reference update

[Leslie Kay]

I am afraid I forgot an important reference in my earlier
post regarding the derivation of the sigmoid nonlinearity
in Freeman's work.  Another reference is:

Eeckman,F.H. and Freeman, W.J. (1991) "Asymmetric sigmoid 
nonlinearity in the rat olfactory system," Brain Research
557, 13-21.

Leslie Kay
lmk2 at garnet.berkeley.edu