Modified radar devices for alarm systems, based on the Doppler effect, have already been employed in monitoring the general locomotor activity of mice [1,3]. These devices can detect movements of subjects in the home cage, thereby avoiding undesirable handling – but the size of the experimental subjects is critical. The apparatus can detect mice very well, but it is not reliable for studying smaller subjects, e.g. cockroaches. This problem arises from the wavelength of the radar emission. The radar employed in alarm systems uses a wavelength of 3 cm, and moving objects smaller than the wavelength are detected only under accurately set levels of sensitivity.

To solve this problem, we used a Lince MW12 microwave sensor on X band (9.9 GHz). In commercial devices, the sensitivity is controlled by a trimmer with only a narrow range of variation, which is therefore not suitable for a fine regulation of sensitivity. The changes we made in the electronics included: (a) substituting the original 22K trimmer with one of 100K, producing a finer resistance scale and greater sensitivity control; (b) zeroing the time constant, to obtain instantaneous responses to each movement; and (c) installing a pulse former in the output for a constant relay-on time, compatible with the reading time of the digital I/O board in the computer.

This modified device was tested on the circadian activity of cockroaches Periplaneta americana. 17 males were housed in individual 15 x 15 x 10 cm Plexiglass cages, with a radar placed above. The animals’ activity was monitored for three days at 18°C and subsequently for three days at 28°C in constant darkness, with food and water provided ad libitum. For each animal, we recorded the number of movements every 10 minutes. Each time series was smoothed, the linear trend removed, and the power spectrum computed by a Discrete Fourier Transform. In all spectra, power peaks with values higher than 2.81 standard deviations above the mean (p < 0.001) [2] were considered significant. A chi-square periodogram (Wintau, code by Refinetti) was used to determine the period during the time intervals 1-8 hrs and 20-26 hrs.

The Q10 values and the circadian free-running period derived both agreed with the data reported by other authors [4,5], thus supporting the reliability of the apparatus. Moreover, the spectral analysis and periodogram revealed the presence of ultradian rhythmicity during the 1-8 hr range.

References

1. Conte, S.; D’Olimpio, F.; Renzi, P. (1995). Uno strumento per la registrazione dell’attività motoria. Congresso A.I.P. 1995.
2. Conte, S.; Ferlazzo, F; Renzi, P. (1995). Ultradian rhythms of reaction time in performance in vigilance tasks. Int. J. Neurosc., 82, 127-133.
3. D’Olimpio, F.; Carola, V.; Rubei, N.; Renzi, P. (1998). L’attività come modificatore dell’espressione dei ritmi circadiani del topo. Congresso A.I.P. 1998.
4. Pittendrigh, C. S. (1966). The circadian oscillation in Drosophila pseudoobscura pupae: a model for the photoperiodic clock. Z. Pflanzenphysiol., 54, 257-307.
5. Harker, J. E. (1956). Factors controlling the diurnal rhythm of activity in Periplaneta. J. Exp. Biol., 33, 224-234.

Paper presented at Measuring Behavior 2002 , 4^th International Conference on Methods and Techniques in Behavioral Research, 27-30 August 2002, Amsterdam, The Netherlands

© 2002 Noldus Information Technology bv