Psychophysical ‘perceptual maps’ of heat and pain sensations by direct localization of CO2 laser stimuli on the skin

Brain activation patterns derived from neurofunctional methods are often implicitly regarded as being directly related to subjective perceptual experience in an iso- or at least homomorph manner, neglecting the operational differences between these two dimensions. This paper (a) introduces a method for assessing ‘perceptual maps’ of stimulation patterns presented to the body surface, providing a means to parametrically relate neural representation and subjective percept, and (b) applies this method to demonstrate the existence of ‘somatotopic maps’ of hot and painful stimulus patterns independent from mechanoceptive co-activation. Brief (90 ms) CO2 laser pulses were presented in an array of multiple stimulation sites on the dorsal forearms (N. radialis area, C7 dermatome) of healthy subjects. Perceived locations were indicated with a 3D tracker without touching the skin, and (mis-)localizations in distal–proximal direction were analyzed. Stimuli were localized with overall mean errors of 22 mm (SD: 16 mm) toward the wrist and 24 mm (SD: 18 mm) toward the elbow. Somatotopic representation of thermal-nociceptive stimuli could be demonstrated in all subjects, independent from mechanoceptive co-activation. The perceptual maps revealed striking individual (mis-)localization patterns, many subjects exhibiting ‘stretched’, some ‘condensed’ somatotopic representations. In estimating the mapping parameters from physical to perceptual space linear regressions generally provided a good fit (adj. R2 > 0.80 in 10 out of 12 subjects). Nonlinear models were advantageous in some subjects only. Our method can be useful in assessing inter-individual differences or experimentally induced shifts in somatotopic processing.