Timing is Everything

Question 1

What is visual persistence?

Answer 1

The apparent persistence of a visual stimulus beyond its physical duration. This is NOT an after image.

Any visual stimulus presented for a duration LESS than 120-150 ms will APPEAR TO LAST for approx 120-150 ms, no matter how short the duration is.

Question 2

Any visual stimulus presented for a duration LESS than 120-150 ms will APPEAR TO LAST for approx ___ ms, no matter how short the duration is.

Answer 2

120-150 ms

Question 3

Broca-Sulzer Effect

maximum brightness under photopic conditions will be from a duration of approx ___ms?
Maximum brightness under scotopic conditions will be approx ___ ms?

Answer 3

Single, brief, suprathreshold flash. The brightness of a brief suprathreshold flash of light depends upon its duration. Short- brighter. Long- dimmer.

Maximum brightness under photopic conditions will be from a duration of approx 50ms.
Maximum brightness under scotopic conditions will be approx 100+ ms.

Question 4

Neural explanation of broca-sulzer effect

Answer 4

Intense stimuli produce photoreceptor overshoot. This produces (via bipolar cells) an initial burst of action potentials in the GCs.

For brief flashes, only the initial burst occurs, so the only info the neurons can use is a high firing rate, which makes the flash appear brighter than when long.

For long flashes, the firing rate AFTER the initial burst signals the brightness. Slower firing rate, so you perceive it as less bright.

Brightness is related to the firing rate of the cells.

Question 5

Brightness is related to

Answer 5

The firing rate of the cells.

Question 6

Temporal masking and what are the three types.

Answer 6

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

Simultaneous: 2 stimuli are presented at the same time. Masking occurs.

Forward: The masking stimulus is presented before the test stimulus. Paracontrast.

Backward: The masking stimulus is presented after the test stimulus. Metacontrast. The masking stimulus must be brighter than the test stimulus in order for the test stimulus to be masked. Neural signals initiated by bright stimuli are transmitted much faster than those initiated by dim stimuli. The brain will “see” the second flash sooner, or at the same time as the first.

Question 7

Paracontrast and metacontrast

Answer 7

Temporal masking.
Forward is paracontrast.
Backwards is metacontrast.

Question 8

Duty cycle

Answer 8

Train of pulses that are equal in duration and equally separated in time. Duty cycle is the proportion of the cycle in which the pulse is “On.”
Ex: 50% duty cycle means light is on 50% of the time,

1 cycle= time for one pulse of light on plus light off.

Question 9

50% duty cycle means

Answer 9

light is on 50% of the time.

Question 10

by combining different flicker rates and duty cycles, we can create a sense of

Answer 10

Motion

Question 11

DeLange functions

Answer 11

Connects modulation depth detection thresholds to the temporal frequency of a flickering light.

• cut off at 60Hz.
• High and low frequency fall off (except no low frequency fall off under scotopic conditions)
• Max photopic sensitivity between 5-20 hz.

Question 12

Troxler Effect

Answer 12

Perceptual disappearance of a stabilized retinal image. A well stabilized image will fade from view within a few seconds of stabilized viewing.
Ex: Purkinje tree is stabilized, so we don’t “see” it unless it moves.

Question 13

Temporal resolution (being able to see flicker) is limited by

Answer 13

Visual persistance

Question 14

Why is temporal resolution (being able to see flicker) limited by visual persistence?

Answer 14

Very brief flashes continue after the stimulus has disappeared for 150-200ms until the cell can return to base line.
Therefore, the point spread function limits spatial resolution.

Question 15

The point spread function limits

Answer 15

Spatial resolution

Question 16

Brucke-Bartley enhancement law:

Answer 16

The flicker frequencies to which we are maximally sensitive are also seen as the brightest. (5-20 hz) under photopic conditions.

Question 17

CFF (Critical flicker fusion frequency)

Answer 17

Max modulation frequency at which flicker can be perceived. Usually considered to be the cut off frequency for the temporal MTF. The frequency of flicker above 60hz will not be detectable. Instead, we will perceive a steady, non flickering light. Ex: fluorescent bulbs are constantly flickering, but we do not detect since it is above 60Hz. Due to the neural response of ganglion cells.

Question 18

The normal photopic CFF is approx

Answer 18

60Hz due to the neuron response of ganglion cells.

Question 19

Flicker frequencies that we are most sensitive to and highest flicker frequency that is detectable

Answer 19

Most sensitive to 5-20hz.

Highest flicker frequency that we can detect is 60hz. Usually the cut off frequency for the temporal MTF.

Question 20

Talbot Plateau Law

Answer 20

Applies to flicker greater than CFF. A light flickering higher than CFF will be equal in brightness to a steady light having the same time-averaged luminance

Question 21

Ferry porter law

Answer 21

Under photopic conditions with stimulus size constant, foveal CFF increases as luminance increases. The increase is linear function on log scale.

Found that processing with M cones are the fastest and S cones are the slowest.

Question 22

Granit Harper Law

Answer 22

If stimulus size does change, results are based on what part of the retina is being stimulated. Ex: Got same CFF for large circular stimulus and annulus of the same size.

Mid peripheral retina has better flicker sensitivity than central retina. The area being stimulated plays a big role in the CFF value.

Question 23

For a stimulus of constant size and luminance.

As you move from the fovea to approx 50 degrees, the CFF will ____. As you move further out in the periphery, the CFF will ___.

Answer 23

Increase (best CFF in mid periphery)

Decrease.

Question 24

Review of laws about Temporal resolution and CFF

1. Brucke-bartley
2. Talbot-Pleateau
3. Ferry porter
4. Granit harper

Bob, the friendly ghost.

Answer 24

1. For frequencies lower than 60Hz, we are maximally sensitive to 5-20hz.
2. For frequencies higher than 60Hz, brightness of flicker= mean luminance of steady stimuli.
3. Photopic conditions, stimulus size constant. CFF increases and luminance increases.
4. If stimulus size does change, CFF changes are based on what part of the retina is being stimulated. Mid peripheral retina has best flicker sensitivity.

Question 25

Flicker perimetry is a test that evaluates

Answer 25

Evaluates an observers ability to detect light/dark stimulus alterations at various locations in the field of view.

There are three different types of flicker perimetry tests:

1. Contrast modulation
2. Critical flicker fusion perimetry
3. Luminance pedestal flicker

Question 26

There are three different types of flicker perimetry tests:

1. Contrast modulation
2. Critical flicker fusion perimetry
3. Luminance pedestal flicker

Answer 26

1. Test stimulus is matched to luminance and color of background. Flicker occurs and the magnitude of contrast modulation of flicker needed to detect the stimulus is determined.
2. How high can I increase the frequency and patient can still detect it?
3. Certain level of luminance in visual field. On top of that, flicker is occurring.