A Conditioned Eyelid Movement Is Not a Blink

23 24 Based on kinematic properties and distinct substrates, there are different classes of 25 eyelid movement described as eyeblinks. We investigate whether the eyelid movements 26 made in response to a conditioned stimulus (CS) are a category of eyelid movements 27 distinct from blinks. Human subjects received 60 trials of classical eyelid conditioning 28 with a tone as the CS and electrical stimulation of the supraorbital branch of the 29 trigeminal nerve as the unconditioned stimulus (UCS). Before and after training, reflex 30 blinks were elicited with the UCS. The kinematics of conditioned responses (CRs) 31 differed significantly from those of reflex blinks. The slope of the amplitude-maximum 32 velocity function was steeper for reflex blinks than for CRs and reflex blink duration was 33 significantly shorter than CR duration. Unlike reflex blinks, for which maximum 34 velocity was independent of blink duration, the maximum velocity of CRs depended 35 upon CR duration. These quantitative and qualitative differences indicated that CRs were 36 a unique class of eyelid movements distinct from blinks and eyelid movements with 37 vertical saccadic gaze shifts. 38