Kinetic and frequency-domain properties of reflex and conditioned eyelid responses in the rabbit

Eyelid position and the electromyographic activity of the orbicularis oculi muscle were recorded unilaterally in rabbits during reflex and conditioned blinks. Air-puff-evoked blinks consisted of a fast downward phase followed sometimes by successive downward sags. The reopening phase had a much long er duration and slower peak velocity. Onset latency, maximum amplitude, pea k velocity, and rise time of reflex blinks depended on the intensity and du ration of the air puff-evoking stimulus. A flashlight focused on the eye al so evoked reflex blinks, but not flashes of light, or tones. Both delayed a nd trace classical conditioning paradigms were used. For delayed conditioni ng, animals were presented with a 350-ms, 90-dB, 600-Hz tone, as conditione d stimulus (CS). For trace conditioning, animals were presented with a IO-m s, 1-k/cm(2) air puff, as CS. The unconditioned stimulus (US) consisted of a 100-ms, 3-k/cm(2) air puff. The stimulus interval between CS and US onset s was 250 ms. Conditioned responses (CRs) to tones were composed of downwar d sags that increased in number through the successive conditioning session s. The onset latency of the CR decreased across conditioning at the same ti me as its maximum amplitude and its peak velocity increased, but the time-t o-peak of the CR remained unaltered. The topography of CRs evoked by short, weak air puffs as the CS showed three different components: the alpha resp onse to the CS, the CR, and the reflex response to the US. Through conditio ning, CRs showed a decrease in onset latency, and an increase in maximum am plitude and peak velocity. The time-to-peak of the CR remained unchanged. A power spectrum analysis of reflex and conditioned blink acceleration profi les showed a significant approximate to 8-Hz oscillation within a broadband of frequencies between 4 and 15 Hz. Nose and mandible movements presented power spectrum profiles different from those characterizing reflex and cond itioned blinks. It is concluded that eyelid reflex responses in the rabbit present significant differences from CRs in their profiles and metric prope rties, suggesting different neural origins, but that a common approximate t o 8-Hz neural oscillator underlies lid motor performance. According to avai lable data, the frequency of this putative oscillator seems to be related t o the species size.