Parallel processing in the nervous system: Evidence from sensory maps

Perhaps the clearest organizing principle in sensory systems is the existence of maps, in which there is an orderly and systematic layout of the stimulus space on a two-dimensional array of neurons. Usually, the layout of the stimulus on the receptor sheet is reproduced so that neurons that innervate adjacent sites on the receptor sheet project to adjacent sites in the central map. Thus, in the visual system there are retinotopic maps in which layout of the visual field on the retina is reproduced across a sheet of neurons in the thalamus or cortex. Somatotopic maps of the body surface in the touch system and frequency maps in the auditory system are similar examples. In the olfactory system, the form of the representation is different in that projections from the olfactory epithelium to the first central olfactory nucleus, the olfactory bulb, are sorted according to olfactory receptor molecules (1) and not position on the olfactory epithelium, but the basic principle of sensory mapping seems to be the same. In most systems, there are multiple repetitions of the map at both subcortical and cortical levels. Because all or most of the receptor sheet is contained in each map repetition, each separate representation is a unit in a system of serial/parallel channels making up the overall system. This organization suggests the hypothesis that each map is performing a different type of analysis on the sensory information from the receptors; perception ultimately involves integration of the information from these separate representations. Much recent effort in sensory neurophysiology has gone into trying to define the types of processing done in each map unit and the interactions among units. For example, the visual cortex consists of a complex series of interconnected units of this type (2) that differ in their responsiveness to color, motion, and …