There are three versions of the paper:

• the version in the Proceedings of IJCNN-91: this was the paper originally submitted to IJCNN-91
• the version in the neuroprose archive: this paper was submitted to IJCNN-91 after the deadline had been extended. It is an improved version of the paper above which it should replace. The IJCNN-91 organizers however, managed, to publish the other (older) version in the proceedings (after accepting both papers for oral presentation 8v) ).
• the version provided here: it is nearly identical to the version in the neuroporose archive. However, two postscript figures which made problems on some printers, have been corrected and enlarged (done in 1995).