In a previous study, Schicatano et al.
8 demonstrated that restraining one eyelid for only 2 hours increased the excitability of OO motoneurons and trigeminal blink circuits ipsilateral to the restrained eyelid in normal subjects. If an excitability difference between the left and right OO motoneurons was the basis of eyelid asymmetry, then eyelid restraint should have modified this asymmetry. We reanalyzed the Schicatano et al.
8 data using our relative blink amplitude measure to determine whether upper eyelid restraint modified eyelid asymmetry. Before eyelid restraint, SO stimulation revealed that three of the five subjects exhibited right eyelid asymmetry (
Figs. 4A ,
4C ; S1, S2, S5) and the other two showed left asymmetry (
Fig. 4C ; S3, S4). Restraining the upper eyelid innervated by the less excitable OO motoneurons for 2 hours modified eyelid asymmetry. For example, before left eyelid restraint, subject 1 exhibited a mean eyelid amplitude ratio of 0.43 (
Fig. 4A ; 4C , S1) indicating a right eyelid asymmetry. After left eyelid restraint, the mean SO eyelid amplitude ratio converted to −0.7, a left eyelid asymmetry (
Fig. 4B 4C , S1). Similarly, subject 2 switched from right eyelid to left eyelid asymmetry after left eyelid restraint (
Fig. 4C , S2). Extremely right eyelid asymmetric subject 5 exhibited reduced right eyelid asymmetry after left eyelid restraint (
Fig. 4C , S5). Reversing eyelid asymmetry was not specific to left eyelid restraint. After right eyelid restraint, subject 3 converted from left eyelid to right eyelid asymmetry (
Fig. 4C , S3). Also consistent with the hypothesis that the nervous system adaptively increased the excitability of the motoneurons innervating the restrained eyelid, restraint of the left eyelid slightly increased left eyelid asymmetry in subject 4 (
Fig. 4C , S4).