Midterm 2: Timing

Question 1

What is visual persistence?

Answer 1

apparent persistence of a visual stimulus beyond its physical duration.

Example: stimulus presented less than 120-150 msec will appear to last 120-150 msec

Question 2

What is the Broca-Sulzer effect?

Answer 2

The brightness of a suprathreshold flash of light depends upon its duration.
As the duration of light flash having a fixed luminance increases, its brightness first increases (predicted by Bloch’s Law), and then decreases (not predicted by Bloch’s Law).

Question 3

Under photopic conditions, how long of a flash will give maximum brightness?

Answer 3

50 msec

Question 4

Under scotopic conditions, how long of a flash will give maximum brightness?

Answer 4

100 msec

Question 5

X and y axis of Broca-Sulzer Effect graph. Units

Answer 5

Y-axis = Comparative brightness
x-axis = time
units = lux

Question 6

Neural Explanation of Broca-Sulzer Effect

Answer 6

• Intense stimuli produce photoreceptor overshoot
• This produces (via the bipolar cells) an initial burst of action potentials in the ganglion cells.
• Brightness is related to the firing rate of the cells. (spikes/second)
• For long flashes, the firing rate after the initial burst signals the brightness
• For brief flashes, only the initial burst occurs, so the only information the neurons in central structures can use is a high
 firing rate, which makes the flash appear brighter than when
 the flash is long.

Question 7

What is temporal masking?

Answer 7

influence that one stimulus has upon another when stimuli are presented at slightly different times.

Question 8

3 Different types of temporal masking

Answer 8

Simultaneous masking- The 2 stimuli are presented at the
 same time. (“Crowding”)

Forward masking - The “masking” stimulus is 
 presented before the “test” stimulus.

Backward masking- The “masking” stimulus is presented
 after the “test” stimulus.

Question 9

______________ (backwards) and ____________(forwards) are masking in which the test flash and masking flash do not overlap spatially on the retina.

Answer 9

metacontrast; paracontrast

Question 10

Describe backward masking effect. How does it work?

Answer 10

A flash of light can influence our perception of one that was presented BEFORE it!. Second light must be very bright.

Brighter stimuli send signal quicker than dim signal so the brain processes them simultaneously or sooner than first flash

Question 11

What is a duty cycle?

Answer 11

Proportion of time with light on vs light off for a given cycle. A cycle is the time for 1 pulse which is addition of both time light is off and on.

Question 12

What two things can combine to make apparent motion?

Answer 12

different flicker rates and different duty cycles

Question 13

How do you find the time averaged (mean) luminance?

Answer 13

average luminance over the TIME of one cycle

Question 14

On a DeLange function, where is the flicker perceived?

Answer 14

region under the curve

Question 15

Examples of Low Temporal Frequency Changes in Illumination or Luminance

Answer 15

Sunset
Minute hand on watch
purkinje tree shadows

Question 16

Describe the troxler effect.

Answer 16

perceptual disappearance of a stabilized retinal image. Image moves exactly with the retina. The image will fade.

Question 17

THE HUMAN VISUAL SYSTEM IS SENSITIVE TO ________ IN LIGHT STIMULATION

Answer 17

changes

Question 18

What do neurons tuned to low spatial frequencies tolerate?

Answer 18

More motion before responding

Question 19

Why does the visual system have more difficulty responding to low temporal frequencies of luminance change?

Answer 19

luminance is increasing, the lateral inhibition of the surround starts signaling to dampen the response. The result, then, is that the changing luminance is not detected.

Question 20

What determines the high temporal frequency cut-off for flicker?

Answer 20

Visual persistance. It limits temporal resolution.

Question 21

Where does maximum brightness occur under photopic conditions during the flicker light test under constant modulation depth?

Answer 21

5-20 Hz

Question 22

What is the Critical Flicker Fusion Frequency (CFF)? What is the normal value?

Answer 22

• The frequency at which the flickering light is no longer flickering and is instead a contact light.
• The maximum modulation frequency at which flicker can be perceived; usually considered to be the cutoff frequency of the temporal MTF.
• 60 Hz

Question 23

What is the Talbot Plateau Law state?

Answer 23

A light flickering at any frequency higher than the CFF will be equal in brightness to a non-flickering light having the same time-averaged luminance.

Question 24

Order of which cones process faster. How do you know this?

Answer 24

• M-cone pathways may be a little faster than within L-cone pathways and the S-cone pathways are the slowest of all.
• since m cones are steeper on Ferry-porter line, they would process faster. S cones are least steep so they process slower.

Question 25

Explain Granit-Harper Law. What does it have to do with?

Answer 25

Influence of Stimulus Size on CFF

-Under photopic conditions
Larger area stimulus is more sensitive to flicker (higher CFF).
Later research showed that it’s not the overall stimulus area that’s critical, but what part of the retina is being stimulated

Question 26

CFF generally ___________ from the fovea to approx. 50 deg., then __________ to the far periphery.

Answer 26

increases; decreases

Question 27

Critical flicker frequency as a function of retinal position appears to ________

Answer 27

decrease

Question 28

Ferry-Porter Law

Answer 28

CFF increases linearly with log increases of mean luminance

Question 29

Granit-Harper Law

Answer 29

• CFF increases linearly with log increases of stimulus area

- Likely a reflection of the portion of the retina that is stimulated.

Question 30

LSD effects on CFF

Answer 30

enhanced CFF while under influence, but persis-
 tently slower dark adaptation and flicker fusion 
 frequency (i.e., lower CFF) and lower sensitivity to flicker in general

Question 31

Sedatives effects on CFF

Answer 31

reduced CFF (can help someone who is too 
 sensitive to flicker).

Question 32

CFF can be used as a measure of _________

Answer 32

alertness

Question 33

Compared to standard automated (static) perimetry, flicker perimetry is generally…

Answer 33

• less affected by normal aging
• more resistant to degradation by optical blur and light scatter
• may be more sensitive for detecting and monitoring early visual field defects related to glaucoma and other optic neuropathies.

Question 34

What does the flicker perimetry evaluate?

Answer 34

A visual field test procedure that evaluates an observer’s ability to detect light/dark stimulus alternations at various locations in the field of view.

Question 35

Test procedures for flicker perimetry

Answer 35

1) Contrast Modulation
2) Critical Flicker Fusion
3) Luminance Pedestal Flicker

Question 36

Describe the Contrast Modulation Flicker Perimetry test

Answer 36

• Test stimulus is matched in luminance and color to the
uniform background.
• Test stimulus undergoes light and dark alternation (flicker) at a predetermined temporal frequency.
• The MAGNITUDE of the CONTRAST MODULATION of the flicker needed to detect the stimulus is determined
at key locations in the visual field to yield a flicker sensitivity map

Question 37

Describe the Contrast Modulation Flicker Perimetry test

Answer 37

• Determines the highest frequency of flicker that can be
 distinguished from a uniform steady stimulus, at different
 locations in the visual field.
• Stimuli typically have a fixed, high contrast modulation (ex.
 100%) because we’re measuring the cut-off frequency of the TMTF.
• Best performed with the luminance modulating equally on
either side of the average luminance of the background.