Lecture 10 - Absolute

What is the balancing act with static perimetry?

test time versus accuracy

1. separation of degrees (spacing/location)

2. threshold (1-4 dB)

uses algorithms to optimize

Why does static perimetry continuously repeat apply spots ~15° temporally?

blind spot located

test for accuracy (gaze tracking and trigger happy patients)

What is static perimetry the best at detecting?

1. small changes in threshold

2. central changes

What is kinetic perimetry the best at detecting?

1. small changes in spatial extent of a defect

2. peripheral changes

3. remaining vision in advanced disease

What are the common uses of static perimetry?

1. glaucoma

2. macular disease

3. visual ability testing

What are the common uses of kinetic perimetry?

1. neuro-ophthalmological conditions

2. peripheral retinal diseases

3. low vision

What is the background luminance of visual field testing?

close to ambient lighting conditions

should in a lighting range where Weber's law holds: small changes in retinal illumination have an equal effect on background and stimulus luminance (constant Weber's fraction)

What is the next generation of dark adaptation testing?

AdaptDx Pro - head-mounted test system, virtual technician provides the patient instructions through the test

How do visual threshold change with scotopic conditions?

detect dimmer light with rod-based vision

threshold is lower (sensitivity is higher), saturate at higher light levels

How do visual threshold change with photopic conditions?

better resolution with cone-based vision

cones recover quicker from bleaching, much more saturation - Weber's fraction lower, acuity is better

Outside of the eye, can rods detect one photon?

yes

Can a human subject detect a flash of light that contains one photon?

no - only about 10% of the photons in original light flash are absorbed by rods

50% of photons in a light flash would get back to the retina (absorbed by cornea, lens, vitreous, etc.), another 40% would pass through (not absorbed) or are absorbed by cones not rods

Assuming only one photon was absorbed by the rod - can we detect that?

no, problem is noise

noise due to spontaneous isomerization of 11-cis retinal to all-trans retinal

roughly happens every 2 minutes in a single rod (120 million rods)

What is the Stiles-Crawford effect?

light entering an adult cone directly, via the center of the pupil, is more likely to be absorbed, and looks brighter

vice versa

light entering a cone at an angle, via the edge of the pupil, is less likely to be absorbed and looks dimmer

Why does peripheral entering light look dimmer? And why is this an advantage?

not completely absorbed by the outer segment

helps with aberrations

What are some reasons why rods are more sensitive than cones?

1. Stiles-Crawford effect (peripheral cones)

2. cones are noisier

3. less signal amplification in cones

4. voltage response to 1 photon in cone only 1/10 of rod

5. rods exhibit better spatial and temporal summation

What is spatial summation?

adding together of responses that occur in different photoreceptors

Which photoreceptor is better at spatial summation? Why?

rods

easier for the rod to absorb 10 quanta (needed to reach threshold) over its larger area

What is Ricco's Law?

threshold is independent of spot size if quanta in spot remain constant

english - small lights need 10 quanta to be seen (reach threshold), larger lights need >10 quanta to be seen

for a light that is 2x width of critical diameter, light would have to contain twice the photon number

What is the test light size required for rods to see a stimulus?

10 mins of arc

What is temporal summation?

adding together of events that occur at slightly different times

Which photoreceptor has better temporal summation?

rods - sum up more information over time than cones

less resolution therefore!

Which stimulus will a rod see?

threshold - 10 luminance

temporal summation threshold - 100 msecs

1. two 5 luminance lights across 120 msecs

2. two 5 luminance lights across 80 msecs

2. two 5 luminance lights across 80 msecs

why? - add lights together, under 100 msec summation threshold

How many lights will a rod see?

threshold - 10 luminance

temporal summation threshold - 100 msecs

two 5 luminance lights 5 msecs apart

rod will see one light whether or not they're above or below (have to add up above threshold) threshold

How many lights will a rod see?

threshold - 10 luminance

temporal summation threshold - 100 msecs

two 10 luminance lights 120 msecs apart

two lights

What is the critical duration for rods?

100 ms

What is the critical duration for cones?

10 ms

What is Bloch's law?

up to the critical time duration, as long as the threshold number of quanta are delivered during this period, it does not matter how they are delivered

english - if the light(s) reach threshold within the critical duration, one light will be seen

if multiple lights outside the critical duration (assuming reach threshold), multiple lights will be seen

What were the conclusions of Hecht et al.?

1 photon was enough to induce a response

100:10:1 rule

100 photons on cornea

10 photons on retina

1 photon minimum number of photon a rod can signal for

What are the absolute threshold for small, brief flashes at scotopic conditions and photopic conditions?

scotopic - 100 photons

photopic - 600 photons

ratio of 6

What are the absolute threshold for large, long flashes at scotopic conditions and photopic conditions?

scotopic - 10^-4 trolands

photopic - 10^-1 trolands

ratio of 1000

rods see 1000 times better dimmer than cones