Absolute Threshold for Light Detection

Question 1

How long does it take for visual system to be sensitive to light in dark room?

Answer 1

• Eye becomes more and more sensitive if have been in dark for some time
• e.g. enter a cinema, wont be able to see for several minutes but with time eyes become adjusted to dark
• Visual system becomes most sensitive to light when rods have not absorbed any light for ~30minutes
  o Photopigments in rods are fully regenerated
  o Dark adaptation is about 6times slower than light adaptation

Question 2

Describe dark adaptation?

Answer 2

• During dark adaptation, both rods & cones have begun adapting with cones adapting much more rapidly over first 5mins
• Rod and cone sensitivity levels cross at point called rod-cone break
  20 degrees nasal or temporal is where the highest concentration of rods are present
  Most sensitive to green colour when there is enough light in the background
  But under scotopic condition, most sensitive to blue-greenish colour (cyan) – peak sensitivity is ~505 nanometres which looks blue-green

Question 3

Describe Bloch’s law (Time-Intensity Reciprocity)?

Answer 3

• States that within critical duration, total number of photons (the quanta – the light energy needed to reach threshold) remains same
• Within critical duration, intensity & duration have a reciprocal relationship – they’re inversely proportional
  o E.g. when luminance is halved within the region, a doubling in stimulus duration will be required to reach threshold
   When luminance is doubled, threshold can be reached in half the duration
   Beyond the critical duration, threshold is only dependent upon luminance rather than product of luminance and duration
• Critical duration for scotopic vision is 100 milliseconds

Question 4

Describe Ricco’s Law (Area-Intensity Reciprocity)?

Answer 4

• Ricco’s Law of Summation relates spatial summation to size of stimulus
  o States that total number of photons required for detection in scotopic condition is constant up to a critical diameter, which is 10 arc minutes
• Area of stimulus and intensity have a reciprocal relationship

Question 5

Describe spatial summation over stimulus size?

Answer 5

• Reciprocal relationship between intensity of spot & its size to maintain a constant no. of quanta
• Threshold number of quanta is constant up to certain diameter but that for larger targets the threshold no. of quanta increases
• 10arcmin diameter – maximum extent of summation of light for scotopic vision so that light will be detected as long as threshold number of photons fall anywhere within this extent

Question 6

Describe the experimental setup for dark adaption?

Answer 6

• Dark adapted the observers for 40minutes to ensure maximum rod sensitivity
• Then they locate test spot/object 20 degrees nasal to fixation so spot can be projected on temporal side of retinal location where there’s highest rod concentration
• They used blue-green test spot for maximum sensitivity according to relative sensitivity of wavelength
• Used a 10arcmin diameter light spot & present light spot for 1 millisecond
• They used artificial pupil to control light influx
• Measured the absolute threshold of light detection using method of constant stimulus

Question 7

Describe the Method of Constant Stimuli (MCS)?

Answer 7

• Classical psychophysical method to measure threshold
• Span psychometric function
  o Select range of stimulus intensities from certainly invisible to certainly visible
  o Pick a few (4-9) intensities in a uniform step from range – will make up constant stimulus set to be tested
  o Present one stimulus intensity at a trial & make observer report if it was seen or not
  o Test each intensity many times (20-100 trials per stimulus intensity) in a random order
  o Calculate proportion of “seen” or “not-seen” responses at each intensity out of the total number of presentations
  o Plot % seen against stimulus intensity – a psychometric function
  o From psychometric function, can calculate threshold ant any performance level
   Threshold = stimulus intensity at which stimulus is detected at a certain percentage (e.g. 60%)
• As increase stimulus intensity, absolute frequency of seeing increases –> absolute frequency
• Estimate loss before retina: ~50% will be lost to absorption by optical media, photopigment &/or reflection off cornea
• Estimated loss at retina: another 40% will fail to activate the rod when photons fall between the rods  incomplete activation
• Only ~10% - 5 to 14 photons will be absorbed by retina within 10arcmin area where about 300 to 500 rods are found
• For ~100 photons hitting cornea, observer will report seeing a flash 60% of time when ~10 photons are absorbed by rods
  o Given this, it’s highly unlikely that 2 photons hit same rod when 10 photons fall randomly, where there are 300-500 rods
  o Given that v small chance that 2 individual photons hit single rod
   Authors of experiment concluded that each photon will land on a single rod to activate & these activations will be summated within this summation area
• Means a single photon should be enough to activate a single rod