QUANTAL ORGANIZATION OF REFLEX AND CONDITIONED EYELID RESPONSES

Upper lid movements and the electromyographic activity of the orbicula ris oculi muscle were recorded in behaving cats during spontaneous and experimentally evoked reflex blinks, and conditioned eyelid responses . Reflex blinks evoked by the presentation of air puffs, flashes, or t ones consisted of a fast downward lid movement followed by late, small downward waves, recurring at approximate to 50-ms intervals. The late ncy, maximum amplitude, peak velocity, and number of late waves depend ed on the modality, intensity, and duration of the evoking stimulus. T he power spectra of acceleration records indicated a dominant frequenc y of approximate to 20 Hz for air puff-evoked blinks. Flashes and tone s usually evoked small and easily fatigable reflex responses of lower dominant frequencies (14-17 and 9-11 Hz, respectively). A basic approx imate to 20 Hz oscillation was also noticed during lid fixation, and r amplike lid displacements evoked by optokinetic stimuli. Five classica l conditioning paradigms were used to analyze the frequency-domain pro perties of conditioned eyelid responses. These learned lid movements d iffered in latency, maximum amplitude, and profile smoothness dependin g on the modality (air puff, tone), intensity (weak, strong), and pres entation site (ipsi-, contralateral to the unconditioned stimulus) of the conditioned stimulus. It was found that the characteristic ramplik e profile of a conditioned response was not smooth, but appeared to be formed by a succession of small waves at a dominant frequency of appr oximate to 20 Hz. The amplitude (and number) of the constituting waves depended on the characteristics of the conditioned stimulus and on th e time interval until unconditioned stimulus presentation. Thus condit ioned responses seemed to be formed from lid displacements of 2-6 degr ees in amplitude and approximate to 50 ms in duration, which increased in number throughout conditioning sessions, until a complete (i.e., l id closing) conditioned response was reached. It is suggested that a a pproximate to 20-Hz oscillator underlies the generation of reflex and conditioned eyelid responses. The oscillator is susceptible to being n eurally modulated to modify the velocity of a given quantum of movemen t, and the total duration of the lid response. Learned eyelid movement s are probably the result of a successively longer release of the osci llator as a function of the temporal-spatial needs of the motor respon se.