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Binocular Summation Flashcards

1
Q

What are the two competing theories of binocular combination (Sensory Fusion)

A
  • Alternation or suppression theory - the monocular image reaching consciousness is alternated RT/LT
    • Never a simultaneous binocular percept
    • Monocular views mutually inhibit each other
    • Occurs with dissimilar monuclar images (binocular rivalry)
  • Fusion theory - similar images to right & left eye are attended to simultaneously
    • 2 eyes images are processed simultaneously
    • Occurs with similar monocular images
    • As the rivalry build up rate is slow - we know fusion theory is TRUE
    • Additionally, we know light & contrast incident on the 2 eyes are added together in the binocular percept
    • Binocular thresholds* would be LOWER than monocular thresholds
      • ex. luminance or contrast detection and perhaps acuity
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2
Q

Define Binocular summation & what are some examples?

A
  • additivity of the information from each eye to yield binocular visual performance (exceeding monocular performance)
  • Example:
    • VA, CS, CV, Light detection, Brightness matching, Reading, CFF, Reaction times etc
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3
Q

What are the different summation between two eyes?

A
  • Neural summation - partial, complete, facilitation, inhibition
    • Signal to noise ratio (SNR) - neural summation increases SNR
  • Probability summation - benefit to two eyes even if they are independent of each other
  • Neural summation and the impact on signal-to-noise ratio (SNR)
    • ​Signal correlated between right and left eye - sums
    • Neural noise differs between the right and left eye and would largely FAIL to sum
    • SNR improves by square root of the number of detectors
    • ex 2 eyes = sqrt(2) = 4
  • We can evaluate extent of summation by measuring visual tasks monocularly and binocularly (comparing)
    • Luminance detection
    • Visual acuity
    • Contrast detection
  • Complete binocular summation - binocular performance is sum of each eye’s monocular performance, i.e. neural summation
  • Partial binocular summation - incomplete summation of each eye’s visual performance
    *
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4
Q

Explain the degrees of binocular summation?

A
  • The degree of binocular interaction between the eyes is characterized by whether binocular performance on a task is
    • greater than
    • equal to
    • or less than the performance of either eye alone (monocular performance)
  • Binocular Facilitation: visual performance is better with both eyes than sum of each eye
  • Binocular inhibition: when used together to the performance of one eye is degraded by the other eye, visual performance with both eyes is less than the sum of each eye
  • No summation: Visual performance of two eyes are no better than one
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5
Q

Describe the first binocular summation experiment?

A
  • Stimulus: Square Wave Flicker
  • Independent flicker signals to right and left eye
    • in phase signals (left)
    • Out of phase signals (right)
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6
Q

Describe the Study on flicker detection, CFF and binocular summation

A
  • Higher perceived amplitude and higher CFF with summation of in-phase signals
  • Lower perceived amplitude and low CFF with summation of out-of-phase signals
  • Degree of binocular summation depends on temporal frequency
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7
Q

Contrast sensitivity plot linear x-axis scale

A
  • Binocular improvement over monocular constant from low to high frequency
  • Factor of improvement = 1.4 (higher sensitivity)
  • Equivalent to 40% improvement
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8
Q

T/F Flicker detection is the only visual function

A

False. It is NOT the only visual function

  • Absolute light detection: thresholds are slightly lower
    • but account for only partial summation
  • Visual resolution: improves binocularly
    • but only partial summation is noted
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9
Q

What are the two theories of binocular summation?

A
  • The independence theory (Probability summation)
  • Interaction theory (Summation theory)
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10
Q

Describe the probablity summation

A
  • Independence theory - even if our eyes work pefectly independently we can expect lower thresholds under binocular conditions
  • Left eye detects: 60% (0.6) threshold
  • Right eye detects: 60% (0.6) threshold
  • This form of probability summation is just one statistical model that may not accurately
    • 0.84 probability of detecting a stimulus is 1.4X or 40% better than 0.60
    • Other statistical models of prediction exist as well. None is accurate
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11
Q

Binocular summation vs probability summation

A
  • Binocular summation occurs ONLY IF both eyes are stimulated within short interval
  • Similarly, Binocular summations occurs ONLY IF both eyes stimulated within corresponding areas
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12
Q

Describe interaction theory (Summation theory)

A
  • True binocular summation has been shown to occur for light flashes and grating stimuli
  • Interaction theory (vs independence theory)
    • predicts an increase to sensitivity (decreased threshold) under binocular conditions greater than probability summation alone
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13
Q

Descibe neural summation

A
  • Neural summation is convergence of monocular information in the visual system, into binocular pathways improves detection threshold under binocular conditions
  • Neural (binocular) summation: occurs under limited conditions
    • Temporal synchrony (OD/OS) - <100ms
    • Corresponding points (OD/OS) or points within panum’s area
    • Similar stimuli (OD/OS)
  • Strabismus, amblyopia, and anisometropia
    • can prevent synchronous stimulation or stimulation within corresponding points & disrupt fusion and binocular summation
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14
Q

Describe the loss of binocular summation in infants

A
  • Binocular vision problems may manifest as loss of binocular summation over a wide range of stimulus conditions
  • Visually evoked potentials (VEP) can be used to evaluate binocular summation in young infants
  • Losses of binocular summation in infants should be noted early, before binocular vision compromise
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15
Q

Is there still a benefit to having two eyes under SUPRATHRESHOLD conditions?

A
  • Studies with increment thresholds, orientation discrimination tasks and grating stimuli reaction time suggest YES,
    • but maybe less benefit than at threshold
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16
Q

Monocular vs Binocular Brightness

A
  • one can also compare brightness of a target seen by one eye to that seen by both eyes
  • ex. when reading a paperback the page does not appear brighter with 2 eyes than with one
17
Q

What is the Fechner’s Paradox - Monocular vs Binocular Brightness

A
  • What if we have targets of different brightness presented to OD & OS
  • OD brighter than OS
  • Target - seen as less bright than OD with both eyes open
  • This is called Fechner’s paradox
  • Fechner’s Paradox: The apparent increase brightness of a figure caused by closing one eye after viewing the target with both eyes open
18
Q

What are the 3 possible explanations for Fechner’s paradox?

A
  • Initially 2 hypothesis, then third
  • Independence hypothesis - eyes are acting totally independence
    • Right eye stimulus brighter
    • Close left eye (with dimmer stimulus)
    • Explanation of this hypothesis
      • Predicts no changes in appearance for right eye (which is not true)
      • ex. no difference in brightness perception under binocular vs monocular conditions
    • However, this conflicts with Fechner’s paradox
    • So, CANNOT BE CORRECT
  • Summation hypothesis - binocular brightness is the sum of OD/OS monocular brightness’s
    • Binocular percept brighter than either monocular percept
    • Hypothesis makes 2 incorrect prediction
      • One binocular percept brighter than monocular percept (which does not occur)
      • Two, it predicts that closing the eye with a filter, dimmer image will result in perceived dimming. (this is the exact opposite of what happens)
  • Average hypothesis
    • ​Visual system averages the perceived brightness of each eye to arrive at binocular percept
      • In case of unequal brightnesses in the 2 eyes, the binocular percept would be midway between the perceived brightness of each eye alone
      • This hypothesis CORRECTLY predicts - that shutting the filtered eye would result in perceived brightness of the target light
19
Q

Describe the Visual Aftereffects

A
  • Aftereffect - biased perception of a target to be seen by fatiguing tuned visual neurons with an adapting (cortically mediated)
  • Motion aftereffects - perceived motion reverses after adapting to motion in a particular direction (occurs binocularly & monocularly). This is a visual illusion.
  • Can demonstrate similar aftereffects by adapting to either
    • orientation (tilt aftereffect) or
    • spatial frequency (size aftereffect)
  • Binocular interaction in visual aftereffects - if visual effects can transfer INTEROCULARLY, then the two eyes are shown to NOT be independent of each other.
20
Q

Do aftereffects transfer interocularly?

A
  • Interocular transfer - present adapting stimulus to one eye and then test stimulus to fellow eye and an aftereffect is still perceived
  • Yes, ex tilt aftereffect transfers interocularly, but not as strong as the monocular after effect
21
Q

Describe the monocular tilt aftereffect

A
  • The aftereffect results from fatiguing tuned visual neurons
  • Results in a biased final percept (e.g. tilt bias)
22
Q

Describe the binocular tilt aftereffect - intraocular transfer

A
  • The aftereffect results from fatiguing tuned visual neurons (in V1 & V2) areas
  • More binocularly driven cells in V2 > than V1
  • Results in a biased final percept (ex. tilt bias)
23
Q

Describe motion aftereffect - intraocular transfer

A
  • Is a visual illusion
  • Exhibits interocular transfer, with the magnitude of the aftereffect being weaker for interocular transfer than that of the monocular aftereffect
  • Results from fatiguing tuned visual neurons (in V1 & V2) corticol areas
24
Q

Describe random-dot motion displays - intraocular transfer

A
  • Exhibits evenmore interocular transfer than do moving grating stimuli
  • Occurs because: Cortical area MT - contains a high concentration of binocularly driven cells (which is instrumental in the computation of motion coherence)
25
Q

Describe Dichoptic Masking

A
  • Visual masking - degradation of test stimulus percept by masking stimulus presented before, with, or after stimulus
  • Dichoptic visual masking occurs when test stimulus is presented to one eye and masking stimulus to fellow eye
  • This effect is STRONGER when mask is presented to the dominant eye
  • Crowding effect (A form of simultaneous masking) - flanking contours near a target reduce visibility of that target (ex presenting flanking contours dichoptically to fellow eye)
    • Ex: Commonly seen in hyperacuity tasks such as vernier acuity. Lines parallel to the vernier target and in close proximity cause vernier thresholds to increase
  • Metacontrast masking - reduction in target visibility by a subsequently presented mask reduces target visibility
26
Q

Describe Interactions in Abnormal BV

A
  • Abnormal Binocular Interactions in stereoblind - occur in pt with strabismus, high anisometropia or aniseikonia, or amblyopia who have lost binocularity
  • The stereoblind are expected to have abnormal binocular summation, but may show probability summation
    • ​However, no binocular summation beyond the predicted probability summation
  • Impact on binocular summation depends on condtion
  • Stereoblind strabismics have been found to be impacted more than anisometropic amblyopes
  • Similarly,interocular transfer of aftereffects can be reduced or lost in stereoblind pt (ex. early onset strabismus - esp high spatial frequency stimuli)
  • Excitatory and inhibitory binocular interactions may be impacted differently in amblyopia
  • ex. amblyopic subjects do not exhibit, subthreshold binocular summation (excitatory)
  • However, dichoptic masking can still occur (inhibitor)
27
Q

Summary of Binocular summation

A
  • Binocular summation is additivity of RT/LT eye information
  • Binocular summation is an advantage over probability summation
  • Binocular summation occurs at and above visual threshold (ex. brightness averaging)
28
Q

What is intraocular transfer

A

aftereffects indicate interaction between 2 eyes at a higher cortical level

Binocular Vision (8 decks)

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