Temporal Properties of Vision

(1)
University of Sydney, Sydney, Australia
 

Overview

1.
The visual system in a changing environment
  • The visual system responds to variation in light over time, allowing instantaneous interpretation of a rapidly changing environment.
  • It concentrates on useful information (e.g., contrasting boundaries of objects, temporal changes in location and magnitude) and discards irrelevant features.
  • Successive retinal images are stored, integrated, differentiated, and erased, resulting in the perception of apparently stable scenes.
  • Temporal responsiveness varies between scotopic and photopic conditions.
 
2.
Temporal resolution of stimuli
  • The visual system is only able to detect stimuli at finite time intervals.
  • Stimuli presented closer together than this finite time are treated as a single stimulus event.
  • The time at which two discrete stimuli are just detected is the temporal threshold or limit of temporal resolution.
 

Temporal Summation and the Critical Duration (Tc)

The duration of a light stimulus influences its:
(a)
Ease of visibility
 
(b)
Subjective appearance
3.
Temporal summation
  • Temporal summation describes the influence of stimulus duration on its visibility [1].
  • It occurs because a longer duration stimulus emits more photons over time than a brief stimulus of the same intensity. Multiple, sequential photons may be required for the light to be seen.
 
4.
Critical duration (Fig. 22.1)
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Fig. 22.1
Schematic representation of stimulus duration influencing detection threshold: Bloch’s law and critical duration (Tc)
  • The critical duration (Tc) is the maximum time period over which temporal summation can occur.
  • Beyond Tc temporal summation ceases and detection depends on luminance alone.
  • For flashes briefer than Tc, the chance of visual detection of a light source is based on luminance (B) and duration (t), described in Bloch’s law [2]:
    $$ Bt=C, $$
    where C is constant
 
5.
Factors that influence the critical duration (Tc)
  • In humans, Tc is approximately 40–400 ms, depending on factors outlined below [1, 3, 4].
    (i)
    Background luminance
    • Tc is greater in dark (scotopic) than bright (photopic) conditions [5, 6].
    • In scotopic conditions, temporal summation enhances sensitivity to low-luminance stimuli; however, in photopic conditions, it interferes with temporal and spatial contrast discrimination [7, 8].
     
    (ii)
    Stimulus size
    • Tc is greater for small stimuli and smaller for large stimuli [9, 10].
    • This applies predominantly to photopic conditions; in scotopic conditions, size has less influence.
     
    (iii)
    Spectral composition
    • Tc is greater for isolated chromatic stimuli than achromatic (mixed wavelength) stimuli [11].
    • For colored lights, it is greater for shorter wavelength hues (blues) than longer (reds) and decreases with increased chromatic saturation [12, 13].
     
    (iv)
    Other factors
    • Tc is greater for complex perceptual tasks and for high acuity tasks [3, 14].
    • It is also influenced by retinal location [12].
     
 
6.
Critical duration and assessment of contrast threshold
  • Thresholds to light detection are measured using flashes longer than Tc, so that flash duration is removed as a variable that may influence threshold.
  • This is important in static perimetric testing: each test stimulus must be present for longer than Tc (see Chap. 23, The Visual Field).
 
 

The Broca-Sulzer Effect (Fig. 22.2)

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Fig. 22.2
The Broca-Sulzer effect
  • Brief stimuli appear subjectively brighter than a longer flash of the same luminance; this is the Broca–Sulzer effect [15, 16].
  • As flash duration increases, there is a transient peak brightness at 50–100 ms.
  • For stimuli of duration longer than this, subjective brightness is decreased and reaches a plateau of subjective luminance [17].
  • The Broca-Sulzer effect is most apparent for bright flashes and is less significant for dim stimuli.

Troxler’s Phenomenon

  • Troxler’s phenomenon is a time-dependant visual adaptive process (see Chap. 21, Luminance Range for Vision).
  • A fixed retinal image fades from perception in a few seconds; it is restored by a slight movement of the image or the eye [18, 19].
  • It demonstrates the visual system’s reliance on temporal as well as spatial contrast to capture visual information.
  • Troxler’s phenomenon is a neural, not photochemical, phenomenon.
  • The decay is slower with larger, brighter, and more central images [20].

Visual Fixation

1.
Control of fixation
  • During fixation, Troxler’s phenomenon is prevented by repetitive small eye movements [21].
  • These include slow monocular drifts, microsaccades, and tremors [22].
 
2.
Saccadic suppression
  • Saccades are brief voluntary conjugate eye movements to bring an object of regard into central view (see Chap. 18, Neural Control of Eye Movements) [23].
  • During saccades (10–80 ms) visual processing is temporarily suppressed and the visual system is unresponsive to visual input, preventing the sensation of movement and blur [24, 25].
  • Between saccades the eyes make fixed pauses of brief duration (200–300 ms) to take in visual information, during which suppression is released [26].
 

Critical Flicker Frequency

1.
Definition
  • When light is turned on and off repeatedly, it appears to flicker.
  • As the speed of the on/off cycle increases, we eventually perceive the flashes as a single fused light.
  • The critical flicker frequency (CFF) is the transition point of perception from flicker to continuous light.
  • The CFF is a measure of the temporal acuity (resolving power) of the visual system.
 
2.
Factors that influence the critical flicker frequency
(i)
Luminance
  • The Ferry–Porter law states that CFF increases linearly with log luminance (Fig. 22.3A) [27, 28].
    A347009_1_En_22_Fig3_HTML.gif
    Fig. 22.3
    The Ferry-Porter law, (a) Mixed light (b) Monochromatic light sources
  • The Ferry-Porter law is only valid in photopic states [28, 29].
 
(ii)
Spectral composition
  • When monochromatic light sources are used, the CFF increases linearly with log luminance, according to the Ferry-Porter law.
  • The increase in CFF with luminance is greater for green light and less for red (Fig. 22.3B) [30, 31].
  • This may be due to differences in signal processing speed between green and red cone pathways.
  • The linear increase of CFF with luminance is least for blue light [32].
  • This is because short-wavelength-sensitive cones (and rods) have slower processing speeds than medium-wavelength-sensitive and long-wavelength-sensitive cones.
 
(iii)
Stimulus size
  • CFF increases with stimulus size.
  • This is the Granit–Harper law, stating that CFF is linearly proportional to log stimulus area [33, 34].
  • It is only valid in photopic states and with stimuli within 10° of central fixation [3].
 
(iv)
Retinal eccentricity (Fig. 22.4)
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Fig. 22.4
Change in critical flicker frequency with increasing retinal eccentricity (Based on Rovamo and Raninen, 1984) [37]
  • CFF increases with retinal eccentricity within the central 50° of the visual field and then decreases with further eccentricity [35–37].
 
(v)
Background luminance/adaptive state
  • In general, CFF increases with greater levels of adaptation.
  • Maximal CFF occurs when background luminance is the time-averaged luminance of the flickering stimulus [3, 38].
  • Photopic flickering lights presented on dark backgrounds results in rod-cone interactions that decrease sensitivity [39].
 
 
3.
The effects of flicker on perception
(i)
Brucke-brightness enhancement effect
  • The apparent brightness of a flickering stimulus varies with the frequency of the flicker, with a maximum apparent brightness at a range 15–20 Hz [40].
  • This is closely related to the Broca-Sulzer effect, as flicker frequency is related to stimulus duration per flicker (Fig. 22.2).
 
(ii)
Talbot-Plateau law
  • The Talbot–Plateau law describes the brightness of an intermittent light source with a frequency above the CFF [41, 42].
  • This law states that above CFF, subjectively fused intermittent light and objectively steady light (of equal color and brightness) will have precisely the same luminance.
  • For example, a flickering stimulus at twice the CFF needs to be twice as bright as a steady stimulus.
 
 

Temporal Contrast Sensitivity

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Oct 28, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Temporal Properties of Vision

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