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Temporal Vision
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- 1Temporal response of visual system has to:* Be able to differentiate between events in changing environment rapidly enough
* Also be able to gather enough light across time and space to enhance sensitivity - 2Concept of integration timeIntegration time is identified by Tc
* longer time - lower intensity
* quicker time - higher intensity - 3Bloch's lawAccording to Bloch’s law of vision, within this critical duration, threshold is reached when the total luminous energy is reached. Bloch’s law states that total luminous energy is a constant value (k), thus threshold is reached when the product of luminance (L) and stimulus duration (t) equals this constant.
- 4Temporal summationTemporal summation refers to the eye’s ability to sum the effects of individual quanta of light over time. However, temporal summation only occurs within a certain period of time, called the critical duration or critical period.
- 5Critical Flicker FrequencyCritical flicker frequency (CFF) is the transition point of an intermittent light source where the flickering light ceases and appears as a continuous light. There are a multitude of factors that determine our perception of flicker that includes the intensity and size of the test stimulus.
* not an abrupt transition
* CFF is usually taken as the frequency where there is 50% probability of the light being seen as flickering - 6Talbot-Plateau LawThe Talbot-Plateau Law describes the brightness of an intermittent light source which has a frequency above the CFF. The Talbot-Plateau Law applies only above the CFF.
Effective luminance = the time average of luminance - 7Factors affecting critical duration* Light level - longer for darker backgrounds
* Size of stimulus - longer for smaller test areas
* Type of task - longer for finer acuity task
* Wavelength - shorter wavelength stimulus are integrated over longer intervals - 8Factors affecting CFF* intensity of the stimulus - CFF increases with luminance (Ferry-Porter Law)
* Spectral composition of the stimulus
* area of the stimulus (eye is more sensitive to flicker with larger areas - Granit-Harper Law) - as stimulus size increases, the Ferry-Porter relation shifts upwards with no change in its slope
* eccentricity - able to see more flickering in periphery - 9Ferry-Porter LawThe Ferry-Porter Law states that CFF is proportional to the logarithm of the luminance of flickering stimulus (L). It can be expressed as
CFF = a logL + b
As the intensity of the test stimulus is increased, our perception of flicker also increases. From a practical point of view, if a stimulus is flickering, such as computer monitor, decreasing the intensity level will eliminate the flicker. - 10Neural basis of Ferry-Porter LawThe level of activity of the inhibitory neurone is dependent upon the excitation it receives. Inhibition can be feed-forward inhibition or lateral inhibition.
- 11Temporal modulation transfer function* variation of intensity light at a point in space in time
* inability to see temporal changes at very low and very high temporal frequences - 12Troxler Phenomenon* blurred borders disappear when fixating on a target
* sharp borders remain even when fixating on target - 13Spatiotemporal Contrast Sensitivity Function* useful for testing parvocellular and magnocellular pathways separately
* Studies of retinitis pigmentosa - loss of sensitivity at higher temporal frequencies
* AMD - loss of sensitivity at mid-temporal frequencies - 14Neural Mechanism of direction selectivityContain neurones that detect direction of motion - only fire when stimulus contains motion in preferred direction
* generated in primary visual cortex
* firing along a direction will cause a chain of cells to fire, if excitatory cell is fires first - preferred direction, if motion is opposite - will cause inhibitory neuron to fire first
* cause post-synaptic cell to have directionality - 15How motion aftereffects are generated and their possible neural basisMotion aftereffect is a result of imbalance between two pathways.
* observing the same motion over a period of time will cause those direction selective neurons to become fatigue
* when redirect gaze to a still background - will cause more activation of the other pathway to create motion - 16What is "aperture problem" in the coding of stimulus motionMotion of a contour across a small area (RF) provides ambiguous information about contour movement
EXAMPLE: think the line is shifting right when actual movement is down and right
* only viewing through small aperture may not give actual motion
Overcoming problem: required to visually process two different areas (2 different RF) and integrate them to perceive overall movement - 17Evidence linking MT directly to perception of motion - electrophysiological and imaging studiesArea V5/MT contains cells that are directionally selective (90%) that have large RF - Pattern cells (respond to overall motion by integrating signals from component cells)
* integrate smaller motion signals from V1 cells - component cells (respond to single gratings moving in preferred direction)
* integrating few gratings together - get overall movement (vector sum of gratings)
Evidence - lesions in MT (in monkeys) resulted in impaired behaviourally measured motion performance - in global dot motion experiment
* electronic micro-stimulation of MT influences monkey's ability to correctly signal direction of motion - 18Problem of the "stability of the visual world"* need to be able to separate perceptual consequences of our own movements from events caused outside our bodies
* require mechanism that separates object movement from observer movement
* visual stability is aided by saccadic suppression, eye blink suppression and taking eye and head movements into account
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- Postgraduate (Visual Functions - OD1) Visual Functions - Optometry Fichas sobre Temporal Vision, creado por Kim Vu el 27/05/2018.
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- Kim Vu
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