Psychophysical examination of the thermal grill illusion

Harper, Daniel Elliott

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March 19, 2019

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Harper, Daniel Elliott
- Affiliation: College of Arts and Sciences, Department of Psychology and Neuroscience

Abstract

The thermal grill illusion (TGI) is a phenomenon in which interlaced warm and cool bars elicit sensations of burning heat, and in many cases pain. Although it was discovered in the late 19th Century, there is still no scientific consensus on its underlying mechanism. The primary goal of this dissertation was to test the validity of TGI theories by manipulating the illusion in ways that put the predictions of the theories at odds with one another. In Experiment 1, the TGI was subjected to conditioned pain modulation (CPM), a phenomenon in which one painful stimulus reduces the painfulness of another. CPM equally reduced the painfulness of the TGI and noxious heat, suggesting that the important signals for the TGI ascend to the brain in nociceptive spinal neurons (i.e. those known to be inhibited by CPM). This result is at odds with a cognitive addition theory, but it is compatible with two others. Therefore, Experiment 2 assessed whether the TGI is the product of a simple addition of warm and cool signals, as suggested by one theory, or if it is rather a result of more complicated interactions, as suggested by another. Subjects were selectively adapted to either the warm or the cool bars of the grill before being exposed to both the warm and cool temperatures simultaneously. Cool adaptation significantly attenuated the TGI while warm adaptation was without effect, indicating that the TGI is not due to simple addition. In Experiment 3, I tested the accuracy of anecdotal evidence that the TGI is more robust when warm precedes cool. The issue is an important one, because any temporal effect could undermine the conclusions of Experiment 2. The protocol of Experiment 2 was altered so as to minimize adaptation while preserving stimulus offset. The results showed no evidence of a temporal order effect, suggesting that the results of Experiment 2 were due to adaptation. Taken together, these experiments suggest that the TGI is a complex product of activity in peripheral afferents that project to nociceptive spinal cord neurons, and that the grill's cool stimulus is particularly important in eliciting their activity.

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