In 1971, Paivio published his book, Imagery and Verbal
Processes, In that work he introduced in print what has come
to be known as the Dual Coding Theory (DCT) of memory
and cognition (see 26.3, 29.2.3). Later, Paivio (1991) said
that DCT evolved from his specific experiments on the role
of imagery in associative learning (e.g., Paivio, 1963, 1965).
Paivio isn’t easy reading, and he is prone to take many pages
to explain his theory, but nowhere does he state it simply.
The closest that he comes is within a summary table (Paivio,
1991) where parsimony is essential: