Exercise 6.257 on page 424 introduces a study showing that exercise appears to offer some resiliency against stress, and Exercise 8.18 on page 507 follows up on this introduction. In the study, mice were randomly assigned to live in an enriched environment (EE), a standard environment (SE), or an impoverished environment (IE) for several weeks. Only the enriched environment provided opportunities for exercise. Half the mice then remained in their home cage (HC) as control groups while half were subjected to stress (SD). The researchers were interested in how resilient the mice were in recovering from the stress. One measure of mouse anxiety is amount of time hiding in a dark compartment, with mice who are more anxious spending more time in darkness. The amount of time (in seconds) spent in darkness during one trial is recorded for all the mice and the means and the results of the ANOVA analysis are shown. There are eight mice in each of the six groups.

(a) Is there a difference between the groups in the amount of time spent in darkness? Between which two groups are we most likely to find a difference in mean time spent in darkness? Between which two groups are we least likely to find a difference?
(b) By looking at the six means, where do you think the differences are likely to lie?
(c) Test to see if there is a difference in mean time spent in darkness between the IE:HC group and the EE:SD group (that is, impoverished but not stressed vs enriched but stressed)

Exercise 6.257

Many studies have shown that people who engage in any exercise have improved mental health over those that never exercise. In particular, even a small amount of exercise seems to confer some resilience to stress. Most of these studies, by necessity, have been observational studies. A recent experiment with mice moves us one step closer to determining a causal association. In the study, mice were randomly assigned to either an enriched environment (EE) where there was an exercise wheel available or a standard environment (SE) with no exercise options. After three weeks in the specified environment, for five minutes a day for two weeks, the mice were each exposed to a €˜€˜mouse bully€€”a mouse that was very strong, aggressive, and territorial. At the end of the two weeks, the mice in the SE group exhibited maladaptive, depressive-like, and anxietylike behavior across a wide spectrum of activities. This was not true of the mice in the EE group; they behaved similarly to mice that had never had the stress-inducing bully experience. In particular, one measure of mouse anxiety is amount of time hiding in a dark compartment, with mice that are more anxious spending more time in darkness. The amount of time spent in darkness during one trial is recorded for all the mice and is shown in Table 6.22 and available in StressedMice. Test to see if mice that have spent time in an enriched environment with options for exercise spend significantly less time in darkness after a stress-inducing experience.

Table 6.22

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Group: IE:HC SE:HC EE:НС IE:SD Mean: SE:SD EE:SD 438 192 196 205 392 231 Source DF Light Error MS 5 481776 96355.2 39.0 0.000 2469.9 SS 42 177835 Total 47 659611 Environment Time in Darkness (seconds) 203 391 280 138 227 Enriched 359 184 231 Standard 394 477 439 428 488 454