Midterm 2: Timing Flashcards
What is visual persistence?
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
What is the Broca-Sulzer effect?
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).
Under photopic conditions, how long of a flash will give maximum brightness?
50 msec
Under scotopic conditions, how long of a flash will give maximum brightness?
100 msec
X and y axis of Broca-Sulzer Effect graph. Units
Y-axis = Comparative brightness x-axis = time units = lux
Neural Explanation of Broca-Sulzer Effect
- 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.
What is temporal masking?
influence that one stimulus has upon another when stimuli are presented at slightly different times.
3 Different types of temporal masking
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.
______________ (backwards) and ____________(forwards) are masking in which the test flash and masking flash do not overlap spatially on the retina.
metacontrast; paracontrast
Describe backward masking effect. How does it work?
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
What is a duty cycle?
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.
What two things can combine to make apparent motion?
different flicker rates and different duty cycles
How do you find the time averaged (mean) luminance?
average luminance over the TIME of one cycle
On a DeLange function, where is the flicker perceived?
region under the curve
Examples of Low Temporal Frequency Changes in Illumination or Luminance
Sunset
Minute hand on watch
purkinje tree shadows
Describe the troxler effect.
perceptual disappearance of a stabilized retinal image. Image moves exactly with the retina. The image will fade.
THE HUMAN VISUAL SYSTEM IS SENSITIVE TO ________ IN LIGHT STIMULATION
changes
What do neurons tuned to low spatial frequencies tolerate?
More motion before responding
Why does the visual system have more difficulty responding to low temporal frequencies of luminance change?
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.
What determines the high temporal frequency cut-off for flicker?
Visual persistance. It limits temporal resolution.
Where does maximum brightness occur under photopic conditions during the flicker light test under constant modulation depth?
5-20 Hz
What is the Critical Flicker Fusion Frequency (CFF)? What is the normal value?
- 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
What is the Talbot Plateau Law state?
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.
Order of which cones process faster. How do you know this?
- 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.
Explain Granit-Harper Law. What does it have to do with?
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
CFF generally ___________ from the fovea to approx. 50 deg., then __________ to the far periphery.
increases; decreases
Critical flicker frequency as a function of retinal position appears to ________
decrease
Ferry-Porter Law
CFF increases linearly with log increases of mean luminance
Granit-Harper Law
- CFF increases linearly with log increases of stimulus area
- Likely a reflection of the portion of the retina that is stimulated.
LSD effects on CFF
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
Sedatives effects on CFF
reduced CFF (can help someone who is too sensitive to flicker).
CFF can be used as a measure of _________
alertness
Compared to standard automated (static) perimetry, flicker perimetry is generally…
- 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.
What does the flicker perimetry evaluate?
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.
Test procedures for flicker perimetry
1) Contrast Modulation
2) Critical Flicker Fusion
3) Luminance Pedestal Flicker
Describe the Contrast Modulation Flicker Perimetry test
- 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
Describe the Contrast Modulation Flicker Perimetry test
- 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.
