BS20. Prepulse inhibition of the blink reflex by thermoalgesic stimuli in healthy humans

Abstract

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 weak conditioning stimulus (prepulse) over the reflex response to a subsequent suprathreshold stimulus. The PPI is a ubiquitous phenomenon, common to many different sensory modalities, it has been described with auditory, electrical and laser stimuli. We considered that thermoalgesic stimuli would also cause PPI, even though these stimuli take relatively long time to reach their peak. Our aim was twofold: 1. Determine when PPI is generated along the stimulus build-up and 2. Determine if conscious awareness (AW) of the stimulus is modified by PPI. In 18 healthy volunteers, we applied electrical stimuli to the supraorbital nerve to elicit the blink reflex. In test trials, subjects had also a thermoalgesic stimulus applied to the skin of the forearm at variable time intervals preceding the supraorbital nerve stimulus. Subjects were requested to tell the minute mark shown by the handle of a Libet’s clock positioned at 1 m distance from the subject’s eyes at the time they felt the stimulus. AW was calculated as the difference between the time defined by the handle position and the real time at which the stimulus was issued. We determined the effects of thermoalgesic stimuli on the blink reflex by measuring the size of R1 and R2 responses in test trials as percentages of the mean in control trials, and the onset of the PPI effect as the time interval in which the R2 response of the blink reflex became less than 40% of the mean baseline. We, then, determined the temporal relationship between PPI and AW of both, the thermoalgesic stimulus, to know if PPI occurred before or after the thermoalgesic stimulus was made conscious, and the supraorbital nerve stimulus, to know if PPI had any effect on conscious AW of the test stimulus. Thermoalgesic stimuli induced facilitation of R1 at a mean of 332 ms (±37 ms) and R2 inhibition at 392 ms (±49 ms). Mean latency of AW was 761 ms (±33 ms) for thermoalgesic stimuli and 370 ms (±20 ms) for supraorbital nerve stimuli. Both were modified at the time when AW of the two stimuli coincided: AW of the supraorbital nerve stimulus was advanced (15.6% ± 14.1% with respect to baseline at the interval of 500 ms) and AW of the thermoalgesic stimulus was delayed (14.5% ± 15.8% with respect to baseline at the interval of 700 ms). Prepulse inhibition is accompanied by a change in conscious perception. The effect is limited to the time when conscious appraisal of the two stimuli coincide in the central nervous system, when subjects tend to consider prepulse and pulse stimuli closer to each other than they really are. Subcortical gating of sensory inputs may modulate not only brainstem responses, but also conscious perception time.