Visual Perception Final

Equations

100

Convert Snellen Acuity to Spatial Frequency

Spatial Frequency = (600 / Snellen Denominator)

Unit = Cycles per degree (cpd)

200

Convert Snellen Acuity to MAR

MAR = Snellen Denominator / Snellen Numerator

300

Convert LogMAR Visual Acuity to Snellen Visual Acuity

Snellen Visual Acuity = 10^(LogMAR)*20

400

What is the Michelson Contrast formula?

Michelson Contrast Formula (in %) = (L_max - L_min) / (L_max + L_min) x 100 = ___ %

500

What is the formula for the Nyquist limit?

Nyquist limit = 1/2 Photoreceptor sampling rate

**Anything beyond/above this Nyquist limit value will be aliased!**

600

Convert LogCS to CT (contrast threshold)

CT = 1/(10^LogCS)

700

What is the equation for negative predictive value?

Negative predictive value (NPV) = (True negative) / (False negative + True negative)

NPV = Probability that subjects with negative screening truly do not have the disease/condition

800

You have a light flash of 50 units on for 20 msec. Assuming Bloch’s law to be valid, if you now double the light flash luminance, what should the TIME DURATION be?

10 msec

Bloch's Law: I*t = K

50 * 20 = 1000 = K

100 * t = 1000. t = 10

900

Calculate the height of the 20/40 optotype for a regular projector chart at 20 ft

17.73 mm

(Problem #30 on Lab #2)

1000

A Yes-No experiment (i.e., asking patient if they can detect stimulus in question) is performed with 310 trials.

There are 50 blanks within this experiment

The subject reports seeing the stimulus 220 times

The stimulus was present but not seen for 50 trials

Calculate the number of 'false positive' responses

10 false positive responses

Duplex Retina

100

The system with overall greater temporal summation

Scotopic system

200

Shape of cone synaptic ending 

Pedicle

300

An environment with a luminance level of 10^-1 cd/m² elicits what kind of vision?

Mesopic vision

400

When the slope of the scotopic light adaptation curve is 1, (i.e., m=1) this law is followed

Weber's Law

Change in increment threshold (ΔI to the background intensity is constant (I_B)

k = ΔI / I_B

500

This law applies to stimulus up to 10 mins of arc in critical diameter

Ricco's Law (spatial summation): With critical diameter of 10 arc mins, total number of quanta needed to reach threshold remains same for different flash duration. 

Size/intensity interchangeable for detection! Reciprocal relationship between stimulus intensity and stimulus area

IA = K

600

Law that applies to stimulus when their diameter is greater than 10 arc mins

Piper's Law

Spatial summation less complete for larger stimuli

A^1/2*I = c

A = area

I = intensity

c = constant threshold value

700

Critical duration of photopic system

10-50 msec

800

50% of the cones recover from bleaching in this time

1.5 minutes

900

A bright light bleaches 100% of your patient's rhodopsin. What period of dark adaptation is required to ensure that 75% of the rhodopsin has regenerated?

10 minutes

1000

Property of the human eye that describes the directional sensitivity of the cone receptors. Light rays that strike cone perpendicularly are perceived as brighter (in photopic conditions).

When a pupil is decentered, cones shift themselves toward so they can still orient toward the pupil center.

This feature may be abnormal for patients who have retinal disease that causes disruption of cone orientation

Stiles Crawford Effect I

Visual Fields

100

Boundary between region of visibility and invisibility

Stimulus can be seen anywhere within it

Represent limits of equal retinal sensitivity (appear as concentric circles)

Isopters

200

State the vision requirements for driving in MA

At least 20/40 distant visual acuity (Snellen) in either eye, sc or cc and not less than 120 degrees combined horizontal peripheral field of vision

300

The peak of the Hill of Vision corresponds to this ocular landmark

Fovea

400

Most diagnostic visual field testing concentrates on the area ___ degrees of fixation

30

500

Type of perimetry where stimulus is moved from a non-seeing region of the visual field to a seeing region of the visual field while measuring its isopter

Kinetic perimetry

600

Short Wavelength Automated Perimetry (SWAP) assesses the function of this photoreceptor that is thought to be affected in the early stages of glaucoma

S-Cone

700

Humphrey Static Perimetry is an example of this type of visual field testing

Qualitative / Diagnostic field testing

800

An optic disc or nerve problem may result in this type of visual field loss

Central visual field loss

900

Background illumination of Humphrey Perimetry test

31.5 apostilbs

i.e., minimum brightness for photopic vision

1000

How many locations are tested in a 24-2 visual field?

54

each location tested at least 2x

Temporal Vision

100

What is temporal acuity?

The temporal frequency (i.e., the upper limit of the CFF) that represents the high temporal resolution limit of the visual system for a given depth of modulation

200

At high luminance, the critical flicker frequency is highest here

Mid-peripheral retina

300

Stimuli that fall inside of the temporal modulation transfer function (TMTF) are seen as...?

Resolved

Perceived as flickering

400

What are the CFF values in both scotopic and photopic vision?

Scotopic vision: 20 Hz

Photopic vision: 70 Hz

500

The Purkinje Tree effect is an example of this visual occurrence

Troxler Phenomenon

600

What does the Granit-Harper Law state?

The High Frequency CFF increases with the log of the STIMULUS AREA

▪ Flicker is more likely to be perceived if the stimulus is LARGE.

700

The low temporal frequency cutoff of the temporal modulation transfer function (TMTF) is caused by what?

Lateral inhibitory processes in the retina

800

An amblyopic having worse visual acuity with a standard eye chart rather than isolated optotypes is an example of this type of masking taking effect

Simultaneous masking

900

Temporal Modulation Transfer Function (TMTF) can assist in the early diagnosis of this ocular condition. 

This suggests loss of temporal sensitivity at moderate and high frequencies may be characteristics of this disease

Glaucoma

1000

Compare the (x) and (y) axes of the CSF/SMTF and the TMTF

CSF/SMTF: Sensitivity (spatial contrast) vs Function of spatial frequency

TMTF: Sensitivity (% modulation) vs Function of temporal frequency

Entopic Phenomenon

100

Most commonly-occurring entopic phenomenon

Floaters or Muscae Volitantes

200

Type of entopic phenomena caused by shock waves in vitreous humor hitting retina, resulting in the appearance of lightning type streaks

Moore's Lightning Streak

300

A form of VITREOUS degeneration in which calcium soaps (phosphates or phospholipids) aggregate in vitreous body and are visible as small mobile WHITE to YELLOW-WHITE opacities (in the vitreous humor).

- More prevalence in older patients with Male to Female ratio 2:1.
- Etiology is not clearly understood.
- Known to occur in humans, dogs, cats, horses.

Asteroid hyalosis 

400

Entopic phenomenon characterized by experiencing seeing light without light actually entering eye

Vague visual sensations

Arising when retina stimulated by energy other than light

Can be directly induced by mechanical, electrical, or magnetic stimulation of the retina or visual cortex, as well as by random firing of cells in the visual system

Common symptom produced = Flashing lights

Phosphene

500

Which vascular beds do "flying spots/corpuscles - white blood cells (i.e., blue field) entopic phenomenon originate from?

Capillary loops of the inner nuclear layer and outer plexiform layer

600

Maxwell's spot is an entoptic visual phenomenon appearing as a small, reddish or dark spot in the center of the visual field, often surrounded by a faint blue halo. 

It occurs because of the selective absorption properties of this macular pigment

(The size of the spot seen depends on how diffuse the Retinal Macular Pigment is for particular subject in question)

Xanthophyll

700

Best known occurrence of pathological haloes are those associated with this ocular condition

(This symptom is key to diagnosis of this condition!)

Angle Closure Glaucoma

800

Corneal edema will NOT occur (i.e., halos will not form in vision) if this particular layer of the cornea is able to accommodate this change

Corneal endothelial layer

900

Haidinger's brushes were NOT visible in light that exceeded this wavelegngth

> 560 nm

1000

Treatment for most severe cases of floaters

Vitrectomy

Visual Acuity / Factors Limiting VA

100

The smallest spatial separation between two nearby spots (points) or lines that can be discriminated or resolved

AKA, the minimum angle of resolution (MAR)

Resolution acuity

200

Resolving limit for fovea

1 arc min

300

This method of acuity is 'relatively' insensitive to optical defocus which makes it not an ideal way to measure resolution acuity in adults

However, it can be used to estimate resolution for patients who have cataracts to estimate their VAs post cataract surgery

Grating acuity

400

The minimum separation between two light sources that may be resolved into distinct objects.

Example: When the separations between two
stars (objects) on the retina is less than half the
spread of the each image, they will appear as a
single star

Rayleigh criterion

500

How would this stimulus be perceived?

Perceived as 'gray' or aliasing

Need at least 2 cones per cycle to perceive accurately/be able to resolve stimulus

600

The visual acuity measure most sensitive to optical defocus

Sensitive to the effect common clinical conditions have on vision

Recognition acuity

700

Ideal value for chart luminance?

Moderate photopic luminance (80-320 cd/m²)

800

A limit of the optical image formation of the eye

Increases defocus of eye. Decreases VA

Poor MTF due to large pupil (>6 mm)

Aberrations 

(spherical or chromatic)

900

This measurement of acuity is not the best descriptor of visual acuity because it's stimulus bound

(Meaning you can change the strength of the stimulus and alter the visual acuity value)

Detection acuity

1000

An observer with myopic astigmatism at axis 180 will have more difficulty distinguishing these letters

Letters with vertical contours

V, Y

Motion Perception

100

What is the formula for Temporal Frequency?

Temporal Frequency = (Velocity of Stimulus) * (Spatial Frequency)

200

This area in the visual cortex is responsible for processing motion

Damage to this area can result in akientopsia

Middle Temporal (MT) area; V5

300

Televisions, bowling alley signs, flip-books, and stop motion movies rely on this visual phenomenon to depict the illusion of motion. 

When sequentially flashed with an appropriate interval between the flashes, sense of motion is elicited

Stroboscopic motion

400

A consequence of direction-selective (DS) adaptation

After prolonged viewing of a moving
stimulus a stationary stimulus appears to drift in the
opposite direction

(literally his favorite picture)

Motion Aftereffect

500

The effect that is well known as an illusion, affecting pilots who fly at night

(Small isolated light in darkness; appears to be moving)

Autokinetic Motion Effect

600

This clinical test can be used to study complex global motion perception. It presents complex first-order stimuli that require the integration of motion cues across the visual space

Random dot kinematograms (RDT)

700

This visual phenomenon has been utilized to enable a type of stereoscopy, or 3D visual effect in visual media such as film and TV to create the illusion of a three dimensional image

The Pulfrish Effect

800

Achieved by selective inhibition of the magnocellular pathway (i.e., suppressing high temporal frequencies), this phenomenon allows us to look from 1 side to another without our vision being "smeared"

Saccadic suppression

(Vision is suppressed shortly before, during, and shortly after saccadic eye movements)

900

This area of the brain uniquely processes BIOLOGICAL MOTION

(biological motion = Ability to perceive object movement by connecting small stimuli; depicts natural movements of humans and other animals)

Posterior Superior Temporal sulcus Cortex (STC)

1000

As stimulus velocity increases, resolution acuity remains constant until the stimulus reaches this velocity

60-80 degrees/second

Spatial Vision

100

The least amount of contrast that can be seen (or resolved by the patient) in a chart. 

Expressed in %.

Contrast Threshold (CT)

200

Contrast sensitivity reaches adult levels within this age range 

Ages 7-9

300

Type of contrast in a letter chart

Weber contrast

400

Formula to calculate contrast threshold of the chart if only given contrast sensitivity (CS)

Contrast of the chart = [1 / CS] * 100 

answer is in %

500

The concept of retinal responses depending more on the local average intensity (i.e., local contrast) rather than absolute luminance

Perception of brightness of an object depends on CONTRAST more than absolute luminance.

Simultaneous contrast

600

The high spatial frequency cutoff (x-axis) of the contrast sensitivity function corresponds to this

Best visual acuity (VA)

700

Adult contrast sensitivity is at its peak at this particular value

4 cpd

800

How do you produce the perfect square wave?

By adding all the odd numbered harmonics (up to infinity)

900

Regions of increased or decreased brightness caused by the response of the visual system to luminance boundaries

It exaggerates the contrast between edges of the
slightly differing shades of gra y, as soon as they contact
one another, by triggering edge detection in the human
visual system Illusion of brightness

MACH bands

1000

Name 2 factors that contribute to the reduction in sensitivity for high spatial frequencies in the human visual system

Optical limitations (e.g., aberrations)

Packing density of photoreceptors (60 cpd cutoff due to spacing of foveal cones 0.5 min arc apart)

Psychophys. Methodology

100

What are the x- and y-values of the psychometric function?

X-Axis: Stimulus intensity

Y-Axis: Percentage of stimuli detected

200

Asking patients what the lowest line they can read on the Snellen chart is an example of this psychophysical method

Method of Descending Limits

300

Subjects with this criterion will have a high hit rate and a high false positive rate

Lax criterion

400

In optometry, we use these scales to grade redness, staining, bulbar roughness, and contact lens fittings

Ordinal scales

500

At low background levels, the human visual system acts as an ideal detector.

What law applies in this situation?

deVries-Rose Law

As background luminance levels increase, threshold change in luminance needed to differentiate test stimulus from background increases approximately as square root of background luminance

600

Automated visual field testing uses this psychosocial method

Staircase method

700

This law depicts a log relationship between intensity and sensation

S = c*log(I)

S = Sensation magnitude

I = Stimulus intensity

c = Constant related to Weber's constant

Fechner's log law

800

Law that states that sensory magnitude is proportional to the stimulus magnitude raised to some power

S = I^c

Steven's Power Law

900

What is the formula for specificity?

Specificity = True negative / (False positive + True negative)

Specificity = Percentage of patients who are healthy and test negative on given measure (i.e., correctly identified by test as NOT having condition)

1000

Name the x- and y- axes of the receiver operating characteristic (ROC) curve

X-Axis: Probability of false positive

Y-Axis: Probability of a hit

Depth Perception

100

Monocular patients manifest robust depth perception largely due to _________ cues

pictorial

200

Objects that produce a smaller retinal image are seen as ____________

further away

300

This monocular cue is used when viewing objects of known size 

Familiar size (size constancy)

Example = Familiar with size of average plane. See plane flying far up in bright blue sky in the distance with no other plane to reference size (i.e., cannot rely on relative size monocular cue in this situation so instead rely on familiar size monocular cue)

400

The angle the object makes with the horizon

Visual system uses this angle to determine object distance

Angular declination below the horizon

500

Kinetic monocular depth cue
Results when moving observer fixates an object while noticing the relative motion of surrounding objects
Relative motion provides information regarding relative distance

MOTION PARALLAX

600

The visual system uses this angle to determine object distance

Angular declination below the horizon

700

Name this optical illusion

Muller-Lyer illusion

Lines equal size but line that appears to form outgoing corner appears farther away than line that appears to form ingoing corner

800

Type of disparity that results in images forming temporal to fovea

Crossed disparity

900

Normal value for stereoacuity

40 arc seconds

1000

A common clinical practice of correcting

One eye corrected for distance, one eye corrected for near

Does result in some reduction of stereopsis

Monovision