  Semantic representations in higher sensory cortices form the basis for
robust, yet flexible behavior. These representations are acquired over the
course of development in an unsupervised fashion and continuously maintained
over an organism's lifespan. Predictive learning theories propose that these
representations emerge from predicting or reconstructing sensory inputs.
However, brains are known to generate virtual experiences, such as during
imagination and dreaming, that go beyond previously experienced inputs. Here,
we suggest that virtual experiences may be just as relevant as actual sensory
inputs in shaping cortical representations. In particular, we discuss two
complementary learning principles that organize representations through the
generation of virtual experiences. First, "adversarial dreaming" proposes that
creative dreams support a cortical implementation of adversarial learning in
which feedback and feedforward pathways engage in a productive game of trying
to fool each other. Second, "contrastive dreaming" proposes that the invariance
of neuronal representations to irrelevant factors of variation is acquired by
trying to map similar virtual experiences together via a contrastive learning
process. These principles are compatible with known cortical structure and
dynamics and the phenomenology of sleep thus providing promising directions to
explain cortical learning beyond the classical predictive learning paradigm.
