Spatial summation of heat-induced pain: influence of stimulus area and spatial separation of stimuli on perceived pain sensation intensity and unpleasantness

Abstract

1. Psychophysical experiments were initiated to determine the possible influence of increasing stimulus size on perceived pain intensity. Six trained human subjects (5 male, 1 female) made visual analogue scale (VAS) ratings for pain-sensation intensity and unpleasantness in response to nociceptive thermal stimuli. Test stimuli consisted of 5-s duration heat pulses (45-50 degrees C in 1 degrees increments) delivered by one, two, or three contact thermal probes (1 cm2 each) applied to the medial aspect of the anterior forearm. 2. The area of skin receiving noxious thermal stimuli was changed by randomly varying the number of thermodes activated. The effects of varying the distance between the thermal probes also were evaluated. In the first series of experiments, thermal-probe separation was kept close to 0; in subsequent experimental series, the thermodes were separated by either 5 or 10 cm. 3. In each experimental series, considerable spatial summation occurred in both pain-sensation intensity and unpleasantness dimensions of pain. This summation occurred throughout the nociceptive thermal range of 45-50 degrees C and was larger at suprathreshold temperatures (greater than or equal to 47 degrees C) than those near threshold (less than or equal to 46 degrees C). Unlike spatial summation of perceived warmth, that of pain was not characterized by systematic changes in power-function exponents but as approximately upward parallel displacements in double-logarithmic coordinates. 4. Thermal-probe separation over a range of 0-10 cm had no effects on spatial summation of pain-sensation intensity or pain unpleasantness. In contrast, increasing thermal-probe separation increased the subjects' ability to discriminate differences in stimulus size and their ability to detect correctly the number of thermal probes activated. 5. Because affective VAS ratings of unpleasantness were linearly related to, but distinctly and systematically less than, VAS ratings of pain-sensation intensity, it was clear that subjects responded quite differently to these two pain dimensions. Affective judgements were not additionally influenced by thermal probe separation and hence by the ability to perceive stimulus size or number of thermal probes activated. 6. The results indicate that powerful spatial-summation mechanisms exist for heat-induced pain. Spatial summation of pain is likely to be subserved both by local integration mechanisms at the level of single spinothalamic-tract neurons and by recruitment of central nociceptive neurons, because spatial summation of pain occurred to approximately equal extents under conditions of thermode separations over a distance of at least 20 cm.

Attentional Modulation of Spatial Integration of Pain: Evidence for Dynamic Spatial Tuning

“…Spatial summation of pain during overall ratings has been repeatedly identified (Price et al, 1989;Greenspan et al, 1997) and is thought to reflect static spatial integration resulting from the intrinsic RF proprieties of nociceptive neurons. Importantly, the division of attention between two stimuli abolished spatial summation and may reflect the initiation of inhibitory processes (Oshiro et al, 2007;Quevedo and Coghill, 2007).…”

“…Spatial summation of sensory information is characterized by the perception of increased intensity when the size of the stimulated area is expanded (Price et al, 1989). This phenomenon provides evidence for spatial integration of afferent information and has been found in different sensory modalities including the visual (Harutiunian-Kozak et al, 2006) and auditory (Buell and Hafter, 1991) systems.…”

“…First, subjects were required to give one overall rating of two simultaneous thermal stimuli. Overall ratings have been widely used to assess spatial summation of pain and require the integration of nociceptive information across large body areas (Price et al, 1989;Nielsen and Arendt-Nielsen, 1997). Second, subjects were instructed to divide their attention by separately rating both stimuli and giving two ratings, one from each stimulus.…”

“…Spatial summation in the somatosensory system is a well documented phenomenon (Nielsen and Arendt-Nielsen, 1997) and has been found in a variety of submodalities including innocuous warmth (Hilz et al, 1999), cool (Hilz et al, 1999), and mechanical perception (Greenspan et al, 1997). Extensive spatial summation of pain occurs during noxious thermal (Price et al, 1989;Coghill et al, 1993;Nielsen and Arendt-Nielsen, 1997), electrical (Andersen et al, 1994) and mechanical (Greenspan et al, 1997) stimulation.…”

Specific Mechanical Pain Hypersensitivity Over Peripheral Nerve Trunks in Women With Either Unilateral Epicondylalgia or Carpal Tunnel Syndrome

“…8,66 The sensitization of nerve trunk nociceptors may trigger spontaneous discharges in sensory nerve fibers, 60 resulting in ectopic discharges in the dorsal root ganglion, 9,35 which may induce changes in the dorsal horn receptive fields, contributing to central hyperexcitability. 11,42 Increased recruitment of central neurons by peripheral nociceptive inputs, 37 enhanced spatial summation, 46 and increased pain intensity 55 have been suggested as potential peripheral mechanisms. The role of sensitization mechanisms was supported by the findings of 2 studies using functional neuroimaging, which demonstrated that cortical remapping of the primary somatosensory cortex S1 and the extent of cortical hand somatotopy found in patients with CTS was related to the intensity of pain.…”