Upper eyelid position and velocity, and the electromyographic (EMG) activity of the orbicularis oculi muscle, were recorded bilaterally in alert cats during spontaneous, reflexively evoked, and conditioned eyelid movements. 2. Spontaneous blinks appeared randomly (0.2-0.5 per min) and consisted of a fast, large downward lid movement followed by a slower up phase. Blinks of smaller amplitude and slower velocity were also observed mainly accompanying behavioral movements, such as during peering and grimacing. 3. Eyelid response to air puffs applied to the cornea and tarsal lid skin consisted of a short-latency (9-16 ms), fast (up to 2,000 degrees/s) downward movement that lasted for 25-30 ms, followed by late, small downward sags that were sometimes still evident after stimulus offset. Blinks outlasted the duration of the stimulus by approximately 150 ms. Blinks elicited by flashes of light or tones showed longer latency (47.3 +/- 6.3 and 53.7 +/- 8.0 ms, mean +/- SD; respectively), smaller amplitude, and a quicker habituation than air-puff-evoked lid responses. 4. For the down phase of the blink, the peak velocity, but not its duration, increased linearly with blink amplitude. Because the rise time of the down phase remained constant, changes in blink amplitude seemed to be the result of increased blink velocity. The down phase of a typical 10 degrees blink was 10 times faster than the up phase of the same blink or than upward and downward lid saccades of the same amplitude. The peak velocity and duration of the up phases of reflex blinks and upward and downward lid saccades increased linearly with lid movement amplitude. 5. The initial down phase of air-puff-evoked blinks decreased in latency, increased in amplitude and peak velocity, and maintained the same rise time for increasing puff pressure. None of these parameters was dependent on puff duration. The duration of the blink also increased linearly with air puff duration. 6. The amplitude of air-puff-evoked blinks was inversely related to lid position, decreasing with further lid positions in the closing direction. In contrast, neither peak nor integrated EMG activity of the orbicularis oculi muscle was affected by lid position, being only a function of stimulus parameters and of the animal's level of alertness. 7. Air puffs > 20 ms and > 1 kg/cm2 evoked two successive bursts (R(ap) 1 and R(ap) 2) in the EMG activity of the orbicularis oculi muscle. Shorter and/or weaker stimuli evoked only the R(ap) 1 response.(ABSTRACT TRUNCATED AT 400 WORDS)
