Monitoring kinetic and frequency-domain properties of eyelid responses in mice with magnetic distance measurement technique.

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Abstract
Classical eye-blink conditioning in mutant mice can be used to study the molecular mechanisms underlying associative learning. To measure the kinetic and frequency domain properties of conditioned (tone - periorbital shock procedure) and unconditioned eyelid responses in freely moving mice, we developed a method that allows adequate, absolute, and continuous determination of their eyelid movements in time and space while using an electrical shock as the unconditioned stimulus. The basic principle is to generate a local magnetic field that moves with the animal and that is picked up by either a field-sensitive chip or coil. With the use of this magnetic distance measurement technique (MDMT), but not with the use of electromyographic recordings, we were able to measure mean latency, peak amplitude, velocity, and acceleration of unconditioned eyelid responses, which equaled 7.9 ± 0.2 ms, 1.2 ± 0.02 mm, 28.5 ± 1 mm/s, and 637 ± 22 mm/s2, respectively (means ± SD). During conditioning, the mice reached an average of 78% of conditioned responses over four training sessions, while animals that were subjected to randomly paired conditioned and unconditioned stimuli showed no significant increases. The mean latency of the conditioned responses decreased from 222 ± 40 ms in session 2 to 127 ± 6 ms in session 4, while their mean peak latency increased from 321 ± 45 to 416 ± 67 ms. The mean peak amplitudes, peak velocities, and peak acceleration of these responses increased from 0.62 ± 0.02 to 0.77 ± 0.02 mm, from 3.9 ± 0.3 to 7.7 ± 0.5 mm/s, and from 81 ± 7 to 139 ± 10 mm/s2, respectively. Power spectra of acceleration records illustrated that both the unconditioned and conditioned responses of mice had oscillatory properties with a dominant peak frequency close to 25 Hz that was not dependent on training session, interstimulus interval, or response size. These data show that MDMT can be used to measure the kinetics and frequency domain properties of conditioned eyelid responses in mice and that these properties follow the dynamic characteristics of other mammals.