Pyschophysical methodology Flashcards
why do we check VA/visual functions as optometrists
- to chekc if pts vision is normal
- to check if a disease has progressed
- to check whether a treatment is effective
- for legal purposes
- for occupational purposes
what are objective tests
examples?
a test that doesnt require any feedback or action for a pt
- ret
- electroretinography (ERGs)
- visual evoked potentials (VEPs)
what are subjective tests
a tests that requires feedback or action from a pt
what does an electroretinography do
the function of the retina is assessed w/ means of electrophysiology
-the pts eye is stimulated by light and the retinal neuron’s electrical impulses are collected w/ electrodes
what does a visual evoked potention (VEP) do
- the electrode collects the electrical activity of the retina which then can be used to assess the retinal function
- fucntion of the visual pathway (from retina to V1) is assesed w/ means of electroretinography and compared to normal values
- pts eye is stimulated w/ patterns of light and the elctrical responses from neurons from the primary visual cortex are collected w/ electrods
why are subjective tests called psychophysics
perceptual processes are involved in subjective testing
-psychophysics is a procedure that correlates a physical stimulus w/ mechanisms of the visual system after a pt has given a feedback or taken an actoin
what are some visual processes that can be asssessed by psychophysics
- detection-the minimum amt of energy so as to perceive light
- discrimination-ability to differentiate btwn 2 wavelengths of light
- recognition-min letter size you can recognize
- speed of response-min amount of time needed to respond to a stimulus
what is the central concept to psychophysics? what does this mean
sensory threshold: the threshold is the idea that mental events have to be stronger than some critical amount in order to be perceived
what are some examples of psychophysical tests
VA vf accomadative tests (perimetry) color vision tests steroscopic acuity refraction
what is absolute threshold or RL (Reiz Limen)
-how do you measure it
the smallest amount of stimulus energy necessary to produce sensation
-measured in spectral sensitivity curves or dark adaptation curves
ex. need at least 1 cd/m^2 of 500 nm stimulus so as to be able to see it
what is the difference threshold?
what do you use to measure it
amount of change in a stimulus required to produce a just noticeable difference (jnd) in the sensation
-measured in light adapatation curve, refers to Weber fraction
ex. need to add 2 cd/m^2 on a stimulus of 10 cd/m^2 so as to see the change
what is threshold
the intensity for which we perceive 50% of the stimuli
what are the classical psychophysical methods for measuring threshold
method of constant stimuli
method of limits
method of adjustment
what is the method of limits
finds the transition or ‘limit’ btwn yes and no responses
2 types: ascedning and descednign
what is the ascending method of limits
on the ascending method of limits, the stimulus characteristic (ex. intensity or size) changes from values that are below threshold to values that are above threshold
-point where there is a transition from yes to no is considered to be threshold
what is the subthreshold
what is the suptrathreshold
subthreshold: stimulus below threshold
suprathreshold: stimulus above threshodl
what are the advantages to ascending limits
what are the disadvantages
advt: does not disrupt the adaptation of the subject (preferred for dark adaptation)
disadv: patient’s anticipation, habituation, practice, fatigue
what are some precautions you must take for the ascending limits
- make sure pt undertsands the test
- test the pt multiple times for reliability
- vary the starting point
what visual test uses the ascending limits
visual fields
what is the descending method of limits
the stimulus characteristic changes from values that are above threshold to values that are below threshold
what are the advantages and disadvantes of the method of descending limits
advantages: the pt is familiarized w/ the stimulus
- obtains practice before reaches the threshold
disadvantages: disrupts adaptattion (not good for dark adapt)
- pts anticipation
- habituation
- fatigue
what are the precautions of the method of descending limits
- test the pt multiple times for reliability
2. vary the starting point
what are some techniques that use the method of descending limits
snellen VA
-test starts w/ letter sizes that are well above threshold and pt is familiarized w/ procedure
by combining the ascending and descending methods of limits, what are cancelled out
habituation and anticipation
-pt isnt habituated in the one direction of responses and they dont anticipate a certain response as they dont know what stimlul will b enext
how do you determine the threshold for the method of limits
for dtermining the difference threshold you usually need to make a comparison btwn a standard stimulus (reference) and a comparison stimulus
what is the interval of uncertainty
the stimulus range over which the subject does not notice a difference btwn the reference and comparison stimulus
-the range of stimulus intensity values in which the subject perceives the values as the same
what is the point of subjective equality
the point where the reference and the comparison stimulus are perceived as equal
-midpoint btwn the lower limen and upper limen
PSE=(LL-LU)/2
what is the lower limen, what is the upper limen
lower limen: the intensity value where the ‘NO’ response transitions to an ‘E’ response or equal (both the comparison and background stimulus are equal)
-upper limen: intensity value where a ‘YES’ response transitions to an E or equal perception
what is the difference threshold
half of the interval of uncertainty
DL=IU/2
what is the point of objective equality
the point where the reference and the comparison stimulus are measuured as equal using an objective technique
ex. testing luminous intensities of a light you could measure when the values of the 2 stimuli are the same w/ a photometer
what is the constant error (CE)
the difference btwn the POE and PSE
CE=POE-PSE
what is the staircase method
the method changes from ascending and descending method automatically dpeending on the pts response
-general principle is if the pt doesnt see the stimulus make it stronger, if pt sees it, make it weaker
how is the threshold determiend for the staircase method
the threshold is determined as the avg of the last 3 or 4 reversals
what is a disadvantage to the staircase method
it can take a long time to reach the first reversal (fatigue or boredom can occur)
what is the parameter estimation by sequential testing (PEST)
the increase and decrease of stimulus intensity isnt constant throughout the test and is dependent on pt’s response
what are the advantages and disadvantages to the parameter estimation by sequential testing (PEST)
advantages: much faster than the traditional staircase method bc you need fewer stimuli to elicit a reversal
- stimulus is close to threshold so that pt’s adaptatoin does not get disrupted
disadvantages: boredom, learning effect
how can you overcome the disadvantages of the staircase method
by using a double random staircase method
-stimuli from the 2 diff staircases are presented randomly to the pt so he/she doesnt know what to epect
what is the forced choice method
prsents a set of diff stimuli and forces the pt to mak ea choice among the options
- effect of subject’s criterion is minimized bc forced to make a choice
- stimuli changes after choice and another is made
- can be separated in space or tiem
what is spatially separated simuli?
what is temporally separated stimuli?
spatially: which stimulus is diff? present stimuli that differ in spatial location ex. choose right or left
temporal: present the stimuli at diff points in time, which was diff, the first or second one?
what techniques use forced choice
jackson cross cylinder
also called a 2 alternative forced choice procedure (2AFC) and preferential looking (in infants)
in the forced choice, how many presentations are required?
from the number of correct responses, what kind of function is plotted
~100 presentations
psychometric function is plotted => % of correct answers as a function of the stimulus characteristic => forms a sigmoid curve
in the forced choice 2AFC, the starting point of a psychometric function is 50% correct response and not at 0, why?
-what is the threshold at?
bc the subject has a 50% chance on guessing correct even if he doesnt see the stimulus
-the threshold is defined at the 75% correct answers (1/2 btwn 50 and 100%)
what is the starting point of a psychometric function for a 4AFC experiment?
what is the threshold
25% and not 0 bc the subject has a 25% chance on guessing correct even if he doesnt see stimulus
-threshold defined as 62.5% (1/2 btwn 25 and 100%)
why does a forced choice method yield lower threshold
bc the subjects are forced to guess
what is the method of constant stimuli
a set of predefined stimuli are shown in RANDOM ORDER
- subject asked whether he saw or not the stimulus responding w/ a yes or no
- usually btwn 5 and 10 stimuli are used, each one is presented in a random order
what does the frequency of seeing curve/psychometric curve plot
can be plotted once you have the percentage of YES responses for each stimulus intensity
-threshold is the stimulus intensity point on the curve where the subject has a YES response 50% of the time
in the method of constant stimuli, why do you have to do prelinary runs
to have an idea of approximately where the threshold is
-you don’t want to use stimuli that are a lot below or above threshold
what are the advantages and disadvantages to the method of constant stimuli
advantages: no habituation (bc stimuli changing randomly)
- no anticipation (dont know whether the next one will be yes or no)
disadvantages: time consuming
the frequency of seeing curve might reveal _____ on the detection of a stimulus even if the threshold is caluclated to be w/in normal limits
abnormalities
- the curve itself will show deviations in responses even if the threshold is normal
- can give us info about disease progression
psychometric function often follow a particular S-shaped function, called….
(FOS curve)
ogive
for measuing the DL (difference threahold) using the method of constant stimuli, what must be shown to the subject
a reference stimulus and a comparison stimulus
-the intensity of the comparison stimulus is varied from trial to trial and he must judge whether this value is higher or lower than the ref sitmulus
in the method of constant stimuli, what ist he lower limen and upper limen and PSE
lower limen: intensity value at 25% higher responses
upper limen: intensity value at 75%
PSE: intensity value at 50% higher responses
how do you control for space errror in method of constnat stimuli
the reference stimulus must be presented on half of the trials in one retinal location and on the other half on the othe rlocation
how do you control for time error for method of constant stimuli
reference stimulus must be presented as first on half of the tirals first and as second on the other half of the trials
the method of counterbalancing spatial location or temporal order is based on the assumption that…
when the results from all the trials are combined the effects of the time and space error will be canclled, providing an unbiased estimate of DL (diff thresholdd)
what are the advantages and disadvantages of the method of adjustment
advant: natural and straight forward
- not boring for the subjects
disadvantages: experimenter loses control
- may take a long time
- cues in the experiment may allow pt to set threshold
what is the method of avg error
where is this used
the pt adjusts the comparison stimulus until it is perceived as equal to the reference sitmulus
-used in method of adjustment to find the DL
what are catch trials
method used to estimate guessing
- trials where the subject must always be able to detect the stimulus or the reverse
- subj unaware of the catch trials and therefore will continue to have the same bias and rate of guessing
what is the guessing rate for 2AFC
4ACF?
50%
25%
using ht emthod of constant stimuli, is there a need for catch trials?
no-
use 2 stimuli, one well below threshold and one well above will give you the guessing rate, if any
what are the 4 diff ways to measure the relationshiop btwn the stimulus strength and the magnitude of sensation
ratio production
ratio estimation
magnitude estimation
magnitude production
what is ration production
subj presented w/ a ref stimulus and asked ot adjuct the test stim so as it appreas to be a ration of the ref stim
ex. adjust test stim so it appears twice as bright as ref
what is ratio estimation
examiner sets the strength of the ref and test stim to be of a certain ratio and the subj is asked to est the ratio
ex. how much brighter is thet est stim
what is magnitude estimation
- obs presented w/ a ref stim and told that has a certain value. series of test stim then presented and the obs needs to assign a value to indicate his perceived mag in relation to the ref stim
- there is no ref stim and the subj prsented w/ a series of stim and asked ot assign a number to them
what is magnitude production
obs presented w/ a ref stim w/ a certain value. test stim then presented and the obs is asked to adjust the stim to some value relative to the ref stim
ex. here is a stim w/ a value of 100, adjust so as it has a value of 25
