BV4 - panum's area and rivalry

what is Panum's area?

an area in the retina of one eye, which when stimulated simultaneously with a point in the retina of the other eye provides single binocular vision

How can you measure the angular size of Panum's area?

moving rod in and out of VM circle until it goes double and recording the break → the area between the inner and outer limits is the size of Panums area

How does Panum's area change in different areas of the retina?

Panum's area is smallest in the fovea and larger in the periphery

What is fixation disparity?

an ocular deviation less than the size of the diplopia threshold

how does the fixation disparity change the Panum's area outside of the fovea?

increases the measured PA (for other eccentricities) by size of PA at the fovea → why still able to see single

english: Panum's area is bigger in the periphery because fixation disparity increases

What is the size (literal) of Panum's area at the fovea?

7 minutes of arc = 1/20th of a degree

What is the diplopia threshold at the fovea?

3.5 mins of arc in each direction (3.5 exo, 3.5 eso) → half of Panum's area

How does spatial frequency affect Panum's area?

high spatial frequency (smaller objects) → smaller Panum's area

low spatial frequency (larger objects) → larger Panum's area

How does temporal frequency affect Panum's area?

high temporal frequency → smaller Panum's area

low temporal frequency → larger Panum's area

How does the shape of Panum's area change with changing temporal frequency?

higher temporal frequency → circular

smaller temporal frequency → elliptical

How does the shape of Panum's area affect us clinically?

elliptical shape (smaller Panum's area vertically) indicates it is easier to induce diplopia with vertical prism than horizontal → we have smaller vertical vergence ranges then horizontal

What does binocular rivalry appear as?

confusion → an overlay of dissimilar targets causing alternating suppression/perception

How is confusion from rivalry eliminated?

by alternate suppression of one eye

What does the suppression area size equal to during suppression in rivalry?

the size of Panum's area

What is the equation for the rate of rivalry?

rate = 1/(tL+tR)

t = time seeing/cycle with that eye

What is Levelt's law?

the duration of impression of one eye (tL or tR) decreases as the stimulus strength in the other eye increases

What is stimulus strength in Levelt's law?

power of contralateral suppression → how quickly it will steal perception back from the other eye

ex: ↑ stimulus strength to L eye determines how fast L eye will steal perception

How can you increase stimulus strength?

1. increase target contrast

2. increase contrast sensitivity of the eye

what influences contrast sensitivity of eye?

1. light adaptation

2. retinal eccentricity

How does the time seeing with each eye and rivalry rate change when you increased contrast in the left eye?

R = decreases

L = no change

rate = increases

therefore relative predominance increases for left eye

How does the time seeing with each eye and rivalry rate change when you increased contrast in the right eye?

R = no change

L = decreases

rate = increases

therefore relative predominance increases for right eye

How does the time seeing with each eye and rivalry rate change when you decrease contrast in the left eye?

R = increases

L = no change

rate = decreases

relative predominance increases for right eye

how does the time seeing with each eye and rivalry rate change when you increase contrast in both eyes?

R = decreases

L = decreases

rate = increases

relative predominance unchanged

How does the time seeing with each eye and rivalry rate change when you decrease contrast in both eyes?

R = increases

L = increases

rate = decreases

relative predominance unchanged

How can you clinically decrease the suppression of the right eye in amblyopia?

reduce stimulus strength of left eye:

-patch OS

-atropine OS

-bangerter foil OS

-optic blur OS

How does rivalry change among the binocular field?

independent across binocular field → one eye can be dominant in one part of the image while the other eye in another part

How does rivalry change depending on field size?

rivalry is asynchronous for large fields

rivalry is synchronous for small fields

what is synchronous rivalry?

figure-ground segregation

"rivalry different in front than background"

What is the dominance of contour?

if contour to one eye and uniform field to another → the uniform field is suppressed

What is Levelt's law of complementary shares?

binocular luminance is the weighted sum of the monocular luminances → weighted more for dominant eye

why is brightness viewing with both eyes the same as with one eye?

luminances of 2 eyes are averaged NOT summed

How is binocular brightness changed when a filter is placed in front of dominant eye compared to over the non-dominant eye?

binocular brightness is less when filter placed over dominant eye

ex: room appears brighter when cover APD eye

When does a luster/chrome appearances appear with binocular fusion of luminance?

if the right and left luminances differ by >10:1 → no longer fusible

When does dichoptic color presentation cause color mixing?

colors will mix if there is no background and a small (<2 degree) field

When do you get chromatic luster in binocular field?

when 2 different colors (one presented to each eye) is on a luminous background and large field

How does the suppression zone in rivalry change when the targets are flashed/flicker? Why?

the suppression zone may not be evident

this is because development of rivalry takes >200msec and Panum's area is small for brief exposures

How does the Cheshire cat illusion work?

movement in front of one eye increases the stimulus strength of that eye and decreases the time looking with other eye → increase in dominance in the eye with movement

ex: look at face with OS and look at mirror with OD while erasing causes face to disappear

how does rivalry suppression eliminate confusion?

when non-fusible images fall on corresponding points one is alternately suppressed

How do patients with strabismus avoid confusion?

1. suppression gets embedded in strabismus → the fixating eye becomes dominant and the rate of rivalry becomes very slow

2. change in ARC (retinal correspondence) to shift Panum's area

3. enlargement of Panum's area

What does harmonious ARC enable and facilitate?

corresponding points shift so common images in each eye now correspond → enables suppression and might enable motor fusion

Why is fusion poor with ARC?

there is a difference in image quality between the two eyes → one eye is at the fovea, the other is shifted

What is a suppression scotoma?

suppression extends into non-corresponding areas