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
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
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
*
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
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)
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
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
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
9
Q
What are the two theories of binocular summation?
A
- The independence theory (Probability summation)
- Interaction theory (Summation theory)
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
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
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
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
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
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
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
- Visual system averages the perceived brightness of each eye to arrive at binocular percept
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
Describe Dichoptic Masking
* **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
Describe Interactions in Abnormal BV
* **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
Summary of Binocular summation
* 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
What is intraocular transfer
aftereffects indicate interaction between 2 eyes at a higher cortical level
