[R] Psychometric curves, two altnerative force choice, glm, and budbworms

Dieter Menne dieter.menne at menne-biomed.de
Thu Jun 20 08:54:46 CEST 2002

Dear R-Listers,

to measure the psychometric curve of pitch discrimination, one sequentially
presents two tones of slightly different pitch to an observer (animal will
do), and asks "which is higher". The pschometric curve is the fraction of
correct responses plotted against the pitch difference. It passes through
50% (pure guessing) at zero and normally approaches 100% at large
To compare two psychometric curves, the conventional way is to fit two
logistic curves and compare the 75% correct "threshold" values (whatever
threshold means).

I want to handle the case similar to the budworm example in MASS
(glm(SF~sex*ldose, family=binomial)). My basic idea is that the 2AFC forced
choice psychometric curve, normally only defined for positive stimuls
differences, could conceptually be continued to negative values by mirroring
the values at (0,0.5) to get the whole binomial/logistic curve. As far as I
can see, the result would be correct if we divide the counts for a stimulus
difference equally between positive and negative branch, with correct/wrong
inverted for the negatives. The solution is not clean if we have uneven

Is this a valid approach? Alternatively, could I force a
glm/binomial/logistic fit to go through 0/0.5?

Dieter Menne

Dr. Dieter Menne
Biomed Software
72074 Tübingen
Tel (49) (7071) 52176
FAX (49) (7071) 55 10 46
dieter.menne at menne-biomed.de

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