Visual perception

1. Prepared By:
Anis Suzanna Binti Mohamad
Optometrist

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

3.  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.

4.  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

5. 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.

6.  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.

7.  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.

8.  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)

9.  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

10.  Low temporal
frequency flicker at
the slow rate. (Figure
A)
 High temporal
frequency flicker at
the faster rate. (Figure
B)

11.  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.

12.  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

14. 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/
Percentag
e
Modulatio
n)
Frequency
(Hz)
Flicker
1 3 10 36 100

15. 1 36103 10
0
No Flicker
Stimuli 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/
Percentage
Modulation)
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

16. • Very slow /gradual
changes are not
seen
Low
temporal
frequency
• High frequency
drop off the TMTF
High
temporal
frequency

17.  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

18.  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

19.  Highest or lowest temporal frequency that can
be resolved at a given percentage modulation

20. 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.
RelativeSensitivity(1/percentageOfmodulation)

21. 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.

22. Log Retinal Illumination
CFF(Hz)
Scotopic
vision
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

23.  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.

24.  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 )

25.  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.

26.  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.

28.  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.

29.  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.

31.  Involves the use of stimulus
 (Mask)reduce the visibility of another
stimulus(target)
 Various types of masking:
- simultaneous masking
- backward masking
- forward masking

32.  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

33. • 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

34.  Mask precedes the target
 Mask reduces the visibility of the subsequently
presented target

35.  A form of backward masking

36.  A form of forward masking

37. ..THE END..