See Descartes and the Pineal Gland :
In the Treatise of man, Descartes did not describe man, but a kind of conceptual models of man, namely creatures, created by God, which consist of two ingredients, a body and a soul. “These men will be composed, as we are, of a soul and a body. First I must describe the body on its own; then the soul, again on its own; and finally I must show how these two natures would have to be joined and united in order to constitute men who resemble us”
The bodies of Descartes’ hypothetical men are nothing but machines [...]. The working of these bodies can be explained in purely mechanical terms. Descartes tried to show that such a mechanical account can include much more than one might expect because it can provide an explanation of “the digestion of food, the beating of the heart and arteries, the nourishment and growth of the limbs, respiration, waking and sleeping, the reception by the external sense organs of light, sounds, smells, tastes, heat and other such qualities, the imprinting of the ideas of these qualities in the organ of the ‘common’ sense and the imagination, the retention or stamping of these ideas in the memory, the internal movements of the appetites and passions, and finally the external movements of all the limbs”
Thus, we have stimuli : "the reception by the external sense organs of light, sounds, smells, tastes, heat and other such qualities", and perception : "the imprinting of the ideas of these qualities in the organ of the ‘common’ sense and the imagination".
The pineal gland played an important role in Descartes’ account because it was involved in sensation, imagination, memory and the causation of bodily movements. [...] He explained perception as follows. When the sensory organs are stimulated, [...] image of the sensory stimulus appears on the surface of the pineal gland. It is this image which then “causes sensory perception” of whiteness, tickling, pain, and so on.
“It is not [the figures] imprinted on the external sense organs, or on the internal surface of the brain, which should be taken to be ideas—but only those which are traced in the spirits on the surface of the gland (where the seat of the imagination and the ‘common’ sense is located). That is to say, it is only the latter figures which should be taken to be the forms or images which the rational soul united to this machine will consider directly when it imagines some object or perceives it by the senses”.
Imagination arises in the same way as perception, except that it is not caused by external objects.
I think that "interpretation" is not part of Descartes' theory of mind.