The saltation illusion demonstrates integrative processing of spatiotemporal information in thermoceptive and nociceptive networks.

In sensory saltation, first reported by Geldard and Sherrick (Science 178:178-179, 1972), a stimulus is displaced towards a second one following closely in time and space as a function of the delay between the stimuli. The distance between stimulus locations is restricted by the extension of sensory fields in the primary somatosensory cortex. Saltation is assumed to reflect dynamic changes in these cortical representations. The present study demonstrates for the first time saltation in thermoceptive and nociceptive pathways with CO(2) laser stimulation. Stimuli were presented to the dorsal forearms of 18 healthy subjects at two intensities. Saltation patterns consisted of a reference stimulus S0 near the wrist, the first test stimulus S1 at the reference location after a fixed onset delay of 1,000 ms, and a second test stimulus S2 at a location 105 mm distant from reference after a variable onset delay of 60-516 ms. Perceived positions were indicated by the subjects without skin contact with a 3D tracker. As expected, subjects mislocalized S1 towards S2. Mean S1 displacement was 51+/-36 mm. Decreasing delays between S1 and S2 resulted in increasing displacements, independent of intensity. However, since no clear-cut discrimination of thermal versus nociceptive activation could be achieved definite conclusions about differences between the two modalities cannot be drawn. In addition, effects of body site on the saltation characteristics were observed. The saltation paradigm constitutes a promising approach to the functional analysis of spatiotemporal dynamics in thermoceptive and nociceptive networks to supplement brain-mapping approaches to cortical sensory fields.