Test 3 Flashcards
Rivalry
When your brain can’t fuse the images. Sensory fusion requires that similar images be presented simultaneously to corresponding points in the two eyes or panums.
Two types of binocular rivalry
- Exclusive dominance.
- Targets are perceived to come from exclusively 1 eye, then the other.
- Small targets, eccentric targets. - Mosaic Dominance.
- Targets are perceived as mosaics/zones.
- Large targets, close to foveal targets.
*not under voluntary control. Images on nasal retina tend to dominate over temporal retina.
Binocular suppression 2 types
Physiological: Suppression in absence of strabismus.
Pathological: Suppression secondary to strab or ocular abnormality.
If the two eyes have equal dominance, then the eye presented the weaker image will be suppressed. Weaker images include
Dimmer, blurred, stationary, or images on nasal retina (weaker than temporal retina).
(Motion can be used to break suppression, strong stimulus)
Why is amblyopia more common in ET than XT?
Nasal retina suppression tends to be deeper than temporal.
3 requirements for rivalry
Dissimilar stimuli for the two eyes
Continuous alteration of dominance within zones
Wavelength specific
3 requirements for suppression
Suppression can occur with or without same stimuli
More constant
Wavelength does not affect suppression.
Da Vinci Stereopsis
Visual system can use rivalry as a cue to relative distance.
The size of zone of suppression decreases with ___ spatial frequencies
Increasing. Smaller details.
Binocular luster
Dissimilarities in color, luminance or contrast appear shimmery
Where does rivalry and suppression originate in the brain
Must be in V2 and beyond because stereo can occur in the presence of rivalry and stereo happens in V1.
Wolfe and blake proposed theory about mechanisms for rivalry
Two monocular channels, one binocular channel.
If corresponding images, the binocular channel is activated.
Does not account for mosaic rivalry
ET binocular suppression scotoma
In the deviated eye from the fovea to avoid confusion and nasal retina to avoid diplopia. Usually very deep. Size is proportional to angle of deviation
XT binocular suppression scotoma
Usually not as deep as ET. Extends from fovea to etire temporal hemiretina of the deviated eye.
Methods to detect/measure suppression
Worth 4 dot Phoria testing Vergence Vectograph 4BO - should see nasal re fixation movement. Bagolini lenses. X is normal.
Monocular cues to depth
Aerial perspective- less contrast farther away Interposition Shadow Retinal image size Motion parallax (moving) Reactive velocity Looming
SOLI SILO
Adding BO can cause pt to perceive images as this. Depends on brain’s interpretation from EOMs
Absolute distance/depth
The distance of a single object from the ego center of the observer. Does not change with vergence. Determines the binocular parallax
Binocular parallax
Vergence angle based on the absolute distance.
Does not depend on vergence angle of the eye, only nodal points.
Absolute disparity
The difference between the binocular parallax of an object and the convergence angle of the eyes. Stimulus for vergence!
Absolute disparities smaller then __ min of arc will not stimulate a vergene movement because it will fall in panums.
6 mins of arc
Relative horizontal disparity
The difference between the absolute horizontal disparities of the two points. Encodes the relative depth between the fixated objects and other objects in the binocular field of view.
Required for stereo.
Angular disparity
Relative disparity between the front and back of and object. Stimulates depth within the object.
Depth constancy
The perception that the depth within a familiar object (or distance between two objects) is unchanged despite the change in binocular disparity.
Absolute vertical disparity
Difference between the angles of elevation measured separately of the two eyes for one point.
Relative vertical disparity
The difference between the absolute vertical disparities of two points. Not affected by vertical misalignment. Do not yield perception of depth.
Disparity gradient
Linear change in horizontal disparity across space. Encodes slant.
Disparity gradient= disparity / separation.
Binocularly linked images
two types
Images of any type that engage the disparity detection system.
Linked corresponding images- correctly matched images.
Linked non corresponding- Incorrectly matched images and produce depth
Disparity in the stereogram
vs
real depth
Disparity in stereogram= 1/ distance
Real depth= 1/ square root of distance
Line or contour stereograms are used to evaluate
Local stereopsis. Individual elements such as lines, edges, or contours are displaced in each half view to yield disparity. Ex: wirt circles.
Random dot stereograms are used to evaluate
Global stereopsis. One half view has some portion of it’s elements displaced laterally relative to those elements in corresponding positions.
Large areas of the binocular view must be matched by finding patterns
Seeing shapes in random dot stereograms requires
Local and global stereo. Takes longer to do this. Object recognition is not a prerequisite
The corresponding problem
information from elements does not restrict the number of possible partings between the left and right eye’s images. Brain must figure out which match is correct.
Depth ambiguity- no obvious solution about which dot each eye should fixate in order to fuse.
Correlation using pixels
Mean luminance is zero
Bright dot is 1
Dark dot is -1
Correlation= 1 x number of pixels / # of positions
When all positions are identically filled = +1
When all are randomly filled = 0
Half are identical, half are random = 0.5
Stereoacuity (Dmin)
Minimal retinal disparity that can be detected.
Threshold for humans is 4-5 ec of arc. Possible to record as low as 2.
98% of pop has stereo better than
40 sec
Wy doesn’t good stereoacuity translate to good stereopsis all the time?
Some people demonstrate asymmetries in the ability to see crossed vs uncrossed disparities.
Disparity angle =
2pd x distance between the two targets
_______________________________
fixation distance ^2
Are fine or large disparities processed first?
large.
Magnocellular and parvocellular system and disparities
Magno sees coarse disparities in motion
Parvo sees static fine disparities.
Best stereoacuity is when
Targets are separated 15-50 sec of arc.
Duration is at least 100 msec
Natural fixation (slight movements)
Stereoacutiy worsens when
Targets are closer than 15 or further than 50 sec of arc
Short duration = threshold increases
Motion more than 2-3 degrees/ sec
Monocular blur
Blue target (S cones have large receptive fields)
How does contrast affect stereoacuity when less than or greater than 10x the threshold
Greater than 10x the threshold
stereo = 1/ contrast ^3
Less than 10x the contrast
Stereo= 1 / contrast ^2
Mechanisms to detect motion/disparity changes in the Z axis
Serial processing- uses both eyes to detect disparity over time,
Parallel procssing- using monocular half images to look at velocity differences.
When stereo fails
Patent Quantitative
Latent Qualitative
Patent- Disparities within and slightly beyond panums. Direction and magnitude can be detected.
Latent- Stereo from a point greater than panums. Can detect direction, but not magnitude.
Dmax (upper limits of stereo) increases with
Retinal eccentricity.
Stereoscopic scotoma vs stereomotion scotoma
Stereoscopic- cannot perceive crossed or uncrossed.
Stereo motion- cannot perceive motion but can see static stereo
Stereo and strabismsus
Worse stereo thresholds (Dmin) due to crowding. Upper limit (D max) is still in tact.
Poor static stereo because esotropia? Can usually appreciate stereo motion better.
Corpus callosum
Detects coarse disparities larger than 2 degrees. along the vertical midline.
LGN layers and ocular dominance
Magnocellular- layers 1, 2 (ventral)
Parvocellular- layers 3-6 (dorsal)
Contra- 1, 4, 6
Ipsi- 2,3,5
V1 libel and wiesel cell types
Type 1- only contra
Type 4- both
Type 7- only Ipsi
OD columns are most clear where
Layer 4C. Highest proportion of monocular neurons.
__% of V1 neurons are binocular. Except in layers 4C
50%
3 broad types of neurons in V1-V3
- Flat neurons. Binocular cells that don’t care about stereo.
- Reciprocal.
- Near and far. - Disparity tuned
- Tuned zero and inhibitory
- Tuned near and far
Area v2 contains the cytochrome oxidase stripes
Thick- magnocellular. Coarse stereo. with high temporal frequency.
Thin- parvocellular. Fine stereo, low temporal frequency.
V5 (MT)
Specific for stereo motion.
Feeds into middle superior temporal area (MST) which gives information about self motion.
Postioer parietal cortex lesion
Impairs stereo, spatial memory and spatial attention
VEP beat responses
Duanes has decreased amp based on what you would expect but better binocular summation.
Difference between positional and phase disparities during cortical coding
Positional- binocular neuron has highest firing rate when stimulus falls on R and L receptive fields with horizontal offset.
Phase- Centers of receptive fields are aligned, but their on/off regions are offset.
Critical periods for retina vs brain
retinal develops earlier in critical period
Spatial and binocular vision takes longest to develop.
Convergence and fine stereo develop faster than
Divergence and fine monocular VA in children
LGN development at birth
Already segregated into layers at birth. Do not need visual stimuli.
Hebbs rule
Connections about synapses.
How does artificially induced strab affect LGN and V1
Slight shrink in LGN parvo cells
loss of V1 binocularity.
Constant strab- more cells in category 1.
Alternating strab- Equal cells in category 1 and 7
What compounds cause cortical plasticity
NMDA
Nerve growth factor
NE
Sherington
First evidence of binocular summation with flicker. Could see more Hz when in phase OU
Do we have better binocular summation/faciliation at low or high temporal frequencies?
Facilitation and low
linear summation at mid
Partial summation at high
We need a slower moving target to see depth.
Contrast sensitivity is better with binocular viewing. The improvement of binocular over monocular contrast sensitivity is a factor of
1.4
Independence and interaction theory
Independece- Brain uses probability summation to detect stimuli wrong.
Interaction-
signal must arrive to the two eyes at same time.
must be on corresponding points on retina
Stimuli must be similar
Fechners
One bright light and one dim light= brain will average it.
