Re: VisionScienceList: Psychophysical method ?

From: Jeffrey B. Mulligan (
Date: Tue Dec 19 2000 - 10:22:05 PST

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    > When using the method of constant stimuli,


    > I often wonder whether it is possible to know a priori
    In general, no.

    I have a few "rules of thumb" that I follow, which usually
    work well, but you always need to run a couple of pilot runs
    to make sure things are reasonable...

    > what is the best compromise between number of trials,

    I like 30 per condition (but I always use a staircase with
    adaptive step size).

    > of levels of the variable

    A general rule is that for things like contrast threshold,
    steps of 0.1 log unit will place 4 or 5 samples on the interesting
    part of the psychometric function. For other types of measures
    (e.g. orientation discrimination), you need to do a little
    trial and error.

    > and number of observers.

    3 or 4 with consistent data is usually enough to satisfy reviewers.
    If you are interested in individual differences you will want more.

    > I'm looking for theoretical reference(s)
    > related to this issue.

    Can't give you too much help here... there was a paper
    in the last few years by Kontsevich on adaptive trial placement
    (an improvement on the QUEST procedure...). This might be of
    interest, or contain some interesting references.

    I see that Tim Meese has already referred you to the work of my
    colleague Andrew Watson... He concluded that a procedure such
    as QUEST is better than a simple staircase, but that is mainly because
    a staircase that starts with the minimum step size can take a while
    to get the the region of interest. I use an adaptive step size that
    is halved on each reversal until the minimum increment is reached.

    My former graduate advisor Donald MacLeod made the observation that
    a staircase with a short memory (e.g. a staircase) is superior
    to one with a long memory (e.g. QUEST) when the threshold
    is unstable during the course of the experiment, because the simple
    staircase will be able to track the fluctuations better.

    My main gripe is that it is still a fairly widespread practice to average
    some number of staircase reversal points to estimate the threshold...
    It is much better to estimate the psychometric function from all the data.
    Reversal points can be moved with a single finger error.


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