Spacetime Body Maps and Somatosensory Evoked Potentials

Abstract

Scalp-recorded somatosensory evoked potentials (SEPs) in humans and dogs suggest that it takes longer for peripheral information to reach the brain from the anogenital perineum than from the distal hind limbs, in spite of a shorter afferent pathway. The delay seems to be centrally determined, rostral to the lumbar segments, rather than by slower conduction velocities in the peripheral pudendal-sacral afferents. Thus the mammalian brain may temporally conserve the spatial sequence of segmental inputs in spite of variable afferent pathway lengths and/or conduction velocities. Human SEPs indicate the arrival timetable in cortex to be ordered: Lips, fingers, toes, genitals, anus; with a delay between lips and anogenital perineum of 30–40 msec. This delay is comparable to the interpair-interval threshold for human detection of two successive tactile events on the fingers, while ambiguous temporal defusion of two successive identical stimuli can be perceived down to objective separations of about 15 msec. It is suggested that, whereas the metric tensor model proposes central correction of asynchronous sensorimotor signals within an approximately 30 msec predictive function by cerebellar circuitry (in order to avoid spatial distortion in a spacetime tensor of sensorimotor processing), the brain may use longer time differences between proximal and distal receptive fields to develop a Cartesian, four-dimensional body map within its adaptive, behavioral world.