Ivan Ezquerra-Romano: Revisiting a classical theory of sensory specificity: assessing consistency and stability of thermosensitive spots
Early work reported that many spots were temperature-specific, eliciting either warm or cool sensations with the corresponding stimulus (6). Crucially, each spot was thought to indicate the presence of nerve endings from a single cutaneous afferent fiber, responding consistently to either warmth or cold (17–22). Thus, thermosensitive spots have provided foundational support for theories of neural specificity—the view that specific sensory qualities are associated with specific classes of afferent fibers (23). Later studies of the loss of sensation during pressure block and anesthetic block showed that cold sensations were carried by thinly myelinated Aδ-fibers, whereas warm sensations were carried by unmyelinated C-fibers, confirming the link between afferent fiber types and sensory qualities (24).
Green et al. (11) developed a two-step search method to identify thermosensitive spots across larger skin areas. Briefly, they used a thermode with a contact area of 16 mm2 to first identify broad thermosensitive sites, followed by a thermode with a contact area of 0.79 mm2 to identify the smaller, classical spots within those sites. They applied this procedure in the human forearm, classifying sites and spots according to the quality of the evoked sensations. They found that the quality of sensation evoked by a thermal stimulus could be inconsistent. Although 96.7% of sites remained sensitive over the experimental session, a surprising 31.8% were associated with different sensations across repeated tests, which presumably meant that their stimulations activated multiple thermosensitive primary afferents. In that case, smaller stimulation areas should produce more consistent sensory qualities – although this prediction was not tested in that study.
Such a study is required for two reasons. First, if thermosensitive spots are shown to be inconsistent and unstable over time, this might question the notion that each spot corresponds to a single afferent unit since the skin locations of afferents’ nerve endings can be assumed to be unchanging. Second, near-threshold stimulation of a single thermosensitive spot can be considered to cause a minimal afferent signal to the brain. Neural specificity theories predict that even minimal afferent signals should consistently evoke the same sensation because the “line” carrying the signal bears a “label” that is read by the brain as defining the sensory quality.