The sketchpad model. A theory of consciousness, perception, and imagery.

Subjective consciousness suggests a unity of the sensing and perceiving self that is difficult to reconcile with the multiplicity of sensory analyzers and the absence of a convergence zone in the brain. This has led--on the one hand--to the dead-end assumption of a unifying sentient homunculus and--on the other--to a denial of conscious unity. The sketchpad model presented here avoids this dilemma by viewing conscious thought as a selfreferent loop of neural activity, rather than as the information content of a fictitious set of output neurons. Use is made of the numerous neural pathways that originate at various cortical and subcortical areas and terminate in the thalamus. The model assumes that primary sensory inputs are modified by such feedback in the thalamic relay nuclei through hill-climbing processes that tend to optimize global responses.