Conditions that affect the thresholds of the components of the eyeblink reflex in humans.

The threshold stimulus intensities for elicitation of the two EMG components of the eyeblink reflex were determined in human subjects under different conditions. In the first experiment subjects sat with eyelids open and were not warned about reflex elicitation. The threshold of R1 was substantially greater than that of R2. In four additional experiments subjects (a) triggered the eliciting stimulus, (b) were warned about the arrival of each stimulus, (c) had a conditioning stimulus presented before reflex elicitation, and (d) had their eyelids closed at the time of stimulus delivery. The conditions of these subsequent experiments reduced the difference between the reflex thresholds largely by lowering the R1 threshold. These results indicate that variations in the testing environmental contribute to the discrepancy between our data showing unequal threshold for elicitation of the R1 and R2 components and other reports showing equal thresholds. The results are also another illustration of the ability of complex psychological events to selectively affect different reflex pathways.