Sensory Role of Impulses Traveling in the Dorsal Columns

In the classical theory of cutaneous sensation, detailed discrimination depends on impulses conducted to cortex over the newly evolved dorsal columns. This idea is no longer tenable because, when this system is destroyed, animals can still discriminate weight, texture, vibration, two points, and position. If all parallel ascending systems are destroyed except for the dorsal columns, the dorsal colunms alone are unable to initiate any behavior. Positive results of dorsal column section are all related to movement: failure of orientation; failure to handle objects in extrapersonal space; and the presence of immobility, especially in the absence of vision. In an attempt to explain these results, it is proposed that dorsal column impulses do not lead to sensation, but act to initiate and control an internal search of messages arriving over other pathways. When this search fails to provide adequate information for identification of the stimulus, an external search or exploration begins, and it is proposed that dorsal columns are necessary for this search too. In these proposals, the new system sets the program on which the older systems operate so that the new system is only essential for sensation when program change and manipulation is essential in the older pathways.