320 ch. 2

study of the relationship between properties of physical stimuli and psychological reactions to those properties

• abrupt change from not being able to detect a stimulus to just being able to do so
• aka absolute limen

• method of limits
• method of adjustment
• method of constant stimuli

observer presented with several alternating ascending and descending series

begin with a stimuli that is clearly noticeable and then present increasingly weaker stimuli until the observer reports that they can't detect it

start with a stimulus clearly below threshold and the present increasingly stronger stimuli until observer reports that they can detect it

• believe that stimulus is likely to be the same as last time, so just keep giving same answer
• tend to keep saying no on ascending series and keep saying yes on descending series

• stimulus is likely to be different from last time so change answer and "jump the gun"
• on ascending series, claim that they can detect stimulus when they can't
• on descending series, claim they can't detect stimulus anymore when they can

• observer adjusts the intensity of the stimulus instead of the experimenter
• observer makes continuous adjustments rather than discrete

• stimuli presented in random order
• experimenter usually selects between 5-9 stimuli, with the weakest stimulus clearly below threshold and the strongest stimulus is clearly above threshold- these values must be chosen after pretesting with a quick method like method of adjustment
• constant stimuli- constant set of stimuli chosen before testing and present these stimuli a constant number of times during testing

• people with high sensitivity are capable of detecting minor changes in those around them or have a low threshold for detecting change
• insensitive people have very high threshold
• sensitivity is inversely related to thresholds

• argues that the thresholds obtained by classical psychophysical methods are composites of 2 separate processes
• 1. observer's sensitivity to the stimulus
• 2. observer's decision-making strategy or criterion
• can separate sensitivty from criterios by examining observer's responses to trials with a signal and trials with only noise
• top-down processing- physical stimulus alone isn't sufficient to determine perception- mental factors also critical

weak physical stimulus is present

no physical stimulus is present, only background noise

willingness to say that you detect a stimulus

• some of the trials will have no tone presented, only normal background noise present
• remaining trials will have a very weak tone presented, added to the background noise

observer responds no on all noise-only trials indicating no stimulus is present

observer incorrectly responds yes to a noise-only trial

when observer responds yes when a stimulus is actually present

observer will occasionally respond no when stimulus is present

• index of sensitivity
• distance between peaks of 2 distributions

• important determinant of the criterion
• rewards and punishments associated with a particular response

• receiver operating characteristic curve
• shows relationship between the probability of a hit and the probability of a false alarm

• two-alternative forced choice procedure
• trial with 2 presentations- one with a target stimulus and one without
• observer's task is to indicate which of the 2 presentations is the target
• effects of expectations and criteria are minimized

• variation of psychophysical methods- determines smallest amount a stimulus must change to be perceived as different
• present a standard stimulus & comparison stimulus

• measures observers' discrimination ability
• smallest change in a stimulus that is required to produce a noticeable difference 50% of the time
• amount of change in a physical stimulus required to produce a jnd in the psychological sensation

value of the comparison stimulus that the observer considers equal to the value of the standard stimulus

standard stimulus remains the same and comparison stimulus is presented in alternating ascending and descending series

ex. determine point of subjective equality by asking observer to adjust the comparison stimulus until it seems to match the standard stimulus

experimenter presents comparison stimuli in random order and may ask observer to judge whether each comparison stimulus is > or < the standard stimulus

deltaI/I=k

• k or constant number
• smaller fractions indicate better discrimination abilities because less change is needed to produce a jnd

S=k log I

exponent that 10 must be raised to equal that number

S=kI^n

observers are asked to give numbers to match (estimate) their impression of psychological magnitude

observers asked to judge one stimulus modality (loudness of sound) by using another modality (brightness of light)

• happens when neurons are stimulated
• change from a neg to a pos state and then return to neg state

extremely small microelectrode is used to measure activity of a single neuron

• electroencephalography
• to study massed activity of many neurons by attaching electrodes to the scalp
• waves often with a distinctive pattern

PET and MRI

• positron emission tomography
• injecting a radioactive chemical into the bloodstream