wk 3 binocular vision

Advantages of binocular vision

1. Have a spare in case of injury or disease

2. Image distortion (reduced VA, contrast sensitivity or brightness perception) in one eye can be masked by other eye

3. Binocular visual field is larger than a single monocular field

4. Improved depth perception

Characteristics of BV (6)

1. Binocularly summation

2. Visual direction: describing location of an object

3. Corresponding retinal points

4. Abnormal retinal correspondence

5. Retinal disparity

6. Physiological diplopia: when double vision is normal

Binocular visual field

2 types:

1. Laterally directed eyes seen in hunted animals (prey)

2. Frontally directed eyes seen in predators

Laterally directed eyes (prey)

Panoramic field of prey (eg rabbits) have small binocular region, both in front and behind and a full 360 degree view

Frontally directed eye of predators

Eg cats give up 80 degrees of their field behind to allow a larger binocular field in front

Binocular visual field of humans

Have an extent of 190 degrees of which ~110 degrees is binocularly

• Overlapping fields increase the chance of detection through binocular summation and provide depth perception

Pictorial depth cues (monocular) - 9

1. Interposition

2. Perspective

3. Compression

4. Aerial perspective

5. Elevation

6. Lighting and shadows

7. Surface shading

8. Specular reflections

9. Size

Pre-requisites for binocularly vision

1. 2 eyes at least 6cm apart

2. Overlapping visual fields

3. Partial decussation (crossing) of visual pathway

4. Fusion of images (motor and sensory)

5. Provides stereopsis (2 flat images converted to perception of 3D)

Evolutionary changes required (4)

1. Eye location - laterality

2. Eye movements - fixation, convergence and divergence

3. Partial decussation

4. Eye-hand coordination

Decussation (crossing) - 3

1. No decussation

2. Complete decussation

3. Partial decussation

No decussation

Visual image from 2 eyes produce a discontinuous representation of the visual world to the visual cortex

Complete decussation

Gives panoramic vision with no stereopsis

Partial decussation

Partial crossing of the visual pathways in human vision

Human development of BV

1. Conjugate fixation from birth to 4-6 wks

2. Convergence is evident at 3 months

3. Accommodation slow in first 2 months

4. Stereopsis evident at 4 months and improves over 3-5yrs, reaches adult levels at 6yrs

5. VA and BV approaches adult levels by 3 yrs but reaches at 6yrs

Visual cliff experiment

Steroacuity develops after birth, dependent on visual experience.

• displayed by the visual cliff experiment as it demonstrates an infant’s ability to differentiate between a small or large drop

Stereoacuity development (human development of BV)

1. Babies show preference for stimulus displays w depth info from 4months.

2. Steroacuity improves over first 3 to 5yrs and reaches adult levels at 6yrs

Physiological evidence for BV (3)

1. Neurons from each eye project to same cell in visual cortex

2. Staining of ocular dominance columns

3. Electrical recordings

Neurons from each eye project to same cell in visual cortex

Cortical cells w similar orientation preferences are grouped together

Staining of ocular dominance columns

Each column represent input from L or R eye

Electrical recording

Some cells respond best when:

1. There is simultaneous stimulation of each eye project to

2. Stimulus is on the plane of single vision

3. Information to two eyes differs slightly

Single cell electrophysiology

1. Micro-electrode techniques are used to directly record the electrical activity of a neuron

2. In cats, some striate cortical neurons are responsive to signals from either eye, while others only respond to the stimulation of one eye

Process of seeing in depth

1. Fixate on object of regard

2. Image on fovea; best spatial resolution

3. Need accurate binocular eye movements

4. Motor fusion

5. Slightly different images seen by each eye

6. Images combined in brain to give sensory fusion

7. Production of 3D depth (stereopsis)

What happens when 2 eyes view an object

Their respective retinal images are from 2 distinct vantage points as the 2 eyes are in different locations in the head, separated by interpupillary distance

What is the difference between monocular views that lead to perception of stereopsis?

1. Ability to see in 3D is due to slightly different retinal images in each eye due to interpupillary distance between each eye

2. 2-5% of ppl have no stereopsis as they have abnormal BV

Objective vs subjective visual space

1. Our perception of space is subjective

2. This means that ppl w abnormal BV will not interpret the visual world the same way as ppl w normal BV

What happens when the info to the 2 eyes is dissimilar?

1. Suppression

2. Diplopia

3. Ocular dominance

4. Binocular rivalry

Binocular summation

Is improvement in visual sensitivity when viewing binocularly.

1. Contrast sensitivity improved in BV of temporally and spatially in phase sine waves

2. Binocular VA superior if acuities in 2 eyes are similar

3. Binocularly reactions times are shorter

4. Binoculars brightness sensitivity is 0.1log units better

Binocular summation - contrast sensitivity improved in BV

Reduced for out of phase sine waves compared to monocular sensitivity

Binocular summation: binocular VA better

1. If acuities in 2 eyes are similar

2. Increased sampling, reduced effects of retinal noise resulting in 1 line improvement on letter chart

3. Binocular VA worse than best monocular VA if VA is reduced in one eye

Binocular summation: brightness thresholds

1. Binocular brightness sensitivity is 0.1 log units better due to increased probability of light detection and summation of info

Fechner’s paradox

1. Neutral density (ND) filter placed over 1 eye during binocular viewing results in unequal brightness for 2 eyes( eye w/o filter is brighter)

2. Binocular brightness has greater brightness than brightness through unfiltered eye alone due to binocular summation

3. Paradox as receive more light viewed binocularly but perception of brightness is dimmer than A alone - indicates averaging process

Visual direction

1. Visual line

2. Visual direction (principal direction)

Perception of direction, objects lying in the same direction will stimulate the same retinal point

Visual line (visual direction)

1. Line connecting stimulus, nodal points and retina

2. Locus of points which all have the same monocularly visual direction

Visual direction/principal direction

1. Location in space in relation to the direction you are foveating

Relative visual directions

The relationship between the principal and secondary visual directions remain unchanged as the eyes change fixation

Hering’s law of identical visual directions

1. Fixate a mark on a window

• objects lying along principal visual direction of each eye when viewed monocularly

• Appear to overlap when viewed binocularly

2. Can also be demonstrated by placing afterimages at fovea of each eye

• always appear superimposed when viewed binocularly, regardless of vergence of 2 eyes

Cyclopean eye

1. Common subjective principal visual directions remain unchanged lies somewhere between the 2 eyes - location depends on ocular dominance

2. Consider the direction objects A and B w respect to an imaginary eye midway between the true eyes - world is viewed binocularly from this single (imaginary) eye

Egocentric directions for haplopia and diplopia

Haplopia = single vision

Diplopia = double vision

Corresponding retinal points

1. Pairs of retinal points (one in each eye) are perceived to have identical visual directions

2. Foveas represent one pair

3. Many other pairs associated with the secondary visual directions

4. The outputs of the corresponding retinal receptors of the 2 eyes merge on one cell in visual cortex

Abnormal retinal correspondence

1. In infantile strabismus, sensory organisation of binocular visual system can be altered

2. Development of abnormal retinal correspondence (ARC) may occur where a point other than foveal is used to fixate objects

3. Eccentricity fixation

4. Eccentricity viewing

Definition of ARC

1. Is an adapted shift in visual directions of the deviated eye relative to the normal visual directions of the fixating eye

2. An eccentric point in one eye is linked to the fovea in the other

3. Sensory adaptation eliminates the visual confusion that occurs in strabismus (common visual directions for fixated and non- fixated objects)

ARC: eccentricity fixation

Neurological remapping of the monocularly principal visual direction

ARC: eccentricity viewing

Purposeful aiming of a retinal locus other than the Foveal towards object of interest

Test for retinal correspondence

Hearing-Bielschowsky after image test:

• vertical line afterimage is generated in LE (Foveal projects to gap in the line, corresponding to a fixation point on the flash strobe) and a horizontal afterimage in RE.

1. In normal retinal correspondence (NRC), a perfect cross is seen

2. In ARC there is a relative displacement of 2 afterimages

Retinal disparity

1. Small differences in images of 2 eyes do not cause diplopia but are fused

2. Images of these objects don’t fall on corresponding points and result in disparity

3. Crossed and uncrossed retinal disparity

4. Too much disparity results in diplopia or suppression

Uncrossed disparity

Gives sensation of farness

Crossed disparity

Gives sensation of nearness

What happens when disparity is large

1. Fused single vision cannot occur which leads to physiologic diplopia, binocularly rivalry or suppression

2. In everyday life we are rarely aware of the diplopia

• occurs mostly in periphery

• Rivalry causes suppression of 1 image or we ignore 1 image

Physiological diplopia

1. Objects falling on non-corresponding retinal points appear to lie in different apparent directions and are seen doubled or in disparity

2. If we fixate on a distance object, nearer one in the same plane will be imaged on the temporal side of each retina and thus be seen in crossed diplopia/dispariy

3. Similarly, when fixating nearer object, distant one appears in uncrossed physiological diplopia/disparity

Suppression

1. If retinal images are very different, image or a section of the image from one eye or both will be suppressed to avoid diplopia

• dependent on form of the image and viewer’s BV system

• Ocular suppression (one eye views a homogenous field when one eye is patched)

Suppression in strabismus

In infantile strabismus, sensory organisation of binocular visual system can be altered

1. Development of suppression may occur (unlikely in the adult visual system)

• sensory adaptation is necessary to eliminate constant diplopia

2. Suppression can involve hemifields or small retinal areas (suppression scotoma)

Ocular dominance

Relative weighting of each eye’s input into the binocular percept, even though the cyclopean eye is situated midway between L&R eye but not exactly central.

1. Ocular dominance can be clinically determined - “sighting dominance”

2. Important when making prescribing decisions

Retinal/binocular rivalry

1. Occurs when very different objects are presented to each eye - strongest when dissimilar contours presented

2. Rhythmic alternating perception between each image - mosaic of 2 images (avg oscillation ~4s)

• fuse 2 squares using either crossed or uncrossed convergence

• Experience rivalry between vertical and horizontal stripes

• In any given region, stripes will sometimes appear vertical and sometimes horizontal (mosaic of images)

Binocular rivalry examples

2 half-views containing orthogonally oriented oblique lines are fused

• percept will consist of continuously changing “patchwork” of oblique lines

• Different regions of the figure are suppressed by one, then the other eye

Binocular luster

Occurs when objects are of the same shape and orientation but different colours or contrast polarities are fused.

• location of contours are the same but their luminance levels are different

• Perception of a shimmering silvery surface, like the reflection of light off a chrome surface