FEMALE rats can normally be conditioned more easily than their male siblings,
but after stressful events females do worse and males do better, say researchers
in New Jersey. They expect to get the same results with humans.
Excessive stress has been shown to impair normal learning in both humans and
animals. In a specific type of learning test, however, in which animals are
conditioned to expect a puff of air to the eye after hearing a tone and blink
automatically, male rats鈥� performance improved after tail shocks. 鈥淏ut no one
had looked at females,鈥� says Tracey Shors, an experimental psychologist at
Princeton University.
With her colleague Gwendolyn Wood, Shors performed the eye-blink test on 44
male and female rats. She exposed some of them to stress beforehand, either by
mildly shocking their tails or making them swim for 20 minutes. A day later,
after conditioning the rats to expect the puff of air after a tone, she tallied
the number of times each rat blinked in response to the tone when no air puff
followed.
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Unstressed males blinked after the tone about half the time and, as expected,
stressed males were more successful, blinking around 80 per cent of the time.
But surprisingly, females showed the opposite response. They learnt better
without being stressed, blinking 75 per cent of the time鈥攎uch better than
the unstressed males. Stressed females reacted to only 30 per cent of the tones
(Proceedings of the National Academy of Sciences, vol 95, p 4066).
Shors and Wood repeated the experiment with females whose ovaries had been
removed. These females did not suffer from the impairment, leading the
researchers to suspect that the hormone oestrogen鈥攚hose levels rise after
stress鈥攎ay play a key role. To confirm this, they gave females with
ovaries a substance that blocks oestrogen receptors and got the same
result鈥攕tress no longer impaired the female rats鈥� ability to learn.
Oestrogen does not always hurt learning or memory, says Shors. It reduces
memory deficits in Alzheimer鈥檚 patients and can slightly improve the memories of
postmenopausal women. Shors thinks the precise level is what matters. Learning
improves as oestrogen levels rise, but only to a point鈥攖oo much is as bad
as too little.
Shors is now testing human subjects for sex-linked differences in learning
after stress. Although individual differences and higher cognitive powers make
the human case more complex, Shors expects the basic result to be the same. The
eye-blink test may not seem like a high form of learning, but she says that
association of events is fundamental to cognition. 鈥淲e use it every second of
every day,鈥� she says.
鈥淪he has uncovered something pretty interesting,鈥� says Bruce McEwen, a human
cognition expert at Rockefeller University in Manhattan. 鈥淚 expect that there
will be some correlates in human behaviour if we start looking.鈥�