Visual Perception Lecture 3: Visual Fields - Concepts

Visual Field

the total area in which objects can be seen in the side (peripheral) vision as you focus your eyes on a central point - the extent of object space visible while the eye steadily fixates a target

What does visual field testing measure?

peripheral vision

visual field for clinical purposes is tested monocularly

Where does the normal (monocular) human visual field extend to?

-60 degrees - nasally

-100 degrees - temporally

-75 degrees - inferiorly

-60 degrees - superiorly

What is the monocular visual field horizontally without eye movements?

150 degrees

Binocular Visual Field

-horizontally - 180 degrees wide (with both eyes open)

-overlapping part - 120 degrees

What is the Massachusetts driving requirement?

at least 20/40 distant VA (Snellen) in either eye, with or without corrective lenses, and not less than 120 degrees combined horizontal peripheral field of vision

What is the incidence of visual field loss in the general population?

-3-5% - incidence increases significantly with age

-the cause of visual field loss may be sight or life-threatening

-in some cases, a visual field defect is the only evidence of a disease or disorder, or other clinical signs are not detected

Why is a normal visual field diagnostically important?

-serves as a baseline measurement (glaucoma, retinal disease)

-helps to follow-up the progression of certain ocular or neurological conditions associated with visual system

-helps to counsel patients on their peripheral vision, especially driving

What is an advantage of visual fields?

allows the clinician to assess the integrity of the visual pathways through noninvasive means

What can damage to the visual pathways produce?

-generalized or localized loss of vision (visual field defect/scotoma)

-careful analysis of the visual field defect can assist the clinician in localizing the damage within the visual pathway and determining its cause

What are the 3 levels of visual field testing?

-visual field screening

-qualitative or diagnostic field testing

-quantitative field testing

Visual Field Screening

-rapid: general or problem specific screening for gross VF defect

-ex: confrontational visual fields, tangent screen

Qualitative or Diagnostic Field Testing

-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

-ex: Humphrey Static Perimetry (Carl Zeiss, Octopus 900, 600 (HAAG-STREIT Diagnostics)

Quantitative 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 are some examples of visual field screening?

-confrontational visual fields: count finger confrontations and color (red-cap) confrontations

-tangent screen

Perimetry

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

How is perimetry tested?

monocularly - the monocular field is 3D (resembles a hill)

What area do most diagnostic visual field testing concentrate on?

area within 30 degree fixation - where most photoreceptors-retinal ganglion cells are located

What are the 2 types of perimetry?

-static

-kinetic

Hill of Vision

-"an island hill of vision in a sea of darkness" - Harry Moss Traquair

-described as a 3D representation of differential light sensitivity at different positions in space

What is the hill of vision like under photopic conditions?

light sensitivity is greatest centrally and falls off in the periphery (looks like a hill)

What does the peak in the hill of vision represent?

fovea (fixation)

What does the pit in the hill of vision represent?

physiological blind spot (absolute lack of sensitivity)

What do the outermost edges in the hill of vision represent?

-the absolute limits of the visual field

-outside these edges - very large/bright objects cannot be seen

What is the hill of vision like under scotopic conditions?

the peak light sensitivity is flattened and suppressed

What is the height or sensitivity of the normal hill of vision affected by?

-age

-the level of ambient light (photopic vs. scotopic)

-stimulus size

-stimulus duration

-associated pathology

Visual Field Defect

-any reduction (or statistical depression) of sensitivity compared to the normal hill of vision

-can be localized (scotoma) or general depression

What are visual field thresholds not affected by?

senile miosis and nuclear sclerosis (cataract)

What does visibility depend more on?

-more on object contrast than on absolute brightness

-Weber Fraction remains constant

Isopter

-areas of equal retinal sensitivity

-in the determination of visual fields, it is the contour line representing the limits of equal retinal sensitivity (same threshold) to a given test target

What is the variant of isopters dependent on?

characteristics of the test spot (size, brightness, etc.)

Where can the stimulus be seen in relation to the isopter?

-stimulus cannot be seen anywhere outside the isopter (the stimulus is infrathreshold outside the isopter)

-the stimulus can be seen anywhere inside the isopter (the stimulus is suprathreshold inside the isopter)

therefore, the isopter is a boundary between a region of visibility and invisibility for a particular stimulus

Threshold

threhold = 1/sensitivity

the minimum quantity of stimulus that can be detected

What is threshold in relation to visual pyschophysics?

-lowest (minimum) intensity at which a stimulus can be detected 50% of the time (also referred as absolute threshold)

-pyschology, physiology: the point at which a stimulus is of sufficient intensity to begin to produce an effect

What is threshold in relation to visual field testing?

the minimum light intensity that can be detected

Where is threshold the lowest in visual field?

lowest in the fovea and increases with eccentricity in all directions

Suprathreshold Stimulus

test stimulus that is stronger (larger or brighter) than the threshold stimulus - stimulus is visible to observer

Infrathreshold Stimulus

test stimulus that is weaker (smaller or dimmer) than the threshold stimulus - stimulus is not visible to the observer

Sensitivity

inverse of threshold: 1/threshold

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

Visual Sensitivity

minimum perceivable brightness

Decibel (dB)

-values refers to stimulus intensity

-logarithmic unit (of stimulus/light - intensity/brightness)

-0 dB: maximum brightness perimeter can produce

-it is relative to the brightness stimulus available on the particular model of perimeter

Apostilb

an old unit of luminance (withdrawn from use in 1978)

What is the current unit of luminance?

candela per square meter (cd/m^2)

3.14 asb = 1 cd/m^2

Why is the decibel scale significantly more convenient to use than actual luminance values?

because the eye appears to perceive differences logarithmically

What is the decibel scale directly related to?

directly related to sensitivity - areas of higher sensitivity on the Hill of Vision are designated by higher decibel values for the threshold stimulus

Where is the highest decibel value in a normal eye?

at the fixation point

highest dB values (perceived) --> better visual sensitivity

Blind Spot

-no rods/cones (optic nerve)

-there are no rods and S cones in the center of the fovea

Kinetic Perimetry

-measuring the isopter for a given stimulus from slowly moving the stimulus from a non-seeing to a seeing region of the visual field

-ex: Goldmann perimetry

What is kinetic perimetry used for?

-one attempts to find locations in the visual field of equal retinal sensitivity

-by joining these areas of equal sensitivity gives an isopter (nasally, superior nasally, superiorly, etc.)

Threshold in Kinetic Perimetry

-the location where the stimulus just becomes visible

-the exact location of the kinetic isopter for a stimulus depends on the rate of movement of the stimulus

Static Perimetry

-visual threshold is measured at a series of fixed points in the visual field

-the brightness of the test spot is varied, but not its location

-threshold is usually plotted relative to normal fields to reveal defects

What is the procedure in static perimetry?

-the size of the test spot and the background illumination are kept constant throughout the testing

-the dimmest stimulus that can be seen is determined (by presenting the test spot at various intensities above the background illumination)

-threshold: weakest (dimmest) white spot that is visible against the background

What happens during static perimetry (photopic condition)?

-the patient views a fixation target while a stimulus is presented in another region of his visual field with various intensities

-similar to light adaption experiment increment threshold is determined at a number of large number of retinal locations

-threshold values are compared to norms established by the manufacturer - software programs allow visual sensitivity of various regions to the visual field to be compared with other regions

Threshold Testing

-provides more diagnostic information than suprathreshold testing

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

Why are threshold testing findings always subject to some variability?

-patients make mistakes

-the visual system itself is subject to certain variabilities

-successful strategies: balance time efficiency with provisions to counteract such errors

How long did threshold testing take in the early days of automated perimetry?

20 min/eye

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

Humphrey Threshold Testing

-start testing at a single location in each quadrant of the visual field

-if a stimulus is seen --> subsequent stimuli at that location are dimmed (usually by 3 or 4 decibels) until they are no longer seen

-conversely, if the initial stimulus is not seen --> then subsequent presentations are made brighter in steps until the patient responds

-uses SITA, the Swedish interactive threshold algorithm

Suprathreshold Testing - Screening

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

-presents the patient with fairly bright stimuli that should be seen if vision is normal, easy to use with patients who have never been tested before

-take considerably less time than threshold tests

What is a disadvantage of suprathreshold testing (screening)?

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

-as a result, suprathreshold testing is used less often in glaucoma diagnosis now

Humphrey Visual Field Analyzer

-gold standard for perimetry

-automated

-static perimetry

What is the stimulus for Humphrey Visual Field Analyzer?

-stimulus of known

-size and intensity projected onto a screen having a known

-brightness for a known

-amount of time at a known

-location in the visual field

What areas does the Humphrey Visual Field Analyzer test?

-mostly the area within 30 degrees of fixation

-30-2 --> 76 locations

-24-4 --> 54 locations

-each location tested at least twice

-pyschophyical procedure - stair case/adaptive technology

What is the background illumination for Humphrey Perimetry?

-projects stimuli against a background with a brightness of 31.5 apostilbs

-this adaptation level was chosen because it approximates the minimum brightness for photopic vision (vision that depends upon retinal cone function rather than on rods)

What is the advantage of testing the photopic visual system in Humphrey Perimetry?

-visibility depends more on object contrast than on absolute brightness as it does in rod vision

-small changes in 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 like in Humphrey perimetry?

-200 milliseconds (1/5 of a second), which is long enough for visibility

-not affected by small variations in duration, but still shorter than the latency for voluntary eye movements (about 250 ms)

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

What is the principal of temporal summation for very short durations?

the visibility of a stimulus increases with duration

What is the principal of temporal summation for long durations (when stimulus lasts more than about 0.5 seconds)?

the stimulus visibility is basically independent of duration

What is the stimulus size and intensity like in Humphrey Visual Field?

-white stimuli

-brightness range: 51 db (5.2 log units), 0.08 asb to 10000 asb

-background brightness: 31.5 apostilbs

What is the total deviation for Humphrey Visual Field indices?

-total deviation probability plots indicate all test locations that are outside normal limits

-general depression of the whole visual field or localized loss

What is the threshold sensitivity of Humphrey Visual Field compared with?

-compared with age-matched normal values at each test point to produce the total deviation (TD) decibel plot

-negative values - indicate sensitivities that are below the median age-corrected sensitivity

-positive values - indicate higher than normal sensitivities

What is the range of sensitivity (variability) in Humphrey Visual Field indices?

-larger in the periphery than in the center of field

-larger superiorly than inferiorly

-therefore, a depression of 5 decibels from age normal may be significant at the center of the field, but totally within the normal range of variability in the periphery of the test area

Pattern Deviation (PD) Probability Plot

-most useful analysis

-shows sensitivity losses after an adjustment has been made to remove any generalized depression

What is the great strength of PD probability plots?

-they ignore variations that are within the normal range and highlight subtle, but significant, variations that might otherwise escape notice

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

What do PD plot primarily highlight?

-primarily highlight only significant localized visual field loss

-if visual field loss is so far advanced then the PD plot will NOT be able to highlight localized loss

Total vs. Pattern Deviation

-if the plots look more or less the same --> then there is little or no generalized loss

-a uniformly depressed TD plot combined with a normal-looking PD plot --> probably indicates cataract

-the opposite pattern - a normal TD plot and an abnormal-looking PD plot --> often trigger-happy patient who has repeatedly pressed the response button when no stimulus was seen loss

Visual Field Index (VFI)

-designed to be less affected by cataract

-provides improved correspondence to ganglion cell loss compared to MC

-VFI is ~100% in normal and ~0% in perimetrically blind fields

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

-MD is 0 db in normal and ~30 db in extreme visual field loss

Pattern Standard Deviation (PSD)

-reflects irregularities in the field - caused by localized field defects

-if PSD small/close to zero --> both in normality and blindness

-peaks at moderate levels of localized loss

Short Wavelength Automated Perimetry (SWAP)

-aka blue-yellow perimetry

-developed to assess S-system function in certain eye diseases - partially glaucoma (at early stages)

What are S cones like in the eye?

-there are no S cones in the center of the fovea, and they are relatively sparse throughout the retina

-ganglion cells that compare S cones to L and M cones carry a blue/yellow signal, and are also relatively sparse

What is blue-yellow perimetry designed to do?

designed to stimulate a small percentage of ganglion cells to make a loss of cells more obvious

What color is the background in SWAP in order to suppress the M and L cones?

yellow rather than standard white

What is the technique used in SWAP?

blue Goldmann size V stimuli are presented on a bright (100 cd/m^2) yellow background

What does SWAP detect?

-progression of field loss in glaucoma patients earlier than standard perimetry

-found to be more sensitive than standard perimetry in detecting neuro-ophthalmic disease, ARMD, migraine, and diabetic macular edema

Central or Peripheral Visual Field Loss

loss may be central/foveal (e.g. an optic disc or nerve problem) or peripheral (along the visual pathways from the optic chiasm back)

Scotoma

-defect surrounded by normal visual field

-may begin as a gradual enlargement of the blind spot and may pass unnoticed by the patient until quite large

Relative Scotoma

an area where objects of low luminance cannot be seen but larger or brighter ones can

Absolute Scotoma

nothing can be seen at all within that area

What are some common causes of visual field loss?

-glaucoma

-retinal conditions

-neurological conditions

What are some common retinal conditions that can cause visual field loss?

-cone-rod dystrophies

-diabetic retinopathy

-retinitis pigmentosa

-detached retina

-papilledema

-optic nerve atrophy

What are some common neurological conditions that can cause visual field loss?

-visual pathway lesions

-neurological damage such as from optic neuritis

-brain damage from stroke, disease, or injury

-head injuries

What are some common causes of central field loss?

-ARMD

-optic neuropathy

-Leber's optic atrophy

-macular holes

-cone dystrophies

-retinal artery occlusion

-a number of rare conditions like Best's disease, Stargardt's disease, and achromatopsia

What are some common causes of peripheral field loss?

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

-retinal detachment

-retinitis pigmentosa

-chorioretinitis

-branch retinal artery occlusion

What are some common causes of artifatual field loss?

-small pupil (<3 mm)

-lensholder, lens rim

-significant uncorrected refractive error

-fatigue

-cataract; media opacities

-fixation losses

Generalized VF Depression

-age

-cataract; media opacities