Lecture 7 - Psychophysics

What is psychophysics?

scientific study of the relationship between a physical stimulus and a perceptual response

What are the methods of limits?

start at or below threshold and increase or decrease stimulus

What are the advantages of the method of limits?

quick

What are the disadvantages of the method of limits?

error of expectation or error of habituation - pt understands they'll eventually see it or not

What is the staircase method?

increase stimulus until threshold passed, then decrease stimulus until threshold passed in opposite direction

How do you determine threshold with the staircase method?

average of the stimulus values at the reversal

image - all red

What is the method of adjustment?

subject control stimulus magnitude and instructed to set the stimulus at threshold

What are the advantages of the method of adjustment?

active subject involvement (less boredom)

What are the disadvantages of the method of adjustment?

affected more by criterion bias and observer guessing than other (depends on reliable patient)

What is the method of constant stimuli?

stimulus value is varied randomly during each trial

named because patient's expectations kept constant

What are the advantages of the method of constant stimuli?

highly accurate

What are the disadvantages of the method of constant stimuli?

requires lots of trials - best if 30+ presentations of each stimulus is used to get accurate percentage

Where is the threshold placed for the method of constant stimuli?

50%

why? - where 50% of participants could see the light

What are the issues with response bias?

1. observers with strict threshold criteria

2. observers with lax threshold criteria

3. criteria may change from trial to trial in the same observer

What is the forced choice method?

stimulus is presented in 1 of 2 (or 4 or more) possible spatial positions or temporal intervals

What are the advantages of the forced choice method?

minimizes criterion bias - person has to guess even if nothing was seen

What are the disadvantages of the forced choice method?

can be time-consuming to generate full psychometric function (like the method of constant stimuli)

Where is the threshold set for the forced choice method?

75%

why? - by chance they'll get 50% correct, so you're evaluating the remaining 50%

For dark adaptation experiments, why is ascending limits the best psychophysical technique?

no bleaching from the actual test

Where is bleaching done during dark adaptation experiments?

20° off fovea

why? - highest concentrations of rods

What does the dark adaptation curve look like?

What is the first phase of the dark adaptation curve?

cone-mediated sensitivity

why? - cones regenerate photopigment quicker, more sensitive

Where does the first phase of the dark adaptation curve end?

cone plateau

What is the color of the stimulus during the first phase of the dark adaptation curve?

seafoam green!

why? - highest sensitivity

Why is the cone phase first in the dark adaptation curve?

photopigment recovers faster than rods

What happens at the rod-cone break?

rods/cones are equally sensitive to light

What is the second phase of the dark adaptation curve?

rod-mediated sensitivity

Where does the second phase of the dark adaptation curve end?

rod plateau

What is the color of the stimulus during the second phase of the dark adaptation curve?

grey/white

why? - rod only mediated vision at this point

Which are more sensitive to dark light?

rods!

How would a weak and strong bleach look differently on the dark adaptation curve?

How does changing the color of the stimulus light during the dark adaptation curve affect the graph?

can shift the graph both vertically and horizontally

How does a blue/green light stimulus during the dark adaptation curve affect the plot?

1. rod-cone break moves left (earlier)

2. deepens the difference between rod and cone sensitivity

why? - rods are more sensitive to blue than cones!

How does orange light stimulus during the dark adaptation curve affect the plot?

1. rod-cone break shifts more right (later)

2. shortens the difference between rod and cone sensitivity

why? - there's almost no difference in sensitivity, so everything just slows

How does red light stimulus during the dark adaptation curve affect the plot?

NO rod-cone break, why cones are always more sensitive!

no rods on graph

What colors are rods more sensitive to than cones?

blue and green

basically everything under Rayleigh's region (>550 nm)

What colors are cones more sensitive to than rods?

red

why? - Purkinje shift, in the dark cones can only mediate the color red which is why red appears darker in scotopic conditions

What color do rods and cones have similar sensitivity to?

orange

What would happen if you used a red bleaching light?

no bleaching of rods and stay dark adapted

why? - cones are more sensitive to red

After full bleach, what is the total time for regeneration of photopigment?

~35 mins

What is the relationship between rhodopsin regeneration and visual threshold?

not a direct relationship (i.e. the number of photopigments are NOT a direct correlation to the threshold)

relationship is exponential and in log units, so as you regain photopigment you dramatically regain sensitivity and decrease threshold

Why do cone photopigment regenerate faster than rod (chromophore)?

alternative pathways!

1. muller cells can regenerate chromophore for cones, not rods

2. some regenerated within cones themselves

as well as RPE cells

Where do drusen collect during AMD (age-related macular degeneration)?

between RPE and Bruch's membrane

What is the difference between dry and wet AMD?

wet - newly-formed abnormal blood vessels grow in macula, fragile and tend to break/leak

What is an early impact of AMD?

night vision before central vision

why? - a lot of rods in the macula (not the fovea)

What is the pathogenesis of AMD?

cholesterol accumulation leads to panmacular deposits between Bruch's membrane and the RPE, eventually become clinically visible drusen, impairs normal transport (like chromophore) across Bruch's membrane

causes localized deficiency of vitamin A, and dark adaptation

Why are rods attacked first during AMD?

cones are able to use both RPE and muller cells to get recycled chromophore

How does AMD affect the dark adaptation curve?

prolongs the rod-cone break

why? - rods cannot regenerate chromophore faster due to waste accumulation between Brush's membrane and the RPE (inhibit transport)

What is the AdaptDx?

first dark adaptometer for rapid routine clinical use

What color stimulus light is used for the AdaptDx?

green - 505 nm

why? - sooner rod-cone break (rod more sensitive to green) & rods most sensitive peak!

What psychophysics method does the AdaptDx use?

staircase method