Thermoalgesic stimuli induce prepulse inhibition of the blink reflex and affect conscious perception in healthy humans

All sensory stimuli produce transient excitability changes in various central nervous system circuits. One example is prepulse inhibition (PPI), which is the inhibition generated by a preceding weak stimulus (prepulse) over the reflex response to a subsequent suprathreshold stimulus. The PPI is a ubiquitous phenomenon, common to many different sensory modalities. However, it has not yet been studied with thermoalgesic stimuli. These stimuli take a relatively long time to reach their peak, which implies some uncertainty in the exact timing of prepulse effects with respect to stimulus onset. In 20 healthy volunteers, we determined when thermoalgesic stimuli cause PPI of the blink reflex and measured conscious awareness (AW) of thermoalgesic stimulus perception using the Libet's clock. In this way, we determined the temporal relationship between AW and PPI. In a second experiment, we investigated whether prepulse effects on blink reflex also involved a change in conscious perception of the supraorbital nerve stimulus. Our results show that thermoalgesic stimuli generate PPI of the blink reflex long before subjects were consciously aware of the stimulus, confirming the already‐known principle that conscious perception is not required for PPI to take place, and that prepulse stimuli induce a change in the time of conscious perception of prepulse and pulse stimuli, in such a way that AW of both stimuli tended to become closer to each other. By using slowly rising temperature (thermoalgesic) stimuli as prepulses for blink reflex responses and Libet's clock to assess conscious stimulus awareness, we demonstrated that prepulse inhibition (PPI) occurs long before the prepulse stimulus reaches conscious perception. We also found a tendency for mutual influence on awareness between prepulse and pulse stimuli during the time the two stimuli reached the central nervous system in close proximity. PPI is, hence, interpreted as an early step in subcortical processing of sensory inputs that contributes to the control of exteroceptive signals in their way to conscious interpretation.