Lecture 1: Visual Psychosis Flashcards
distinguish absolute vs difference threshold
Threshold - demarks border between seeing and not seeing
absolute: minimum quantity of a stimulus that can be detected against a DARK bg
difference threshold: minimum quantity of a stimulus that can be detected against a bg that is NOT DARK
discuss factors resulting in variability in threshold results
(1) background luminance (eg. a star at midnight vs at twilight)
(2) perfect vs non perfect observer (humans with presence of neural noise - interferes with ability to perceive stimulus
(3) subject’s concentration (attention, motivation, fatigue)
state the psychophysical methods
method of adjustment, method of limits, staircase method, & constant stimuli
explain method of adjustment
SUBJECT adjusts stimulus (ascending) from NS to S or (descending) S to NS
quick for estimating threshold but vulnerable to:
(1) habitation - eg. responding to seeing 3 steps past threshold every time
can be overcome thru clear instructions, demos & practice runs
(2) anticipation - eg. prematurely reporting seeing the stimulus before threshold
can be overcome by changing starting intensity for every new trial
explain method of limits
(ascending) STIMULUS adjusted from NS to S, (descending) STIMULUS adjusted from S to NS
subject to respond every time stimulus shown, “NS” or “S”
QUICK to estimate threshold & (descending) FAMILIARISE px with task
take average of threshold values from both ascending and descending for better accuracy
vulnerable to habitation & anticipation
explain staircase method
modified ver. of method of limits combining asc & desc
stimulus starts as NS until S, then goes backwards until NS
stimulus intensity at these reversal points (3-4) averaged to get threshold
explain method of constant stimuli
stimulus intensity presented randomly (good for controlling anticipation) & subject responds whether S or NS
time consuming - same stimulus presented multiple times
explain common flaw of psychophysical methods
px uncertain to make a response -> high threshold/low sensitivity obtained
explain adaptive psychophysical methods
obtains threshold more efficiently by taking account the subjects previous response
(1) force choice tracking -> when subject responds correctly 3 times, stimulus decreases by 1 step. incorrect response results in 1 step increase. narrow range of stimuli level obtained
(2) maximum likelihood -> require prior info about population’s distribution of threshold. then probability distribution function constructed
state the two psychophysical procedures
yes/no & forced choice procedure
explain forced choice procedure
subject shown MULTIPLE windows (2/4/6 choices) where one contains stimulus. subject to indicate which window contains stimulus
gives better threshold than other PM because possibility of reduced threshold (subjects own strictness) is less likely
to get threshold:
determine probability of correct answer (chance performance) > 100-chance performance divide by 2 > ans + chance performance
