Temporal aspects of vision


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  • 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
  • In comparison,
  • Enables us to examine a visual stimulus in detail such that it can be identified
  • Temporal aspects of vision

    1. 1. Prepared By: Anis Suzanna Binti Mohamad Optometrist
    2. 2.  Introduction  Stimulus considerations  Temporal modulation transfer functions  Critical flicker fusion frequency  Other temporal visual effects  Masking
    3. 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. 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. 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. 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. 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. 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. 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. 10.  Low temporal frequency flicker at the slow rate. (Figure A)  High temporal frequency flicker at the faster rate. (Figure B)
    11. 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. 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
    13. 13. 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
    14. 14. 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
    15. 15. • Very slow /gradual changes are not seen Low temporal frequency • High frequency drop off the TMTF High temporal frequency
    16. 16.  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
    17. 17.  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
    18. 18.  Highest or lowest temporal frequency that can be resolved at a given percentage modulation
    19. 19. 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)
    20. 20. 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.
    21. 21. 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
    22. 22.  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.
    23. 23.  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 )
    24. 24.  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.
    25. 25.  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.
    26. 26.  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.
    27. 27.  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.
    28. 28.  Involves the use of stimulus  (Mask)reduce the visibility of another stimulus(target)  Various types of masking: - simultaneous masking - backward masking - forward masking
    29. 29.  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
    30. 30. • 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
    31. 31.  Mask precedes the target  Mask reduces the visibility of the subsequently presented target
    32. 32.  A form of backward masking
    33. 33.  A form of forward masking
    34. 34. ..THE END..