Synthetic heat at mild temperatures

Area of Science:

• Neuroscience
• Sensory Physiology
• Psychophysics

Background:

• The heat grill illusion (synthetic heat) is perceived when warm and cold stimuli are adjacent.
• Existing models propose this involves warm stimulation inhibiting cold neurons, unmasking pain signals.

Purpose of the Study:

• To test the "unmasking model" of synthetic heat perception.
• To investigate if mild, non-painful temperatures can induce synthetic heat.
• To determine if lowering base temperature enhances noxious heat sensations.

Main Methods:

• Experiment 1: Combined mild warm (35-40°C) and cool (≥27°C) stimuli.
• Experiment 2: Paired near-threshold cooling with mild warmth at varying base temperatures (33°C, 31°C, 29°C).
• Participants reported sensations, including heat, burning, and stinging.

Main Results:

• Mild warm and cool stimuli induced non-painful synthetic heat.
• Cooling by 1-3°C from 33°C evoked heat in over 33-75% of trials.
• Lowering base temperature increased heat and burning reports but not significant pain.

Conclusions:

• Synthetic heat can be induced by mild, non-painful thermal stimuli.
• Findings challenge the "unmasking model" and suggest integrated neural activity.
• Heat perception may involve convergence of warm and cold fiber signals on neurons in the spinothalamic tract.