The Reverberations of Fear
Memories of fearful experiences can straddle the generations.
Posted Sep 19, 2014
Fear is the most elemental emotion. A threat to self-preservation triggers a panoply of sympathetic nervous system activities: our senses go on ‘red alert,’ our pupils dilate, muscles are tensed, and respiration is increased. The body prepares to fight, flee, or freeze. (The latter is not as well known a response but is amply illustrated in the animal kingdom, e.g., “playing possum.”)
The part of the brain instigating all this activity is the hypothalamus. In moments of anxiety or threat it prompts the adrenal glands to release the hormones adrenaline and noradrenaline. The hypothalamus also stimulates the pituitary gland to release a hormone called ACTH, which, carried to the adrenal glands, causes the release of still other hormones. One of these is cortisol, which serves to marshal bodily energy (and also has an effect on the immune system over time). This entire stress response system is known, in shorthand, as the hypothalamic-pituitary-adrenal (HPA) axis.
You’ll see why the HPA axis is germane to sensitivity in just a bit. For now, let’s consider how extreme fear registers not just in a given animal but can actually be transmitted to its offspring – and even to its offspring’s offspring.
The study was conducted by a team at Emory University, and it’s set off shock waves throughout the behavioral science community. In the first phase, mice were trained to be afraid of acetophenone, a fruity smell that’s used in cherry, jasmine, honeysuckle and almond flavorings. The researchers paired this fragrance with an electric shock, so that the scent soon became a warning signal. Their noses adapted accordingly, generating more of a particular kind of neuron keyed to the smell, and so did their brains, which grew an expanded recognition area for it. So far, all of this is basic Pavlovian conditioning and neural adaptation.
The offspring of these mice, however – who had never before been exposed to the smell – also showed increased fear and startle responses to it. Somehow the learned association was transmitted from one generation to the next. The brains of these offspring also had more of those same neurons, so their noses were more sensitive to the fruity smell. Amazingly, even the third generation of mice was similarly affected.
The researchers took care to verify that the results did not stem from the mouse pups learning anxiety by modeling it after an anxious parent. The mice showed no fear reaction to other scents or other types of warnings. To confirm this, the scientists took sperm from the first set of mice, implanted it in females from another lab, raised them in isolation, and still found an increased sensitivity to the original scent.
Identical results have been affirmed by other researchers in Switzerland and Israel. That such changes happen over the span of a single generation indicates that random DNA mutations are not involved, since such Darwinian adaptation takes place slowly, over many generations. Epigenetics, a relatively new concept in which environmental influences alter gene expression, seems to be at work.
If the same findings hold for human beings, we’re left with the amazing implication that fearful experiences can make a man’s children – and even his children’s children – more sensitive to the same stimuli that evoked fear in him. His progeny may also be more highly reactive in general, given studies linking trauma with an intensified startle reflex.
If this is the case for fathers, imagine what effect a pregnant mother’s fearful experiences might have on the child she is carrying. After all, she and her child are "locked in the closest of biological embraces" via the placenta. (Nathanielsz, Peter W. Life in the Womb: The Origin of Health and Disease. Ithica, New York: Promethean Press, 1999, 24.) (By the by: as an indicator of just how keenly children are primed to register maternal fears, a new study demonstrates that mouse pups quickly learn to fear that which their mothers came to fear even before they were pregnant. The fearful response is conveyed through an ‘alarm odor’ given off by the mother.
To return to the HPA stress response, it’s entirely possible that a pregnant mother’s fearful reactions – especially if chronic – could effectively ‘program’ her child to be anxious, high reacting, and environmentally sensitive. Alternatively (as argued in a previous post), it’s conceivable that any number of things affecting a mom during pregnancy – infection, injury, an autoimmune illness, general deprivation, exposure to environmental toxins – could cause her child’s brain to become hyper-connected, setting the stage for pronounced sensitivities. The conjunction of neural connectivity, fear, and sensitivity has been clearly demonstrated in rats with overly connected neural networks. Although they learn faster than normal rats, they’re also more anxious, showing aberrant social behavior and increased repetitive behaviors (much like humans with an Autism Spectrum Disorder). Furthermore, while ordinary rats become scared of an electrified grid when a shock is paired with a particular sound, hyper-connected rats come to fear not just the sound, but the whole grid and everything connected with it – including colors and smells.
It’s worth noting here that Kai, the son of autism researcher Henry Markram (who we met earlier), displayed a veritable terror of trying new foods when he was young. He can recall, to this day, exactly where he was sitting at a certain restaurant when he once tried to force himself to eat a salad. Similarly, a researcher who worked with highly gifted children observed that several of them “were terrified of germs and would visualize them flowing through their bodies, destroying their health.”
Just as most people can vividly remember where they were and what they were doing when the World Trade Center was attacked on September 11, 2001, I propose that fear – that most primal emotion – can ‘imprint’ certain people with a virtual snapshot of what was taking place around them when something traumatic occurred. Their brains and their bodies may be especially sensitive to begin with or, as suggested by the Emory study on fear imprinting in mice, their parents or grandparents may have had a particularly fearful experience.
Child prodigies manifest this process to a great degree. In my next post, I’ll relate some especially stunning examples of what they remember.