Characterized temporal vision 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 Procedure repeated for a large number of temporal frequencies to obtain TMTF
This figures displays TMTFs under various levels of retinal illumination. The increasing background illumination has different effect on relative
Enables us to examine a visual stimulus in detail such that it can be identified
Temporal modulation transfer functions
Critical flicker fusion frequency
Other temporal visual effects
Temporal aspect of vision is a time related
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.
Temporal vision is frequently studied with
stimuli with luminance that varies sinusoidally
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
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
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.
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.
When depth of
modulation is low,
light source appears
steady. (Figure A)
When depth of
modulation is large,
light source resolve
and seen flickering.
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)
frequency flicker at
the slow rate. (Figure
frequency flicker at
the faster rate. (Figure
CFF represent the high frequency resolution
limit of the visual system for a given depth of
Typically given in the Hertz (Hz)
1 Hz = 1 cycle per second.
Frequency of the light stimulation at the which it
becomes perceive as a stable and continuous
That frequency depends upon various factors:
Ind. view a light
source that is
depth is very
first sees flicker
1 3 10 36 100
1 36103 10
Stimuli fall outside
TMTF are seen as
Stimuli fall under
TMTF are temporally
resolved, perceived as
Temporal Modulation Transfer
•Relative sensitivity as function of temporal frequency
•Band pass shape
•Sensitivity for detection of flickers FALLS OFF at both low and high temporal
• Very slow /gradual
changes are not
• High frequency
drop off the TMTF
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
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
A frequency is reached can not be response because
of limitation of neural response
Highest or lowest temporal frequency that can
be resolved at a given percentage modulation
1 3 4 10 36
Frequenc y ( Hz )
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.
( 1 / percentage
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
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
Graphical presentation of Ferry Porter Law
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
Higher cortical areas (forming
the cortical what system )
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
This phenomenon is associated with temporal
summation and explains the leveling off of
brightness to a plateau.
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
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
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.
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.
Involves the use of stimulus
(Mask)reduce the visibility of another
Various types of masking:
- simultaneous masking
- backward masking
- forward 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
• 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
• Mask transmitted along the neural pathways at
a relatively rapid rate
• This enables it to surpass the preceding target
and interfere with its detection
Mask precedes the target
Mask reduces the visibility of the subsequently