[R] Fitting a cumulative gaussian

[Gamer, Matthias]

Dear R-Experts,

I was wondering how to fit a cumulative gaussian to a set of empirical 
data using R. On the R website as well as in the mail archives, I found 
a lot of help on how to fit a normal density function to empirical data, 
but unfortunately no advice on how to obtain reasonable estimates of m 
and sd for a gaussian ogive function.
Specifically, I have data from a psychometric function relating the 
frequency a subject's binary response (stimulus present / absent) to the 
strength of a physical stimulus. Such data is often modeled using a 
cumulative gaussian function. I have tried to implement such a fitting 
algorithm in R, but unfortunately, I was not successful. Maybe anyone on 
the list already coded a script for such purposes or could help me 
otherwise???

Thanks in advance,
Matthias