In Posner & Raichle's (1994) book, two essential and strictly related
limitations of cognitive neurophysiology are not sufficiently enhanced
: (1) The problem of ''coding,'' namely the capability of a natural br
ain to redefine its own ''basic symbols'' as a function of a changing
environment; (2) the inadequacy of a Hebbian rule to reckon with compl
ex computational problems such as those solved by real brains.