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This week we see why

Mama's Boys are Braver


A mother's presence, a new study suggests, can go a long way toward making the world less frightening. Photograph by Socar Myles, used by permission.





by David Dobbs, Editor, Mind Matters

The child who seems to prefer having Mom around during the journay through youthdom is a sort of icon of caution and hesitance -- the Mama's boy. The study under review here, however, suggests there may be a critical stage of childhood or adolescence in which Mom's presence can make the difference between learning to approach the world fearfully and learning to approach it confidently. The study, "Maternal Presence Serves as a Switch Between Learning Fear and Attraction in Infancy" (by Stephanie Moriceau and Regina Sullivan, in Nature Neuroscience, 9 July 2006), discovered a fascinating period of rat-pup development in which maternal presence acted, as the paper's title suggests, like a switch between fear-learning circuits and preference-learning circuits in certain situations. Our reviewer, Marc G. Berman, explains this clever study and explores questions from whether a mother's odor might mask the smell of peppermint to whether the dynamics revealed in this study might explain why some people stay in abusive relationships.



Between Fear and Attraction, a Mother's Presence

by Marc G. Berman University of Michigan
Cognitive Neuroscience Laboratory

In their recent Nature Neuroscience article, researchers Stephanie Moriceau and Regina Sullivan explore learned olfactory preference and aversion as mediated by maternal presence. In this study rat pups were exposed to a peppermint odor that was paired with a light (0.5 milliamp) shock -- a classic conditioning exercise that taught the pups to associate the odor with the shock. It has long been established that a mature rat exposed to such conditioning will learn to avoid the associated odor. It is also well established -- but less well known -- that an extremely young rat pup (less than 8 day old) exposed so such conditioning will actually become attracted to the odor. This study by Moriceau and Sullivan, however, discovered that there is a period during rat youth during which the reaction to the odor-shock conditioning -- that is, whether the rat learns to prefer or avoid the odor -- depends on whether the pup's mother is present during the conditioning. It's a fascinating finding that raises intriuging questions about human behavior. A Path from Young and Innocent to Old and Wise First some background. Previous research by Sullivan and others had revealed that young pups (eight-day-olds) still in the nest show "paradoxical learning" -- that is, they are attracted to odors that are paired with light shocks. This goes along with a general tendency of rat pups at this age to show enhanced preference learning, even to aversive stimulation, and attenuated aversion learning. They easily learn to like something, even if that something is not pleasant, and don't readily learn to fear. This does not reflect either a higher pain tolerance or an inability to feel pain. Rather it seems to reflect an underdeveloped amygdala (a part of the brain essential to learning conditioned fear), which in pups this young fails to be engaged or activated in odor-shock or other fear-conditioning scenarios. Moriceau and Sullivan refer to this time period in a pup's life, when pups cannot yet walk and must stay near the nest, as the sensitive period. It ends around day 10, when the pups begin to walk and move around outside the nest. Until then, the mother's presence or absence has no effect on the pup's reaction to odor-shock conditioning: whether mom is there or not, the pup will show preference for a smell associated with a shock. During the weaning age (21 to 23 days old), when pups grow more independent and spend less time in the nest, they will begin to show classical aversive conditioning, learning to avoid the odor that is paired with the shock. One they are that old, maternal presence fails to affect their reaction to odor-shock conditioning: whether she's there or not, they quickly learn to avoid a smell associated with a shock. A Brief Window in Which Mother's Presence is Critical Moriceau and Sullivan, however, found in this study that there is a period between the sensitive and weaning periods -- the "post-sensitive" period, when pups are 12 to 15 days old -- when the mother's presence makes the difference between learning attraction and learning aversion. At this age, the pup will come to prefer an odor paired with a shock if the mother is present but will learn to avoid an odor paired with a shock if the mother is absent. In addition to these behavioral effects, the authors also found the neural mediators of those effects. In post-sensitive-aged pups whose mothers were absent, the authors found increased amygdala activation (as assessed by a measure of glucose uptake known as the 2-DG method, reflecting the learning of a new fear. Those pups alos showed higher corticosterone levels, indicating stress. Pups who were conditioned with their mothers, however, showed increased olfactory bulb activation, reflecting the learning of a new odor preference. To try to verify the amygdala's role in this learning, the authors attempted to condition some pups after temporarily inihibiting activation of the pups' amygdalas. (They did this with muscimol, which triggers the inhibitory neurotransmitter GABA, which thus quells the amygdala's normal reactions.) When they did so, the pups would learn to prefer the odor even if conditioned without their mother present. On the other hand, when the authors injected corticosterone 30 minutes before conditioning (to increase corticosterone levels, inducing a heightened state of stress), pups learned to avoid the odor even in the presence of their mother. These experiments data dramatically indicate the power of a mother's presence in providing stress relief and in affecting fear conditioning in postsensitive age pups -- making the difference, in fact, between learned preference or and learned avoidance. Implications Fascinating and Grim This implication-rich findings. Pups of this age are in a transitional period where they are moving away from dependence on their mother to independence --- perhaps akin to a human teenager. As the authors write, "The effects of maternal presence on odor-pain conditioning may ensure that pups continue to only learn approach response to her odors, whereas in her absence they learn complex contingencies required for survival outside the nest." Does the criticality of the mother's presence help the pup learn about the world while still allowing it to prefer the mother's presence? That's one of many interesting questions this study raises. I would love to know, for instance, if this effect occurs only if the adult rat who is present is the mother, or if the presence of any adult rat would yield similar effects. Is there something special about a mother that makes an environment feel safer, or does the sheer presence of any other adult individual make the environment seem safer? Would the other individual even have to be an adult? I wonder too if this research is confounded with a young pup's inability to distinguish a single odor from other odors in the environment. For example, perhaps a pup can distinguish a peppermint odor well when other odors are not present but might have difficulty distinguishing the peppermint odor when the mother's odor is present. As the rat olfactory system gets more sophisticated with age, the rat might be able to decipher the smell of the peppermint odor equally well with the mother present or not (pups in the weaning stage). So it could be that the presence of the mother does not really lower stress levels in post-sensitive pups, but simply makes the peppermint odor less salient and more difficult to learn as a predictor. Arguing against this idea, however, is the fact that corticosterone levels were lowered when the mother was present -- a strong indication that the mother's presence does, in fact, reduce stress levels in the pups. Regardless, I would have liked to see some mention of the development of the olfactory system in rat pups to put some of these other worries to rest. Relation to Humans These data clearly bear some interesting possible relations to human behavior. For example, could these findings help explain why some people stay in abusive relationships, learning to prefer experiences that one would expect to be aversive? Might levels of cortisol (the human equivalent of corticosterone) help determine whether people remain in such relationships? In a somewhat related 2003 study, psychiatrist Carl E. Schwartz and colleagues found that human toddlers who were identified as inhibited grew up to be adults who showed increased amygdala activity for novel faces versus familiar faces compared with adults who were uninhibited toddlers. These data suggested that neural differences that contribute to personality and preferences might persist from childhood into adulthood. When viewed in light of the Moriceau and Sullivan study, such findings suggest that some learned preferences may be preserved throughout childhood and into adulthood, possibly causing some people to prefer environments or situations that are quite harmful.

Marc G. Berman is a doctoral student in cognitive psychology and industrial and operations engineering at the University of Michigan.



-- Edited by David Dobbs at 01/02/2008 8:39 AM