Exercise 42 Detecting rhythmicity in a data set

This exercise uses the program Rhythm to detect rhythmicity in various data sets. As explained in Section 3.3, the program uses the chi square periodogram procedure to evaluate the presence of statistically significant rhythmicity.

Jan Jul Jan Jul Jan Jul Jan Jul Jan

1999 2000 2001 2002 2003

FIGURE 4.62 Ups and downs in Chicago. The graph shows the mean daily temperature in Chicago (Illinois) from January 1999 to January 2003. There is a clear annual rhythm with higher temperatures in the summer and lower temperatures in the winter. (Source: National Weather Service, U.S. National Oceanic and Atmospheric Administration.)

Jan Jul Jan Jul Jan Jul Jan Jul Jan

1999 2000 2001 2002 2003

FIGURE 4.62 Ups and downs in Chicago. The graph shows the mean daily temperature in Chicago (Illinois) from January 1999 to January 2003. There is a clear annual rhythm with higher temperatures in the summer and lower temperatures in the winter. (Source: National Weather Service, U.S. National Oceanic and Atmospheric Administration.)

1. Double-click on the Circadian icon to open the program banner, then click on Rhythm (the fourth icon from the left).

2. Open the Data subfolder and then select the file A30 in the Source File panel. This file contains the values of mean daily temperature in the city of Chicago during a 4-year interval. Refer to the graph provided in Figure 4.62.

3. Rhythm requires equally spaced data with no missing points (and no time tags): A30 complies with these requirements.

4. In the Data Resolution panel, set the Bin size to 1 and click on the Days option button (because the file contains data collected once a day).

5. In the Target Periodicity panel, set the Period to 12 and click on the Months option button (because inspection of Figure 4.62 clearly suggests the existence of 12-month rhythmicity).

6. Click on Execute. The Results panel reports that a statistically significant 12-month periodicity exists in the data, as expected. Details are also reported, including the mean level of the rhythm (10.5°C, or 51°F) and the range of oscillation (51.2°C, or 124°F). Note the statement that "no harmonics were detected." Higher frequency oscillations (such as weekly or monthly) might have been present in the data set, even though a search for only yearly rhyth-micity was requested.

7. Checking for lower harmonics is especially important if the wrong target periodicity is specified. Select the data file A04. As shown in Exercise 3.3, this file contains the records of running-wheel activity of a golden hamster maintained in constant darkness for 29 days. If you don't remember the data set, you may want to open the program Plot and inspect the data in the actogram format before proceeding.

8. In the Data Resolution panel, enter the correct information (Bin size is 6 and unit is Minutes). Set the Target Periodicity to 48 and Hours (rather than 24 and Hours, or 1 and Days, as it would be reasonable to do). Then click on Execute.

9. What happened? The program identified significant 48-hour periodicity, but it was smart enough to check also for 24-hour periodicity and to warn that the significant 48-hour periodicity may be just an artifact. If you think about it, a process that repeats itself every 24 hours is also rhythmic in a 48-hour scale (that is, it repeats itself exactly twice every 48 hours).

10. Now change the Target Periodicity to 24 hours and click on Execute again. This time, the program shows that there is significant periodicity but that no harmonics were detected. The true periodicity is 24 hours (as inspection of the actogram clearly suggested).

11. Keep in mind that the program looked only for broad periodicity. When Rhythm indicates that 24-hour periodicity exists in data set A04, it is actually indicating that periodicity occurs between about 23 and 25 hours. The exercises in Chapter 5 deal with programs designed to determine the exact period of a circadian rhythm.

0 0

Responses

  • ANGELICA
    What do u mean by detecting rhythmicity?
    1 month ago

Post a comment