line segment, the direction of motion is ambiguous -- many different directions of motion could create the same perception in the viewing aperture. Line terminations and grouping with other image features (often using Gestalt principle of common fate) can reduce this ambiguity.
our motion through the world creates optic flow. But sometimes coordinated motion in the world around us is real, e.g., a train going past.
our eyeballs are constantly moving in our head, even if our head is holding still. This creates image motion that should not be perceived as object motion or scene motion (or optic flow).
Brain areas: Motion encoding
Area MT (medial temporal lobe) and nearby MST (medial superior temporal) are strongly involved in motion perception.
When presented with a field of randomly moving dots, MT responds more strongly as the motion coherence increases (as more dots move in the same direction)
MT encodes direction of motion (in the monkey), but neurons do not respond to optic flow patterns
MST neurons respond to different kinds of optic flow.
responds most strongly to biological motion, Structure from motion is the general term describing the Gestalt-like effect in which we perceive a solid geometrical shape
a copy of the motor signal sent to the eyes to move them
Motion caused by eye rotation can be disambiguated from motion caused by real image motion (or optic flow) if a comparator performs a logical or operation between the corollary discharge and the retinal image motion
created by ego motion, moving through our environment
When we're moving toward something, the target is the only thing that looks stationary (it's called the focus of expansion).
Perception & Action
Perception helps correct for errors in motor commands
selects targets for action, and helps us correct errors as we execute actions
As we act (move, pick up things, turn our head), our perception changes. We often use action (egomotion) to change our perspective to get better visual information, or to turn our head to hear something better, or to reach out and touch something to see if it's hot
Parahippocampal place area
area for navigation (both in recognizing places, and in recognizing landmarks)
respond to an action whether the brain-owner or someone else is doing the action, neurons that encode actions/behaviors, independent of agent
some neurons in parietal cortex respond to the visual aspects of a given action, while others respond to motor commands
loss of vision in the periphery, tunnel vision, blindness
Wet macular generation
new vessels grow under macula and leak fluid
Dry macular generation
generation of photoreceptors and pigment epithelium
missing info for chunk of visual field caused by retinal, optic nerve LGN or cortical damage, blind spot in vision (skotos = darkness)
an enzyme involved in oxidative metabolism (which means that blobs consume more oxygen than other parts of cortex
Physical stimuli are transduced to neural impulses by sensory neurons. For vision, the
physical stimulus is a photon reaching the back of the retina and causing a conformation
(shape) change in retinol, which is a photopigment derived from Vitamin A.
For each sensory modality, there is a different relationship between stimulus intensity
and perceived magnitude. It is rare that the relationship is linear (e.g. line length
estimation). It is more likely that the function is compressive (brightness estimation) or
expansive (pain estimation).
selects individual image regions, features or objects out of the
hundreds of possible stimuli in each scene. Change blindness demonstrations illustrate
how much of a given scene we are unaware of; inattentional blindness illustrates how
we can miss even very obvious aspects of our environment
At the neural level, we know that the rate of action potentials generated by neurons
primate primary visual cortex can be modified by scene organization (information that is
only available to higher visual areas with larger receptive fields), but only when the
monkey is aware of the scene structure, e.g. figure‐ground segmentation
Color Opponent theory of color vision
color opponent structure of the ganglion cell responses support this theory-red, green opponency means that when we stare at something red, then look away toa white background, we see green after image because of the L photoreceptors have adapted, reducing the response of red/green ganglion cells
Extrastriate visual areas: V2, V3, and V4
other "early" visual areas. These are right next to V1 in the back of
the brain. Neurons have larger and more complex receptive fields than V1 (not
simple oriented lines), but are still stimulated best by local features
responds to moving
stimuli. In monkey, MT is not selective for direction of motion (optic flow), but
nearby MST (medial superior temporal) is.
Lateral occipital complex (LOC)
a cluster of visual areas on the lateral side of the occipital cortex that responds better to pictures of objects than to scrambled
pictures. Analogous to some regions of monkey inferior temporal cortex. Damage
to this region (as well as to inferior temporal regions) can result in an inability to recognize the shape of objects
Extra‐striate body area (EBA)
a small region in ventral temporal cortex that responds well to things that are shaped like human bodies, but not to faces
Superior temporal sulcus (STS)
will respond well to "point‐light" walker stimuli (biological motion)
Fusiform face area (FFA)
this region of inferior temporal cortex responds preferentially to
Attention and salience
At first glance, the salient parts of an image are the ones that we look at. After a few hundred milliseconds, however, we start to understand the scene and start looking at meaningful objects (using a top‐down schema). Looking is usually (but not always)correlated with attention
brings stimuli to awareness. help us with the binding problem-experiments show that perceiving the conjunction of two features, ex. green and horizontal when describing short colorful line
identity information, object detection
a single line segment could move
many many different different ways ways and and cause cause the the same same apparent apparent
Retinal prosthetics require intact
ganglion cell layer in eye. Cortical prosthetics bypass optics entirely and stimulate brain
What we typically call color‐blindness is a color deficiency, usually a
red/green deficiency due to lack of L (long wavelength, red) or M
(medium wavelength, green) cones that results in an inability to
distinguish some reds & greens
you can feel your eyes moving together
(converging) as you track something moving closer to you
static monocular depth cues
Static depth cues with just one eyeball: occlusion, relative height/size & texture gradient, perspective convergence, familiar
size, atmospheric perspective, shadows
dynamic monocular depth cues
Dynamic depth cues with just one eyeball: motion parallax & accretion/deletion
3D stereo stimuli are created by presenting different images to
each eye, where the differences have the correct disparity for a
the correspondence problem
Seeing in 3D with stereo vision requires understanding how the
different images in the different eye correspond to each other.
Failure = binocular rivalry.
Contrast sensitivity: adult observers are most sensitive to gratings
at 4 cycles per degree. Infants peak sensitivity is less than 1 cpd.
These specialized visual areas work together to create a
distributed representation of scene information:
interconnectivity is the key.
Without attention, we perceive scene gist and our eyes are drawn to points of high salience, but we miss many details (as
demonstrated by change blindness)
neural correlates of recognizing objects
Object‐selective neurons do not respond to their preferred object when you look at it but don't see it
EgoMotion: motion of the body and the eyes`
Observer motion and eye motion both produce optic flow (and
need to be disambiguated from ego motion).
Motion processing in the brain
Neurons in MT/MST are sensitive to motion coherence; MST is
notably responsive to optic flow.
use of objects
Synesthesia occurs when a stimulus of one type (e.g., tactile)
elicits a sensation of another type (e.g., auditory).
Retinal prosthetics (currently in clinical trials) require intact
ganglion cell layer in eye. Cortical prosthetics (not fully
developed) bypass optics entirely and stimulate brain.
three kinds of attention
object‐based, feature‐based or location‐based.
compares eye motion and image motion in brain