Fitting a distribution to an array containing zeros

I have data from stimulation of subjects with different intensities (say we have 54 different intensities), and what follows is the percentage of them recognizing the respective stimulation:

    x =    [0  0  0  0.50  0  0  0  0  0  0  0.5  0  0  0  0  0  0  0  0.125000000000000  0  0.333333333333333  0  0  0.111111111111111  0  0.428571428571429  0  0.285714285714286  0.166666666666667  0  0.1  0  0.400000000000000  0.5  0.4  0.25  0.6  0.727272727272727  0.714285714285714  0.25  0.666666666666667  0.777777777777778  1  0.75  0  1  0.9375  1  1  1  1  1  0.92  0.92]

Say the first index is the weakest stimulation, and the last index the strongest, as visible the stronger the stimulation the more likely the subject recognizes it. I want to fit now a distribution to these values, to get something called a psychophysical curve, usually a logistic function (looks like [this][1]).

What I have tried is:

    pd = fitdist(x,distribution);

but this throws an error, I assume because of the 0's in the x array. What could I do alternatively?

[1]: http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/psychophysics/psychophysics%20slides/Slide7.jpg

[Jos (10584)]

What is the exact error you get?

What is the exact distribution you are fitting?