Human Sensitivity to Light Flashcards
How does the pupil control light influx?
- Pupil constricted under bright illumination
- Pupil dilated when light v low
- Pupil diameter ranges from ~2mm to 8mm
o Factor of 4 -> amount of light that can be adjusted by pupil response is about an order & a ½ of magnitude – 4 is a diameter (ratio) here & then have to square this to get the error
Error is 16
Describe the duplex nature of retina?
- Scotopic System:
o Mediated by rods
o Slowly activated under low light condition
o Provides maximum sensitivity to light
o No colour vision
o Compromises spatial resolution (6/60) - Photopic System:
o Mediated by cones
o Quickly operates under bright light condition
o Provides finest spatial resolution (6/6)
o Colour vision
o Compromises absolute sensitivity to light
Describe spatial summation by rods?
- Mechanism to generate an action potential in a cell by combining multiple neuronal signals from neighbouring cells
- Anatomically cones found exclusively within fovea whereas rods will be found mostly in fovea whereas rods will be found mostly outside fovea
- Sensitivity to light is known to be v high outside fovea – large group of rods converge on a single GC
- 10 photons needed to signal detection of light
o If amount of light is sub-threshold (<10 photons) – GC would not recognise light - If two groups of 5 photons on 2 sides of retina separated by a distance however under scotopic condition GC will add them up, as long as the distance is withing the coverage of this GC summation
o Consequence of this summation is poor spatial resolution - If 2 spots of light which fall on right and left side of retina, separated by same distance as above but this time each spot contains 10 photons
o Will be merged together to signal detection of a single light instead of a 2-light event
In expense of spatial resolution
Reason why scotopic has excellent sensitivity to light but poor spatial discrimination
Describe spatial summation by cones?
- Photopic system has very limited summation range compared to scotopic system
- Red cones sensitive to red colour, green cones sensitive to green
o Also have cones sensitive to blue but red & green are majority & blue cones are quite rare - 10 photons of light needed for GC to detect light
- Assume two spots of light which are both sub-threshold (5 photons each) fall on right and left side of retina
o Here there will be no summation signal light detection because the range of summation is quite limited, but they are just separated so no light detection event - If two spots of light each with 10 photons each fall on right and left side of retina, system will signal 2 separate light events
o Reason behind the fine resolution of photopic system but worse sensitivity to faint light
Describe light adaptation?
- Photopic and scotopic systems are known to regulate range of light intensity by way of adaptation
- Light adaptation occurs as level of background illumination is slowly increased
- Light Adaptation Experiment:
o Measuring difference threshold of light as a function of background luminance
o Px 1st needs adapted to background luminance for >30minutes
Background luminance is not completely dark
o Reverse process of light adaptation dark adaptation which takes more time than light adaptation
o Once px is dark-adapted, luminance of centre will gradually increase until observer sees the spot measuring difference threshold or just noticeable difference (JND)
o Once increment threshold is measured for given background luminance, then luminance of background increases slightly
Then measure this different threshold again & repeat same steps over a wide range of background intensity
Describe the light adaptation curve
- Log background luminance on x-axis, log increment threshold on y-axis
- (1): region where detection is limited by internal noise – spontaneous, random neural activity = absolute threshold
- (2): log increment threshold starts to increase with a slope of 0.5 – where fluctuation in light source affects increment threshold – relationship known as deVries-Rose law
- (3): region where Weber’s law holds for scotopic vision > 4 orders of magnitude
- (4): rod-cone break: rods saturated, not responsive to any luminance difference (completely overwhelmed by brightness of background); cones start to take over
- (5): region where Weber’s law holds for photopic vision > 4 orders of magnitude
Section (3) and (5) are regions where log increment threshold increases as a function of log background luminance with a unit slope.
These are the regions where Weber’s law holds.
Give values of Weber’s fraction for rods and for cones?
- Scientist found that Weber’s fraction for rods is ~0.14 whereas 0.02 for cones
- Means that in case of scotopic system, for e.g., if background luminance is 100 unit, then at least a 14 units of increment to background which is 114 is needed to detect stimulus
o If background luminance increases to 1000 units, then the increment must be 140
This way our relative sensitivity is kept constant, but our absolute sensitivity decreases as background luminance increases - As our threshold is increased from 14 units to 140 units
Describe Weber’s Law and what is the equation?
ΔꞮ / Ɪ lower b = k
OR ΔꞮ = k* Ɪ lower b
Weber’s law maintains that amount of change in stimulus magnitude, intensity or strength required to detect change from initial impression is dependent on initial strength of original stimulus or background and ratio between these two quantities is constant
Perceptual sensitivity is maintained constant relative to initial level of impression to which we adapted
* Ɪ lower b: baseline intensity, magnitude, strength, or quantity of stimulus being compared
* ΔꞮ: minimum difference required for detecting change in perception from the Ib (just noticeable difference JND)
o JND will increase as background intensity increases get more sensitive as background intensity decreases
* k: sensory modality specific constant (Weber fraction)
Amount of change in stimulus strength required to invoke the change in sensation from initial impression is contingent upon initial stimulus strength
Weber’s law is one of v few laws in psychophysics generalised over wide range of sensation and perception applies to audition or tactile sensation as well as vision
Describe threshold vs intensity (T v I) Curve?
- Increment threshold is a linear function of a background luminance with a constant factor known as Weber’s fraction
- If plot increment threshold (ΔI) as a function of background intensity, then will see a straight line with constant slope of K
o Constant slope can be shown differently when ratio between increment threshold and background intensity is plotted against the background intensity -> will see a flat line across all background intensity
Describe Sensitivity Regulation?
- Trade-off between relative and absolute sensitivity
- We are more sensitive to relative difference in luminance (i.e. contrast) than absolute levels of luminance
- Weber’s contrast revisited: difference in light level, divided by overall light level:
C lower W = ΔL / L lower b = (L lower t - L lower b) / L lower b
