  The quantum model of the brain proposed by Ricciardi and Umezawa is extended
to dissipative dynamics in order to study the problem of memory capacity. It is
shown that infinitely many vacua are accessible to memory printing in a way
that in sequential information recording the storage of a new information does
not destroy the previously stored ones, thus allowing a huge memory capacity.
The mechanism of information printing is shown to induce breakdown of
time-reversal symmetry. Thermal properties of the memory states as well as
their relation with squeezed coherent states are finally discussed.
