Chapter 3: Spatial Vision (Part the First) Flashcards
What happens if ALL of the ganglion cell receives light??
NOTHING
Ganglion cells are unaffected by average light intensity, and are most responsive to _____ .
DIFFERENCES in light intensity.
Cortical visual pathways
- Retina
- Optic Nerve
- Optic Chiasm
- Optic Tract
- LGN
- Optic Radiation
- Striate Cortex
Acuity
The smallest spatial detail that can be resolved
Visual Acuity: Oh Say, Can You See?
The Bars
At some point your perception of the black and white bars turns to grey.
This threshold is a result of your visual acuity.
Measuring visual acuity:
Eye doctors use distance (e.g., 20/20)
Vision scientists use the smallest visual angle of a cycle of a grating.
Concept of visual angle:
Cycle is one black + one white stripe.
The smallest part of the pattern.
The visual system “samples” the grating discretely.
This is just like digital cameras…
Remember the arrangement of cones at the back of the retina…
Each cone takes up some room, and a cycle will only be perceived if it as at least the width of 2 cones.
If cycle is the width of one cone, the visual system won’t be able to encode it.
CONES at the fovea have a separation of 0.008 degrees of visual angle. PRETTY SMALL!
Normal Human Visual acuity is 0.017degrees. Twice the separation of cones!!
Herman Snellen invented method for designating visual acuity in 1862
LETTER is 5 times as large as the strokes that form the letter.
So, what does it mean to be 20/20?
It means you can see like most people:
the distance at which you can just identify the letters.
divided by
the distance at which a person with “normal” vision can just identify the letters.
not perfect but normal vision
20/20
The physiological limit of human vision, (based on cone density).
20/8
legally blind in the US
20/200 while wearing corrective lens is legally blind in the US
Landolt Rings
In other countries:
Spatial Frequency:
The number of cycles of a grating per unit of visual angle (usually specified in degrees)
Contrast
The difference in illumination between a figure and its background
If you were standing at a distance for which the double arrow was one degree of visual angle, then…
Our ability to see these patterns actually depends on BOTH contrast and SF…
Why does an oriented grating appear to be gray if you are far enough away?
This striped pattern is a “sine wave grating.”
The visual system “samples” the grating discretely.
Sensitivity
A value that defines the easy with which an observer can tell the difference between either
The presence or absence of a stimulus.
The difference between stimulus 1 and 2.
Just Noticeable Difference (JND)
AKA a difference THRESHOLD
The smallest detectable difference between two stimuli.
The minimum change in a stimulus that enables it to be correctly judged as different.
The Relationship between
Threshold and Sensitivity
Thresholds are inversely proportional to sensitivity.
Thresholds and sensitivity are inversely related.
James Bond
Low pain sensitivity
High pain threshold
Carleton Banks
High emotional sensitivity
Low emotional threshold
Cycles per degree
The number of dark and bright bars per degree of visual angle
Contrast Sensitivity Function (CSF).
Developed by Otto Schade.
How much lighter must the light patch be for you to see the grading (not gray)?
Contrast Sensitivity is the inverse of the contrast Threshold.
For 1 cycle per degree, the light must be 0.01 lighter than the background dark region (surface reflecting 1000 photons, the background reflecting 990).
A contrast of 100% (All light vs No light) corresponds to a contrast sensitivity of 1.
We can see about 60 cycles / degree at that sensitivity. 60 cycles per degree is 0.017.
Contrast Threshold
minimum difference in contrast for you to detect a pattern.
If a pattern is 1000 photons (brighter) alternating with 990 photons (darker), and this is the smallest difference in contrast for you to see this pattern, then
CT= (1000-990)/1000 = 0.01.
Contrast Sensitivity =
inverse of Contrast Threshold.
What is a Contrast Sensitivity of 1?
A contrast of 100% (All light vs No light) corresponds to a contrast sensitivity of 1.
We can see about 60 cycles / degree at that sensitivity. 60 cycles per degree is 0.017 (as mentioned before).
contrast sensitivity function (CSF)
Graph Details
The contrast sensitivity function (CSF) is a plot of the threshold contrast to detect the grating (as opposed to seeing a uniform gray) as a function of spatial frequency.
contrast sensitivity function (CSF)
Graph Details
The contrast sensitivity function (CSF) is a plot of the threshold contrast to detect the grating (as opposed to seeing a uniform gray) as a function of spatial frequency.
For photopic (daylight) vision, the CSF peaks around 2-4 cycles per degree. Note how sensitivity is reduced for mesopic (twilight) or scotopic (nighttime) vision.
Contrast sensitivity varies dramatically among different species.
mesopic vision
twilight vision
photopic vision
daylight vision
scotopic vision
nighttime vision
Contrast Sensitivity Function
over a Lifetime
Contrast sensitivity is quite poor at birth, but it improves gradually with development.
Contrast sensitivity is reduced with aging, primarily for high spatial frequencies.
Retinal cells like spots of light.
…
Ganglion cells respond well to stripes!!
Which stripes?
Depends on their Receptive Field!
The smaller the RF, the higher the Freq they like!
Phase
The phase of a grating refers to its position within a receptive field.
Not only is the spatial frequency important, but so is the phase.
Responses depend on the PHASE of the grating…
i.e., how the lines align with the RF
Two lateral geniculate nuclei (LGNs)
Axons of retinal ganglion cells synapse here.
Relays between retina and cortex.
6 layered structure.
Geniculate means bent.
Bottom 2 layers have larger cells: MAGNOCELLULAR layers.
Top 4 layers are PARVOCELLULAR (“small” in latin).
PART OF THE THALAMUS.
INHIBITION DURING SLEEP.
Opening your eyes won’t make you SEE at night!
Two types of layers in LGN:
Magnocellular and. Parvocellular
MAGNO:
M Ganglion Cells (Parasol cells)
Fast, large moving objects
PARVO:
P ganglion cells (Midget cells)
Details of static objects
Magnocellular
M Ganglion Cells (Parasol cells)
Fast, large moving objects
Parvocellular
P ganglion cells (Midget cells)
Details of static objects
The world is divided at the LGN:
Each layer:
input from ONE eye
organized map of half of the visual field…
i.e.,TOPOGRAPHICAL MAPPING
TOPOGRAPHICAL MAPPING
organized map of half of the visual field
LGN is not only a “relay” between eyes and visual cortex, it also….
….receives information from a number of other brain areas, functioning as a GATE to the cortex.
Ipsilateral
Referring to the same side of the body (or brain).
Contralateral
Referring to the opposite side of the body (or brain).
Striate cortex
Also known as primary visual cortex, Brodmann’s Area 17, or V1
Striate cortex
Also known as primary visual cortex, Brodmann’s Area 17, or V1.
Major transformation of visual information takes place in striate cortex.
V1 has about 200 million cells!
Retina Recap:
100 million photoreceptors
1-1.5 million ganglion cells
Retina Recap:
The Numbers
100 million photoreceptors
1-1.5 million ganglion cells
V1 has about ____ cells!
200 million
Major transformation of visual information takes place in _____ .
the striate cortex
2 important features of striate cortex:
Topographical mapping
Cortical Magnification
Topographical mapping
The organization of sensory surface matches the organization of the sensory world.
Neighboring ‘stuff’ in the visual field will be processed by neighboring cells.
Cortical Magnification
The dramatic scaling of information from different parts of visual field.
1 degree of visual angle at fovea is processed by 15 times more neurons than 1 degree of visual angle just 10 degrees away from fovea. Why?
ALL we need is high resolution detail from a very small amount of space. If more detail is needed, we simply focus eyes there!
Both the LGN in the Thalamus and the Striate Cortex are made up of …..
…6 layers
Visual acuity declines in an orderly fashion with ….
…eccentricity
One consequence of cortical magnification is that images in the periphery have much lower resolution than images at fixation.
This can lead to visual crowding: the deleterious effect of clutter on peripheral object detection.
Stimuli that can be seen in isolation in peripheral vision become hard to discern when other stimuli are nearby.
This is a major bottleneck for visual processing.
When we can’t see an object due to crowding, we have to move our eyes to look directly at it with our high acuity foveal receptive fields.
visual crowding
the deleterious effect of clutter on peripheral object detection.
Stimuli that can be seen in isolation in peripheral vision become hard to discern when other stimuli are nearby.
fMRI
functional magnetic resonance imaging
measures blood oxygenation level–dependent (BOLD) response.
David Hubel and Torsten Wiesel
DISCOVERY:
ELONGATED RECEPTIVE FIELDS.
(Not circular spots of light).
(1958). Nobel Prize.
Selective Responsiveness
Orientation tuning
Orientation tuning
tendency of neurons in striate cortex to respond optimally to certain orientations, and less to others.
More cells are responsive to Horizontal and Vertical than to Oblique lines.
How are the circular receptive fields in the LGN transformed into the elongated receptive fields in striate cortex?
Hubel and Wiesel: Very simple scheme to accomplish this transformation
circular receptive fields
LGN
elongated receptive fields
striate cortex
