Cognition, according to Ulric Neisser, ‘refers to all processes by which the sensory input is transformed, reduced, elaborated, stored, recovered, and used. It is concerned with these processes even when they operate in the absence of relevent stimulation, as in images and hallucinations’ (Neisser, 1966). To discover some of the neural mechanisms operating in these processes remains one of the great challenges of neuroscience.
Significant progress has been made in analyzing the mapping and coding processes that occur in the first few stages of mammalian vision. These processes of so-called early vision involve neural mechanisms that sort out geometrical features, heighten contrast, emphasize contours, and help determine shape and motion of objects (Marr & Poggio, 1979; Marr & Hildreth, 1980; Marr, 1982).
Cognition, as Neisser defines it, must go beyond these early processes, utilizing not only innate circuitry, but bringing the full richness of stored experience to bear on the new sensory information. The time scale of such processes must extend from the order of a hundred milliseconds to seconds and beyond.
Cognition and reafference
These higher and later cognitive processes are usually relegated to cortical association areas. In the present model, however, we assume that significant changes in the afferent sensory information are brought about at relatively peripheral sensory levels – the lateral geniculate nucleus in the case of vision – and that these changes play an important role in the transformation, reduction, and elaboration of the sensory input.