A Psychophysical Test of Labeled-Line and Across Fiber Models of Innocuous Thermal Perception Restricted; Files Only
Ali, Hanifah (Spring 2023)
Abstract
Despite over a century of systematic research on the perceptual coding of warmth and cool, the mechanisms are still debated. There is some evidence for both labeled-line and across-fiber models. Under a labeled-line model, innocuous thermal perception of coolness and warmth are encoded via the activity of distinct primary afferents and central pathways that selectively respond to cooling or warming of the skin, respectively. According to across-fiber models, the brain compares the pattern of activity of multiple sensory afferents (e.g., ratio of warm to cool signals) to generate a given perception. For example, a recent animal study suggested that cool fibers are necessary for warmth perception because knocking out cool receptors made mice unable to detect the warming of a contactor, an idea that is at odds with at least several lines of evidence in humans. The diverging and, at times, conflicting evidence necessitates further systematic inquiry into how cool and warm fibers contribute to innocuous thermal perception. The present study was driven by three aims. The first aim explored whether people can detect the warming of a contactor from a cool baseline without necessarily feeling warmth, and the cooling of a previously-warmed contactor without feeling coolness. The results confirmed that thermal change detection in this scenario is often achieved by detecting less of a prevailing sensation or thermal neutrality, rather than the opposite qualitative thermal percept. The second aim investigated whether cool and warm fibers are necessary for the opposing sensation, or if they are sufficient for their own thermal quality. Selectively adapting cool or warm fibers resulted in unilateral deficits to cool and warm detection thresholds when measured from skin temperature. Additionally, starting from a cool baseline, detection of a neutral sensation occurred before warmth detection, and warmth detection aligned with temperatures in which warm fibers become active. The third aim addressed whether, as some previous evidence in humans suggests, warm fiber activity masks cool fiber activity. Adapting individuals to a warm stimulus and measuring cool sensitivity from that baseline resulted in subjects reporting cool sensations at more elevated temperatures compared to conditions without adaptation to a warm stimulus. Findings from the first two aims suggest that across-fiber interactions are not necessary for innocuous thermal perception, which fits a labeled-line model. Findings from the third aim suggest that an across-fiber interaction occurs specifically in temperature ranges where warm and cool fiber activity overlap. In such conditions, warm fiber activity inhibits cool perception, so removing warm fiber activity unmasks cool sensations. Overall, the findings demonstrate that both labeled-line and across fiber coding of thermal perception exist, and the application of each is highly nuanced. Further research into innocuous thermal perception utilizing regions of the skin that are devoid of a given thermal sensory afferent can further elaborate upon the extent to which each model explains thermal sensation.
Table of Contents
Introduction and Background....................................................................................................................................................1
Labeled-Line Theory..............................................................................................................................................................2
Across-Fiber Models. .............................................................................................................................................................4
Warmth-Insensitive Fields......................................................................................................................................................7
Study Objectives....................................................................................................................................................................9
Materials and Methods............................................................................................................................................................12
Participants..........................................................................................................................................................................12
Equipment............................................................................................................................................................................12
Questionnaires and Surveys....................................................................................................................................................13
Baseline Measurements and Thermal Testing...........................................................................................................................14
Training in Terminology and Tasks...........................................................................................................................................14
Experiments...........................................................................................................................................................................16
Experiment 1 - “22℃ Change Detection Task”........................................................................................................................18
Experiment 2 - “Adaptation Tasks”.........................................................................................................................................20
Experiment 3 - “40℃ Change Detection Task”........................................................................................................................23
Experiment 4 - “Adapt + 40℃ Cool Task”................................................................................................................................24
Experiment 5 - “40℃ Cool Task”...........................................................................................................................................27
Experiment 6 - “Neutral Task”...............................................................................................................................................28
Experiment 7 - “Warmth Task”...............................................................................................................................................29
Second Study Visit: WIF Testing and Experiments......................................................................................................................31
Data Analysis..........................................................................................................................................................................34
Results.......................................................................................................................................................................................36
Study Sample Characteristics.....................................................................................................................................................36
Experiment 1: 22℃ Change Detection Task.................................................................................................................................37
Experiment 3: 40℃ Change Detection Task.................................................................................................................................39
Experiment 2: Adaptation Task..................................................................................................................................................41
Experiment 4: Adapt + 40℃ Cool Task and Experiment 5: 40℃ Cool Task......................................................................................42
Experiment 6: Neutral Task and Experiment 7: Warmth Task........................................................................................................44
WIF Searching and Characterization...........................................................................................................................................45
Modifications made to Experiments 1 and 3.................................................................................................................................47
Baseline Perception.................................................................................................................................................................47
Visual Aid...............................................................................................................................................................................51
Discussion...................................................................................................................................................................................53
Aim 1: Subjective Experience of a “Warming” Task.......................................................................................................................53
Experiment 1: 22℃ Change Detection Task and Experiment 3: 40℃ Change Detection Task.........................................................54
Aim 2: Are Cool Fibers and Warm Fibers Necessary for Opposing Sensations or for Their Own?........................................................59
Experiment 2 “Adaptation Task”................................................................................................................................................59
Experiment 6: Neutral Task and Experiment 7: Warmth Task.......................................................................................................61
Aim 3: “Warm Inhibits Cool”- Does Warm Fiber Activity Mask Cool Fiber Activity?..........................................................................63
Experiment 4: Adapt + 40℃ Cool Task and Experiment 5: 40℃ Cool Task....................................................................................64
Warmth Insensitive Fields..........................................................................................................................................................67
Limitations and Other Considerations.........................................................................................................................................68
Study Sample..........................................................................................................................................................................68
Ascending and Descending Method of Limits............................................................................................................................70
Training of Subjects................................................................................................................................................................71
Sensory Adaptation.................................................................................................................................................................72
Conclusions..............................................................................................................................................................................74
References..................................................................................................................................................................................76
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File download under embargo until 19 May 2025 | 2023-04-10 11:18:00 -0400 | File download under embargo until 19 May 2025 |
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