Week 3

Question 1

how does the retina change its sensitivity to light

Answer 1

Two different photoreceptor systems handle different ranges
Change in pupil size
Change in concentration of bleached and unbleached photo-pigment
Neural adaptation mechanisms / dark adaptation

Question 2

photopic

Answer 2

day vision

Question 2

scotopic vision

Answer 2

night vision

Question 3

mesopic range definition

Answer 3

The range where the two mechanisms are working together (region of 0.01-3 cd/m2)

Question 4

what is dark adaptation

Answer 4

significant proportion of photopigment is bleached and recovery from bleach contributes to visual sensitivity

Question 5

what are the factors that can change dark adaptation curve

Answer 5

amount of adapting light
test spot size
wavelength
position on retina

Question 6

photochromatic level

Answer 6

The range of light intensity that is sufficiently high to stimulate rods but not cones in the retina of the eye and that therefore produces vision without colour

Question 7

how is luminance threshold meaured

Answer 7

function of time in darkness

Question 8

what is luminous efficiency

Answer 8

effectiveness of illumination of different wavelengths on the eye

Question 9

effects of amounts of adapting light

Answer 9

More intense/longer duration adapting light makes the cone branch more prominent and prolongs dark adaptation

Question 10

effects of Test spot size

Answer 10

A larger test flash generally yields a more prominent rod branch of the dark adaptation curve

Question 11

effects of wavelength

Answer 11

distribution of light used and photochromatic level

Question 12

effects of position on retina

Answer 12

rod portion of the curve is more prominent if the test flash stimulates a retinal region with high rod density

Question 13

what is light adaptation

Answer 13

Process that changes visual systems sensitivity in response to increased light levels
Each system must adjust to increases in background light
Changes in sensitivity of the visual system to lights superimposed on a steady background

Question 14

steps of the increment threshold experiments

Answer 14

Observer must judge whether a target is visible against the background

The difference is that the background is purposefully adjusted by the experimenter to different light levels
visual sensitivity is measured at each background level

Threshold is plotted against background intensity NOT over time

Question 15

explain increment threshold function

Answer 15

1. Absolute threshold
   For low background levels, threshold is independent of background
   Internal noise in the visual system is greater than the background noise
2. When the background level increases above internal noise, threshold increases in proportion to the square root of the background intensity
   Vries-rose law
   3.As background light increases to more visible level, increment threshold curve steepens so that now threshold is directly proportional to background intensity
   Weber’s law
   4.Saturation
   higher background light levels, thresholds for rods cannot be determined as all Na+ channels are closed once about 10% of photopigment has been bleached

Question 16

what is weber’s law

Answer 16

visual system is adapted to extracting relative information and it enables the visual world to appear constant despite huge variations in lighting conditions

Question 17

how can contrast constancy be expressed

Answer 17

Change in L/L = constant