TEMPORAL ASPECTS OF VISION

Prepared By:Anis Suzanna Binti Mohamad

Optometrist

Overview:

Introduction Stimulus considerations Temporal modulation transfer functions Critical flicker fusion frequency Other temporal visual effects Masking

What is temporal aspects of vision?

Temporal aspect of vision is a time related vision.

It is concern the analysis of changes in luminance over time.

Example: task involves detection of flicker produced by a flashing light.

Temporal vision is closely related to the ability to perceived motion.

Stimulus considerations

Temporal vision is frequently studied with stimuli with luminance that varies sinusoidally over time.

A temporal sinusoid manifests a sinusoidal change in luminance over time.

From the sinusoidal graph we must take in mind the two stimulus consideration: Depth of modulation Temporal frequency

Graph stimuli with luminance that varies sinusoidally over time

• Luminance profile for a stimulus with luminance that is temporally modulated in a sinusoidal manner over time.

• A computer screen that turns on and off with a sinusoidal time course would produce a illimunance profile similar to that given in this figure.

• “A” refers to amplitude of modulation.

Stimulus considerations

A temporally modulated stimulus is produced by a light source for example the computer monitor that will turn on and off.

To produce temporal sinusoid, the light source must turn on and off with a sinusoidal course.

Depth of modulation

The visibility of a temporally modulated stimulus is related to its depth of modulation.

May be two of modulation: Low depth of modulation steady field High depth of modulation flicker.

Depth of modulation

When depth of modulation is low, light source appears steady. (Figure A)

When depth of modulation is large, light source resolve and seen flickering. (Figure B)

Percentage of depth of modulation

The percentage of depth of modulation of a temporally modulated stimulus is given by:

Where A= amplitude of modulation Iave = time-averanged

luminance

Percentage modulation = A(100) Iave

Temporal frequency Low temporal

frequency flicker at the slow rate. (Figure A)

High temporal frequency flicker at the faster rate. (Figure B)

Temporal frequency

CFF represent the high frequency resolution limit of the visual system for a given depth of modulation.

Typically given in the Hertz (Hz) 1 Hz = 1 cycle per second.

What is critical fusion frequency (CFF)?

Definition Frequency of the light stimulation at the which

it becomes perceive as a stable and continuous sensation.

That frequency depends upon various factors: Luminance Color Contrast Retinal eccentricity

TEMPORAL MODULATION TRANSFER FUNCTION

(TMTF)

How to determine TMTF??

Ind. view a light source that is modulated at given temporal rate

Initially, modulation depth is very low;

screen appear steady

Slowly increase—until subject report

flickering

Threshold??–

modulation at which person

first sees flicker

Relative sensitivity

(1/PercentageModulation)

Frequency (Hz)

Flicker

1 3 10 36 100

1 36103 100

No FlickerStimuli fall outside TMTF are seen as fused/steady;

not temporally resolved

Flicker Stimuli fall under TMTF are temporally resolved, perceived as flickering

Relative sensitivity

(1/PercentageModulation)

Frequency (Hz)

Temporal Modulation Transfer Function (TMTF)

• Relative sensitivity as function of temporal frequency• Band pass shape• Sensitivity for detection of flickers FALLS OFF at both low and high temporal

frequencies

Reduction sensitivity for..

Low temporal frequency

•Very slow /gradual changes are not seen

High temporal frequency

•High frequency drop off the TMTF

Reduction sensitivity for low temporal sensitivity

Very slow /gradual changes are not seen Ie: sun set

We aware the changing illumination, we don’t actually perceived the change itself

Ie: minute hand on a watch

Origin of low frequencies TMTF drop offs Due to time lags inherent lateral inhibition within

retina Low temporal frequency stimuli maximize these

inhibitory interaction with a resultant reduction in sensitivity

Reduction sensitivity for high temporal sensitivity

High frequency drop off the TMTF Ie: household incandescent light bulb

Bulb modulated at 60Hz Because we are sensitive to high-frequency

temporal modulation, bulb appear steady rather than flickering

Origin of low frequencies TMTF drop offs Due to neural limitation in coding high temporal

frequency information A frequency is reached can not be response

because of limitation of neural response

Critical Flicker Fusion Frequency

Highest or lowest temporal frequency that can be resolved at a given

percentage modulation

Example 1.00

0.25

0.01 1 3 4 10

36 100

Frequenc y ( Hz )

CFF CFF

For 4.0 percent modulation, relative sensitivity of ¼, or 0.25, a line extended from this point intersects the TMTF at two points at 4 and 10 Hz, represents the low and high frequency. Stimuli < 4 Hz or >10 Hz are seen as fused, not resolved and appear steady.

Rel

ativ

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ensi

tivity

( 1

/per

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age

Of m

odul

atio

n )

Effect of Illumination on the Critical Flicker Fusion Frequency

Relative sensitivity ( 1 / percentage Modulation )

1 3 10 36 100 Frequency ( Hz )

≈ 100 td

≈ 1000 td

≈ 10 td

The increasing background illumination has different effect on relative sensitivity for low and high temporal frequencies. In low frequency, increasing the illumination has no effect on relative sensitivity.

Log Retinal Illumination

CF

F (

Hz)

Scotopicvision

Photopic Vision

70 Hz

20 Hz

For high-frequency, relative sensitivity increases with CFF increasing approximately with the log of retinal illumination. Probably related to a general speeding up of retinal processes that occurs at increasing level of light adaptation.

Graphical presentation of Ferry Porter Law

Effect of Stimulus Size on the Critical Flicker Fusion Frequency

Granit – Harper Law CFF increases with the log of the stimulus area.

For a given percentage modulation, flicker is more likely to be perceived if the stimulus is large.

The extrafoveal retina ; better in detecting the flicker and movement than the

foveal retina contribute the “where” system which alert us to the presence of visual stimuli that

require immediate attention. Retinal parasol ganglion cells display high sensitivity to

high temporal frequencies and may contribute to the peripheral retina’s superior sensitivity to the stimuli.

Once a stimulus is detected by the where system, it is examined with foveal vision.(display highly developed visual acuity)

Involving the midget (parvo) ganglion cells, Most concentrated in the fovea.

Midget cells Parvo layers

Striate cortex

Higher cortical areas (forming the cortical what system )

BROCA SULZER EFFECT

The Broca-Sulzer effect, which describes the apparent transient increase in brightness of a flash of short duration. Subjective flash brightness occurs with flash durations of 50 to 100 milliseconds.

This phenomenon is associated with temporal summation and explains the leveling off of brightness to a plateau.

BROCA SULZER EFFECT

When the light is turned on, time is required for temporal summation to reach threshold for light of low luminance. Light of high luminance reach this threshold very quickly. As flash duration increases, brightness levels off to a plateau as temporal summation begins to breakdown according to Bloch’s law after the critical duration.

The apparent transient peak in brightness is probably due to an underlying neural mechanism.

BROCA SULZER EFFECT

BRÜCKE-BARTLEY EFFECT

The Brücke-Bartley effect is the phenomenon in which a flickering stimulus appears brighter than the same stimulus presented unflickering.

The Brücke-Bartley (brightness enhancement) effect is a phenomenon related to the Broca-Sulzer effect. When the frequency is gradually lowered below the CFF, the effective brightness of the test field begins to rise.

Not only does the brightness reach a value equal to that of the uninterrupted light, but the brightness even transcends it, reaching a maximum when the flash rate is about 8 to 10 Hz.

TALBOT-PLATEAU LAW

The Talbot-Plateau Law describes the brightness of an intermittent light source which has a frequency above the CFF.

This law states that above CFF, subjectively fused intermittent light and objectively steady light (of equal colour and brightness) will have exactly the same luminance. In another words, brightness sensation from the intermittent light source is the same as if the light perceived during the various periods of stimulation had been uniformly distributed over the whole time.

The Talbot-Plateau Law applies only above the CFF.

MASKING

INTRODUCTION

Involves the use of stimulus (Mask)reduce the visibility of another

stimulus(target) Various types of masking:

- simultaneous masking

- backward masking

- forward masking

Simultaneous masking

Mask and target present at the same time

Both frequencies share the same spatial frequency channels causes reduction in the visibility of the target gratings

More pronounced in patients with amblyopia

Crowding phenomenon- low acuity viewing row of letters rather than viewing isolated letters

Backward masking

• Target precedes the mask• Even though the mask occurs after the

target,it reduces the visibility of the target

• Typically occurs when mask is brighter than target

• Mask transmitted along the neural pathways at a relatively rapid rate

• This enables it to surpass the preceding target and interfere with its detection

Forward masking

Mask precedes the target Mask reduces the visibility of the

subsequently presented target

Metacontrast

A form of backward masking

Paracontrast

A form of forward masking

..THE END..