Visual Perception: Visual Fields

What is visual field?

Refers to the total area in which objects can be seen in the side (peripheral) vision as you focus your eye on a central point

What does visual field testing measure?

Peripheral vision

In normal (monocular) human visual field, how many degrees does our vision extend to? (Nasally, temporally, inferiorly, and superiorly)

60-nasally

100-temporally

75-inferiorly

60-superiorly

Without eye movements, the monocular visual field horizontally is about how many degrees?

150 degrees

Visual field for clinical purposes is tested how?

monocularly

How wide (degrees) is our binocular visual filled horizontally? Overlapping part is how wide?

Horizontally-180 degrees

Overlapping-120 degrees

What is the driving requirement?

20/40 at distant and not less than 120 degrees combined horizontal peripheral field of vision

Visual field loss affects how much of the general population? It increase with what?

affects 3-5% and it increases significantly with age

The cause of visual field loss may be because of?

greater clinical significance: sight or life-threatening, sometimes visual field defect is only the evidence

What are the 3 reasons to use test visual fields?

1. Serves as a baseline measurement

2. used as a follow-up the progression of certain ocular or neurological conditions associated with visual system

3. Help to counsel patients on their peripheral vision

What the 3 advantages to visual field testing?

1. Allows us to assess the integrity of the visual pathways through noninvasive means

2. We can see if the visual pathways produce generalized or localized loss of vision

3. We can localize the damage within the visual pathway

What are the 3 levels of visual field testing?

1. Visual Field Screening

2. Qualitative or Diagnostic field testing

3. Quantitive Field testing

What is Visual field screening? Example?

Rapid-general or problem specific screening for gross VF defect

Example: Confrontational Visual Field, Tangent Screen

What is Qualitative or Diagnostic field testing? Example?

Designed to determine the characteristics of a VF defect, such as the location borders, shape, and size, homonymous defect etc to determine the cause and location of underlying condition

Example: Humphrey Static Perimetry

What is Quantitive Field testing?

To fully quantify a known or suspected VF defect so that future changes can be detected. Also performed to establish some baseline VF

What is involved in Confrontational Visual Field testing?

1. Counter finger confrontations

2. Color (Re-cap) confrontations

What is Perimetry?

The clinical procedure in plotting (or the science of measuring) the visual fields

How is Perimetry tested?

Monocularly, the monocular field of vision is 3D (resembles a hill)

Most diagnostic visual field testing concentrates on the area within how many degrees of fixation?

30 degrees

What are 2 types of Perimetry?

Static and Kinetic

What is the name of the 3D hill used in Perimetry?

Traquir named it the Hill of Vision aka an island hill of vision in a sea of darkness

What does the hill of vision represent

differential light sensitivity at different positions in space

What happens to the hill of vision in Photopic condition?

Light Sensitivity is greatest centrally and falls off in the periphery (looks like a hill)

How does the hill correspond to the eye?

peak- fovea (fixation)

pit in the hill-bling spot (absolute lack of sensitivity)

Outermost edges- absolute limits of the visual field, outside of the edges very large/bright objects can't be seen

What happens to the hill under scotopic conditions?

the peak sensitivity is flattened and suppressed

What affects the height/sensitivity of the normal hill of vision? (4 things)

1. Age

2. The level of ambient light

3. Stimulus Size

4. Stimulus duration

So how is the visual field defect found in the hill of vision?

Any reduction (or statistical depression) of sensitivity compared to the normal hill of vision, can be localized or general depression

What is a Isopter?

Areas of equal retinal sensitivity

What is a isotopes in the determination of visual fields?

It is the contour line representing the lists of equal retinal sensitivity (same threshold) to a given test target

What is Sensitivity? What is Sensitivity equal to?

The ability of an organism or part of an organism to react to stimuli

Sens=Inverse of Threshold

What is light sensitivity?

the minimum perceivable brightness

In Dim light, is the threshold high or low? sensitivity high or low? Is it detected? Overall Visual sensitivity?

Low threshold, high sensitivity, detected, visual sensitivity is excellent

In Bright light, is the threshold high or low? sensitivity high or low? Is it detected? Overall Visual sensitivity?

High threshold, low sensitivity, not detected, and visual sensitivity is poor

What is threshold and what is the equation?

Its the minimum quantity of stimulus that can be detected or the lowest minimum intensity at which stimulus can be detected 50% of the time (absolute threshold)

Threshold=inverse of sensitivity

For a visual field test, the threshold is?

the minimum light intensity that can be detected

What is decibel? What kind of unit is used?

The values refers to stimulus intensity. Logarithmic unit

What is the maximum brightness a perimeter can produce?

0 db, it is relative to the brightness stimulus available on the particular model of perimeter

What is a apostilb?

old unit of luminance, units are candela per square meter

3.14 asb=1 cd/m2

Why is the decibel scale used more than luminance values?

1. The decibel scale is significantly more convent to use than actual luminance values, because the eye appears to perceive differences logarithmically.

2. decibel scale is directly related to sensitivity

For a normal eye, what is the decibel value?

The highest decibel value is at the fixation point, high db values means better visual sensitivity

Most diagnostic visual field testing concentrates on what area?

within 30 degrees of fixation-where most photoreceptors/ retinal ganglion cells are located

Blind spot of course has no rods/cones (optic nerve)

What is kinetic perimetry?

Example?

Stimulus is slowly moved from a non-seeing to a seeing region of the visual field

Example: Goldmann perimetry

What happens during kinetic perimetry?

one attempts to find locations in the visual field of equal retinal sensitivity, which leads to a isotoper

What are some items that don't affect visual field thresholds?

-senile miosis and nuclear sclerosis (which cause reduction in retinal illumination)

What remains constant during kinetic perimetry?

Weber fraction

What is static perimetry?

During photopic condition, the patient views a fixation target while a stimulus is presented in another region of his visual field with various intensities. It is similar to light adaptation experiment, except increment threshold is determined at a number of large number of retinal locations.

How are threshold values compared?

-They are compared to the norm established by the manufacture

What are the 2 types of static perimetry?

-Threshold testing and Suprathreshold testing

Which one provides more diagnostic information?

Threshold

What is threshold testing?

-to determine the minimum stimulus intensity (visual sensitivity of the patient) that can be seen at each tested location

Why are the findings of TT have some variability?

1. Patients make mistakes

2. The visual system itself is subject to certain variabilities

Successful strategies: balance time efficiency with provisions to counteract such errors

Why did automated perimetry didn't work?

1. Took too long, 20 min/eye

2. Test strategies were not very efficient

Efficiency improved: by using test results at a measured point to determine the initial stimulus brightness at adjacent points and so on

How is the Humphery testing conducted?

1. Starts at a single location in each quadrant of the visual field

2. If a stimulus is seen, then a subsequent stimulus at that location are dimmed until they are no longer seen

3. If the initial stimulus is not seen, then subsequent stimulus at that location is made brighter until the patient responds

Uses SITA (Swedish interactive thresholding algorithm)

What are some characteristics of Suprathreshold testing compared to TT?

-Has a different goal compared to TT

-referred as screening

-brighter than threshold

-takes less time than threshold

- do not provide quantitative data and they are not as sensitive to early glaucomatous field loss

-used less often in glaucoma diagnosis

What is the objective of Supra?

-establish whether or not sensitivity is abnormally low at any location in the visual field

What happens during Supra Testing?

1. Presents the patient with fairly bright stimuli that should be seen if vision is normal

What is the stimulus of the Humphrey Visual Field?

-A known size/intensity, brightness, amount of time, and location

What is being tested during Humphrey?

-Mostly the area within 30 degrees of fixation

-30-2, 76 locations

-24-2, 54 locations

-Each location tested at least twice

-Psychophysical procedure: Stair case/ Adaptive technology

How bright is the background illumination in Humphrey Perimetry?

-31.5 apostilbs

Why was 31.5 chosen as a background?

-it approximates the minimum brightness of photopic vision (vision that needs cones)

What is the advantage of testing photopic visual system?

-Visibility depends more on object contrast

-pupil size or clarity of media have little or no effect on test results

-Small irregularities in background brightness can be remedied by commensurate adjustments in stimulus brightness to keep stimulus contrast at the desired level

What is the stimulus duration for the Humphrey Perimetry?

200 milliseconds

-Which is long enough for visibility

- not affected by small variations in duration

-shorter than the latency for voluntary eye movements

-patient does not have time to see a stimulus in the peripheral visual field and look toward it

What is the principle for temporal summation for short and long duration?

short: visibility of a stimulus increases with duration

long: stimulus lasts more than about 0.5 seconds, the stimulus visibility is basically independent of duration

What is the stimulus size and intensity for the humphrey visual field?

It is a white light stimuli

Brightness range: 51 db (5.1 log units) (0.08 asb to 10000 ask) Think of the box Decibels vs Stimulus Luminance-Higher lum and low decibel

Background brightness: 31.5 apostilbs

What does total deviation plots indicate?

all test locations that are outside normal limits such as general depression of the whole visual field or localized loss

What is threshold sensitivity compared with? What do negative and positive values indicate?

age-matched normal values at each test point to produce a total plot

Negative- below the median age-corrected sensitivity

Positive- higher than normal sensitivities

Is the range of sensitivity (variability) large in the periphery or the center of the field?

Periphery and also larger superiorly than inferiorly

What is pattern deviation?

Most useful analysis, it shows sensitivity losses after an adjustment has been made to remove any generalized depression

What is the greatest strength of the PD?

they ignore variations that are within the normal range and highlight subtle but significant variations

Beginning field defects regularly show up earlier in the probability map than in grayscale printouts

What does the PD plots primarily highlight?

only significant localized visual field loss, but if it is so far advanced then the PD plot will not be able to highlight localized loss

What are the 3 main comparisons of total vs pattern deviation?

1. If the plots look almost the same then there is barely any generalized loss

2. depressed TD and normal PD probably means there is a cataract

3. abnormal PD and normal TD means trigger happy patient

What is a visual field index?

Designed to be less affected by cataract. Provides improved correspondence to ganglion cell loss compared to Mean deviation

100% normal and 0% in perimetrically blind fields

What is mean deviation (MD)

shows how much on average the whole visual field departs from age-matched normal. Used to assess changes over period of time.

Normal: 0 db and -30 to -35 in extreme field loss

What is pattern standard deviation?

Reflects irregularities in the field-caused by localized field defects

small/close to 0 are found in normality and blindness

What is the Short wavelength Automated Perimetry (SWAP) do?

-aka blue-yellow perimetry

-assess S-system function in certain eye diseases (particularly glaucoma)

What does SWAP suppress and what does the background look like? Technique?

surpasses M and L cones and the background is yellow

Technique: Blue Goldmann size V stimuli are presented on a bright (100 Cd/m2) yellow background

SWAP has been known to find what because of its sensitivity?

1. Glaucoma

2. neuro-ophthalmic disease

3. ARMD

4. Migraine

5. Diabetic macular edema

What are 3 types of visual field loss?

1. Central-optic disc or nerve problem

2. Peripheral- along the visual pathways from the optic chiasm

3. Scotoma (Relative-low lum. can't be seen/Absolute-nothing)-defect surrounded by normal visual field. It may begin as a gradual enlargement of the blind spot and may pass unnoticed by the patient until quite large

Common causes of Visual Field loss

1. Glaucoma

2. Retinal conditions

3. Neurological conditions

Common causes of Central field loss

1. Age-related macular degeneration

2. Optic neuropathy

3. Leber's optic atrophy

3. Macular holes

4. Cone dystrophies

5. Retinal artery occlusion

6. A number of rare conditions like Best's disease, Stargardt's disease and achromatopsia

Common causes for peripheral field loss?

Glaucoma (angle-closure glaucoma and open- angle glaucoma)

Retinal detachment

Retinitis pigmentosa

Chorioretinitis

Branch retinal artery occlusion

Common causes of Artifactual field loss

Small pupil

Lensholder, Lens rim

Significant uncorrected refractive error

Fatigue

Cataract; media opacities

Generalized VF depression

Age and cataract/media opacities