Fusion and failure to fuse generate two quite different effects

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Chapter 4 Summation and Rivalry
Fusion and failure to fuse generate two quite
different effects
1. Fused targets Binocular Summation As
clinical experience with binocular vs. monocular
visual acuity testing will affirm, people
generally have slightly superior vision with two
eyes than with one. We call this slight
advantage binocular summation. 2. Unfused
targets Binocular Rivalry When two different
images arrive at a single location in the visual
cortex, we might imagine several possible
perceptual outcomes we might see a blend of the
two images, we might see only through our
dominant eye, or we might see part of the image
through one eye and part through the other. The
latter is generally true, but it is always
changing, and we refer to this alternation in
suppression as binocular rivalry.
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The phenomenon of rivalry use Red/green glasses
to view this demonstration of rivalry. First
view with either R or L eye, notice that each
eyes sees different orientations. What will you
see when both eyes view the target?
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Use mirror to perceptually superimpose two of
your classmates. Neural images of the two faces
arrive at the same location in the brain. What
do you see? A hybrid? One face (seen by your
dominant eye)? Alternating faces? Fragmented
faces? Fragmented faces that are unstable?
Experiment
Equipment Needed One small mirror.
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Rivalry
Free-fuse these pairs of gratings (diverge so
that your R and L eyes fixate the right and left
gratings, respectively)
The effect of contrast on predominance
The effect of size on fragmentation
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Rivalry Waves
Free-fuse these pairs of gratings (diverge so
that your R and L eyes fixate the right and left
gratings, respectively). Take time to see if you
can observe a wave of dominance moving around
this stimulus.
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During rivalry, only one of two non-fusible
stimuli are seen. There are two simple
hypotheses consistent with this observation H1
There is true binocular rivalry, and when one
stimulus appears to dominate it is because neural
signals from the other eye are suppressed. H2
Binocular rivalry is really a suppression of the
neural response generated by one of the two
non-fusible stimuli. In this scenario, if for
example, the right eye sees vertical stripes and
the left eye sees horizontal stripes, then the
rivalry suppresses neurons responding to either
vertical or horizontal orientations.
A simple experiment, designed by R. Blake allows
us to discriminate between these two hypotheses.
In the above movie, the right and left grating
targets are switched. If the right eye is
looking at the right target and the left eye at
the left (under converged), then this stimulus
change switches the stimuli seen by the R and L
eyes. If hypothesis 1 is correct, and, for
example, the RE is dominant, then as the RE
stimulus is changed from up/left to up/right
diagonals, the dominant RE will see this.
Alternatively, if hypothesis 2 is correct, and
up/left neurons are suppressing up/right neurons,
then as the targets are changed, whatever
stimulus is dominant will continue to be seen
irrespective of which eye the signal arrives
from. Result as the stimulus changes, we see it
change. Conclusion..Complex logic, so think
this through carefully.
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• Properties of Rivalry/Suppression
• Rivalry takes about 1 second to develop
  (initially both stimuli are seen simultaneously).
• Supression during rivalry in normals is more
  likely as RE and LE stimuli are more dissimilar
• Suppression in strabismics is more likely when
  the RE and LE stimuli are more similar.
• Suppression reduces VA and CS.
• Rivalry suppression in normals does not reduced
  contrast adaptation, tilt after-effect, and the
  motion after-effect, and thus seems to accur
  after V1
• Strabismic suppression prevents contrast
  adaptation, and thus probably occurs in V1.

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The neural basis of rivalry. Where in the brain
does it occur?
The surface field potentials measured above the
occipital lobe (Visually Evoked Potentials, or
VEP) show that under conditions of rivalry, the
right and left eye VEPs amplitudes are inversely
correlated (when one increases the other
decreases). This is evidence that rivalry occurs
in the visual cortex or before. It is difficult
to locate the precise site of rivalry with this
approach.
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The neural basis of rivalry. Where in the brain
does it occur?
In humans, when two different orientations are
seen by each eye, each VEP scalp response
recorded above V1/V2 oscillate in opposition and
in synchrony with perception. In monkeys viewing
rivalrous stimuli, when recording responses from
single cells, there are no oscillations in the
LGN, small fluctuations in lower visual areas (V1
and V2, blue lines), but complete suppression of
responses in the inferotemporal cortex (red
trace) during perceptual suppression phase.
V1
IT
perception
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BOLD fMRI data also confirm suppression in higher
cortical areas. Record FACE response from the
fusiform gyrus and the BUILDING response from the
parahippocampus
Perceptual Change
Perceptual Change
face
house
face
house
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Summation
Since we have two eyes, do we see twice as well?
The general answer is no, we tend to see
slightly better, and in some cases (1.41
or 41) better.
Contrast Sensitivity Function (CSF)
Binocular
Contrast Sensitivity (1/contrast threshold)
Monocular
low
high
medium
Spatial Frequency (c/deg)
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Binocular Summation Rules
This graph shows how much right (R) and left (L)
eye contrast is required to maintain a constant
binocular signal (B) as the proportion of right
and left eye contrast is varied from only RE
through typical binocular viewing (RL) to only
LE..
YX line, RELE, normal binocular viewing
Monocular LE threshold
1
0.8
When binocular thresholds are lower by 1.41, then
binocular sensitivity is higher by 1.41
0.6
Left eye contrast
0.4
0.2
BM/2
0
1
0
0.2
0.4
0.6
0.8
Monocular RE threshold
Right eye contrast
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Mechanisms for Binocular Summation 1.
Probability Summation Statistical basis,
assumes complete independence of the two eyes,
observed with unfused targets. 2. Neural
Summation Neural signals from R and L eyes
converge and sum at binocular neurons in visual
cortex resulting in better than probability
summation, observed with fused targets (more than
41 improvement)
Example of probability summation To see a
target either the RE only sees it, the left eye
only sees it, or both eyes see it
simultaneously. Thus, probability that target is
seen when both eyes look is 1-p(neither eye sees
it). For example, if the probability that the RE
or LE alone can see the target is 0.6, what is
the probability that the target will be seen when
both eyes look? P(bin) 1 - (p neither eye see
it) 1 - (0.4 0.4) 1-0.16 0.84. Thus,
with two eyes looking, you can perform a little
better even without any neural summation.
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Monovision on and in eye corrections for
presbyopia
Distance Correction
Near Add, e.g. 1.5
One eye is corrected for distance and one for
near. Success occurs when the patient only sees
the focused image (blur suppression).
Eye
Right
Left
Eye
Near Object
Eye
Right
Left
Eye
Monovision works best with low add powers or
early presbyopes with residual accommodation.
Distant Object
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Worth 4 dot test
Clinical test for suppression
Colored filters
Stimulus
LE supp
RE supp
Examples of appearance during suppression
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Worth 4 dot test
Normal Fusion
Alternates R/G fusion rivalry
Eso deviation w/ diplopia
Exo deviation w/ diplopia