Flashcards
Visual Psychophysics
Cards in this deck (27)
- 1Define psychophysicsbranch of psychology that relates physical stimuli with mental phenomena
- black box analogy (presenting light to a black box and based on the output of the box, work out what is going on inside the box) - 2Role of psychophysics in clinical optometryDetermine the how light interact with the eye to produce a perception
- 3Threshold psychophysicsMinimum amount of light required to detect a stimulus or a difference in stimulus from another
- 4Supra-threshold psychophysicsinvolves observer making a subjective judgement about a stimulus that is well above threshold (clearly visible)
- 5Thresholdsmallest stimulus intensity required for detection or discrimination (able to detect that they are different from one another)
threshold=1/sensitivity (expressed in log 10) - 6Why use log scale?Represent large range on a smaller scale to make it more manageable
- 7How to measure threshold?- method of adjustment
- method of limits
- staircase (modified method of limits)
- method of constant stimuli - 8Two sources of major errors when measuring threshold?- error of habituation (continuously saying no - overshoot threshold)
- error of anticipation (feel like saying yes because they said no too many times - undershoot threshold) - 9Error of habituationHave a habit of responding a certain way
- started at threshold too low to begin with
- cause overshoot threshold - 10Error of anticipationPatient anticipates when they think they will reach threshold by responses
* undershoots threshold - 11Method of constant stimuli* randomly present stimulus at different intensity levels
* data obtained to construct psychometric function - 12Psychometric functionGraph showing percent stimuli detected vs stimulus intensity
* s-shaped curve
* threshold is defined in experiment as intensity of the stimulus when 50% of trials are correct - 13Classical Threshold TheoryEarly theory
* measured threshold = estimate of neural threshold
* sharp transition between sensation and no sensation
* stimulus must cause a critical amount of neural activity for threshold to be exceeded
* threshold varies with stimulus property and adaptive state
* repeats (identical) will produce same response - 14Primary assumptions of classical threshold theory* threshold varies over time
* stimulus only generates a response if it exceeds the momentary threshold
* if momentary thresholds are normally distributed, psychometric function = cumulative normal (ogive)
* Mean and SD of distribution of momentary threshold can be calculated from psychometric function - 15Changes in shape of momentary threshold graph* difference in mean threshold - shift graph along x-axis
* difference in SD of threshold - changes steepness of the graph - 16Signal detection theory* signals are alway detected against a background of activity/noise
* external noise (stimulus noise) and internal noise (spontaneous activity)
* SDT models discrimination of signal+noise from noise alone
* brain has to discriminate between signal and noise - 17Signal detection theory * Receiver-Operating Characteristic (ROC) curve* plots relationship between the probability of reporting "yes" when the stimulus is present and the probability of reporting "yes" when it is absent
* Plots hit rate as a function of false alarm rate
* Each curve represent data for one stimulus intensity
* Each point on curve represents a different criterion
* Larger the separation - higher threshold - 18SDT - Sensitivity* d' : is the separation between means of N and S+N
* as d' increases, greater separation between the two distributions - 19Method of adjustment* allows subject to adjust the stimulus until they can just see it or not
* useful in obtaining quick threshold estimate
* not criterion-free estimate: false alarm rate
* potential problems: with repeated measurement, may be able to do the task with eyes closed - 20Method of limits*Modify stimulus from seeing to non-seeing (descending ) and from non-seeing to seeing (ascending)
* repeat and take average to determine threshold
* cancels out both errors
* cons: less efficient if start off too low or too high - 21One-way method of limitsmodifying stimulus from non-seeing to seeing (ascending)
* used for measuring dark adaptation
modifying stimulus from seeing to non-seeing (descending)
* testing visual acuity - 22staircase methodpresents a stimulus to an observer in discrete steps of increasing and decreasing visibility
* start off ascending and then switch to descending once observer sees stimulus
* switch again once observe can no longer see stimulus
* avoid redundant stimuli - 23Psychometric function graphShape of the psychometric function depends on the observer
* subject with liberal criterion - "think" you see (low threshold but doesn't bottom out at zero - more false positives)
* subject with strict criterion - threshold is higher, less false positives - 24determining psychometric functions for 2 alternative forced choice (2AFC)* contain only two options - forced to pick one
* more objective measure of threshold
* if they are unable to see - take guess (50% chance of getting right) - 25Forced choice experiment* psychometric function generated by measuring responses to multiple stimuli: subject must always respond
* subject given a number of choices (2AFC or 4AFC)
* threshold measure is criterion-free
* guessing determines the lowest probability correct (lower asymptote = false alarm rate, hence threshold percentage is altered - 26Supra-threshold psychophysics* specifically dealing with stimulus above threshold
* asking subject for a judgement on a relative sensation
* Fechner and Stevens both provide laws to predict the association between perceived stimulus intensity and actual intensity - 27Stephen's Power Law* log transformation - linearise data
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- Postgraduate Visual Functions - Optometry (Visual Functions - OD1) Flashcards on Visual Psychophysics , created by Kim Vu on 24/05/2018.
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- Kim Vu
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