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Even larger sex-differences tend to emerge when facing more difficult operant conditioning tasks. A two–way avoidance task was used in Dalla’s study (2008) in which rats had to pass though the doorway twice to avoid shock (controlled stress). During training, rats were coupled to another rat that was shocked regardless of completion of the task (uncontrolled stress). When males were exposed to the new, more difficult task, most males trained under uncontrollable stress took longer to learn to escape, and thus exhibited learned helplessness. In some cases, males would not return to the place they received their first shock and thus could not pass through the doorway twice and never learned to avoid the shock. In contrast, females were able to learn to escape the novel task relatively quickly, regardless of the prior type of stress they were exposed to (Dalla, 2008).
Studies have also shown that females will retain escape information longer than males. Van Haaren (1990) showed that females placed in the shock chamber without any shock presentation escape sooner than males. This suggests that extinction of the learned behavior is slower to occur in females.
Sex differences in operant conditioning are at least partially related to differences in performance. Averse stimulation caused female rats to respond actively while male rats typically froze in place which inhibited escape (Dalla, 2009). Females also are generally more active (Hyde, 1983) and more sensitive to shock (Van Haaren, 1990) than males, which may result in their passing through the doorway before males and an increased incentive to figure out how to escape the shock.
Barret et. al. (2009) examined the effect of age on spatial learning for female Sprague-Dawley (an outbred strain) and male and female Dark Agouti (an inbred strain) rats. Rats at 6, 11, 14, 17, 20, and 26 months of age were tested for performance. The trials consisted of 3 trials/day for 10 days and performance was based on the ability to reach criterion, which was classified as less than 2.5 errors/trial average on any single day of testing.
Barnes maze vs. MWM:
The MWM has been criticized because rats are submerged in water causing high stress levels and it requires a high level of physical fitness, which is detrimental to aging rat performance.
Instead, the Barnes maze places subjects on an open, unprotected circular platform where they have to find the correct escape hole on the edge of the platform (see photo below). Since the hole is relatively easy to find, it is important that un-trained subjects cannot see previous trials and that the platform is thoroughly cleaned after each trial to avoid visual cues on the platform that would alert the subject to the correct escape hole.
Age and Performance:
At six months, all rats reached the learning criterion. For AD males, performance impairment began at 11 months. DA females experienced performance impairment at 14 months, and SD females experienced impairment at 17 months. The oldest groups was tested at 26 months, and at this age only 70% of SD females, 33% of DA females, and 57% of DA males reached criterion.
In all groups, the percentage of non-learners increased with age, but substantial strain and gender differences were present, highlighting the potential error of comparing age data for different groups (Barret, 2009).
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