  We identify the presence of typically quantum effects, namely 'superposition'
and 'interference', in what happens when human concepts are combined, and
provide a quantum model in complex Hilbert space that represents faithfully
experimental data measuring the situation of combining concepts. Our model
shows how 'interference of concepts' explains the effects of underextension and
overextension when two concepts combine to the disjunction of these two
concepts. This result supports our earlier hypothesis that human thought has a
superposed two-layered structure, one layer consisting of 'classical logical
thought' and a superposed layer consisting of 'quantum conceptual thought'.
Possible connections with recent findings of a 'grid-structure' for the brain
are analyzed, and influences on the mind/brain relation, and consequences on
applied disciplines, such as artificial intelligence and quantum computation,
are considered.
