1. 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 approximate to 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/cm(2) 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. Both R(ap)1 and R(ap)2 responses contributed to the generation of the initial down phase of air-puff-evoked blinks. 8. The latency and amplitude of reflex blinks were dependent on the skin receptors activated by the air puff. Air puffs directed to the cornea and tarsal lid skin evoked blinks of shorter latency and larger amplitude than air puffs directed to periorbital skin areas. Topical anesthesia of corneal receptors blocked the appearance of the R(ap)2 component, reducing by approximate to 1/2 the amplitude and peak velocity of the initial down phase of the blink. Air-puff-evoked blinks are thus the result of the activation of fast-conducting, low-threshold receptors, located in the lid skin, that produce the R(ap)1 response, and of more slowly conducting, higher-threshold mechanoreceptors, located in the cornea, sclera, and tarsal skin, that produce the R(ap)2 response. 9. The late downward sags that followed the initial fast down phase of air-puff-evoked blinks were also dependent in their latency and amplitude on air puff pressure, However, these sags occurred at a dominant frequency of approximate to 25 Hz, which was independent of stimulus parameters, suggesting that this frequency is a property of the neural circuit controlling reflex blinks. 10. Neither lid saccades nor slow lid movements evoked by ramp optokinetic stimulation were accompanied by any detectable EMG activity in the orbicularis oculi muscle. The gain of upward slow lid movements during optokinetic stimulation was larger than that of downward lid movements, particularly at higher ramp speeds. 11. Animals were classically conditioned with four different paradigms. The unconditioned stimulus (US) always consisted of a long, strong air puff applied to the left eyelid. Conditioned stimuli (CSs) differed in sensory modality (air puff or tones), intensity (weak or strong air puffs), or presentation side (air puffs ipsilateral or contralateral to the US). 12. For ipsilateral weak and strong air puffs, and tones used as CSs, the conditioned response (CR) reached a 95% criterion during the second (weak and strong air puffs) or fourth (tone) conditioning sessions. Weak air puffs applied contralaterally as a CS never reached >40% of CRs. The latency of the CRs decreased slowly, whereas their peak velocity and time to peak amplitude increased to saturation at the fourth to fifth conditioning sessions. The CR amplitude also increased until a complete closure of the lids was reached. 13. The latency of the CR was a function of the stimulus used as CS (tone or air puff) and of the side where the CS was presented (ipsilateral or contralateral to the US). For the four conditioning paradigms, the latency of the CR was always in the range of the corresponding reflex response and did not depend on CS-US interval. It is concluded that CRs are initiated in the same brain stem circuits that produce the unconditioned response, i.e., the reflex response, according to CS sensory modality, duration, strength, and presentation side. 14. The peak velocity of the down phase of the CR was also linearly related to CR amplitude, but with lower gains (1/5-1/10, depending on CS sensory modality and strength) than those described for air-puff-evoked blinks. 15. The CR appeared as a downward lid movement radiating from its onset, in a close temporal proximity to the CS, toward the US, where it reached its maximum amplitude. The CR was formed by successive small downward sags similar in amplitude to the late components observed in reflex blinks. These sags were the result of small but conspicuous spikes present in the EMG of theorbicularis oculi muscle. The number of these sags increased, and their duration and amplitude decreased, with successive conditioning sessions until straight and dumped lid CRs were reached. At the same time and as the proximal cause of these motor changes, the orbicularis oculi muscle changed from a phasic EMG activity to a tonic firing that was maintained along the CS-US interval.
