Block 11 Flashcards
Study of the relationship between the physical stimuli and perceptual responses
Psychophysics
The minimum quantity of a stimulus that can be detected
Threshold
Percentage of stimuli detected is plotted as a function of stimuli intensity to produce
FOS curve (frequency of seeing) or (psychometric function)
Type of observer that’s manifests an unambiguous threshold (never seen below threshold, above threshold is always seen)
Ideal observer
Type of observer: As the intensity of stimulus in creased, the probability of seeing it increases
Real observer
Type of observer: no clearly defined intensity below which the stimulus is never seen and above which it is always seen
Real observer
A diseased visual system in _______ than a healthy one
Noises
A noiser visual system makes the FOS curve
Less steep (more flat)
6 mothers to measure the threshold
Ascending limits Descending limits Staircase Constant stimuli Adjustment Forced choice
How the the method for finding threshold determined
By the nature of the experiment or procedure
Stimulus is initially not visible, intensity increased until visible, done several times and averaged to determine threshold
Method of ascending limits
Advantage of method of ascending limits
Dark adaptometry
Disadvantage of method of ascending limits
Observer anticipation (Begin each trial at a different intensity)
Reverse method of ascending limits (stimulus clearly visible and decreased until no longer seen)
Method of descending limits
Advantage of method of descending limits
Determination of VA
Disadvantage of method of descending limits
Observer anticipation
Combo of ascending and descending limits
Staircase method
Advantages of staircase method
Quick and reliable
Psychophysical experimentation
Automated visual field testing
A “yes/no” procedure where the stimulus is varied randomly and the curve is plotted based on response
- involved blank trials, false positive and false negatives
Method of constant stimuli
Advantage of method of constant stimuli
Maintains observers expectation at same level
Diadvantage of method of constant stimuli
Time consuming
False pos/neg affect curve and must be adjusted
When no stimulus is presented, even though the observer is asked
Blank trials
Observer reports seeing the stimulus during a blank trial
False positive
Stimulus is above threshold and the subject reports the stimulus is not seen
False negative
The subject adjusts the stimulus intensity until it is barely visible/invisible
Method of adjustment
Advantage of method of adjustment
Pretty quick
Disadvantage of method of adjustment
Repeatability is not as high as other methods
What are the 3 observer’s criterion involved in forced choice method
1- not all use the ‘same criteria’
2- threshold criteria may vary trial to trial for same observer
3- observer Amy use one set of criteria for one type of stimulus and another set for a different stimulus
Criterion: do not report seeing a stimulus until absolutely certain (give high threshold)
Strict threshold criterion
Criterion: do report seeing a stimulus even though they may have a great deal of uncertainly (give low threshold)
Lax threshold criterion
Effects of observer’s criteria are minimized by forcing a choice
Forced choice method
Example of a 2 forced choice method
Vector vision CSV chart
Threshold for a 2AFC (2 alternative forced choice)
75%
Threshold is assumed to be the point midway between what
Chance performance and perfect performance
How to determine chance performance
The chance that they will get it correct
Threshold for 4AFC
62.5%
The steeper the curve, the more or less confident?
More
Advantages of forced choice method
- lower thresholds
- used effectively to determine the VA and other visual capabilities of infants
- when forced to guess, they do well despite claiming they cannot see the stimulus
4 things that influence the determination of threshold
Decision criteria
Attention
Motivation
Internal neural noise
What does signal detection theory provide
A useful model to predict the effects of certain factors
How are the neural signal produced by the stimulus and neural noise related
Superimposed
What is the task of the observer
To differentiate the signal and noise combo FROM background noise
Present in the absence of a stimulus
Neural noise
Randomly distributed over time and randomly fluctuates over time
Neural noise
Stimulus causes a constant level of neural activation to be added to the noise
Noise + signal
As the signal becomes larger, what happens to the noise and noise+signal
Get further apart
As the signal becomes larger, what happens to detectability
Increases
It is easer for observer to determine noise+signal if the signal is: large or small
Larger
When there is a large detectability (d), how does a viewer see it
No uncertainty regarding whether it was presented or not
How strong is the signal when there is overlap of the N and N+S
Weak stimulus
If the stimulus is delivered what the noise is low, what is the resulting level of neural activation
Ambiguous
- no way to be certain if the stimulus is absent or present
If the stimulus is delivered when the noise is high, what is the resulting level of neural activation
Unambiguous
- neural noise only occurs when stimulus is present
When the level of neural activation is above the criterion line, will the subject see the stimulus?
Yes
When the level of neural activation is below the criterion line, will the subject see the stimulus?
No
The subjects response is determined by what
The level of neural activation
Is the subjects response determined by whether the stimulus is presented or not?
No
When the stimulus results in a neural activity that exceeds the threshold criterion
Hit
When the activity does no exceed the criterion
Miss
When the neural activity exceeds the threshold criterion in the absence of a stimuli
False positive
When the activity is below the criterion during a blank trial
Correct reject
Large number of hits, few misses, but lots of false positive
Lax threshold criterion
Few hits, more misses, but few false positives
Strict threshold criterion
Curve showing the probability of a hit as a function of the probability of a false positive
ROC curve (receiver operating characteristic)
Probability of a high is high, so the probability of a false positive is high
Lax criterion
Probability of a hit is low, so the probability of a false positive is low
Strict criterion
Weber’s law deals with
Contrast
Which system is more sensitive when it comes to contrast
Photopic (.015…….scotopic is .14)
Discriminate between the combination of stimulus+background and background alone
Weber’s law
Can be understood using an increment threshold experiment
Weber’s law
The threshold increment is referred to as
JND (just noticeable difference)
As the background intensity increases, what happens to the JND
Increases
What happens to the ratio of JND to background intensity as the background intensity increases
The JND increases as well, so the ratio remains constant
The ratio of the optotypes to the background luminance (contrast) is the same, no matter the illumination
Lightness constancy
Why is the appearance of the E the same under both dim and bright lights
Because of lightness constancy (visual system detects contrast, no absolute luminance)
What is the appearance of a stimulus of constant luminance when viewed against backgrounds of various luminance
Brightness depends on background
The phenomenon regarding the brightness something appears when viewed against backgrounds of various luminances
Simultaneous contrast
Key factor in predicting the brightness of a stimulus (its appearance)
The contrast of the stimulus
When no stimulus is presented, even though the observer is asked if they saw
Blank trial
during a VF test, pt is reluctant to signal that she sees stimulus even when she may see it. The threshold criteria used by patient is
Strict
When determining a clinical threshold, which psychophysical method most minimizes the importance of the patients criterion
Forced choice
Common used during automated VF testing
Staircase
Which forced choice clinical procedure is expected to result in least steep (flatter)
2AFC
Response in which subject reports not seeing a clearly visible, Supra-threshold stimulus is referred to as
False negative
If sub threshold and they see
False positive e
If sub threshold and they dont see
Correct reject
The JCC test for measuring astigmatism is an example of which method
Forced choice
If the chance performance of 4AFC is 25%, what is threshold
62.5%
When a patient adopts a strict criterion during clinical tests, the result is
Few false positives and few hits
Ocular disease is expected to cause the slop of function to
Decrease (get flatter)
Weber’s law predicts as background intensity increases, contrast threshold
Remains constant
When measuring VA with snellen, which method will provide most reliable results
Descending with forced choice
As the intensity of background increases, the brightness of a patch inside the background
Decreases
According to signal selection theory, a stimulus produces a normal activity that is
Constant
Ass the probability of a hit increases, the probability of FP
Increases
The method of constant stimuli is designed to keep what constant from trial to trial
Stimulus intensity
Subject’s expectations
As stimulus size increased, the detectability
Increases
The retinal analyzes data and encodes it into what
A neural signal
What cells in the retina produce action potentials
Ganglion and amacrine
Small light on center of ganglion receptive field does what to the AP firing
Increases firing
Small light on surround of receptive field of ganglion cell does what to AP firing
Decrease firing
Large light that covers the entire center of the ganglion cells receptive field does what to AP firing
Increases firing
Why is there an increase in AP firing with the light covers the entire center of the ganglion cells receptive field
Due to spatial summation in the field’s center
Stimulus that covers the entire receptive field of the ganglion cells will do what to AP
Nothing, acts the same as if there were no stimulus
What type of ganglion cells do not respond well to diffuse illumination
Spatially antagonistic GCs
What is an example of a strong stimulus for ganglion cells
Sine wave grating
The the bright bar of the sine wave grating falls on the center and the dark bars fall on the surround, what does that do to APs
Increase firing
Spatial grating does what to a ganglion cell
Vigorously excites
When do you reach max firing of AP in a ganglion cell
When the entire center of the receptive field is covered
When the stimulus covers the entire receptive field of a GC plus some more area, does this have an affect on the cell’s response
No
First cell to respond with an action potential in phototransduction
Amacrine cells
What is the order than info is sent in the phototransduction path
PR Horizontal Bipolar Amacrine Ganglion
Which cells convert light quanta into electrical activity
PRs
