>Psychtoolbox>Quest

q=QuestRecompute(q [,plotIt=0])

Call this immediately after changing a parameter of the psychometric
function. QuestRecompute uses the specified parameters in “q” to
recompute the psychometric function. It then uses the newly computed
psychometric function and the history in q.intensity and q.response
to recompute the pdf. (QuestRecompute

QuestCreate saves in struct q the parameters for a Weibull psychometric function:
p2=delta*gamma+(1-delta)*(1-(1-gamma)*exp(-10.^(beta*(x-xThreshold))));
where x represents log10 contrast relative to threshold. The Weibull
function itself appears only in QuestRecompute, which uses the
specified parameter values in q to compute a psychometric function
and store it in q. All the other Quest functions simply use the
psychometric function stored in “q”. QuestRecompute is called solely
by QuestCreate and QuestBetaAnalysis (and possibly by a few user
programs). Thus, if you prefer to use a different kind of
psychometric function, called Foo, you need only create your own
QuestCreateFoo, QuestRecomputeFoo, and (if you need it)
QuestBetaAnalysisFoo, based on QuestCreate, QuestRecompute, and
QuestBetaAnalysis, and you can use them with the rest of the Quest
package unchanged. You would only be changing a few lines of code,
so it would quite easy to do.

“dim” is the number of distinct intensities that the internal tables in q can store,
e.g. 500. This vector, of length “dim”, with increment size “grain”,
will be centered on the initial guess tGuess, i.e.
tGuess+[-range/2:grain:range/2]. QUEST assumes that intensities outside
of this interval have zero prior probability, i.e. they are impossible
values for threshold. The cost of making “dim” too big is some extra
storage and computation, which are usually negligible. The cost of
making “dim” too small is that you prejudicially exclude what are
actually possible values for threshold. Getting out-of-range warnings
from QuestUpdate is one possible indication that your stated range is
too small.

If you set the optional parameter ‘plotIt’ to 1, the function will plot
the psychometric function in use.

See QuestCreate, QuestUpdate, QuestQuantile, QuestMean, QuestMode,
QuestSd, and QuestSimulate.